Interrupting Rating of end user equipment

[ahladas]

I work for a large computer manufacturer and we are going to be coming out with a piece of gear that has 480V/150KVA input power on one power cord. One of the engineers was at one of our install sites chatting with the facilities engineer and the facilities engineer wanted to know if he had to assure the fault current was not capable of exceeding 10KA, he had planned to coil wire to do this. He claimed he was required to limit the fault current to 10KA unless the end equipment was rated for higher. Nobody here has heard of rating our equipment for an interrupting capacity. Cannot find in the NEC or UL950 (the end equipment safety standard we use) where is mentions anything either.

It is reasonable to expect this computer will be installed on a circuit with >10KA fault capacity, but I am not aware of anything we can/should do to rate the equipment as such. Where in the code can I go to dismiss this concern?

P.S. The equipment has an AC compartment with a line filter after it which feeds a bunch of power modules, each power module has fuses rated for 100KA interrupting.

 

[Jraef]

As of 2005, the NEC article 409 has required what is called an SCCR for all equipment. SCCR stands for Short Circuit Current Rating, and there are a number of ways of attaining it. The usual route is to have your system evaluated by an NRTL (Nationally Recognized Testing Lab) such as UL. In that process, they will look at the SCCR ratings of all devices in your power circuit, and the overall rating without testing will be that of the lowest rated device. So for example, if you have a main breaker rated for 35kAIC, but it feeds a load center using breakers that are only rated for 10kAIC, then you cannot attain an SCCR rating higher than 10kAIC (without expensive testing).

In some cases however, you can check with manufacturers of your power devices to see if they have "series ratings" with regards to SCCRs, many do (it's that expensive testing mentioned earlier). I used to work for Siemens and for example, if you use a Siemens main breaker rated 65kAIC and then certain Siemens feeder breakers and motor controllers rated at lower levels individually, as long as they are configured correctly you can attain a 65kA SCCR rating on the entire system. This will not typically work for mixed manufacturers however, because why would one manufacturer test other people's stuff?

So what that SCCR rating means is that if, by default or simple evaluation, you end up with 10kA SCCR, the end user will be required to limit their Available Fault Current (AFC) to 10kA, which can be problematic if theirs is significantly higher than that. I don't think "coiling some wire" is going to cut it by the way. They would need a current limiting reactor, but also they may be able to deal with it by just using current limiting fuses. It needs some engineering attention.

Assuming you have a standards compliance engineer on staff, that would be their job by the way....

 

[ahladas]

Assuming you have a standards compliance engineer on staff, that would be their job by the way....

Well we do, and he just looks at me dumbfounded on this one. Probably because he is a computer product safety engineer, not a facilities engineer. It would not appear that Article 409 would apply to a computer as an end product, since it is not an industrial control panel. Does a regular PC from Dell have to have an SCCR? Our dilemma is the same, albeit on a larger scale, since it is reasonable to expect the circuit fault current to exceed 10KA for our machine, but not likely for a regular PC.

 

[Jraef]

Well we do, and he just looks at me dumbfounded on this one. Probably because he is a computer product safety engineer, not a facilities engineer. It would not appear that Article 409 would apply to a computer as an end product, since it is not an industrial control panel. Does a regular PC from Dell have to have an SCCR? Our dilemma is the same, albeit on a larger scale, since it is reasonable to expect the circuit fault current to exceed 10KA for our machine, but not likely for a regular PC.

A "regular PC" doesn't take 480V/150kVA!

 

[ron]

A regular PC doesn't have "AC compartment with a line filter after it which feeds a bunch of power modules, each power module has fuses rated for 100KA interrupting".
Maybe it is not a control panel, but it surely isn't a pc

 

[ahladas]

Maybe it is not a control panel, but it surely isn't a pc

It sure is'nt, you should see the price tag!

But my point is that Art 409 is stated as specifically applying to control panels and not end user equipment. If I were installing a 150KW chiller, would it come with an SSR? If so, then our machine should as well.

If this can be done with a simple review of the hardware and interrupting capacity of protectors, it will be no big deal. We in the computer industry tend to be heavily persecuted by the safety agencies however, so I can see this turning into destructive testing of some number of (expensive) units along with hefty fees for the testing agency. I'd even side with the agencies on this one to need to test because the connection to the equipment is by a detachable line cord, and I have no clue what that connector would do if exposed to the 100KA the fuses are rated for.

 

[jim dungar]

Art 409.110 is simply about providing a label on the equipment so that it is easier to comply with 110.10.

110.10 is applicable to everything covered by the NEC, not just control panels. 110.10 says that "... the component short-circuit ratings..." need to be considered so that a fault can becleared without "extensive damage to the electrical components of the circuit". It also says that "Listed products" used within their listing are considered to meet these requirements.

So the answer may depend on how your equipment is Listed and applied.