High Resistance Grounding
- Thread starter danilo
- Start date
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ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: High Resistance Grounding
Many HRG installation that I've seen, use the fact that OCPD's don't trip during a ground fault, as an opportunity to not fix ground faults for days .... weeks at a time, making it an unsafe place to work.
Why do you want to change?
Many HRG installation that I've seen, use the fact that OCPD's don't trip during a ground fault, as an opportunity to not fix ground faults for days .... weeks at a time, making it an unsafe place to work.
Why do you want to change?
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: High Resistance Grounding
I guess my first question would be: Do you meet the criteria of 250.36, particularly items 1 and 2?
By the way, I agree with Ron 100%
I guess my first question would be: Do you meet the criteria of 250.36, particularly items 1 and 2?
My next question would be...240.4(A) alows you to eliminate overload protection if you are in a building that would allow a high impedance system. Would this be an option?
By the way, I agree with Ron 100%
rcwilson
Senior Member
- Location
- Redmond, WA
Re: High Resistance Grounding
A solidly grounded system supplies the maximum fault current to the first ground fault. This trips the ground fault or overcurrent protection quickly. The trip shuts down the circuit and could shut down the process or the plant.
The solid ground exposes the electrician to high levels of arc flash hazard from single ground faults.
High resistance grounding limits the fault current to about 5 amps to keep the fualted circuit operating wihtout damage. A fault detector alarms when the first ground fault occurs. If the fault is not corrected soon, a second one is more likely to occur on another phase creating a phase-phase-ground fault with more fault current than a "normal" solid ground fault.
We put timers to trip the circuit in 24 hours if the fault was not cleared. A siren and a rotating light also call attention to it. So we had good success with clearing the faults before the second one occurred.
Our systems had a tracing circuit that pulsed the fault current from 5 to 10 amps about once a second. A clamp-on ammeter placed on the conduits or cables leaving the MCC would show the pulsing current on just the faulted circuit. We could follow the conduits right to the fault location. This allowed faster location and repair. We documented savings approaching $1.5M a year by not shutting down the refinery.
The high resistance ground also means that the first ground fault cannot create an arc. The phase-to-phase arc flash hazard is still there, but when a screw drops between ?A? phase and ground, nothing happens except an alarm. (NFPA 70E does not recognize this benefit because the arc flash the phase-to-phase fault hazard is still present and there's teh hazard of a second fault.)
One disadvantage is no phase-neutral loads on a high resistance grounded system. 480-480/277 isolating transformers are needed if you have any 277 V lights.
A solidly grounded system supplies the maximum fault current to the first ground fault. This trips the ground fault or overcurrent protection quickly. The trip shuts down the circuit and could shut down the process or the plant.
The solid ground exposes the electrician to high levels of arc flash hazard from single ground faults.
High resistance grounding limits the fault current to about 5 amps to keep the fualted circuit operating wihtout damage. A fault detector alarms when the first ground fault occurs. If the fault is not corrected soon, a second one is more likely to occur on another phase creating a phase-phase-ground fault with more fault current than a "normal" solid ground fault.
We put timers to trip the circuit in 24 hours if the fault was not cleared. A siren and a rotating light also call attention to it. So we had good success with clearing the faults before the second one occurred.
Our systems had a tracing circuit that pulsed the fault current from 5 to 10 amps about once a second. A clamp-on ammeter placed on the conduits or cables leaving the MCC would show the pulsing current on just the faulted circuit. We could follow the conduits right to the fault location. This allowed faster location and repair. We documented savings approaching $1.5M a year by not shutting down the refinery.
The high resistance ground also means that the first ground fault cannot create an arc. The phase-to-phase arc flash hazard is still there, but when a screw drops between ?A? phase and ground, nothing happens except an alarm. (NFPA 70E does not recognize this benefit because the arc flash the phase-to-phase fault hazard is still present and there's teh hazard of a second fault.)
One disadvantage is no phase-neutral loads on a high resistance grounded system. 480-480/277 isolating transformers are needed if you have any 277 V lights.
Re: High Resistance Grounding
in our building which is a mixed use,power reliability and continuity of it is the top priority. With our solidly grounded system(btw,we dont have 277 system),several ATS have been tripping one at a time every year caused by ground fault. And so, when a vendor introduced his product as a solution to this unnecessary power system shutdown,i bought his idea but not the product yet maybe until such time i get professional opinions from you guys.
appreciate more inputs from you.
thanks,
Danilo
hi,
in our building which is a mixed use,power reliability and continuity of it is the top priority. With our solidly grounded system(btw,we dont have 277 system),several ATS have been tripping one at a time every year caused by ground fault. And so, when a vendor introduced his product as a solution to this unnecessary power system shutdown,i bought his idea but not the product yet maybe until such time i get professional opinions from you guys.
appreciate more inputs from you.
thanks,
Danilo
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: High Resistance Grounding
Danilo,
I wouldn't use the HRG as a band-aid for cable failure problems. Is there a reason you are experiencing cable failure problems?
Maybe the effort needs to be forensic in nature, rather than system modification.
Danilo,
I wouldn't use the HRG as a band-aid for cable failure problems. Is there a reason you are experiencing cable failure problems?
Maybe the effort needs to be forensic in nature, rather than system modification.
Re: High Resistance Grounding
thanks..cable problem?yeah maybe..but my concern is more than fixing cable problems but more of providing a safe,reliable and trouble-shooting friendly system thats why i opt to try this HRG out.
what do you mean by forensic in nature?
regards,
Danilo
hi Ron,
thanks..cable problem?yeah maybe..but my concern is more than fixing cable problems but more of providing a safe,reliable and trouble-shooting friendly system thats why i opt to try this HRG out.
what do you mean by forensic in nature?
regards,
Danilo
T
taylorp
Guest
Re: High Resistance Grounding
danilo:
Buenas noches, Yo soy Pablo.
I am not a big fan of High-Resistance grounding systems UNLESS you ABSOLUTELY must have continuity of power. Since you said that continuity of power is the top priority, here are my pros and cons:
PROS:
Power stays on during first ground fault.
Somewhat reduced arc-flash hazard.
Reduces damage to equipment compared
to solidly grounded systems.
CONS:
No ground reference for test equipment.
Arc-flash hazard is still present from phase
to phase, so protective clothing is still
required (at least in the U.S.)
HRG systems require greater training and
troubleshooting skills on the part of your
electrical maintenance personnel. (No more
quick fault locating.)
HRG systems expose electricians to increased shock
hazards (in my opinion) because all troubleshooting must be done with the equipment energized.
HRG systems cannot serve line-to-neutral
loads. Isolation transformers must be used
for line-to-neutral circuits.
Ground detectors are required on the HRG
system, electricians must respond QUICKLY to
clear any faults.
Faults are harder to find unless you install a
tracing circuit and/or equip your electricians with
flexible-current probes and meters for use with
pulsers, as RCWILSON has pointed out.
HRG systems do not fix your ground-fault
problems, they only buy you a little time to
find the problem, as RON has pointed out.
But hey at least you are still running.
HRG is limited to systems operating from
380 V up to 5 kV. Above 5 kV, more careful
evaluation is required due to the probability
of increased capacitive charging currents on
the system.
In conclusion:
If you must have continuity of power, and your
system is 380 V to 5 kV, then HRG is probably
your only solution. But I would question you as
to why your selective co-ordination is not keeping your ATS's from tripping on [other] ground faults? Sounds like you might have a design problem and this vendor is trying to sell you a HRG system so he can make money????
danilo:
Buenas noches, Yo soy Pablo.
I am not a big fan of High-Resistance grounding systems UNLESS you ABSOLUTELY must have continuity of power. Since you said that continuity of power is the top priority, here are my pros and cons:
PROS:
Power stays on during first ground fault.
Somewhat reduced arc-flash hazard.
Reduces damage to equipment compared
to solidly grounded systems.
CONS:
No ground reference for test equipment.
Arc-flash hazard is still present from phase
to phase, so protective clothing is still
required (at least in the U.S.)
HRG systems require greater training and
troubleshooting skills on the part of your
electrical maintenance personnel. (No more
quick fault locating.)
HRG systems expose electricians to increased shock
hazards (in my opinion) because all troubleshooting must be done with the equipment energized.
HRG systems cannot serve line-to-neutral
loads. Isolation transformers must be used
for line-to-neutral circuits.
Ground detectors are required on the HRG
system, electricians must respond QUICKLY to
clear any faults.
Faults are harder to find unless you install a
tracing circuit and/or equip your electricians with
flexible-current probes and meters for use with
pulsers, as RCWILSON has pointed out.
HRG systems do not fix your ground-fault
problems, they only buy you a little time to
find the problem, as RON has pointed out.
But hey at least you are still running.
HRG is limited to systems operating from
380 V up to 5 kV. Above 5 kV, more careful
evaluation is required due to the probability
of increased capacitive charging currents on
the system.
In conclusion:
If you must have continuity of power, and your
system is 380 V to 5 kV, then HRG is probably
your only solution. But I would question you as
to why your selective co-ordination is not keeping your ATS's from tripping on [other] ground faults? Sounds like you might have a design problem and this vendor is trying to sell you a HRG system so he can make money????
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