NGR-Arc Flash
NGR-Arc Flash
(OP)
Can NGR reduces arc flash on the secondary side of delta-wye grounded transformer?
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RE: NGR-Arc Flash
RE: NGR-Arc Flash
RE: NGR-Arc Flash
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.
RE: NGR-Arc Flash
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.
RE: NGR-Arc Flash
RE: NGR-Arc Flash
So, in this situation NGR would limit the arc fault current since it is same as L-G fault .right?
RE: NGR-Arc Flash
RE: NGR-Arc Flash
RE: NGR-Arc Flash
transformer is 4MVA,600 secondary.
RE: NGR-Arc Flash
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.
RE: NGR-Arc Flash
With a high-res grounding system, it's not really possible to have much of a line-to-ground arcing fault. Not enough current.
RE: NGR-Arc Flash
RE: NGR-Arc Flash
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.
RE: NGR-Arc Flash
Didn't we discuss a little while ago, that the fastest we've seen was the around 50mS?
RE: NGR-Arc Flash
RE: NGR-Arc Flash
What type of trip unit does the seconday breaker have?
Is the primary a fused disconnect switch? What feeds the switch?
RE: NGR-Arc Flash
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.
RE: NGR-Arc Flash
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.
RE: NGR-Arc Flash
RE: NGR-Arc Flash
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RE: NGR-Arc Flash
The only option is finding a way to reduce the arc flash. primary has a fuse disconnect switch combination., however available short circuit rating is more than switch rating to open under fault.
despite the other practical situation, I need more fault current here to operate the fuse faster.
RE: NGR-Arc Flash
Cant you use a remote operator?
http:
And a remote racking system?
http://www.remoterackingsolutions.com/
RE: NGR-Arc Flash
there is no electrical charging mechanizm on this breaker. so it needs to be manually charged.
Thanks for the links.
RE: NGR-Arc Flash
with close and trip coil option. Isnt joke, Im serious.
Its simple and cheaper.
Regards.
Slava
RE: NGR-Arc Flash
Best I can tell from info in your posts you have an (indoor?) substation with a transformer of unknown size and type, delta-wye connected, I assume MV/LV. It sounds like you have a fused disconnect switch(?) on the primary fed from an unknown source. The secondary of this transformer feeds a main breaker of unknown type with unknow protective devices to some design of switchgear lineup.
Your problem is a problem at every industrial plant I have been in and there are several solutions, the more details about your actual problem the more we can help recommend a solution. We understand the coord issue you are having, it is very common.
Help us help you.
RE: NGR-Arc Flash
Or work it dead. See my posting in thread238-210239: Arc Flash Reduction Methods On Secondary of Sub-unit for a suggestion for getting around the catch 22 involved in de-energized work in this situation.
RE: NGR-Arc Flash
RE: NGR-Arc Flash
RE: NGR-Arc Flash
While stevenal's method is safer for the voltage check and grounding part, you still have a breaker operation on the seconday side.
Common practice in the field is also to ground and test both the primary and seconday side of the transformer prior to work, grounds on the primary side of a transformer may not protect a worker on the seconday side from a backfeed on the secondary.
RE: NGR-Arc Flash
NFPA allows a reduced amount of PPE for breaker operation with doors closed or covers on. If still unacceptable, downstream loads can be removed one by one. Or move upstream to find a suitable device. Or use the remote switch operator you suggested. Once the primary is opened, tested, and grounded, assuming there are no other sources, the manual recharging can occur without PPE.
RE: NGR-Arc Flash
I am not speculating anything, you need to open the main breaker before you open the primary switch. Thats why I suggested the remote operator in the first place!!!
After I suggested the remote operator you came in with the work it dead idea (Which as I pointed out is a fine idea but you still have the problem on the LV side for opening the breaker)then you suggested removing an interlock as a safety suggestion??
RE: NGR-Arc Flash
However, the breaker needs to charge mechanically, so it means working on a breaker withing the boundary even if I use remote trip device.with high calory incident energy.
Is charging the breaker allowed without using PPE?
RE: NGR-Arc Flash
RE: NGR-Arc Flash
As far as chraging the breaker goes, that is a grey area, the tables refer to breaker operations, which implies per IEE def of operations opening and closing, no mention of charging. The 2008 70E will use the term "Interact with switchgear: which implies charging is interacting. I have asked the 70E committe (Some members) in an open forum about charging operations and got some answer like "If it presents a hazards the PPE should be worn". What dies that mean? I dont know.
I am on a special risk assessment group that will be looking at these types of items, I will report our findings. But I believe (Again, my opionon) that charging would not produce a signifigant arc flash hazard, I believe racking from disconnect to removed and vice versa does not either. It is the making and breaking of a circuit that poses the hazard.
Now there is a possibility that when charging the breaker it could close on you (Charging latch failure or misadjustment), it is a long shot but possible.
What working distance are you assuming for the switchgear (LV side)Ei calcs?
RE: NGR-Arc Flash
My handbook only says to perform an analysis, not specifying how the analysis is done. The tables imply the presence or not of covers or doors and the nature of the task may come into play. Not as clear cut as it could be, I agree.
Seems Humble does not need to open the breaker first.
My comments regarding the speculated interlock were not meant for anyone with a wrench, but for engineers familiar with evolving codes and safety issues qualified to make the call.
If work practices mandate testing and grounding all sides of a transformer prior to clearance, the catch 22 will remain.
RE: NGR-Arc Flash
I have seen plenty of EE's overide or remove key interlocks in substations.