Short Circuit Withstand Capacity of LV Panelboards (Arc Flash Calculation)

[Gonenc]

Hi Everyone;

I have two question about arc flash analysis;

1- I am doing arc flash analysis for one of our customer. I would like to learn how can we assume short circuit capacity of low voltage switchboard. There is no information on the related project and those switchboard are really old.

2- Let us assume I calculte short circuit current for one of distrubition bus. For this bus let me say 16kA max sc value. Incomer of this switchboar has 36kA therefore ıncomer of cb is quite enough. But this switchboar also have many feeder such as socket and lighting. As you know those lv cb has generally 3-6kA breaking capacity. If one fault occur, one of socket's lv cb outgoing terminals, this lv cb can not break that short circuit value. What can I say to customer. Should they chance all lv cb such as socket. I think there is no sense. But I could not find any information on IEEE-1584/2018 standard.

Best Regards,

 

[dpc]

As you probably know, the short circuit rating of the equipment is not directly related to the incident energy calculation, so it is not covered in IEEE 1584. The short circuit rating of the upstream protective device is important from a practical standpoint because the incident energy calculation uses an arc time that is generally based on thee time-current curve of the upstream device. If it is underrated for the available fault current, this is a concern, obviously. But it is somewhat more complicated because the arcing current will be less than the bolted fault current.

The short circuit rating of the bus is less important for arc flash. If you are unable to determine the short circuit rating of the upstream device, this should be noted in the study. But generally if you have sufficient information to come up with a time-current curve, you should be able to determine the short circuit rating.

Cheers,

Dave

 

[mparenteau]

If you're talking about the actual calculation of short circuit current, then you'll need a considerable amount of data:

1) Utility contribution
2) Cable sizes / lengths
3) Motor sizes / subtransient reactances

to name a few. For more information and the standard method, refer to IEEE red book (141).

Mike

 

[wbd]

Q1: This can be done in several ways. One way is to remove the panel cover and look for the label inside which contains the SCCR of the panel. Another way is to look at the AIC ratings of the breakers that came installed from the factory as sometimes those can be used. Or assume a low rating such as 10kA for a panel. Then when you look at the actual available fault current, you can determine if the breakers and panel are properly rated.

Q2: How are you calculating the short circuit current? The proper way is to obtain the available short circuit current from the utility and use that to determine the short circuit current throughout the system.

 

[wbd]

But generally if you have sufficient information to come up with a time-current curve, you should be able to determine the short circuit rating.

can you explain this? How do you determine the AIC rating of a breaker from a TCC?

 

[dpc]

wbd,

If you have a manufacturer's TCC, you have to know the breaker model, type, and current ratings. If you have that data, you can find the short circuit rating.

 

[wbd]

If you have a manufacturer's TCC, you have to know the breaker model, type, and current ratings. If you have that data, you can find the short circuit rating.

I would urge caution in that approach as it will probably work the majority of the time. Take Square D FA breakers for example: There are two versions of the breakers. One is rated at 14kA @480V and the other is rated at 18kA @480V. Both breakers have the same TCC's, so without inspecting the breaker you don't know if you have the 14kA or 18kA version.

 

[dpc]

There are sometimes multiple TCCs as well, depending on vintage. Worst case, take the lower of the two AIC ratings.

 

[Parchie]

Gonenc,
Look for as-built drawings and see from the plans the rating of the switchboard. Else, look for the ratings of the breakers installed, take the smallest rating of the elements inside the switchboard!