TESTING GROUNDING SYSTEMSOriginally Developed by:

RONALD J. RENOWDEN - CMSPDenver Field Office Supervisor, MNM MSHA

Rocky Mountain DistrictDenver, Colorado

Enhanced by:

Roy W. Milam

Electrical Engineer/Instructor

National Mine Health and Safety Academy

Beckley, West Virginia

TESTING GROUNDIN

G SYSTEMS30 CFR §56/57.12028

As required by:

56/57.12028 the REG.

“Continuity and resistance of grounding systems shall be tested immediately after installation, repair, and modification; and annually thereafter. A record of the resistance measured during the most recent test shall be made available on request by the Secretary or his duly authorized representative.”

WHY DO WE TEST ?

Ensure that continuity & resistance tests are conducted on a specific schedule (at least annually)

Alert mine operators if there is a problem in the grounding system

Problem may not allow the circuit protective devices to quickly operate when faults occur

WHY DO WE TEST?

With exception of fixed installations - Numerous fatalities and injuries have occurred due to HIGH resistance or LACK of continuity in equipment grounding systems

Proper testing and maintenance of grounding systems can prevent electrical accidents

This all sounds so good that we made it the policy regarding 12028.

56/57.12028 the POLICY

The intent of this standard is to ensure that continuity and resistance tests of grounding systems are conducted on a specific schedule. These tests will alert the mine operator if a problem exists in the grounding system which may not allow the circuit protective devices to quickly operate when faults occur. With the exception of fixed installations, numerous fatalities and injuries have occurred due to high resistance or lack of continuity in equipment grounding systems. These accidents could have been prevented by proper testing and maintenance of grounding systems.

Grounding systems typically include the following:

1. equipment grounding conductors - the conductors used to connect the metal frames or enclosures of electrical equipment to the grounding electrode conductor; 2. grounding electrode conductors - the conductors connecting the grounding electrode to the equipment grounding conductor; and 3. grounding electrodes - usually driven rods connected to each other by suitable means, buried metal, or other effective methods located at the source, to provide a low resistance earth connection.

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Operators shall conduct the following tests: 1. equipment grounding conductors - continuity and resistance must be tested immediately after installation, repair, or modification, and annually if conductors are subjected to vibration, flexing or corrosive environments;

2. grounding electrode conductors - continuity and resistance must be tested immediately after installation, repair, or modification, and annually if conductors are subjected to vibration, flexing or corrosive environments; and

3. grounding electrodes - resistance must be tested immediately after installation, repair, or modification, and annually thereafter.

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56/57.12028 the POLICY

Conductors in fixed installations, such as rigid conduit, armored cable, raceways, cable trays, etc., that are not subjected to vibration, flexing or corrosive environments may be examined annually by visual observation to check for damage in lieu of the annual resistance test. When operators elect to conduct this visual examination as a method of compliance with 30 CFR56/57.12028, MSHA will require that a record be maintained of the most recent annual visual examination.

56/57.12028 the POLICY

Grounding conductors in trailing cables, power cables, and cords that supply power to tools and portable or mobile equipment must be tested as prescribed in the regulation. This requirement does not apply to double insulated tools or circuits protected by ground-fault-circuit interrupters that trip at 5 milli-amperes or less.

56/57.12028 the POLICY

Testing of equipment grounding conductors and grounding electrode conductors is not required if a fail-safe ground wire monitor is used to continuously monitor the grounding circuit and which will cause the circuit protective devices to operate when the grounding conductor continuity is broken.

56/57.12028 the POLICY

A record of the most recent resistance tests conducted must be kept and made available to the Secretary or his authorized representative upon request. When a record of testing is required by the standard, MSHA intends that the test results be recorded in resistance value in ohms.

That’s all the policy! But what does it all mean ?

TYPICAL GROUNDING SYSTEMhas three parts

EQUIPMENT GROUNDING CONDUCTORS(the most important part!!!)

Grounding Electrode Conductors

Grounding Electrodes

EQUIPMENT GROUNDING CONDUCTOR

The conductors used to connect the metal frames or enclosures of electrical equipment to the grounding electrode conductor

Life Wire; Life LineMost important electrical safety

item in the electrical system People protector

Equipment Grounding Conductors

GROUNDING BUS maybe in MCC

Motor 1 Motor 2 Motor 4

Motor 3

GROUNDING ELECTRODE CONDUCTOR

Conductor that connects the grounding electrode and the transformer to the equipment grounding conductor Main Ground Bus

maybe in the MCC

Earth

Grounding

Electrode

TRANSFORMER

GROUNDING ELECTRODESUsually driven rods connected to each

other by suitable means, buried metal plates, or any other effective methods located at the power source to provide a low resistance earth connection.What is actually in contact with Earth.

GridRod

EQUIPMENT GROUNDING CONDUCTORS

Test for Continuity and Resistance

Immediately after -INSTALLATION,REPAIR, ORMODIFICATION, ANDANNUALLY, IF conductors are

subjected toVIBRATIONFLEXINGCORROSIVE

ENVIRONMENTS

GROUNDING ELECTRODE CONDUCTOR

Test for Continuity and Resistance Immediately after -

INSTALLATION,REPAIR, ORMODIFICATION, ANDANNUALLY, IF conductors are

subjected toVIBRATIONFLEXINGCORROSIVE

ENVIRONMENTS

GROUNDING ELECTRODES

Testing

Test for Resistance (Earth)

Immediately after -INSTALLATION,REPAIR, ORMODIFICATION, ANDANNUALLY thereafter.

Are there EXCEPTIONS ? YES !!!

In lieu of doing annual resistance tests, the Grounds may be examined annually by visual observation to check for damage….. BUT ONLY IF:The conductors are in fixed locations such as;

RIGID CONDUITARMORED CABLERACEWAYSCABLE TRAYS, ETC (there’s more)

MORE EXCEPTIONSThese FIXED LOCATIONS

MUST NOT be SUBJECTED TO:

HOW MUCH IS TOO MUCH ?

The inspector makes the final determination concerning areas acceptable to visual examinations

HOW MUCH IS TOO MUCH ?

IF VISUAL EXAMS ARE CONDUCTED

MSHA policy requires that a WRITTEN RECORD be maintained of the most recent ANNUAL VISUAL EXAMINATION, can be electronic

EQUIPMENT GROUNDING CONDUCTORS IN:

Trailing cables

Power cables

Cords supplying power to tools

Cords and cables supplying power to portable or mobile equipment

MUST BE TESTED AS PRESCRIBED IN THE REGULATION

When Is Testing NOT Required?IF a fail-safe ground wire monitor is used. The monitor will cause the circuit protective device to open when the grounding conductor continuity is broken. It continuously monitors the continuity of the grounding conductor.

Double insulated tools are used. There is no grounding conductor to test.

If the circuit used incorporates a GFCI that trips at 5 milli-amps or LESS. There are 110v and 220v devices now available.

RECORDS

RESISTANCE VALUES IN OHMS

Most recent tests Must be available for review by

MSHA upon request No specific format required

Be legible Visual exam records

A little ADVICE ? Should have electrical knowledge Know how to test and use the

instrument/PPE Know what kind of electrical system Check circuit protective device vs.

Ohms test Check with power off Proper size ground wire Never let metal framework or earth be

the primary grounding conductor Never use peg grounding

NATIONAL ELECTRICAL CODE

Section 250-51-EFFECTIVE GROUNDING PATH: The path to ground from circuits, equipment, and metal enclosures for conductors shall:

Be permanent and electrically continuous

Have capacity to conduct safely any fault current likely to be imposed on it, and

Have sufficient low impedance to limit the voltage to ground and to facilitate the operation of the circuit protective

devices.

The earth SHALL NOT be used as the sole equipment grounding conductor

OK, now that the operator has tested his grounding circuit and recorded the results, what do we do with them?

In other words……

How do you know if the ground wire resistance is okay?

Can apply good old OHM’S LAW: E = IR E = VOLTS

I = AMPS (current)R = RESISTANCE

(ohms)

E I =

R

EE

II RR

BUT …..FIRST !Remember OHM’S LAW ! ( E = IR )Circuits are protected by fuses/breakers against

SHORT CIRCUITS AND GROUND FAULTS IF GF occurs, we want the fuse/breaker to clear

ASAP!

Question? Where does the current go when a ground fault occurs?

GROUNDING CONDUCTOR carries the fault current back to the SOURCE (current does not return to earth, but sometimes travels thru it).

FAULT CURRENT takes path of LEAST resistance?

CONTINUED

IF ground wire is missing or high resistance, current will flow in dangerous places to return to the source.

TOUCH AND STEP POTENTIALS MAY EXIST ON THE FRAMES OF EQUIPMENT AND ON THE EARTH

HAZARDOUS!!!!! : SHOCK, BURNS, ELECTROCUTION……..DEATH

EXAMPLE:

The operator’s record of resistance testing (in OHMS) shows the following:

Crusher motor = 1 ohmScreen Deck = .5 ohmStacker main motor = 1.5 ohmConveyor 1 = 1 ohm

Example:What’s Next ? Need: Circuit/System VOLTAGE:

Motors supplied with 3 phase, 460 volt AC Grounded WYE Service; Ø-Ø=460v; Ø-

grd=265v

Need: Fuse/Circuit Breaker Size (for each) 100hp, 125amps, 3-fuses @ 300amps each

(crusher motor)

40hp, 52amps, 3-fuses @ 100amps each (screen)

25hp, 34amps, 70 amp breaker (stacker) 10hp, 14amps, 30 amp breaker (conveyor 1)

GROUNDED WYE Grounded WYE Service; 460 V, 3-PHASE

ØA-ØB=460v; ØA-grd=265VØA-ØC=460v; ØB-grd=265V ØB-

ØC=460v; ØC-grd=265V

A B

C

GroundO C

O A

O B

Ground

How good is the equipment grounding?

CONVEYOR 1: 1ohm, Ø-grd=265v. How much FC will flow if GF at motor frame? To find current (ohms law) I= E/R.

265v I = =265 amp FC; On 30 amp 1ohm

breaker

265a.(FC)/30a(bkr) = 8.83...What’s this mean?Breaker will TRIP almost instantaneous level

ASAP GOOD GROUNDING!

LET’S CHECK ANOTHER ! SCREEN: .5 ohms, Ø-grd=265v

265vI = =530 amp FC; On 100

amp .5ohm fuses

530a.(FC)/100a.(fu) = 5.30...What’s this mean?Fuse will not TRIP near instantaneous level

(ASAP) GOOD GROUNDING ? Questionable! Review

trip curve chart for fuse to find trip time….ADVICE: Check connections; loose, dirty, wire

size? CORRECT/ REPAIR! Screens- flexing and vibration!

LET’S CHECK ANOTHER ! CRUSHER: 1 ohm, Ø-grd=265v

265vI = =265 amp FC; 300 amp

fuses 1ohm265a.(FC)/300a.(fu) = .88 ...What’s this

mean?Fuse will not BLOW (must see at least 125%

to begin)DANGEROUS GROUNDING

UNACCEPTABLE! ADVICE: Check connections; loose, dirty,

wire size? CORRECT & REPAIR! Crushers - flexing and vibration

Let’s make a small change!!!CRUSHER: 1 ohm, Ø-grd=265v, but this time

we’re going to use a 100 Amp, Magnetic Trip Circuit Breaker with a trip range of 150-480 amps, set on LOW, as protection.

265vI = =265 amp FC; CB set 150 Amps

1ohm265a.(FC)/150a.(Inst.) = 1.77 ...What’s this mean?

Breaker will TRIP instantaneously.

GOOD GROUNDING!

Corner Grounded Delta Service; 460 V, 3-PHASE ØA-ØB=460v; ØA-grd=0V ØA-ØC=460v; ØB-grd=460V ØB-ØC=460v; ØC-grd=460V

Corner Grounded Delta

C

B

A O C

Ground

O A

O B

Corner Grounded Delta

CONVEYOR 1: 1ohm, Ø-grd=480v. How much FC will flow if GF at motor frame? To find current (ohms law) I= E/R.

480vI = =480 amp FC; On 30 amp

breaker 1ohm

480a.(FC)/30a(bkr) = 16.0...What’s this mean?

Breaker will TRIP at instantaneous level ASAP GOOD GROUNDING !

Corner Grounded Delta

SCREEN: .5 ohms, Ø-grd=480v480v

I = .5ohm =960 amp FC; On 100 amp

fuses

960a.(FC)/100a.(fu) = 9.60...What’s this mean?

Fuse will TRIP near instantaneous level (ASAP)

GOOD GROUNDING !GOOD GROUNDING !

Corner Grounded Delta CRUSHER:CRUSHER: 1 ohm, Ø-grd=480v

480vI = =480 amp FC; 300 amp

fuses 1ohm480a.(FC)/300a.(fu) = 1.6 ...What’s this mean?Fuse will start heating up. (must see at least 125% to

begin) Will blow on very long delay.DANGEROUS GROUNDING

UNACCEPTABLE! ADVICE: Check connections; loose, dirty, wire

size? CORRECT & REPAIR! Crushers - flexing and vibration

FLOATING or UNGROUNDED SYSTEMSBE AWARE!

HAZARDOUS- IF NOT MONITORED FOR GROUNDS AND MAINTAINED

ELECTRICAL SHOCK

THERMAL ARC FLASH BURNS

ELECTROCUTION

FIRES

Now let’s look at some of the test instruments that can be used to test the grounding system

Instruments used to test the grounding electrode

This is a Biddle Earth Tester that is battery powered. It uses 6 “C” cells. It is used to test the grounding electrode resistance.

Measuring the resistance of the equipment grounding conductor for this piece of equipment.

This a type of Biddle Earth Tester that is battery powered. It is powered by 4 “C” cells. It is used to test the grounding electrode resistance.

This is a Biddle Earth Tester that is used to test the resistance of the grounding electrode. It is powered by the use of the hand crank on the side of the device.

This is the Field Kit that is used with an Earth Tester to measure the resistance of the grounding electrode. It contains 2 metal stakes, and 3 conductors of various lengths.

Earth

Grounding Electrode

C1 P1 G P2 C2

Test Rods

Set-Up for Testing Grounding Electrode

Resistance

This person is driving a test rod to be used while testing the grounding electrode resistance.

This is the method used when testing the grounding electrode resistance with a Biddle Earth Tester.

The results obtained during the testing of the grounding electrode resistance are plotted on a graph. The actual resistance of the electrode is the flat part of the graph.

Another method of testing the resistance of the grounding electrode is using a voltage source, such as a vehicle battery, a voltmeter and an ammeter. We do not recommend this method as it can be extremely dangerous.

This is one possible set-up for testing the grounding electrode resistance.

This is the method used when testing the grounding electrode resistance with a Biddle Earth Tester.

Use OHM’S LAW to calculate the

resistance values your tests found.

R=E/I

The results obtained during the testing of the grounding electrode resistance are plotted on a graph. The actual resistance of the electrode is the flat part of the graph.

This is the ECOS Pow-R-Mate Tester. It is used to test the resistance and impedance of the grounding electrode. It is both battery powered and plug-and-cord connected.

This is a Biddle Insulation Tester that is used to test the insulation value of conductors, motors, transformers, and cables. It is powered by the hand crank on the side of the device. This device should not be used to test the grounding electrode nor the equipment grounding conductors.

Instruments used to test the grounding electrode

conductor and the equipment

grounding conductors

This is a Fluke Graphics Multi-Meter which can be used to measure the resistance of the equipment grounding conductors.

This is a Fluke Multi-Meter which can be used to measure the resistance of the equipment grounding conductors.

This is a Simpson Multi-Meter which can be used to measure the resistance of the equipment grounding conductors. It is reading an open conductor now.

This is a Simpson Multi-Meter, with test leads attached, which can be used to measure the resistance of the equipment grounding conductors. It is reading an open conductor now.

This is a Simpson Multi-Meter, with test leads attached, which can be used to measure the resistance of the equipment grounding conductors. It is reading Zero ohms resistance as the test leads are contacting each other. The meter is being zeroed.