HRGU w/ Ground Fault Enabled on Breakers

[mboutal]

Hi All,
Have you ever seen a High Resistance Grounded Unit System w/ ground fault enabled on breakers. What I know is with radial ground fault CT's in breakers, you cannot detect that small current (5-10A). You usually need to disable those CTs and have an external CTs with a relay. I need to know why is required sometimes to have GF protection on breakers with HRGU. HRGU is there to reduce the amount of current at the time when fault occurs. Could you please describe how HRGU works scientifacly with examples? I need to make sure I understand this system theoretical, please help! I also want to know why some they want with ground fault protection, you trip the breaker responsible on the zone where the fault happens.

Thanks a lot,

 

[Ingenieur]

Common in mining
the reason to trip is personnel protection
a person may be parallel to the v-g fault path
say your nrg is 10 A and ground loop R is 4 ohm
fram v would be 40 v and not extremely dangerous

 

[mboutal]

Common in mining
the reason to trip is personnel protection
a person may be parallel to the v-g fault path
say your nrg is 10 A and ground loop R is 4 ohm
fram v would be 40 v and not extremely dangerous

Thank you very much for your response.

So why they need HRGU system then, they can get just the regular GF protection on breakers?

 

[Ingenieur]

Thank you very much for your response.

So why they need HRGU system then, they can get just the regular GF protection on breakers?

it limits fault damage and performs 2 functions
prevents equipment damage (cables, motors, etc)

lowers shock current exposure
assume a 1000 ohm person
if they cause a 277 mA to gnd fault the current is 277 mA
the CB would need set very low 250 or less to trip
nuisance tripping due to charging current, cable leakage, etc.

if set at 10A which is unlikely to have nuisance tripping
ground loop 1 ohm
person 1000
if he is in parallel with the fault, say 277 A
he would see 1/1000 x 277 or 277 mA until it tripped, if quick enough he would likely survive
if he caused the fault (in series) he still sees 277 mA (<10 A) so no trip and he probably does not survive

in my jurisdiction we are re-writing the law to set the gf at 125 mA (down from 6 A on a 15 A ngr)

 

[Tony S]

I think you’ll find that in mining transformers the leakage current is limited to stop any possibility of arcing at connections in the ground loop. It has nothing to do with the risk of shock to operatives, more to do with stopping gas explosions.

 

[Ingenieur]

I think you’ll find that in mining transformers the leakage current is limited to stop any possibility of arcing at connections in the ground loop. It has nothing to do with the risk of shock to operatives, more to do with stopping gas explosions.

I do not agree
there are 2 areas in a mine
the inby or face which is considered/classified, where the coal is mined
outby or open with no exp risk, worked out areas or travelway

in face areas everything is xp or is
ventilation is used to keep atm < lel
for methane 5%
if it hits 2 all power is disconnected back at the open area
that is how explosions are prevented
no xfmrs at the face are exposed to atm
and most are outby

the enclosures are designed to contain an explosion
say the seal fails
gas ingress occurs somehow >5%
contactor arc ignites mixture
the enclosure flame paths cool hot gas to < ignition temp of methane or coal

I work as the managing electrical engineer for one of the oldest governmental mine safety bureaus in the world

Currently gf protection is trip power on fault
primarily equip protection but affords some personnel protection

all ckts are also monitored for egc integrity
if the ckt goes open the cb trips
the cb can't be set until continuity is verified

 

[Ingenieur]

This is a combo vfd/motor used for a longwall chain drive
1500 hp 4160
perhaps $500k
it is currently in the approval process

 

[Ingenieur]

Many portable cables in mining
gf are typically 80-90% of all faults
ph-ph, 2 ph-gn, etc the balance
limiting the gf current to 15 A (actually 40% or 6 A due to the protective relaying) essentially eliminates arc flash hazard
at least greatly reduces it
mining involves a lot of troubleshooting equipment
unfortunately sometimes it must be done hot
a 480 ckt
which would you prefer
a gf to a 0.1 egc
or a 18.4 ohm ngr
???

 

[Sahib]

I need to know why is required sometimes to have GF protection on breakers with HRGU. HRGU is there to reduce the amount of current at the time when fault occurs. I also want to know why some they want with ground fault protection, you trip the breaker responsible on the zone where the fault happens. ,

Another function of HRG is to reduce arc flash hazard. The design may be to retain that function only but to trip the system immediately on ground fault by using ground fault protection in the breaker.

 

[jim dungar]

Another function of HRG is to reduce arc flash hazard.

HRGU does absolutely nothing for reducing the arc flash hazard.
An HRGU system reduces the probability of an arc flash occurring and therefore the risk.

 

[Ingenieur]

HRGU does absolutely nothing for reducing the arc flash hazard.
An HRGU system reduces the probability of an arc flash occurring and therefore the risk.

in a way it does
assume working live 480
inadvertant gf initiated by an errant tool
egc several 1000 A
ngr 15 A
One could say it mitigates
one could say the risk/probability remains the same but the consequence/magnitude is lessened
I guess it's semantics

 

[Sahib]

HRGU does absolutely nothing for reducing the arc flash hazard.

Wrong. Out of all hazards of line to line, three lines, line to neutral etc faults, the line to ground fault is most frequent. By reducing its occurence to zero, a HRG makes a system less hazardous.

 

[wwhitney]

The terminology Jim is using distinguishes between how bad an event would be (the hazard) and how likely it is to occur (the probability). Risk is then defined by Risk = Hazard * Probability.

So as the worst case scenario is the same in an HRGU system then the hazard is the same. But as the probability of that scenario is reduced, because a line to ground fault no longer causes it, then the risk is reduced.

Sure, it's semantics, but I believe it is the standard semantics of risk assessment.

Cheers, Wayne

 

[Sahib]

HRGU does absolutely nothing for reducing the arc flash hazard.

Right. The personnel looking for a ground fault should wear fully rated PPE even in a HRG system.

 

[Ingenieur]

The terminology Jim is using distinguishes between how bad an event would be (the hazard) and how likely it is to occur (the probability). Risk is then defined by Risk = Hazard * Probability.

So as the worst case scenario is the same in an HRGU system then the hazard is the same. But as the probability of that scenario is reduced, because a line to ground fault no longer causes it, then the risk is reduced.

Sure, it's semantics, but I believe it is the standard semantics of risk assessment.

Cheers, Wayne

both hazard and probability go down
the intensity or hazard is reduced
and so is the probability since a gf is 80-90% of all events and they cease to be an arc flash event since 6 A (in my example) is not capable of creating a hazardous flash (at 277)

 

[jim dungar]

both hazard and probability go down

The hazard does not go down. The IEEE1585 arcing fault incident energy calculations are only concerned if the system is solidly grounded or not. There is no single consensus methodology for dealing with single L-G faults, especially for <600V.

This is not a case of simple semantics.
NFPA70E uses specific definitions, although these definitions are likely to be different slightly different than what you might expect.

Hazard. A source of possible injury or damage to health.
Risk. A combination of the likelihood of occurrence of injury or damage to health and the severity of injury or damage to health that results from a hazard.

I agree that HRG does lower the 'Arc Flash Risk'.

 

[GoldDigger]

Both the hazard and the risk of a line to line arc fault remain unchanged except to the extent that some line to line faults start as ground faults. In the latter case, once again hazard is not changed but risk is reduced.

 

[Ingenieur]

The hazard does not go down. The IEEE1585 arcing fault incident energy calculations are only concerned if the system is solidly grounded or not. There is no single consensus methodology for dealing with single L-G faults, especially for <600V.

This is not a case of simple semantics.
NFPA70E uses specific definitions, although these definitions are likely to be different slightly different than what you might expect.

I agree that HRG does lower the 'Arc Flash Risk'.

The hazard does go down
as far as code/regulatory requirements they remain the same
all 3 go down: hazard, probability and as a consequence risk

 

[Ingenieur]

Both the hazard and the risk of a line to line arc fault remain unchanged except to the extent that some line to line faults start as ground faults. In the latter case, once again hazard is not changed but risk is reduced.

If you you remove 80% of all faults from the set of 100 probability goes down
if for those faults the energy level is exp reduced the hazard goes down
try doing a calculation for 277/6 A, I doubt most sw will do it
the min is usually 500-700 A range
If both go down risk must go down

 

[GoldDigger]

If you you remove 80% of all faults from the set of 100 probability goes down
if for those faults the energy level is exp reduced the hazard goes down
try doing a calculation for 277/6 A, I doubt most sw will do it
the min is usually 500-700 A range
If both go down risk must go down

But if you measure hazard (by itself) as the magnitude of the worst possible event (in this case line to line or three phase fault) then the reduction of the magnitude and probability of smaller faults does not change the hazard number.