Electrical Safety

By Anu SinglaAssociate ProfessorDepartment of ElectricalEngineeringChitkara UniversityPunjab Campus

Safety Precautions in Handling ElectricalAppliances

Following safety precautions must be taken while working with electrical installations or while handling electrical appliances:Make sure that all metallic parts of the electrical equipment are effectivelyearthed. Broken switches, plugs, etc., should be replaced immediately.Use a ‘line tester’ to check whether a terminal is live. Still better is to use a‘test lamp’, as the line tester can show a glow even with a small voltage.Before replacing a broken switch, plug or blown fuse, always put off the mainsupply.Never use equipments and appliances with damaged or frayed lead wires.Never insert bare wires in the holes of a socket, for taking a connection. Alwaysuse a proper plug.Use rubber- sole shoes while repairing/testing electrical equipments. If this isnot possible, use some dry-wooden support under your feet, so that your bodyhas no direct contact with earth.Use rubber gloves while touching any terminal or while removing insulationlayer from a conductor.Always use well insulated tools (such as screw- drivers, pliers, cutters, etc.).

Safety Precautions in Handling ElectricalAppliances contd..

Never touch two different terminals at the same time.Be careful that your body does not touch the wall or any other metallic framehaving contact with earth.While repairing an electrical appliance (such as table fan, iron, heater, geyser,etc.), be sure that its plug has been taken out from the socket. Switching off maynot be sufficient, since leaky insulation can given serious shock.Strictly follow all the precautions and instructions given on the ‘name plate’ ofthe machine you are working.In case of electric fire, use only ‘soda- acid’ fire – extinguished. Do not throwwater on live conductors or equipments. Best remedy is to first disconnect to theelectric supply and then throw sand on fire.While working on an electric pole or tower, use safely – belt and a rubberpadded ladder.It is preferable to work in the presence of an ‘assistant’, so that he canimmediately disconnect the supply whenever needed.

What is Earthing?

By earthing, we generally mean an electricalconnection to the general mass of earth, the latterbeing a volume of soil/rock etc., whose dimensionsare very large in comparison to the electricitysystem being considered.

An electrical equipment or appliance is said to beearthed, if its outer frame and its other parts notcarrying any current are connected to the earth soas to attain as nearly zero potential as possible.

Objectives of Good Earthing System

• To limit voltage in electrical distribution system to definitefixed values. To ensure that no part of equipments other thanthe live parts should assume a potential that is dangerouslydifferent from that of the surroundings.

• To limit voltage to within insulation ratings.

• To provide a more stable system with a minimum of transientover voltage and electrical noise.

• To provide a path to ground in fault conditions for quickisolation of equipment with operation of ground faultprotection.

• To provide grounding of all conductive enclosures that may betouched by personnel, thereby eliminating shock hazards.

• To provide protection from large electricaldisturbances (such as lightning) by creating a lowresistive path to earth.

• To reduce static electricity that may be generatedwithin facilities. In some industrial (petrochemical,refineries, where explosives or volatile chemicals arepresent) premises the earthing system is required tocontinuously discharge the build up of static charge,and thus prevent a fire or explosion risk.

• Many power supplies now include a connection toearth, through which residual and harmonic currentsare dispersed to ground.

The circumstances that make electric shock accidents possible are :

a) Relatively high fault current to ground in relation to the area ofground system and its resistance to remote earth.

b) Soil resistivity and distribution of ground currents such that highpotential gradients may occur at points at the earth’s surface.

c) Presence of an individual at such a point, time, and position that thebody is bridging two points of high potential difference.

d) Absence of sufficient contact resistance or other series resistance tolimit current through the body to a safe value under circumstancesmentioned at a to c.

e) Duration of the fault and body contact, and hence, of the flow ofcurrent through a human body for a sufficient time to cause harm atthe given current intensity.

Effect of current thro’ human body

Effect of electric current through the vital parts of humanbody depend on the duration, magnitude and frequency ofthis current. Most dangerous consequence could beventricular fibrillation, a condition of incoordinate action ofmain chambers of the heart, resulting in immediate arrest ofblood circulation.

Currents at 50 Hz about 0.1 A can be lethal. But human cansustain larger currents at 25 Hz and five times higher DC or atfrequencies in the range of 3,000 - 10,000 Hz.

Current depends on voltage applied and body resistance.Resistance is mainly offered by skin. Skin resistance increaseswith thickness and diminishes with moisture / perspiration.Except for skin; blood vessels, intravascular spaces etc. offerconduction system.

Tolerable current for human body

As per studies by Dalziel, 99.5% of all persons can safely withstand withoutventricular fibrillation, the passage of current (IB) for duration rangingfrom 0.03 to 3.0 sec and is related to energy absorbed by the body as performula: SB = (IB)2 x ts

Value of SB = 0.0135 for person weighing 50 kg

i.e. IB = 116 mA for 1 sec.&

SB = 0.0246 for person weighing 70 kg

i.e. IB = 157 mA for 1 sec.

Ts IB (50 kg) IB (70 kg)

0.2 sec 259 mA 351 mA

0.5 sec 164 mA 222 mA

1.0 sec 116 mA 157 mA

Human Body Resistance

Body resistance including skin ranges from 500 to3000 Ω which reduces by damage or puncture ofskin at contact point.

For earthing design, resistance of human body fromhand-to-feet (1100 Ω), foot-to-foot and also hand-to-hand(2300 Ω) is considered as 1000 ohm as per IEEE80-2000.

Components of Earthing system

Earthmat

Earth Electrodes

Risers for equipment to Earthmat connections

Bonding

Electrical bonding is the practice of intentionally electricallyconnecting all exposed metallic items not designed to carryelectricity in a room or building as protection from electric shock. Ifa failure of electrical insulation occurs, all bonded metal objects inthe room will have substantially the same electrical potential, sothat an occupant of the room cannot touch two objects withsignificantly different potentials. Even if the connection to a distantearth ground is lost, the occupant will be protected fromdangerous potential differences. Any exposed conductive metalwork which can be touched is connected together via bondingconductors. In industries, bonding of exposed metalwork wouldnormally ensure that an electrical fault to the frame of one machinedid not create a potential difference between that and earthedmetalwork on an adjacent machine.

Bonding

7

Grounding Cable

Grounding Bus

or Electrode

Bonding Cable

Proper grounding and bonding is used to address the dangers of static electricity. In order for grounding to protect, all surfaces must be bonded together and groundedto earth. Static electricity is thereby released to earth as it is generated, preventing theaccumulation of dangerous charges that may ignite flammable / hazardous substances.

Container Bonding and Grounding(Static Electricity)

Earth Electrode

The earth electrode is the component of theearthing system which is in direct contact withthe ground and thus provides a means ofreleasing or collecting any earth leakagecurrents.

The material should have good electricalconductivity and should not corrode in a widerange of soil conditions. Materials used includecopper, copper bonded mild steel rod,galvanised steel, stainless steel and cast iron.

Earth Electrode

Rod Electrode Pipe Electrode Pipe –in- Pipe Electrode

Rod and Pipe Electrodes

Rod electrodes shall be at least 16mm in diameter of steel or12.5mm in diameter of copper.

Pipe electrodes shall be larger than 38mm in diameter ofgalvanized iron or steel and 100mm in internal diameter of CIor mild steel.

The length of rod or pipe electrode not less than 2.5m, whichshall be driven to a minimum depth of 2.5 m.

Where rock is encountered at depth less than 2.5m, it can betilted by an angle of 35o to vertical.

*The distance between two adjacent electrodes should not beless than twice the length of electrodes.

Plate Electrode

The size of copper plate shall not be less than600mm x 600mm x 3.15mm and that of iron andsteel plates not less than 600mm x 600mm x 12mm.The top edge of the plate shall be at a depth not lessthan 1.5m from surface of ground.

When two plates are connected in parallel theminimum distance of 8m shall be kept between thetwo plates.

Typical Equipment Earthing

Main Terms

IS 3043 defines

Earth Grid: A system of grounding electrodes consisting ofinter-connected connectors buried in the earth to providea common ground for electrical devices and metallicstructures.

Earth Mat: A grounding system formed by a grid ofhorizontally buried conductors and which serves todissipate the earth fault current to earth and also as anequipotential bonding conductor system.

Ground Potential Rise (GPR): The maximum electricalpotential that a substation grounding grid may attainrelative to a distant grounding point assumed to be at thepotential of remote earth. This voltage, GPR, is equal to themaximum grid current times the grid resistance.

Touch potential is the difference in voltage between the objecttouched and the ground point just below the person touching theobject when ground currents are flowing.

Step Potential is the difference in voltage between two feet, whichare one metre apart along the earth when ground currents areflowing.

Mesh Voltage: The maximum touch voltage within a mesh of aground grid.

Metal-to-Metal Touch Voltage: The difference in potential betweenmetallic objects or structures within the substation site that maybe bridged by direct hand-to-hand or hand-to-feet contact.

Ground Systems

Touch and Step Potential

Basic Shock Situations in Substations

Typical metal-to-metal touch situation in GIS

As per the Indian Electricity Rule no. 67 (1) inevery E.H.V./ H.V. installations :

(a) Touch voltage and step voltage shall be keptwithin limits.

(b) The ground potential shall be limited to atolerable value.

RESULT: Higher Tolerable Step and Touch Voltages

High Resistivity Surface Material:

High Resistance Surface Material

Laying of High Resistivity Material

The black metal is used to provide high resistivitylayer. The resistivity of the black metal is taken as 3000Ohm-m for calculation of the tolerable touch voltagesin most of the designs of earth mat of sub-station.Crushed stone, i.e. the black metal, of the size of 30 to40 mm for a layer of 100 mm is recommended by theCBIP.

Granite, Gneiss - 25000 Ohm-metreBolder Gravel - 15000 Ohm-metreLime Stone - 5000 Ohm-metreMoran Gravel - 3000 Ohm-metreBase Rock Hard - 1190 Ohm-metreRock, Hard - 1150 Ohm-metreBoulders - 477 Ohm-metre

The range of the values of the resistivity is wide. It is, therefore,essential to know the source of the rock from which the blackmetal is obtained so that the idea of the resistivity of the blackmetal can be had prior to laying of the metal.

The values of resistivity of the different types of rocks

Black metal is spread in the substations to provide highresistivity layer :

To avoid formation of pools of oil in case of leakagesfrom Transformers and Circuit Breakers

to eliminate spreading of fire to keep reptiles away to control the growth of grass and weeds to maintain moisture in the soil to discourages persons running in the switch-yard and

saves them of the risk of being subjected to possiblehigh step voltage

Importance of gravel metal layer in substation switchyards

Parameters Influencing The EarthingDesign

The earthing resistance of an electrode is made up of:

Resistance of the (metal) electrode

Contact resistance between the electrode and thesoil, and

resistance of the soil from the electrode surfaceoutward in the geometry set up from the flow ofcurrent outward from the electrode to infinite earth.

The most important factor influencing the impedanceof the earthing system is the impedance of themedium in which the earth electrodes are situated,i.e. the soil.

Earth is a poor Conductor of Electricity. Typical Resistivity (ρ) of soil is 100 ohm-metre, and

for copper is 1700 micro ohm-metre, Two main constituents of soil are silicon oxide and

Aluminium oxide which are insulators, Soil becomes conductive due to salts and moisture

embedded in between them, Surface of soil layers-clay and moisture with

decayed vegetable material. When dry this doesnot conduct. With moisture contain, it conducts.

Cont..

Soil Properties

Soil under the surface of earth is non-homogenous, hence resistivity values in widerange between 1 ohm metre to 1,00,000 ohmmetres depending on type, nature of soil &physical and chemical properties.

Sandy soil drains faster, solid rock does not retainwater and have high ρ.

Soil resistivity measurement is important fordesign of earthing system.

Soil Properties

Moisture in the soil is the most importantelement determining its conductivity / resistivity.Conditions which increase / decrease distributionof moisture content in the soil resultcorresponding changes

• Resistivity goes seasonal changes as per moisturein the soil, due to climate conditions,

• Values of resistivity are minimum in rainy seasonand maximum in summer / dry season,

• For safe design of earth mat, measurements indry season are adopted.

cont..

Soil Properties

Resistivity for Different Soils

Type Resistivity (Ohm metre)

Sea water 0.1 - 1

Garden soil/alluvial clay 5 - 50

clay 5 - 100

Clay, sand and gravel 40 - 250

Porous chalk 30 - 100

Quartzite/crystalline limestone 300+

Rock 1,000 - 10,000

Gneiss/igneous rock 2,000+

Dry concrete 2,000 - 10,000

Wet concrete 30 - 100

Ice 10,000 - 100,000

Effect of Moisture on Soil resistivity

Effect of salt on Soil resistivity

Effect of Temperature on Soil resistivity

Conventional: Salt, Charcoal, WaterDisadvantage: electrode corrosion.Bentonite: High moisture, Swelling to High volume, Moisture retains to long time, No maintenance required.Marconite:Gypsum:

Low Resistivity Materials

Electronic Equipment Earthing

The single point earthing (earth connection of all cabinets areconnected to the power system earth electrode at one point)is used for earthing electronic equipment operating at lowfrequencies, say up to 300 kHz. This method is effective inpreventing circulating earth currents which can producecommon mode noise.

For equipment operating at high frequencies, multiple-point earthing system should be used. The signal common ofelectronic equipment is tied to the metallic cabinet of theequipment. Each cabinet is further connected to earth at thenearest point. At times, a signal reference grid (SRG) isinstalled beneath the area where cabinets are placed forfacilitating implementation of multiple –point earthingsystem. SRG is a local closely meshed grid tied to mainearthing system.

Measuring the impedance of EarthElectrode Systems

Measurement of the ohmic value of a buriedelectrode is carried out for two reasons:-

• To check the value, following installation and prior toconnection to the equipment, against the designspecification.

• As part of routine maintenance, to confirm that thevalue has not increased substantially from its designor original measured value.

The most common method of measuring the earthresistance is the “fall of potential” method (section37.1 of IS 3043:1987).

Measurement of Earth Resistance

The measurement of earth resistance is done using threeterminal earth meggars or four terminal earth meggars.

Four Terminal: Four spikes are driven in straight line into theground at equal intervals. The two outer spikes are connectedto current terminals of earth meggar and the two inner spikesto potential terminals of the meggar. Then the earth resistanceis measured by rotating the meggar till a steady value isobtained.

Three Terminal: Two temporary electrodes are spikes aredriven in straight line one for current and the other voltage at adistance of 150 feet and 75 feet from the earth electrode undertest and ohmic values of earth electrode is read in the meggar.

Combined earth resistance shall be the same at every earthpit unless it gets disconnected from the earth mat

Measurement of Earth Resistivity

a) Power stations 0.5 ohms

b) EHT Stations 1.0 ohms

c) 33KV SS 2 ohms

d) DTR(distributed transformer) 5 ohms Structures

e) Tower foot resistance 10 ohms

Permissible values of earth resistance

Testing for Earth Continuity Path

This test is conducted to ensure proper earthing of all the metallic parts used in theinstallation. For this test, we need an instrument called earth continuity tester, which iscapable of measuring low- value resistances. Before the test is conducted, we must ensurethatthe main switch is in OFF position,the main fuse is taken out, or main MCB is put OFF,all other fuses are in their position, or all other MCBs are ON,all switches are in ON position, andall lamps are in their holders.

Now, one terminal of the earth continuity tester is connected to an independent earth(the one which is not used in the installation) and the other terminal to a metallic part ofthe installation (say, the conduit or the main switch – board). The resistance as indicatedby the tester under these conditions should not be more than 1 . A high value of theresistance measured is indicative of improper or poor earthing of the installation.

Colour coding of wires:

Three phases, L1, L2, L3: Red, Yellow, BlueNeutral: Black

Ground/Protective earth: Green/yellow striped orgreen

Any Queries