Insufficient Short-Circuit Rating - Arc-Flash

[TheFist]

I work in San Francisco and from what I understand many buildings were originally designed before the redundancy power-grid was put into effect by the utility. The power-grid uses multiple large transformers in parallel which directly increases the available fault current. So now we have a bunch of buildings with low-resistance busducts distributing power up through the core and a bunch of insufficiently rated panelboards and overcurrent protective devices. Now I have two questions:

A) Can an Arc-Flash analysis be properly calculated if these devices aren't rated to handle a full fault?

B) How do people genrally feel about underrated devices?

Thanks
Fist

 

[zog]

I work in San Francisco and from what I understand many buildings were originally designed before the redundancy power-grid was put into effect by the utility. The power-grid uses multiple large transformers in parallel which directly increases the available fault current. So now we have a bunch of buildings with low-resistance busducts distributing power up through the core and a bunch of insufficiently rated panelboards and overcurrent protective devices. Now I have two questions:

A) Can an Arc-Flash analysis be properly calculated if these devices aren't rated to handle a full fault?

No, an arc flash analysis requires a clearing time of the protective device, if the device is not rated for the available fault current in the system the analysis is void.

B) How do people genrally feel about underrated devices?

Thanks
Fist

They should be replaced with properly rated devices. Upgrade ASAP.

 

[jim dungar]

Yes calculations can be performed, but their validity depends on the protective device functioning properly. A protective that fails, especially violently, is not functioning properly. The NEC requires devices to be properly rated for the bolted fault current they experience.

We annotate our reports with verbiage that says the results are only valid if 'underdutied' equipment is replaced with correctly rated devices. This allows the customer to replace most equipment without having to have the study results updated.

This is why arc flash studies should always include a device short circuit evaluation.

 

[weressl]

They should be replaced with properly rated devices. Upgrade ASAP.

The statement should not be made as a blanket solution. There are ways to reduce the available energy that may be more cost effective than replacement.

Reduction of the hazard magnitude is my primary choice.:smile:

 

[jim dungar]

No, an arc flash analysis requires a clearing time of the protective device, if the device is not rated for the available fault current in the system the analysis is void.

One example: say the available bolted 3-phase fault current is 65kA and the device is only rated for 42kAIC, and it is applied at the beginning of a circuit that has a 'through fault' arcing fault current of 18kA, the results of the study may still be valid for that downstream location.

But regardless the 'overdutied' device needs to be replaced. It is in violation of the NEC and NFPA70E 210.5.

 

[zog]

The statement should not be made as a blanket solution. There are ways to reduce the available energy that may be more cost effective than replacement.

Reduction of the hazard magnitude is my primary choice.:smile:

Good point, for the description of the OP I was thinking MCCB's which is an easy swap. Depending on the details a current reduction solution may be better.