No, Arc flash hazard is calculate from a 3 phase short circuit. In the IEEE 1584 equation, there is a factor involving if the system is grounded or ungrounded(NRG included), and the energy of the incident will be greater if the system is ungrounded
Just to be more specific, the system will be considered ungrounded if it's ungrounded or HRG (high resistance grounded). The arc flash result will be greater in theses cases per IEEE 1584.LRG grounded is considered as a grounded system.
IEEE Std 141-1993 (Red Book) section 7.2.4 states "A safety hazard exists for solidly grounded systems from the severe flash, arc burning, and blast hazard from any phase-to-ground fault." For this reason, IEEE recommends resistance grounding.
IEEE Std 142-1991 (Green Book) section 1.4.3 states "The reasons for limiting the current by resistance grounding may be one or more of the following:
1) To reduce burning and melting effects in faulted electric equipment, such as switchgear, transformers, cables, and rotating machines.
2) To reduce mechanical stresses in circuits and apparatus carrying fault currents.
3) To reduce electric-shock hazards to personnel caused by stray ground-fault currents in the ground return path.
4) To reduce the arc blast or flash hazard to personnel who may have accidentally caused or who happen to be in close proximity to the ground fault.
5) To reduce the momentary line-voltage dip occasioned by the clearing of a ground fault.
6) To secure control of transient over-voltages while at the same time avoiding the shutdown of a faulty circuit on the occurrence of the first ground fault (high resistance grounding).
IEEE Std 141-1993 (Red Book) section 7.2.2 states "There is no arc flash hazard, as there is with solidly grounded systems, since the fault current is limited to approximately 5A."
As you can see, it is best to not only ground the neutral, but ground through High-Resistance (typically 5A) for all systems < 600V and most systems > 600V to 5kV. For systems > 5kV, Low-Resistance Grounding (typically 200A or 400A) is used.
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.
What dpc said. All of the things that Humble2000 point out go to reducing the likelihood of an arc flash occurring, but if one does occur, the presence of the HRG does nothing to diminish the hazard.
Sure, I realize that. The situation I have right know is due to low arc fault current the clearing time for upstream fuse is long and therefore it exceeds cat 4 requirements. If I do the calculation based on three phase faults then the clearing time would be fine and the incident energy would be reduced since the fuse would clear the fault instantaniously.
So, in this situation NGR would limit the arc fault current since it is same as L-G fault .right?
Once established the arc will involve all three phases and the arcing current will be seen by your phase protection. The arc current will not be limited by the NGR.
Humble, what fault current do you take? Equation are only made for a 3 phase bolted short circuit. If in this case the clearing time is very long, you can take 2 sec. max as per IEEE 1584.
I could reduce it to 1.2 second using faster fuses , but thats it. however since the arcing current is only around 1200Amps on the 13.8 side the cal is around 42.
transformer is 4MVA,600 secondary.
As dpc and others have repeatedly said, NGR does not come in play for L-L faults or 3 ph faults. Since the NGR does not have any effect on L-L fault currents, the PPE requirement do not change with grounded or ungrounded or impedance grounded systems.
Yes, in case of juat L-G fault the fault current would be less and flash/burn hazard would be lower, but basis of IEEE recommendations is that a L-G fault can quickly turn in to a L-L or 3 phase fault.
Also the impedance grounding is inteneded to limit the damage to the equipment not to the people necessarily. It may limit burn hazard to a person as long as he/she is not part of the fault circuit. But, if a human comes in direct contact with a live part and a grounded surface, the system with a NGR is no less dangerous than a solidly grounded system, as it will produce a current not enough to trip a protective device but more than enough to kill a person.
If you are trying to follow IEEE 1584 calculation procedure, only three-phase faults are considered. The equations are not even valid for single-phase faults.
With a high-res grounding system, it's not really possible to have much of a line-to-ground arcing fault. Not enough current.
There are two problems, first the primary protection is fuse , so I can not trip it in case of light sensor detection. the second is that the secondary breaker does not have shunt trip and I dont want to alter the wiring to trip it using only one trip circuit.
I suggested using fault fiter S&C product and adjust the curve as close as possible to tx inrush current. However I am vconcerened about the hot load pick up. This subject is discussed on the other post.
We discussed arc flash maint switches for LVCB's, this is a light sensing arc flash relay, ABB claims 20ms trip times but as we discussed earlier, the relay is only as fast as the breaker it uses.
When I read "reduce you clearing time to 20 ms." I knew that is misleading. Clearing time and repsonse time of a relay are two different things.
No intention to blame you, but just to avoid any confusion. Especially to someone who doesn't have much experience in this area. They'll think the clearing time is 20mS. No it is not.
I understand and agree with you, it can be misleading, this ABB literature say "Extremely short overall operating time <2.5ms" That can be misleading. The breaker will still take 3-8 cycles depending on voltage, vintage, and type.
However, it is a good solution for those low arcing currents that take longer to trip a overcurrent relay, because they operate based on the light from the flash, not the fault current.
