Power and Grounding

Presenter• Tom Kehr, CTS-D, CTS-I, ISF-C, LEED Green

Associate, Network+

• Since 1986, Tom has been involved in either designing, installing, operating or purchasing audiovisual systems.

• Tom has also been heavily involved in church audio, leading ministry teams and training volunteers in both permanent and portable venues. In addition, he owned a regional sound reinforcement company for over 16 years.

• Prior to joining InfoComm, Tom was a senior designer for a large Washington, D.C. area firm. He is a member of Syn-Aud-Con and AES and is a licensed amateur radio operator.

Agenda• To recognize best practices for power

and grounding systems for AV systems

• To be able to discuss why these are best practices

Outline• Current pathways• System Grounding• Equipment Grounding• Ground Faults• Rack Grounding• What to specify• Ground Loops• Noise Defense

Authority Having Jurisdiction• Enforces codes and

regulations (Required)

• We can also apply:– Standards

• Industry• Client

– Best Practices

Avoid Ambiguous Terms• Dedicated power• Dedicated ground• Clean power• Dirty power• Quiet ground• Low or high voltage• Technical power• Technical ground

Use Electrician (NEC) Terms• Panelboard• Individual branch circuit• Auxiliary Ground• Isolated Ground

Panelboard• Main service

panelboard• Sub-panelboard

Circuit Theory• Electricity seeks to go to ground

Circuit Theory• “Electrical current is the unseen

movement of electrons that flow from the power source through the electrical circuit of the appliance or equipment and then return to the power source. The complete path the electrons take is called the electrical circuit.” – Mike Holt, “Basic Electrical Theory”

Electrical Circuit

Electrical Circuit

Circuit Theory• All of the electrons

leaving the source of power go through each component in the circuit and return to the source

Electricity Goes To Ground?• Let’s explore

Complete The Circuit

• Why have ground?

Circuit Theory• Electricity always takes the path of

least resistance

Circuit Theory• While more current

will flow through the path of lesser resistance, current will take any and all available pathways

• Path could be an unintentional path– Safety

100,

000

Ω

1,00

0 Ω

10 Ω

I = V / R1.2 mA = 120 / 100,000120 mA = 120 / 1,00012 A = 120 / 10

System Grounding• The Grounded Conductor is frequently the Neutral and it

is the return path back to the electrical source

Lightning/Surges On Power Line

System Grounding• Lightning, line surges and

unintentional contact with higher voltage lines

• Reduce fires• Protects the electrical

system

Equipment Grounding• NEC: Non-current-carrying conductive materials … shall be

connected to earth

Ground Fault• Current path during a ground-fault• Connection to earth is not involved

Ground Fault• Interruption of

Ground-Fault Current Path– Improper use of

Grounding Adapter

• Not a “Ground Lift Adapter”

– Not to be used to solve hum and buzz problems

Circuit Theory• While the majority of

the current will flow through the path of lesser resistance, current will take any and all available pathways

• Path could be an unintentional path– Safety

100,

000

Ω

1,00

0 Ω

10 Ω

I = V / R1.2 mA = 120 / 100,000120 mA = 120 / 1,00012 A = 120 / 10

• Common ground stud for all gearGrounding Racks

Grounding Racks

Grounding Racks

Grounding Racks

Review -• Current seeks to return to the source• It will take any and all pathways• System Grounding

– One of the current carrying conductors• Equipment Grounding

– Non-current carrying conductive materials• Current pathway for a Ground Fault

– Do not use Grounding Adapters• Proper Rack Grounding

Auxiliary Ground• Auxiliary Equipment

Grounding Conductor– Insures low

impedance equipment ground

– Avoids depending upon conduit installation

Auxiliary vs. Isolated Ground

Specify -• Branch circuits

for “Audiovisual Use Only” with auxiliary equipment grounding conductor

Ground Loops• A ground loop is a continuous conductive loop

– Formed by using the Equipment Grounding (safety) Conductors of any two electronic devices and the cable shield connecting those two devices

– Formed by the shields of two interconnecting cables between any two electronic devices

– Normal, common– Not always problematic

Ground Loops

Ground Loops

Noise Defense - Shielding• Shield

–Metallic partition–Works bidirectionally

• Contains• Keeps out

–50 / 60 Hz–Nearfield

Noise Defense - Shielding• Magnetic Fields

– Uses any magnetically permeable material to absorb and conduct magnetic lines of flux

• Magnetically permeable materials include:– Iron, steel, nickel alloys

• Iron, steel, nickel alloys vary in their permeability– “Permeability” – Ability to conduct magnetic

fields

Noise Defense - Shielding• Electric Fields

– Uses any electrically conductive material to absorb and conduct electrical lines of force

• Electrically conductive materials include:– Gold, silver, copper, aluminum, etc.

• Gold, silver, copper, aluminum, etc. vary in their conductivity– “Conductivity” – Ability to conduct an electrical

current– Conductivity is the opposite of resistance

Shielding

Shielding

Noise Defense• Balanced Interfaces

• CMR Specifications– Lab measurement vs. “real world”– Component tolerances create slight impedance

imbalances– CMR specifications should be in accordance

with IEC Standard 60268-3

Noise Defense

Differential-Mode Signal Common-Mode Noise Common-Mode Noise with Differential-Mode Signal

Review -• Specify

– Individual branch circuits with auxiliary equipment grounding conductors

• Ground loops– Normal, expected– Not all are problematic

• Noise Defense– Shielding - What works and what doesn’t at 60 Hz– Specify steel conduit– Use balanced interfaces with quantifiable CMRR

specifications

What We Have Covered:• Current pathways• System Grounding• Equipment Grounding• Ground Faults• Rack Grounding• What to specify• Ground Loops• Noise Defense

Questions?