Fault Current

[kingpb]

It seems a solution was already identified, but I thought I would mention, since no one else did, that you could use current limiting reactors.

 

[brian john]

The customer contacted the manufacture and he tells me the panel exceeds the required rating and he will replace the CB's.

Thanks for the responses.

 

[templdl]

Consider a current limiting reactor

Consider a current limiting reactor

I would suggest that a current limiting reactor engineered an sized to limit the fault current available to the switchgear be considered. Current limiting reactors are commonly applied when this issue occurs.

 

[mull982]

So am I understanding correctly that a current limiing fuse can protect a panel only (without breakers) without the fuse and panel being sereis rated? Do they even sereis rate panels and fueses?

However if the panel contains OCPD's then these OCPD's must be series rated along with the upstream fuse to comply? This is mainly due to the fact that the OCPD's are the weak link in the panel and therefore must have a sufficient rating?

Does anyone have a link to the CH article mentioned earlier?

 

[jim dungar]

So am I understanding correctly that a current limiing fuse can protect a panel only (without breakers) without the fuse and panel being sereis rated? Do they even sereis rate panels and fueses?

Bus bars in panels usually do not have any 'stand alone' fault current rating. Bussing in switchboards, switchgear, and motor control centers is usually labled with a withstand or 'bus bracing' value.

However if the panel contains OCPD's then these OCPD's must be series rated along with the upstream fuse to comply? This is mainly due to the fact that the OCPD's are the weak link in the panel and therefore must have a sufficient rating?

It is not all because the circuit breakers are the "weak link". All protective devices must be rated for the fault current they are trying to open, this is even true of a non-current limiting fuse. The series rating "testing" requirement comes from the interaction between an upstream fuse or breaker, the interconnecting conductors, and the downstream breaker. This series-combination of equipment is different than what was used during the standard UL test that provided the device's original AIC rating. In a series-combination test, there is no requirement that the upstream protective device clear the fault, in fact is is not uncommon for the lower rated downstream device to clear the fault before the upstream device operates.

 

[dkidd]

Does anyone have a link to the CH article mentioned earlier?

Try http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/1c96944h02f.pdf

There is a good article at

http://www.iaei.org/magazine/?p=2302

 

[mull982]

Bus bars in panels usually do not have any 'stand alone' fault current rating. Bussing in switchboards, switchgear, and motor control centers is usually labled with a withstand or 'bus bracing' value.

So then must this "withstand" or "bus bracing value" always be greater than the avaliable fault current on the bus? Can a current limiting device be used in cases where it is not and there are no other OCPD's in the equipment?

 

[jim dungar]

So then must this "withstand" or "bus bracing value" always be greater than the avaliable fault current on the bus? Can a current limiting device be used in cases where it is not and there are no other OCPD's in the equipment?

Yes, this is a valid application for the let-through current of a current limiting protective device (be it a fuse or a breaker).

 

[templdl]

Current limiting reactors are typically used to reduce the fault current level downstream of the reactor allowing electrical equipment components such as contactors, cables and other elements to be designed for the reduced fault current level.

Having worked for a dry type transformer manufacturer I sold numerous 480vac CLRs to a motor control center manufacturer for installation in their MCCs to reduce the amount of fault current available to down stream devices.
Essentially the CLR is an air core reactor which is sized to add sufficient impedance to reduce fault current.

 

[ron]

Essentially the CLR is an air core reactor which is sized to add sufficient impedance to reduce fault current.

I understand that an air core reactor, due to the fact that it has no core, you must keep it clear of ferrous material and communications circuits for a good amount of distance.
An iron core reactor, doesn't have that problem, but may exhibit voltage drop on large step loads.
Is that your experience too?

 

[steve66]

If the fault current study was performed using infinite current on the primary, I'd redo the study and take credit for some of the HV cable on the primary of the transformer.

Chances are the real fault current is less than the 25000 amp rating.

 

[brian john]

What affect would installing a 480 delta to 480/277 transformer have on the available fault current at the secondary terminals or maybe a better statement would be is it feasible to consider this option?

 

[Steve O]

I will agree that is a viable option and will greatly reduce the available fault current at the 480V panel board(s), but is it the most cost effective solution?

Size of Transformer
Qty of transformers needed for overdutied equipment
Energy
Just to name a few

I would review the study again for accurate information.

Was series combination ratings ruled out?