available fault current for arc flash study 1

[stevenal]

So the first request has arrived. For equipment sizing, we figure an infinite bus limited only by the service transformer impedance. But this request is regarding arc flash and PPE requirements, so the bus becomes finite. This particular location can be fed from two different substations. Between the two distribution feeds and all the possible combinations of transmission and sub-transmission arrangements and generation possibilities, the combinations are nearly endless. What figure to give? One with a bunch of disclaimers? Or overload them with data? What do you other utility side people do?

 

[davidbeach]

Steve, Obviously I'm not of the utility side of the fence, but from my days on the consultant side of the fence I would have found the following information from a utility to have been of assistance:

1. Maximum possible fault current for equipment sizing. This has often (always?) been infinite bus, service transformer, and service laterals. I'd think that it would be safe for the utility to stick in the impedance of a substation transformer, even two in parallel, between the infinite bus and the service transformer, but haven't seen that done.

2. Fault currents for the 2 or 3 most likely operating conditions, for study and analysis purposes only.

3. The lowest reasonably conceivable fault currents that might be expected, for study and analysis purposes only.

The consultant would then need to size the equipment based on 1. and analyze the arc flash implications of the other currents. In many cases, the worst arc flash conditions may be found using the current of 3.

I don't know what you provide, but I'd like to see utilities provide SLG fault currents as well as X/R for both fault currents. Too often all one gets, without much trouble, is a three phase current.

 

[rbulsara]

This question was discussed in one the good classes I went to for arc flash calculations. There is no one answer but it is important to run the study for, as davidbeach indicated, 2-3 most probable scenario and report/lable for the worst case. As with any similar studies, the report must state assumptions made.

It was also suggested that for more complex systems, one fits all lable may not suffice and a work procedure book may be necessary for each electrical room or major equipment which the operators and the repair crew will have to follow. It may be advisable to insist on performing repairs only in certain configuration (such as only when fed from certain sources)and with certain PPE.

 

[dpc]

I would give them your best approximation of the maximum fault current to be expected. If there are likely system configurations that could result in significantly lower fault current, I would give them that also.

For situations where the customer's equipment is 480 V and you own the step-down transformer, the utility fault current generally doesn't have a big impact on the 480 V fault current and arc-flash anyway, unless you have a very weak distribution feeder. The transformer impedance will nearly define the 480 V fault current. And the transformer impedance is what it is.

 

[jghrist]

The transformer tap is probably a more significant factor than the primary distribution configuration. Without test reports for the transformer, you don't know what the impedance is at each tap. From test reports I've seen (although what I've seen have been power not distribution transformers), the impedance does not vary with the tap in a predictable manner.

 

[stevenal]

DPC,

You've described the situation. However, they did not ask for the 480V fault current, they asked for the 12.47kV fault current and X/R at the location as well as transformer impedance. The alternate substation feeder would be fairly weak compared with the primary substation feed. And transformer impedance is what it is until it changes. Our first mission is to keep the lights on, so transformers may get changed. Less often on the bigger ones, but it happens. Through faults can alter a transformer's impedance also.

 

[dpc]

Like I say, I would give them the most accurate answer I could based on the current equipment and system. Things can change but there is no way to predict how or when. The IEEE equations for calculating arc-flash energy levels will probably be changing faster than your system.

It seems to me that you have the same problem whether or not you provide the fault duty at 480 V or your distribution voltage.

We do a lot of arc-flash studies and all I expect is a reasonably accurate estimate what the system fault duty is or could be under reasonable alternate feed conditions. The changes within a facility are a much bigger issue than changes to the utility impedance when it comes to arc-flash.