It has been a while since I posted anything about the IEEE 1584 125 kVA study exception. IEEE 1584-2002 states:

[INDENT=1]Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply.[/INDENT]

Now that many years have gone by since this was first introduced and study practices have evolved, it is time to take another look at how this exception is used today. Here is this week’s question:.

What transformer size do you use for the IEEE “125 kVA exception�?

• 125 kVA (112.5)
• 75 kVA
• 45 kVA
• 30 kVA
• We don’t use the exception.

Depending on the facility and what kind of results we have been achieving; I will calculate the HRC on some sample transformers of smaller size. If the results indicate there is a hazard, we dig deeper and label everything. If the results indicate minimum HRC we can save a lot of time and effort by not being required to calculate this. Some of the distribution networks feeding from 240V sources include very low power instrument supply that could easily double the already high cost of modelling a complete plant distribution system. Overcooking this will by requiring all calculations to be completed will accomplish nothing except drawing resources from other areas.

The Problem is the low impedance transformers from the utility, A (3) 2% impedance 75well over kVA transformers connected in wye on a 120.208 volt system can create a short circuit current of over 30,000 amperes, and a possible Arc of many kVa.

I saw on small pump station where the circuit breakers did not function and the power company fuses cleared the fault in a heater. One problem is that one assumes in an accident the circuit breakers will operate, the station in question had faulty Circuit Breakers on the main and branch circuit breakers, as determined by field testing, and were replaced.