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Transformer HRG Resistor Values
Posted by Erik from Wellsboro, PA, US on May 21, 2008
What size Resistor should be specified for a 2500KVA Transformer with a 480V WYE connected secondary? What happens if the value is smaller or larger than the “correct” value? What happens if you have two paralled 2500KVA Transformers each with a HRG? Should the HRG Value be adjusted then transformers are paralleled?
The size of the resistor is not dependant on the capacity of the transformer. The resistor size is mostly dependant on the system charging current. This can be either calculated or measured. Please go to www.i-gard.com and look at the application guides for information on measuring the charging current.
If the value of resistance is smaller than the "correct" value, then more current will flow through the point of fault. If this is still a low value, <10 A. , no harm will be done to the equipment.
If the value of resistance is larger than the "correct" value then less current will flow through the point of fault. If the magnitude of current is too low, and thre is an arcing fault, the system is susceptacle to extreme overvoltages.
IEEE & NFPA formulas
Posted by Michael from Portland, ME, US on May 8, 2008
The problem is though, IEEE and NFPA formulas are based on 3-phase bolted faults. HRG doesn’t even factor into the equation! Until they rewrite the formulas, there’s not much we can mathematically do.
According to IEEE 1584, using High Resistance Grounding System increases the Incident Energy by
approximately 30%. This is the only mathematical proof we have that High resistance Grounding
works. Now according to the Red Book 7.2.2 and the Green Book we can reduce the number of
Incidences that an Arc Flash may occur.
The purpose of IEEE 1584 is to calculate the Incident Energy in an Arc Flash. We can use the
information from the Red, Green, Buff Books to reduce the number of incidences.
An Arc Flash can occur on a High resistance Grounded System. When it does, we can calculate the
Incident Energy that we need to protect ourselves from, and wear the appropriate PPE. High
Resistance Grounding reduces the probability of a single phase fault escalating to a three phase fault
Existing installation
Posted by Jorge from Mexico, MX on May 6, 2008
What about existing installations? is just installing HRG that we solve possible risks? What else we should consider in a industrial installation?
HRG will limit ground fault currents to a low value. You should have an Arc-Flash Hazard assessment performed so you know what the hazards are. You should acquire the appropriate PPE and insure you have good training programs and safety policies. This is just a start.
compare with GFCI
Posted by Mark from Buffalo, NY, US on May 5, 2008
On it’s most basic level, is this the same principle as a GFCI outlet in a residential application?
No, I would equate a GFCI with a fuse. If the leakage current is greater than 5 mA then the fuse, GFCI would open clearing the fault from the system. High Resistance Grounding actually limits the fault current to a desired value. The GFCI does not limit , it only detects and isolates.
Arcing faults
Posted by John from Cleveland, OH, US on May 1, 2008
I would say that use of HRG will greatly reduce the risk of arcing faults on a 480 V system, since the large majority of faults begin life as a line-to-ground fault. But what the others are saying is that it does NOT reduce the hazard that can exist for a three-phase arcing fault, so the required PPE is still based on the worst-case three-phase fault. This is pretty clearly addressed in IEEE 1584. There is still an arc-flash hazard since the HRG has no effect on phase-phase or three-phase faults.
You are correct. Arc Flash calculations are based on the Arc Flash current, Time, Distance and Bolted Fault Data. There is no way that the grounding system can reduce the Arc Flash Hazard Analysis.
The only thing a High Resistance Grounding system can do is limit the fault current of a single phase to ground fault. Now that the fault current is limited to 5 A., the probability of that fault escalating to a phase to phase fault or a three phase fault is greatly reduced. In contrast a single phase to ground fault in a Solidly Grounded System has the highest probability of escalating to a three phase fault.
The Red Book is stating that most faults in the electrical industry start as Ground Faults. The probability of that fault escalating to a three phase fault in a Solidly Grounded System is High, whereas, the probability of that fault escalating to a three phase fault in a High Resistance Grounded System is Low. If the Statement 7.2.2 is interpreted this way, then the statement is true.
