Download - The Complete Acromag Guide to Industrial Electrical Grounding

Acromag, Incorporated

30765 S Wixom Rd, Wixom, MI 48393 USA

Tel: 248-295-0880 • Fax: 248-624-9234 • www.acromag.com

Copyright © Acromag, Inc. April 2014 8500-993b

White Paper: Electrical Ground Rules Best Practices for Grounding Your Electrical Equipment

A look at circuit grounding and its importance to you, as well as the US AC power system and its use of earth ground (Part 1 of 3)

Tel: 248-295-0880 Fax: 248-624-1541 [email protected] www.acromag.com

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This paper is part one of a three part series that takes a look at grounding and its role in protecting personnel, protecting equipment, and ensuring the integrity of electrical signals. In this part, we will review circuit grounding and its importance to you, as well as the US AC power system and its use of earth ground.

Part two of this series will look at Ground as a means of protection, ground faults, and the operation of the ground fault circuit interrupter (GFCI).

Part three of this series will review ground and its role as a voltage stabilizer and transient limiter. It will offer some tips on what you can do to improve your connection to ground to realize benefits to safety and signal integrity.

BACKGROUND When wiring or connecting circuits, electrical equipment, and electrical instruments, there is a connection that you probably don’t give much thought to, and one that consequently reigns as one of the greatest sources of instrument error and malfunction. The connection I am referring to is your connection to Ground. For the purpose of this discussion, unless otherwise specified, the term “ground” will refer to a connection to earth ground, or an extension of a connection leading to earth ground. In some circuits, ground may also refer to a connection to the chassis or frame of the device, which is sometimes used in the absence of a connection to earth ground as a convenient reference point for signal measurement.

Electrical systems must be grounded in order to work properly. The earth often serves as an ideal ground because of its large mass and ability to absorb charge, but ground can be any electrical connection that is able to freely conduct electricity, and grounding a circuit does not always refer to making a physical connection to earth ground. For example, airplanes will connect their conductive metal shells and frame to ground for all the electronic components inside the aircraft, both generated AC and DC. The aircraft is only actually earth grounded when it’s on the ground to drain static build-up and during refueling. Parts of an aircraft that are made of plastics or other composite materials are often protected with a metal mesh or fibers to minimize charge buildup. Even some aircraft tires will have a copper mesh molded into the nose-wheel or other conductive material molded into the tire rubber to contact earth and dissipate charge. Helicopters also pick up large amounts of static charge and they often employ an earth-contact lead to contact the ground or water to dissipate charge before contacting personnel, perhaps during an air or water rescue. Likewise automobiles can’t pass electrical charge to earth through their insulating rubber tires.

So ground in a car or airplane may be inclusive of its chassis and this is sometimes used as a return path for current, back to the negative terminal of its battery. The battery current flows from its positive terminal through the devices (lights, radio, etc.), then out of the devices to the negative terminal of the battery, which is usually connected in common to the chassis of the vehicle. It is generally not good practice to use the chassis as a return path for load current, as this really isn't a safe practice in most cases, and for vehicles, it would be a source of noise for any devices that happen to make their ground connection via the chassis or frame (instead, the chassis should only connect to ground at one point).

For AC powered devices, earth ground is never used as a return path for load current, except in the case of a ground fault. That is, with modern 3-wire AC powered devices, ground is usually a third conductor to the device that is held in reserve for special situations (ground faults) and it does not normally carry load current (serving to keep its potential the same along it, or “equipotential”). For these devices, AC


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neutral is the wire used to conduct charge back to the source and complete the circuit during normal operation.

For applications where power to the device is from an isolated source, perhaps via an isolated AC-to-DC converter, a direct path to earth ground may not be present for the powered circuit. Many times, the isolated output power supply will connect earth ground at its output DC minus lead, and it may include a hidden path to earth ground on its AC power side via an isolation capacitor to output DC minus. In cases where an earth ground connection is not evident, ground is usually chosen as the common return path from the power supply to the equipment (isolated output DC minus), and in some applications, it may even be necessary to hard-wire a path to earth ground at the DC minus output of the isolated power supply powering your equipment.

WHAT IS GROUND? When you first learned about ground, you may have been told something like “…ground in a circuit is a reference point of 0V”. While that might represent an “ideal” ground, it is very misleading in practice. It is true that ground in a circuit can refer to a reference point from which all voltages are measured, and for the purpose of simplifying your measurements, it’s convenient to think of this ground as having a potential of 0V. This certainly does simplify your voltage measurement and analysis. But the reality is that ground could be any potential. And unfortunately, the simple fact is that all conductors have some impedance, including the conductors used for ground. Once you start pumping charge through any conductor, it’s going to look very different than a perfect conductor at 0V.

For example, as ground conducts current, a voltage difference will develop across it, forcing each connection to it to occur at a slightly different potential. So in practice, if all connections to ground were really made at 0V, we wouldn’t have ground faults or ground loops. And therein lies the problem with ground—your ability to mimic its idealized behavior as a reference connection to 0V really depends on how well you can minimize its effective impedance, chiefly its resistance and inductance. Generally at 50-60Hz power line frequencies, the resistive component of your connection to earth is more significant than the reactive component (inductance). At higher frequencies, the reactive component gains significance, as the inductance of your connection to ground raises its impedance to transient energy. The bottom line is that you need to minimize the effect both components have on raising ground impedance by keeping both the resistance and inductance of your connection to ground to a minimum.

THE IMPORTANCE OF GROUND A “ground” connection to local earth is normally provided for your AC electrical system and for the equipment owned by the utility. This ground connection must be of low impedance, or with regard to the generally lower frequencies of AC power, its resistance must be low. For example, the US National Electric Code (NEC) specifies an acceptable limit for ground impedance is 25Ω. IEEE Standard 142 recommends a resistance between 1 and 5Ω for the connection between a systems main ground node and earth for commercial and industrial power systems. Your utility will typically target 6-8 ohms for the pole connection to ground. Soil conditions vary widely and will greatly impact ground impedance. In any case, for the protection of yourself and any powered equipment, you want this resistance to be as low as possible, and most electricians will shoot for a measured ground impedance less than 5Ω, typically 2-3Ω. In fact, troubleshooting problematic power systems usually starts with measuring the impedance of the system’s connection to earth ground using special equipment, like a Fall-of-Potential Tester. In some cases, the remedy might involve bonding additional ground stakes to the system ground.

http://en.wikipedia.org/wiki/Ground_(electricity)


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The principal purpose of connecting your system to earth ground is:

To stabilize the voltage to earth during normal operation (think of earth like an anchor to the system voltage).

To limit the voltage rise created by lightning, line surges, and unintentional contact with higher voltages.

For example, a low-impedance connection to earth will limit the voltage that develops if high voltage conductors fall down onto lower-voltage conductors, which are usually mounted lower to the ground in modern power distribution systems, and this helps to minimize the potential shock to any powered equipment. This low impedance path to earth also helps if a failure occurs within the utility’s distribution transformers. When all conductive objects are bonded to the same earth ground system with low impedance, the risk of electric shock is minimized, as the voltage is absorbed by the earth and its energy (charge) is dissipated in its large mass.

However, in practice, there are multiple connections between the utility ground and the ground of system powered equipment that can still lead to “stray voltage” problems. System piping, swimming pools, and other equipment can still develop harmful voltages that can destroy the equipment, or put human life at risk for electric shock. Often these problems can be difficult to resolve, as they may originate from places other than the system premises.

In wiring instruments and electrical equipment, we can derive three main purposes for correctly applying Ground:

We connect to Ground to provide an alternative path for fault current to flow (safety reason).

We connect signals to ground to stabilize them & keep them from floating (to limit the voltage and its variation).

We connect to Ground to limit the voltage-rise induced on powered circuits, typically via lightning, line surges, or unintentional contact with higher-voltages (limiting the induced voltage magnitude).

To this I would add another very important side-benefit to providing a good connection to earth ground:

We ground our circuits to gain EMC benefits that result in lower noise and radiated emissions.

AC POWERED SYSTEMS To really understand ground, it’s important to have a basic understanding of the modern AC power system, as this is where your connection to earth ground is usually established. For example, consider AC power wired to your home. This power is transmitted from your utility over very long distances, using very high voltages to help minimize losses. It is then stepped down using a series of transformers to lower voltage levels before it connects to your home or business.

For power connection right to your home, your utility usually provides three large diameter wires coming from your utility pole to the service entrance of your house—these include two heavily insulated wires from the transformer (two line phases), and a third wire that serves as an earth grounded AC neutral wire. The most common residential service in North America is 240V single-phase (also called 240V split-phase), in which one phase of a higher voltage is stepped down to 240V. This 240V phase winding is center-tapped, which is then grounded and becomes the AC neutral conductor. One line of these windings to neutral powers your 120V loads, and line-to-line they power your 240V loads. The waveforms of each 120V line-to-neutral are offset by one-half cycle, 180 degrees out of phase, although

http://en.wikipedia.org/wiki/Stray_voltage


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they originate from a single phase 240V secondary. This allows you to power both 120-volt loads in your home (lamp, toaster, etc.) and 240-volt appliances (electric stove, air conditioner, etc.). The two heavily insulated wires typically pass through a watt-meter near your service entrance used to measure your consumption of power. The watt-meter also establishes your premise’s connection to earth (from the third utility wire with additional ground stake(s) near the meter). From the meter, two thick wires connect to the service panel, along with a neutral wire (neutral has been bonded to earth ground at your utility pole and one or more earth ground stakes near your meter). The inside of the service panel to your home or business may resemble the following:

Note the three thick insulated wires that connect to your breaker panel. Two thick insulated wires connect to the breakers of your service panel through a lower “mains” panel breaker (the big switch at the top). A third thick wire is the AC neutral wire (the thick white banded cable in the example photo at left). The mains breaker typically consists of two circuit breaker handles joined together as a Double-Pole Service Disconnect. The mains breaker is the switch that controls the utility power from energizing the individual circuit breakers of the service panel. The mains breaker also identifies the amperage capacity of your electrical panel, and will have a number on it like 60, 100, 150, 200, or 400, signifying its total amp capacity. Each circuit of your home or business branches off from the service panel through smaller capacity individual circuit breakers, which will cut off electricity to their branch circuits in the event of an overload.

The two thick hot service wires feeding the mains breaker each carry the 120 volts from the electric meter to two "Hot" electrical bus bars in the panel. Individual breakers contact one or two of

these bus bars, according to the voltage of their branch circuit. Connections to these hot bus bars are made in such a way as to balance the load with half the circuits on split phase and the other half on the other split phase.

This is why you may experience some power failures for only a portion of the powered devices in your home--when only one of your two phases loses power. A circuit breaker will connect to one bus bar for 120 volts (a single-pole breaker), or both bus bars for 240 volts (a double-pole breaker). The individual breakers then feed their loads via a black insulated “hot” wire that connects to your power outlet, and energizes the connected electrical device (light bulb, motor, etc.). The circuit returns back to the panel through a white insulated neutral wire that connects to the Neutral bus bar of the service panel, which also connects to all the individual white neutral wires from all the branch circuits and ties them in common.

The neutral bar connects to the main circuit neutral wire from the power meter and returns the current back to the electric utility from each of the branch circuits. Likewise, the bare copper ground wires of


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each branch circuit (assuming that your home or business is wired using 3-wire cable) similarly connect to a ground bus bar in the panel which also connects in common to the neutral bar. The grounding bus bar may be part of the Neutral bar, or separate from it, and collects all the ground wires from the various branch circuits and ties them back to the Neutral bar. This is an important point—that ground is connected in common with neutral at the service entrance.

The neutral/ground connection also connects to one or more earth ground rods driven into the ground near the service meter similar to the Australian example grounding rod shown at left. The ground circuit may additionally connect to the cold water pipes that service the building. This is how each branch circuit gets its connection to earth ground (the AC neutral coming from the utility pole is also earth grounded near the pole).

Do not be fooled into thinking that by grounding the water pipes to your premises, you are somehow protected from electrocution if an AC appliance were to fall into your bath tub while

you were bathing. Consider that many homes utilize plastic piping for portions of their plumbing, making this an unreliable connection to ground. The reality is that if there is no path to earth ground or neutral completed by your body, no electrocution can occur. In this “hot tub” scenario, the water would likely cause a short between the AC hot wire of the appliance and AC neutral and excess current would flow and trip the breaker that connects to that circuit. However, if the pipes to the tub were metallic and also earth grounded, and you happened to be touching the faucet or drain while in the water, there is the possibility that you could complete the hot-neutral path for at least a portion of the fault current and be electrocuted. If the appliance were connected to a GCFI outlet, then this could protect you from electrocution (more on this in Part 2 of this series).

Each circuit of your house includes a circuit breaker that connects in series to the black hot lead of your AC wiring. The circuit breaker of each branch hot lead is designed to fail safely. Without a breaker, when a circuit draws more current than it is designed to handle, the wiring will get hot and could start a fire. The breaker curbs this potential for fire by preventing excess current from flowing. It does not protect you from potential electrocution. A Single-Pole Breaker provides 120 volts to the branch circuit and typically comes in ratings of 15 amps (for household lighting) to 20 amps (household outlets). Double-Pole Breakers provide 240 volts

to the branch circuit and come in ratings from 15 amps to 50 amps. Double-pole breakers like the example shown at left usually serve circuits dedicated to a single load, such as large appliances, electric dryers, stoves, or air conditioners.


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Electricity must have a complete conductive path to flow. When you look at an AC power outlet, you generally see two vertical slots and a rounded hole between them. The larger vertical slot is AC neutral, and it usually connects to your service entrance via a white insulated wire from your service panel. The smaller vertical slot is AC hot, and it connects to your service entrance via a black insulated wire. The rounded hole between the slots is AC or safety ground and it connects back to your service entrance via a bare copper or green insulated wire. Note that AC neutral and AC ground are separate conductors at your outlet/load, but are actually bonded together at your service entrance.

In the circuit wiring to your load device, you would measure a voltage between AC hot and AC neutral and both lines will carry current to/from your load. AC neutral carries the load current back to your service panel during normal operation, but AC ground does not normally return load current. AC ground only returns current for fault conditions. Thus, you would not measure much voltage between AC neutral and AC ground, as only a small IR voltage drop would exist as a result of AC neutral conducting load current through its resistance. In practice, bonding AC neutral to ground at your service entrance is what allows a circuit breaker to de-energize the entire circuit by simply interrupting the AC hot wire connection upon an overload condition. If AC neutral was not bonded to earth ground, you would have still have a potential for shock by simply changing a light bulb, even with the light switch off, as a path to neutral remains with the switch OFF. Opening the AC hot line via the light switch allows all the conductive wires and fixture metal past the switch to rest at AC neutral, which is “anchored” to the potential of earth ground (nearly 0V), making these conductors safe to handle with the light switch OFF.

This is an important point, that AC neutral is the return current wire of a loaded circuit and it connects to ground at only one point—at the service panel or breaker box. AC ground is a safety wire that normally carries no current. The AC ground wire generally makes contact to the appliance shell or chassis, and may also be connected to other grounds in the facility, such as water or gas piping. If a hot wire to the appliance (AC hot or neutral) breaks and makes contact with a grounded metal chassis, then load current would travel in the AC ground wire back to its source at the service panel (a ground fault). In the American 3-wire system, the white AC neutral wire is held in common to earth ground for 115V power wiring, and also held at earth ground for 230V wired loads (driers, stoves, air conditioners, etc.). The power company supplies 115V on a black insulated wire, 180 degrees out of phase with 115V on the


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other hot wire (typically a red-insulated wire), so that a voltage between them measures 230 volts. Note that this hot-neutral voltage is nominal and appliances generally work from 110V to 125 VAC, such that the same voltage is sometimes referred to as 110V, 115V, 120V, or 125V.

To reiterate, the AC neutral wire is a return wire for the load current in an electrical circuit. AC neutral does connect to AC ground at the service entrance. But do not confuse neutral with the ground wire, which is also a return wire, but only returns current in the event the connected appliance shorts out, and in this way acts to protect the user from electrical shock (an inadvertent hot connection to ground will cause the load current to quickly rise and trip the hot breaker, shutting off current flow). In the wiring to your home or business, the neutral comes from the power plant via the utility pole and is also earth grounded at the utility pole, AC ground comes from a ground rod, typically below your power meter. In many older homes, the ground and neutral were connected to the same bus bar in the breaker box. But in newer homes, they have their own separate bus bars, but these bars actually connect in common at some point at your service entrance.

An interesting thing about AC neutral, is that if you were to test a live circuit using a static charge meter to measure charge, neutral would not show any charge accumulation (it is charge “neutral”), while the hot wire would show charge present. If you had this circuit controlling some device (maybe a light fixture) and the light fixture was in the ON position, and you cut neutral open, you would notice the two neutral wires would spark when you touch them together, suggesting that charge is present. Likewise, if you were to complete this neutral circuit, perhaps with your body, you would also get shocked, maybe even electrocuted. But if the device’s switch (perhaps the light switch) is in the OFF position, you would be safe. This is because the neutral circuit is essentially “anchored” to ground (nearly 0V). Still, never take chances when working with electrical equipment and always turn power OFF before working on its circuitry.

AC Power Outside the US: We have been focused on AC power systems of the US, where American appliances are generally powered from 115V, 50-60Hz. You should note that European outlets deliver 230V at 50Hz. Many modern AC appliances in the US will still be compatible with both voltages, and may run from 115-230V, or will include a switch to optionally power them from 230V. Still, European outlets do not accommodate the two flat prongs of the US plugs and will require an adapter to connect to power. For example, British and Irish outlets use three rectangular prongs, and Continental European outlets utilize two round plugs. Likewise, if your appliance does not accommodate 230V AC power, you will need a voltage converter with an adapter, otherwise the 230V, 50Hz European power outlet would likely reduce your 120V AC appliance to junk very quickly, and could even start a fire. Other countries have other AC standards and these should be checked if travelling abroad with US appliances (some example outlets and plugs are shown below).


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THE GROUND LOOP In an electric circuit, potential difference or voltage is the force that drives current flow. A ground loop refers to unwanted current flow that occurs when a circuit is grounded at more than one potential. Specifically, it is the unwanted signal pickup that results from a shared path to ground. In a way, you could say that a ground loop results when you have too much ground (specifically more than one connection point to ground). In any circuit that covers some distance, the chance that more than one ground point could be made is very high, as all conductors have some resistance, and current flowing through this resistance will always produce a voltage difference along that conductor. The important thing to remember about ground loops is that they usually result in unwanted noise and interference, and also drive measurement error. Severe ground loops can even create the potential for electric shock. There are only two remedies for combating the negative effects of ground loops: signal isolation and/or the use of star grounding in wiring multiple ground paths (more on this later). Usually, ground loops are created accidentally and extra connections to earth ground are not always obvious. For example, devices that connect to a USB port of a personal computer will make a connection to earth ground through the computer, which has earth ground of its power plug connected in common to its chassis and to the USB signal and shield ground. Likewise, double-shielded Ethernet Cable may also connect to earth ground at the network interface of a personal computer. You may inadvertently connect earth ground at more than one point in your circuit via a grounded scope probe.

Ground loops are covered in more detail in another Acromag whitepaper, The Importance of Isolation (8500-988). You can download this and other information at www.acromag.com.

In Part 2 of this series (see document 8501-020), we will look at the aspects of earth grounding as a means of protection from shock and fire via ground faults. Part 2 will also review how ground fault current interrupters (GFCI) work to protect you from fatal electrical shock.

ABOUT ACROMAG Acromag has designed and manufactured measurement and control products for more than 50 years. They are an AS9100 and ISO 9001-certified international corporation with a world headquarters near Detroit, Michigan and a global network of sales representatives and distributors. Acromag offers a complete line of industrial I/O products including a variety of process instruments, signal conditioners, and distributed fieldbus I/O modules that are available with a 2-year warranty. Industries served include chemical processing, manufacturing, defense, energy, and water services.

For more information about Acromag products, call the Inside Sales Department at (248) 295-0880, FAX (248) 624-9234. E-mail [email protected] or write Acromag at 30765 South Wixom Road, Wixom, MI 48393 USA. The web site is www.acromag.com.

http://www.acromag.com/





Copyright © Acromag, Inc. April 2014 8501-020


Examining our use of ground as protection, and how ground fault circuit interrupter devices operate to protect us from severe shock (Part 2 of 3)


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This paper is part two of a three part series that takes a look at grounding and its role in protecting personnel, protecting equipment, and ensuring the integrity of electrical signals. In this part, we will examine our use of ground as a means of protection from ground faults, and how ground fault circuit interrupter (GFCI) devices operate to protect us from severe shock.

In part one of this series (8500-993) we looked at the concept of grounding, the AC power system and its use of ground, and gave three main reasons why we ground electrical equipment: for safety, to stabilize electrical signals, and to limit transient voltages and current.

Part three of this series will review ground and its role as a voltage stabilizer and transient limiter. It will offer some tips on what you can do to improve your connection to ground to realize benefits to safety and signal integrity.

GROUNDS AS A MEANS OF PROTECTION In Part 1 of this series, we said that one reason we connect to ground is for safety because it provides an alternative path for fault current to flow. The voltage in a circuit will force current to travel all available conductive paths back to the source. The majority of this current will follow the path of least resistance. We normally provide an earth ground connection as an alternative low-resistance path for this current flow to limit shock (as an alternative path to using your body to return current to AC neutral and earth ground). For AC powered circuits, a ground fault is any event that causes an imbalance in current flowing in the AC hot lead with current flowing in the AC neutral lead. The ground fault current refers to the difference in current flowing between AC hot and AC neutral, and it could be the current flowing in the earth ground lead or the current taking another path back to the AC source, like through a body to earth ground (because AC neutral and earth ground are tied together at the source).

For AC powered devices, appliance manufacturers go to great lengths to make sure that you do not become the path of least resistance for a fault condition, which allows current to flow from AC hot back to AC neutral/earth ground via your body (Remember that AC neutral and earth ground are held in common at the source of the AC voltage). They do this by properly insulating their product and/or by providing a remote connection to earth ground at the product (via that third rounded conductor at your power outlet). That is, a conductive enclosure or housing of the device will typically connect to earth ground. Then if the AC hot or AC neutral wire to the device should fray or break, it could contact earth ground and complete the circuit, allowing the breaker to trip when its rated limit is reached, possibly preventing a fire, and hopefully without electrocuting yourself in the process. Of course, some appliances are constructed using insulating materials and do not connect their chassis to earth ground (they connect to AC using two-prong power cords). Under normal operation, the current to the device flows from between AC Hot and AC Neutral, through your load, and returns to your breaker box and utility meter power connection. The third wire, or earth ground wire, will become the return path for at least part of that current, should a fault occur at the powered device. Without this third conductor or alternate path held in reserve, your body could become the return path of least resistance to earth ground, and this could drive serious injury. Thus, for most 3-conductor AC wired equipment, its connection to earth ground is primarily to provide a safe path for current to flow in the event of a circuit fault, and normally, no current flows in the ground wire. By limiting current flow in the ground wire to only under certain conditions (ground faults), this helps to ensure that there will be no potential difference across this conductor, and it can better act like an extension of earth ground itself (i.e. nearly 0V).

http://www.acromag.com/page/white-paper-electrical-ground-rules


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So what does a wired earth ground really protect you from? Does it protect you from electrocution? And what if your appliance does not include a connection to earth ground and you become the path of least resistance for some sort of ground fault? The answer to these questions is that a wired ground does not protect you from fatal shock. Rather, a GFCI can protect you from fatal shock.

THE GFCI AND ITS USE OF GROUND AS A MEANS OF PROTECTION FROM GROUND FAULTS

To further illustrate how earth ground works as a means of protection, let’s examine the behavior of a device that you may already be familiar with--the GFCI outlet. It looks similar to the picture at left, and usually includes a “Test” and “Reset” button. However, a GFCI protected outlet may look identical to a standard outlet if it happens to be connected downstream to the load side of a GFCI outlet.

You probably have several of these in your home, perhaps in your kitchen, laundry room, basement, bathroom, garage, or at an outdoor outlet. You probably know that these outlets are used to help protect you from severe shock, and are generally required in “wet” areas, but you may not have given much thought to how this device actually works with ground to help protect you from harmful levels of shock.

For example, as ground conducts current, a voltage difference will develop across it, forcing each connection to it to occur at a slightly different potential. So in practice, if all connections to ground were really made at 0V, we wouldn’t have ground faults or ground loops. And therein lies the problem with ground—your ability to mimic its idealized behavior as a reference connection to 0V really depends on how well you can minimize its effective impedance, chiefly its resistance and inductance. Generally at 50-60Hz power line frequencies, the resistive component of your connection to earth is more significant than the reactive component (inductance). At higher frequencies, the reactive component gains significance, as the inductance of your connection to ground raises its impedance to transient energy. The bottom line is that you need to minimize the effect both components have on raising ground impedance by keeping both the resistance and inductance of your connection to ground to a minimum.

In addition to GFCI outlets, there are Ground Fault Circuit Interrupter (GFCI) Breakers similar to the examples shown at left. These act similar to GFCI outlets, but will protect an entire circuit from ground faults, while the GFCI outlet only protects at its outlet and any downstream outlets for daisy-chained “load” outlets. The advantage of a GFCI Breaker is that you do not need local GFCI outlets. GFCI Breakers are typically used on circuits where the likelihood of fatal shock is much higher (like a public swimming pool), and like a GFCI outlet, they can typically stop current flow within 25ms of detecting a ground fault condition on their branch circuit.

http://homerepair.about.com/od/termsgn/g/ground_fault.htm


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As we noted in Part 1, the AC outlets in your home commonly provide 3-wires or conductors for connection to your appliance—a line hot connection (typically via a black insulated wire that connects to the shorter vertical slot of the outlet), a line neutral connection (typically via a white wire that connects to the taller vertical slot of the outlet), and a ground connection (typically via a green insulated or bare copper wire that connects to the round hole of the outlet at the top or bottom between the two vertical slots). In any circuit, current will seek all conductive paths back to its source, but most of this current will flow the path of least resistance. In your home, when you plug an appliance into an AC outlet, you give current a low-resistance path to flow through and power your appliance, and then return back via the outlet to its source. The source in this case is where your home or business connects to the power line, usually at your power meter and service entrance. Remember, although AC neutral and AC ground are separate conductors at your outlet, they do connect together at your service box and also connect to earth ground near this box, usually via one or more grounding rods driven into the earth. Load current normally flows out the line/load hot conductor, through your appliance, and returns to earth ground via the neutral conductor, and doesn’t normally flow in the ground conductor.

GFCI refers to a type of power outlet called a Ground Fault Current Interrupter. The GFCI outlet will usually provide a second pair of screws for a load cable connection to allow it to connect to additional non-GFCI outlets in daisy-chain fashion, connecting them through itself to power (making it act like a GFCI breaker for downstream loads). This type of outlet can trip and quickly stop the flow of current through it and downstream from it to prevent serious injury if a Ground Fault occurs anywhere in the circuit. A Ground Fault would occur if the electricity (current) is allowed to take some other path back to the source, than neutral (including through a person’s body), as the source is where neutral and ground connect to the earth. A GFCI outlet works to protect you by measuring the amount of current flowing out the “Hot” wire circuit into the appliance, and monitoring the return current flowing out of the appliance back through the “Neutral” circuit.

If the current returned in neutral does not match the current in the hot line, the GFCI will trip and interrupt the current flow out of the hot wire. Of course, the “lost” current doesn’t really go “missing”, but has temporarily found some other path back to the source, perhaps through your self to earth. This errant path is the ground fault we refer to and the GFCI feeding this circuit will trip very quickly to prevent serious shock or electrocution via current in this errant path.

The figure at left depicts one example of a ground faulted circuit. It shows one phase of an AC line (like that sourced from a service panel) powering a hand drill through a GFCI outlet or breaker. The black AC hot wire to the hand drill makes contact with its metallic housing (causing a ground fault), and a workman standing on the ground and holding the drill completes a path to earth for a portion of the circuit current. Because this circuit was made through a GFCI outlet or breaker, the small leakage current triggers a relay in the black hot lead to the

drill to open and stop the flow of current, protecting the workman from severe shock.

It’s important to recognize that a GFCI does not protect against circuit overloads (too much load current), short circuits between hot and neutral, or minor electrical shocks. The man in the example above would still feel a small shock when he makes contact with the hand drill. And, even with a GFCI protected circuit, you can still be shocked if you touch bare wires while standing on an insulating surface, such as a


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wood floor, and you happen to complete the neutral current path. The GFCI only protects against current flow that does not completely follow its normal safe path along neutral to earth ground at your service panel, but uses some other errant path to reach the earth instead. The other path could be through a person’s body to earth ground, or even through the ground wire to earth (perhaps via a hot or neutral short to the grounded chassis of the load device, like in the example above).

Because the GFCI outlet trips to interrupt current when it detects a ground fault (a mismatch in hot and neutral current), it also contains a reset button used to reset its trigger mechanism. This reset has a lockout feature that keeps it in reset for any of the following:

There is no power being supplied to the GFCI.

The GFCI has been miswired by reversing the line-in (source) and line-out (load) lead connections.

The GFCI fails an internal test, indicating that it may not be able to provide protection from a ground fault.

The appliance that you plug into your AC outlet typically connects its other metal parts to the ground connection (any other conductive parts other than its AC hot and neutral circuits). It does this so that if its own AC Hot circuit was to break or fray and accidentally come in contact with another of its metal or conductive surfaces (like its shell or chassis), that surface would instead become a path to ground. This path to ground essentially shorts AC hot to AC ground and causes the circuit breaker or fuse to quickly overheat and open the circuit, interrupting the current flow. The normal level of current to cause a typical breaker to trip is 15A or 20A, but the amount of current that can put the human heart into fibrillation is less than 100mA. So even if a person handling that faulty appliance happened to be barefoot and standing on wet ground, a portion of that branch current would also flow through that person to ground and this could still be fatal.

Generally, in a ground fault situation, the body path would provide a higher resistance path to earth than the grounded chassis and would only take a portion of this current. And just like two different resistive loads in a circuit, current is distributed along both resistive paths, but mostly along the path of least resistance. In the example of a grounded chassis, one path is through the ground wire and another through the body. But, even only a small portion of that 15A or 20A of branch current that flows just before the breaker trips is enough to stop the human heart. In the example of the hand drill, without GFCI protection, the workman may be killed. However, if the branch circuit happens to be passing through a GFCI protected outlet, the workman might still feel a momentary shock, but at a much lower current and of a shorter duration, as the GFCI will interrupt the hot current flow much more quickly than a circuit breaker alone.

In fact, a GFCI protected outlet or breaker will trip if the measured difference in current between the “hot” and “neutral” conductors is as little as 4-6mA. Momentary contact with 120V at 6mA will shock you, but will not likely put your heart into fibrillation, as the GFCI typically trips within 1/30th of a second as soon as it detects a hot/neutral imbalance, a small enough duration to protect you from fatal levels of shock.

Also note that for the purpose of discussion, we tend to think of and talk about AC current as flowing from the hot lead, through the device, and then returning on the neutral lead. However, AC current does not really flow in one direction or with respect to one polarity. Rather “AC” denotes Alternating Current, and alternating current actually flows back and forth through the load (alternating its polarity), 50-60 times a second.


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A GFCI outlet typically uses a differential or toroidal transformer to couple the line to the load, similar to the illustration at left. The current of the hot wire is magnetically coupled to the toroid and then passes to the load. The neutral wire from the load also passes through the toroid in the opposite direction and back to the service entrance of the house, where the line originates. These two wires are mutually coupled via the toroid or transformer magnetic material, such that for normal operation, the current that flows through the hot wire balances with the current that flows back through the neutral wire. But if these currents do not balance, this imbalance triggers a sensor circuit that drives relay contacts to open the Line hot circuit (black wire), interrupting its flow. Of course, the only way the hot and neutral line currents would not balance is if

an external source is adding current to the circuit, or the line current is being leaked through some other external path (perhaps if line current is leaking to earth ground). A hot/neutral imbalance as small as 5mA is enough to quickly interrupt the flow of line current to the load, potentially protecting you from fatal levels of electric shock. In this way, the GFCI interrupts the flow of current much more quickly than the service entrance fuse or circuit breaker, in as quickly as one-thirtieth of a second.

Do not be fooled into thinking that a fuse or breaker alone can protect you from electrical shock. Rather, it’s the purpose of a GFCI to protect you from electrical shock, while the purpose of a fuse or breaker is to protect its premises from electrical fire by limiting the maximum current and duration of excess current to a lower level, but not a low enough level to protect you from shock. Consider that there are many scenarios that could cause a hot wire to accidentally come in contact with the neutral or ground wire, causing a large amount of excess current to flow from hot to neutral. For example, an animal might chew through the wire insulation, a person might accidentally drive a nail or screw through the wire, or the cord connecting to an appliance might get chewed up after being pinched in a doorway or sucked up by a vacuum cleaner. The potential fault modes are endless and difficult to prevent. These faults will all cause excess current to flow and heat up the fuse or breaker and cause it to burn or break open, interrupting the flow of current, possibly preventing the wire from starting a fire. The problem with relying on a circuit breaker or fuse for shock protection is that they are tripped at much higher levels of current (15 or 20A typical), and after longer periods of time, both of which could be fatal if you happen to be in the path of this fault current.

We started this discussion by trying to show how a GFCI outlet works with ground to protect yourself, but the reality is that a GFCI outlet would still protect you from electrical shock due to a ground fault, even without a hard-wired connection to earth ground. That is, if the GFCI happened to be wired using 2-wire AC cables (hot and neutral but no earth ground), it still provides ground fault protection. This is because a GFCI outlet monitors for current imbalances between hot and neutral, not ground. The only possible difference in a 2-wire GFCI powered application is the ground fault current along the errant path could potentially reach a higher level without the third wire ground path present, but the outlet would still detect the hot-neutral imbalance and break the hot wire connection in time to protect you from severe shock.

We spent a lot of time talking about ground as a “safe” path for current, and this is because personal safety should be your number one priority, but we stated at the beginning of this paper that we have


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two other important reasons for connecting to ground: to stabilize signals and to limit transient voltage and current. In Part 3 of this series, we will discuss the other two reasons we connect to ground and give some tips for improving your connection to ground to realize the benefits of increased safety and signal integrity for your wired equipment.



Electrical Grounding Rules Part 1 is available for download: www.acromag.com/page/white-paper-electrical-ground-rules




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This paper is part three of a three part series that takes a look at grounding and its role in protecting personnel, protecting equipment, and ensuring the integrity of electrical signals. In this part, we will examine ground and its role as a voltage stabilizer and transient limiter, as well as offer some tips on what you can do to improve your connection to ground to realize benefits to safety and signal integrity.

In part one of this series (8500-993) we looked at the concept of grounding, the AC power system and its use of ground, and gave three main reasons why we ground electrical equipment: for safety, to stabilize electrical signals, and to limit transient voltages and current.

In part two of this series (8501-020) we examined the use of ground as a means of protection from ground faults. We also looked at how ground fault circuit interrupter (GFCI) devices operate to protect us from severe shock.

GROUND AS A VOLTAGE STABILIZER Think of voltage as a force that causes current to flow in any conductor—a greater voltage results in greater force that drives higher levels of current. High levels of current can drive errant circuit behavior, possibly damage equipment, and may even lead to personal injury. We want to ground signals to stabilize them and keep them from floating, and we do this to limit voltage magnitude and variation.

In practice, connecting to ground helps stabilize signals during normal operation, acting like an anchor that limits the magnitude and variation of voltage. On the other hand, like a boat without an anchor, an ungrounded signal will “float”. Floating a signal will generally make it more susceptible to common-mode noise interference. A common-mode signal is a signal that appears “common” to a set of floating points. Common-mode noise signals can be inductive or capacitive coupled from external sources, or they may be driven by the circuits themselves. All electronic circuits are limited in their ability to filter or reject common-mode noise, especially if the potential of a measurement point is allowed to float outside the limits of the circuitry. The end result is that common-mode noise can drive spurious measurements or spurious output behavior. One example of the importance of grounding is with respect to differential mode measurements, such as that used for some types of

instruments, like thermocouple amplifiers. If you do not earth ground one lead and anchor it from floating, you will likely note that the measurement appears noisier and more widely variant, and that is assuming that a point of signal measurement doesn’t float outside of the common mode range of the amplifier, at which point it cannot be measured or processed by the circuit properly. This is why you will note that many connection diagrams for differential input pairs will show one lead (usually the minus lead) making a connection to earth ground.

Most electrical equipment and industrial instruments utilize differential filters and transient suppression devices at their wired connections to shunt potentially destructive energy from one lead to another and to steer this energy to ground. This same energy ultimately seeks a path to earth ground where it originated and can typically be dissipated more safely. Failure to apply ground to the circuit at the designated connection will leave the circuit vulnerable to damage, as the circuitry must then absorb and dissipate this transient energy in the absence of a clear path to ground. This connection to ground is very important and will help to extend the life of your equipment—always be sure to identify these




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connections to earth ground and make sure that you provide a low impedance path to ground at these points to protect your equipment from damage.

For electrical equipment, all connections to power are usually grounded at some point. The device may be optionally DC powered, but a conversion from AC to DC still occurs and a path back to earth ground usually exists. Isolated power sources usually ground their DC output power minus terminals. Inside electrical equipment, the power connection is often isolated from other parts of the circuit, such as its inputs, its outputs, its network connection, etc. Noise exists in each of these isolated circuits and takes many forms. In many applications, the DC power supply to the circuit itself will provide a path to earth ground at its DC minus terminal. Many instrument manufacturers recognize this and will often employ isolation capacitors connected inside their own circuits between the various isolated reference planes and the DC minus connection to the device, which is often indirectly earth grounded via the power supply. These capacitors significantly reduce radiated emissions from the device by providing a path to earth ground where transient energy on each of the isolated planes can be shunted through the capacitor on its way to ground. In this way with these devices, the earth ground connection at the power supply often serves as a kind of default path to ground for harmful energy, even if the other parts of the circuit have not been properly grounded. Still, do not be tempted to float isolated portions of your device and rely only on these isolation capacitors to provide protection, as they can never compete with a direct, hard-wired connection to earth ground. It’s always best to refer to your connection diagrams and wire ground connections as recommended.

GROUND AS A TRANSIENT LIMITER Modern powered circuits are awash in transient energy from many sources, coupled via many paths, as illustrated at left. Thus, the potential for encountering unintended voltage rise in electronic equipment is ever present via its connection to power, its exposure to ESD, and even its proximity to other electronic devices (by conductive, inductive, capacitive, or radiated noise coupling). Our connection to ground acts to make our circuits safe and will help to stabilize our signals. This ground connection also limits the potential voltage rise induced on our circuit, typically via lightning, line surges, and even during unintentional contact with higher-voltage.

To help filter the effects of unintended voltage signals, most electronic equipment will utilize differential filters, capacitors, and other transient suppression devices at their wired connections. The purpose of these devices is to shunt

potentially destructive energy from one lead to another, usually in an attempt to squelch the imposed voltage and steer the resultant destructive current or charge to earth ground where it can be dissipated more safely. If you fail to connect ground to a designated wired terminal, you leave this energy with no place to go except through your circuit where harmful voltage levels and high transient current levels can wreak havoc and drive damage. So you should think of your connection to ground as an integral part of your circuit’s transient protection. Without it, you leave your equipment unprotected and exposed to potential destruction.


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For example, lightning occurs when atmospheric charge finds a path to earth. Any circuitry in this path, or in the presence of this path to earth, can be easily destroyed by the high voltages that are developed. Providing a low impedance path to earth for powered equipment will help to minimize the potential destruction of a lightning strike by keeping the resultant voltage increase above earth to a lower potential. Without a connection to ground, the energy will continue to develop its high voltage across a circuit, possibly resulting in damaging levels of current that may ultimately destroy the circuit. A low impedance connection to ground will instead help carry this energy into the earth before it destroys the circuit it is otherwise distributed across in its transfer along a path to earth ground.

Earlier I mentioned that a side benefit of a connection to ground is that it offers EMC benefits by lowering system noise and radiated emissions. It does this the same way that it works to squelch the effects of unintended voltage signals sourced by lightning and other sources—by stabilizing voltages and limiting voltage variations, and by providing a low impedance path to earth ground where transient energy can be safely dissipated. Without a clear path to earth ground, this energy will be forced through the circuit and drive signal error, erratic behavior, and potentially damage the circuit.

IMPROVING YOUR CONNECTION TO GROUND At this point, you should recognize the importance of providing a good connection to ground—for personal safety and protection from electrical shock, to stabilize signals and minimize fluctuations, and to limit the magnitude of induced voltages and peak currents. As an engineer for a manufacturer of industrial instruments, I am often called to task for lowering a product’s emissions or raising its EMC immunity with respect to ESD, EFT, and other interference. I can honestly say that most of the time, the solution to these problems lies in the correct application of earth ground. So how do you improve your connection to ground to help realize these benefits in your applications?

To go about improving your connection to ground, you can start by calibrating the way you think about ground. Specifically, you need to think of your connection to ground as a drain that you flush all the unwanted energy in your electrical system down (ground faults, electromagnetic interference, ESD strikes, fluctuations caused by nearby lightning, power line surges, transient noise, etc.). You want this drain to quickly accept unwanted electrical charge from your circuit. Now you wouldn’t connect the drain of your home through a straw, or unwanted waste would back up and contaminate your home. Instead, you would want a wide-open pipe leading to your waste-water drain, and you would avoid angles and changes in direction, keeping this pipe as short and straight as possible to help prevent backup. It’s the same way with ground—you want a wide-open, short and direct drain to earth that doesn’t back “charge” up into your circuit. And just like the drain from your home, you can improve your connection to ground by making it short as possible and by increasing its diameter. Chiefly, with your connection to ground, your goal should be to reduce its resistance and its inductance by using a larger diameter or “thicker” conductor, and by keeping its path as short as possible. Because inductance and resistance both restrict the flow of current (as current through inductance cannot change instantaneously), you want to minimize both the resistance and inductance in your connection to ground so that it can more quickly drain transient energy from your circuit and dissipate it into the earth.

All conductors have resistance and voltage across the conductor acts as a force to drive current through the conductor. When you push current through a conductor, you establish different potentials at different points along that conductor related to the IR voltage drop through the conductor. Ideally, you want your ground to deliver nearly the same potential across it (ideally an equipotential voltage of 0V), such that any tie to ground will see the same ground potential. If you fail in this regard, you give rise to


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unwanted ground currents (ground loops) with possible negative side effects of increased noise and interference in your system. In practice of course, an ideal ground is impossible to attain, but there are still some things that you can do to approximate the ideal. Specifically, you need to pay close attention to how you are making your connection to ground. For example, you can avoid having your circuit grounds connect at different potentials by using a “star” grounding technique.

Star grounding is a concept where each ground connection (represented by each leg of a star) connects outward from the same point (the center of the star). When you wire ground to your circuit, perhaps to each isolated part of your circuit (like input, output, power, etc.), you strive to bring these connections together via separate ground returns to one point (this is the center of your star ground), using short and thick cables to minimize path resistance and inductance effects. The center of your star is usually chosen as the ground return of the power supply to the circuit. It is sometimes chosen as the common chassis connection where a conductive chassis makes its single connection to earth ground.

SOME BASIC GROUND RULES FOR WIRED EQUIPMENT Consideration of ground can be very complex and application specific. But in many of these applications, when we make wired connections to ground and to electrical equipment, there are a few rules of thumb that are helpful:

For isolated power applications where a connection to earth ground is not apparent, ground should be chosen as the common return path from power supply (DC minus). It may be necessary to hard-wire earth ground to this point if an earth ground connection is not already made by the power supply.

Do not ground a signal at more than one point. Typically a signal is grounded at its source (including its shield).

In general, as stated above, we try to never ground a cable at both ends. But one possible exception to this rule is when we are grounding cable shields in small signal applications. For most applications where only small differences in potential exist between grounds at each end of the cable, our equipment will work better when its shield is grounded at each end of the cable (at a minimum, ground it at the end closest to the noise source). Another exception is where your equipment connects to power, as DC powered equipment will often connect earth ground at the power supply minus terminal, but you should additionally include a connection to ground local to the instrument. This is done not only to stabilize applied voltages, but also because internal suppression devices in the instrument need a local, low resistance, low inductance path to shunt potentially destructive energy.

For EMC purposes, a wired signal between devices should have earth ground applied at the end of the cable nearest the noise source of the signal, or nearest the noisiest device. Failure to provide a path to ground at the “origin” of the noise may result in the cable and/or its shield becoming an antenna for this noise, increasing its power and spread into other areas of the circuit, as well as potentially increasing system emissions.

Do not use the chassis of the device as the ground conductor (i.e. make only one ground connection to the chassis). Note that many devices are required by code to have a safety ground connection to their metallic chassis or enclosure, but the chassis should never be used as a return path for load current to the device (for “safety” ground, it is sometimes used only as a return path for fault current). Note that the chassis connection to earth ground is sometimes used as the center of a star grounding scheme for the enclosed circuit.


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Many instruments are housed in plastic enclosures and may not make a connection to earth ground via their chassis. These instruments usually rely on direct-wired connections to earth ground at their terminals, as directed in their connection diagrams. In general, signal connections to these devices should be earth grounded at the end of the I/O cable nearest the instrument. This is because the instrument needs a low-impedance/low-inductance path to earth ground locally, to allow its various filters, capacitors, and transient suppression devices to shunt potentially destructive energy to earth ground without being impeded by high levels of inductance and resistance in the path to earth.

Do not bundle noisy or high-energy signals or power with low level signals. Route all AC power wires away from sensitive signals and signal paths.

Do not duplicate ground connections to the main power line at different points—try to connect all AC powered devices to the same power outlet when possible and safe. Similarly, use a star-grounding concept when making ground connections to your circuit.

Do not combine or bundle isolated signals in the same shield or conduit.

Do not allow conductive material to float unattached to any ground (it should connect to ground at one point).

Do not leave unused shielded conductors in a bundled cable disconnected from ground. Ground unused conductors of a bundle at the load. In general, ground the cable shield at the signal source (or at both ends).

Minimize the length and loop area of the wires that break-out from a bundled or shielded cable, just before the wires make their connection to the equipment.

CONCLUSION By now, you should have a heightened awareness of the importance of ground to the safety of personnel and the operation of your equipment. Never float signals or neglect to make ground connections as shown in the connection diagrams for your device, or you increase your risk of electrical shock and may even damage your equipment. Grounding signals will help to stabilize them and help limit induced transient voltages and current. Many electrical problems can trace their generation to a poor, improper, or a missing connection to earth ground. Don’t neglect this important connection to realize benefits of increased safety and signal integrity for your wired equipment.



Electrical Grounding Rules Part 1 and Part 2 is available for download: www.acromag.com/page/white-paper-electrical-ground-rules



Download - The Complete Acromag Guide to Industrial Electrical Grounding

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