Transcript testing grounding systems

TESTING GROUNDING
SYSTEMS
Originally Developed by:
RONALD J. RENOWDEN - CMSP
Denver Field Office Supervisor, MNM MSHA
Rocky Mountain District
Denver, Colorado
Enhanced by:
Roy W. Milam
Electrical Engineer/Instructor
National Mine Health and Safety Academy
Beckley, West Virginia
TESTING
GROUNDING
SYSTEMS
As required by:
30 CFR §56/57.12028
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.
P
O
L
I
C
Y
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;
P
O
L
I
C
Y
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.
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 SYSTEM
has 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 Line
Most important electrical safety item
in the electrical system
People protector
Equipment Grounding Conductors
GROUNDING BUS maybe in MCC
Motor 1
Motor 4
Motor 2
Motor 3
GROUNDING ELECTRODE
CONDUCTOR
Conductor that connects the grounding
electrode and the transformer to the
equipment grounding conductor
T
R
A
N
S
F
O
R
M
E
R
Main Ground Bus
maybe in the MCC
Earth
Grounding
Electrode
GROUNDING ELECTRODES
Usually 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.
Grid
Rod
EQUIPMENT GROUNDING
CONDUCTORS
Test for Continuity and Resistance
Immediately after INSTALLATION,
REPAIR, OR
MODIFICATION, AND
ANNUALLY, IF conductors are
subjected to
VIBRATION
FLEXING
CORROSIVE ENVIRONMENTS
GROUNDING ELECTRODE
CONDUCTOR
Test for Continuity and Resistance
Immediately after INSTALLATION,
REPAIR, OR
MODIFICATION, AND
ANNUALLY, IF conductors are
subjected to
VIBRATION
FLEXING
CORROSIVE ENVIRONMENTS
GROUNDING ELECTRODES
Testing
Test for Resistance (Earth)
Immediately after INSTALLATION,
REPAIR, OR
MODIFICATION, AND
ANNUALLY 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 CONDUIT
ARMORED CABLE
RACEWAYS
CABLE TRAYS, ETC
(there’s more)
MORE EXCEPTIONS
These FIXED LOCATIONS
MUST NOT be SUBJECTED TO:
HOW MUCH IS TOO MUCH ?
HOW MUCH IS TOO MUCH ?
The inspector makes the
final determination
concerning areas
acceptable to visual
examinations
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
E
R
I=
R
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 paths 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 ohm
 Screen Deck = .5 ohm
 Stacker main motor = 1.5 ohm
 Conveyor 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, 3PHASE
ØA-ØB=460v;
ØA-ØC=460v;
ØB-ØC=460v;
ØA-grd=265V
ØB-grd=265V
ØC-grd=265V
OA
OB
B
A
C
OC
Ground
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.
I=
265v
1ohm
=265 amp FC; On 30 amp
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:
I=
.5 ohms, Ø-grd=265v
265v
=530
.5ohm
amp FC; On 100 amp
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
265v
I=
=265 amp FC;
300 amp fuses
1ohm
265a.(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.
265v
I=
=265 amp FC; CB set 150 Amps
1ohm
265a.(FC)/150a.(Inst.) = 1.77 ...What’s this mean?
Breaker will TRIP instantaneously.
 GOOD GROUNDING!
Corner Grounded Delta
Corner Grounded Delta Service; 460 V, 3PHASE
B
ØA-ØB=460v; ØA-grd=0V
ØA-ØC=460v; ØB-grd=460V
ØB-ØC=460v; ØC-grd=460V
OA
OB
C
OC
A
Ground
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.
480v
I=
=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:
I=
480v
.5ohm
.5 ohms, Ø-grd=480v
=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 !
Corner Grounded Delta
CRUSHER: 1 ohm, Ø-grd=480v
480v
I=
=480 amp FC;
300 amp fuses
1ohm
480a.(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
SYSTEMS
BE 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.
Set-Up for Testing Grounding Electrode
Resistance
C1 P1 G P2 C2
Earth
Grounding
Electrode
Test Rods
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 MultiMeter 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.