Utility AIC Increase

[beanland]

If an electric service was installed based on the available fault current provided by the utility (say 10kA) then the utility increases the available fault duty to above 10kA (e.g. increases transformer size, changes to lower impedance transformer), does anyone (utility,owner) have a responsibility for upgrading service equipment to meet the new available fault current? Though the equipment met NEC when installed, it no longer meets NEC. What if it fails to clear a fault?

 

[benaround]

Re: Utility AIC Increase

I would imagine the POCO is on top of this. With law suits and the like,cya.

jmo frank

 

[ron]

Re: Utility AIC Increase

I would guess that whomever knows of the potentially dangerous situation can be liable, unless they pass the liability baton to some other more responsible party. CYA

 

[celtic]

Re: Utility AIC Increase

Originally posted by ron:
I would guess that whomever knows of the potentially dangerous situation can be liable, unless they pass the liability baton to some other more responsible party. CYA

I would think:
....that whomever is responsible for creating the potentially dangerous situation can be liable.

 

[molotov27]

Re: Utility AIC Increase

who is responsible...I would guess that the utility for making the change.

A transformer & impedance change will make a difference depending on where on the system does the utility makes it and where on the system you are connected. If you are talking residential, at 240/120 25KVA-333KVA and probably even higher KVA service, it will not make any difference at all. However, if you are talking about a large industrial or commercial customer, then it might affect them.

Also, very seldom does utilities make such "drastic" changes, the ones that affect large customers...why, because of what you implied....that many customers will be affected. Plus, the utility would have to make changes to all the fusing, recalibration of all relays and breakers, protection schemes upstream & down stream, their SCADA system....and that costs a bunch of $$$$$$. If they are going to add capacity, they should crunch the numbers and come up with a solution that does not have such a drastic problem

 

[ron]

Re: Utility AIC Increase

molotov27,
Two basic examples of a utility upgrade that happens in residential could be going from a high impedance type transformer (4%) to a low (1%). Going from a small pole mount (15kVA) to a slightly larger pole mount (50kVA), when more houses are added nearby.
It may bring the fault current from 8,000A to 12,000 (numbers just picked somewhat randomly), which would require an AIC higher than the standard 10kA residential loadcenter.

 

[molotov27]

Re: Utility AIC Increase

ron....thats true...it can happen, but perhaps I should have gone a little deeper into the subject.

Being extremly familiar with utility purchasing, contract supply management & utility specifications I can tell you that the very vast majority of the utilities purchase transformers under single or multi-year contracts. When a utility prepares a specification for their purchases, they clearly specify an allowed range of impedance. And most of the time they apply the same range to both pole & pad-mounted transformers from 15KVA to 333 KVA, plus they have kept on purchasing that same range for the longest time.

What utilities typically do is call for the same impedance range for all KVA ratings....and as I said, this is for small pole & pad-mounted units for residential purposes.

For example they might call for 4.00-4.50% range for all their 15 KVA to 333 KVA ratings. The reason is to avoid the a situation of the increased AIC. That is the way they "control" the AIC issue. Utilities don't just buy transformers with random impedances....it is well planned, to precisely try to avoid the AIC issue.

 

[charlie]

Re: Utility AIC Increase

Generally speaking, the pole mounted transformers through 75 kVA are 1?% to 2% and the small service drops will keep the available fault current at the service low enough for the 10 kAIC overcurrent devices to be used.

We post on the Internet our Goldbook with its drawings. The drawing GB7-060 gives the standard fault current available for the transformer used. If this information is used, a larger transformer will not normally have enough fault current to "blow up" the equipment. Our reasoning is that the parameters given are, "The standard calculated available fault currents are given in amperes, RMS symmetrical, at the secondary bushings of the Company's transformer, assuming an infinite bus and a bolted fault." We are assuming that we will not have the lowest impedance transformer, we do not have an infinite bus, and the fault will not be bolted. If the engineer or electrician moves in on those margins and the system blows apart, it will not be the fault of the serving electric utility but will be the fault of the designer even if we replace the transformer bank.

Normally, when a service is upgraded, everyone knows what the new bank is going to be and may plan for the increased fault duty. Under these conditions, the building owner may not wish to replace the existing equipment and would not be required to replace it under most laws. In some cases, that is where 240.86(A) comes in. It may be applied if the circumstances permit.