Is minimum Arc Flash calculation even possible w/o Zsource or primary fault information?

[Murloc]

I've recently have been trouble talking w/ Entergy over getting any information on the primary side of the utility.
They claim that their updated policies won't allow them to provide any information on the primary side of the utility.
I got the same response from 3 different employees.

However, this info has been given in the past. What would probably be a safe way to do an arc flash calc then if info is not provided.
I guess setting Zsource = 5% could be a solution, but could be insufficient since the source impedance might be higher or lower.

I can't get.
3 phase primary fault
L-L primary fault
X/R ratio primary
Z source

 

[petersonra]

I don't know how the utilities expect reliable arc flash calculations without giving reliable data about what is upstream of the service point.

My guess is they expect you to do some kind of worst case assessment, versus reality. This is almost certainly going to end up with far higher levels of energy than is really there.

 

[David Castor]

If this is a utility-owned transformer, you can ask them for the fault current on the secondary side. You don't really need the primary side if you aren't required to do arc-flash calculations for any primary equipment.

If you really need the primary data, you could, as a last resort, develop a reasonable approximation based on the distance to the utility substation and their substation transformer size. For 15 kV systems, it's unlikely to exceed 10,000 A, and probably less. You can't just assume an infinite bus, since that will not always provide the worst-case incident energy (due to the time factor).

 

[Murloc]

If this is a utility-owned transformer, you can ask them for the fault current on the secondary side. You don't really need the primary side if you aren't required to do arc-flash calculations for any primary equipment.

If you really need the primary data, you could, as a last resort, develop a reasonable approximation based on the distance to the utility substation and their substation transformer size. For 15 kV systems, it's unlikely to exceed 10,000 A, and probably less. You can't just assume an infinite bus, since that will not always provide the worst-case incident energy (due to the time factor).

hmm conceptually what you say about substations makes sense, but I 'm not sure how I can get that info unless I work for the utility company

 

[Murloc]

I don't know how the utilities expect reliable arc flash calculations without giving reliable data about what is upstream of the service point.

My guess is they expect you to do some kind of worst case assessment, versus reality. This is almost certainly going to end up with far higher levels of energy than is really there

not sure what requirements are for arc flash, in the past they said just a general arc flash warning is fine, but I think the NEC requires a more specific calculation. I could make a worse case scenario, but that would just be me saying that each time someone opens a panel they need PPE level 4 --- full space suit or constantly worry about nuisance tripping.

 

[JoeStillman]

In my arc-flash study experience, data for the utility transformer serving you is key. If you know the impedance and typical X/R, assuming an infinite source won't be too conservative. More important than the available primary fault current is the primary fusing. It will significantly affect the AF energy on the line-side of the service main. Can they tell you their fusing at least?

 

[LarryFine]

When we do fault-current studies, we are to consider the primary to be "infinite bus" (unlimited), making the transformer's impedance the only factor (in addition to everything following it, of course.)

 

[steve66]

When we do fault-current studies, we are to consider the primary to be "infinite bus" (unlimited), making the transformer's impedance the only factor (in addition to everything following it, of course.)

But that can make the arc flash energy look lower than it actually might be.

With a lower, more realistic fault current, fuses and breakers frequently take longer to open or trip, resulting in a much higher energy.

That's why its important to get the utility to give a reasonable estimate of the actual source impedance or fault current.