Service Entrance Arc Flash Hazard

[Sunny_92]

How do most of you typically handle calculating incident energy at the service entrance gear?

My local utility typically provides SCC and X/R at either the primary or secondary terminals of the utility transformer. They also provide the upstream fuse, which I assume is located in the cutouts at the drop (for a pad mount XFMR). What they don't usually provide is the SCC at the fuse or the primary cable info.

Do you usually calculate the incident energy at the service entrance panel by modeling up to the utility fuse? It seems that I'm lacking information if I wanted to do it that way.

In the past I've just assumed an incident energy greater than 40 cal/cm^2 and used a label stating "Exceeds Max PPE Arc Rating". More recently, I've modeled up to the XFMR primary or secondary (wherever they provided the SCC at) and assumed a fault clearing time of 2 seconds. This method likely inflates the incident energy in most cases, but it seems like a safe bet given the known utility data, and it gives you a value for IE which I think is more valuable than just knowing it's above 40.

What are your thoughts?

 

[wbdvt]

You need to be careful with values that the utility will give for fault current on the secondary side of a transformer. Typically these are standard values based on the size of the transformer and a standard impedance that they purchase and is usually the value with an infinite bus. This information is usually from the Customer Service person that is the first contact. This will not give you a correct incident energy value.

What I do is ask the utility for the following:
1. Transformer data (size, %Z, connections, Bay-O-Net fuse)
2. Riser cable data (insul, size, CU/AL, length, conduit)
3. Cutout fusing (K or T, size)
4. Actual 3 phase and SLG fault current and associated X/R at the riser fusing.

I then model this, including the Bay-O-Net fuse if there is one, to the service entrance equipment. This would include the secondary cabling. Therefore, it is possible that the fault will cleared by either the Bay-O-Net or riser fuse prior to 2 seconds. If not then use the 2 second limit. Since you are sure that you are using actual utility fault current and not maximum possible from a infinite bus, you will have realistic values of IE.