Short Circuit Current from Utility

[DMG_1]

I have a question regarding Short Circuit Current from Utility. When the Utility gives me the available short circuit current at the primary side of a transformer serving a building, how do they derive this value? Is this considered to be the maximum value that the upstream substation transformer can supply (infinite source on primary) or is a complete study done considering what the actual value of short circuit current available at the primary side of the substation transformer? Thanks.

 

[mivey]

You will have to ask to find out. When I make a coordination study, I include the impedance starting with the impedance on the transmission side of the substation down to the primary side of the transformer. These values are then posted to the system map.

For general questions from customers about fault current, I generally provide the values using an infinite primary bus and typical transformer impedance for the size they have or might have. We often change the source feeds, change the service transformers, re-conductor, and change station transformers so it is supposed to represent the maximum fault (for conservative equipment sizing).

However, that infinite bus is not what is needed for an arc fault study and you would need to specifically request data for an arc fault study.

You need to determine what you want to do with the data and specifically what data they provided you. With no other info, I would assume you have the site transformer with an infinite source bus.

 

[templdl]

Usually they know what the available fault current is on the primary of the transtormer and then will calculate what it ends tp be onthe secondary of the transformer.

 

[DMG_1]

The reason I posed this question is because I do a lot of short circuit studies on existing installations. In most cases, when we do the studies we have equipment that is failing due to insufficient IC Rating. I would think that during the intial system design that a short circuit study had been performed in order to select the proper IC Rating for the equipment. With that being said, I am trying to come up with an explanation as to why the equipment would fail now but not before?

 

[templdl]

Again, the POCO establishes what the available fault current is to the primary of the transformer which services tha facility when knoing the transformer impedance can determine what the available fault current to the facility is. Basically you have to request it from the POCO and trust their number.
But, it doesn't hurt to double check by knowing what the KVA of the POCO transformer is and fiuring your own fault current by assuming 100% fault current on the primary with 100% motor contribution and see where that ends up for grins. One would anticipate that your figure would be much higher than theirs. If theirs seems reasonal I wouldn't question theirs.

 

[kwired]

The reason I posed this question is because I do a lot of short circuit studies on existing installations. In most cases, when we do the studies we have equipment that is failing due to insufficient IC Rating. I would think that during the intial system design that a short circuit study had been performed in order to select the proper IC Rating for the equipment. With that being said, I am trying to come up with an explanation as to why the equipment would fail now but not before?

If original calculations accounted for actual transformer feed and did not assume infinite input abilities to the transformer then POCO makes distribution changes the IC likely goes up after their changes, unless they lost some load, but usually they make changes because of increases in load to assure reliability of their system. Substation capacity is increased, more/heavier distribution lines are run, etc. over time. Even the transformer feeding your facility may have been changed if more load was added to it at some time.

 

[mivey]

I am trying to come up with an explanation as to why the equipment would fail now but not before?

For starters:

We often change the source feeds, change the service transformers, re-conductor, and change station transformers

 

[jim dungar]

I would think that during the intial system design that a short circuit study had been performed in order to select the proper IC Rating for the equipment. With that being said, I am trying to come up with an explanation as to why the equipment would fail now but not before?

My experience

No. 1 reason, the installers did not consider the fault current. The NEC has contained 110.9 and 110.10 for more than 30yrs, but I still hear: "this is not the service entrance so fault current does not matter".

No. 2 reason, the service to the facility has gotten larger. Very few expansion projectss consider the impact on the existing equipment, this also results in outdated one-lines.

No.3 reason, the utility has changed their method of providing fault values. 30 years ago we got specific values from the utility based on the installed transformer, about 20yrs ago the utility started to provide "ultimate design" values instead.

 

[ron]

My experience

No. 1 reason, the installers did not consider the fault current. The NEC has contained 110.9 and 110.10 for more than 30yrs, but I still hear: "this is not the service entrance so fault current does not matter".

No. 2 reason, the service to the facility has gotten larger. Very few expansion projectss consider the impact on the existing equipment, this also results in outdated one-lines.

No.3 reason, the utility has changed their method of providing fault values. 30 years ago we got specific values from the utility based on the installed transformer, about 20yrs ago the utility started to provide "ultimate design" values instead.

I'll just add a little to Jim's #1 reason. I've consulted for many consultants that primarily use fused switches and because of their high rating out of the box, they've thought that they didn't have to think about it.