Interrupt rating of a UPS

[Strathead]

So, here is my situation. I have a job spec that the panels are to be series rated. All well and good. However, the distribution goes from a Siemens main panel, through a UPS (equipped with bypass) then to another provided Siemens panel. Obviously when the system is in bypass it series rates with the up stream panel. My instinct says that a 15KVA UPS can't possibly have an AIC rating higher than 10K, but I can't find that information, or the impedance of the output circuit in the documentation. The documentation lists the SCCA at 30KA and the overload capacity as 150% of 41.6A via the battery or normal operation, and 800% for 500ms for bypass. Do I just assume that the KAIC can't be higher than 10K or is there something else that is my responsibility?

P.S. Speak slowly because I have resisted wrapping my head around a deep understanding of AIC calculations.

 

[ron]

You said the documentation lists the SCCA at 30KA. Is that documentation of the UPS? I thought that is what you were trying to guess at.

 

[Strathead]

You said the documentation lists the SCCA at 30KA. Is that documentation of the UPS? I thought that is what you were trying to guess at.

No, the SCCA is the maximum allowed amperage on the input side. I think I am looking for the maximum current that can be produced by the output circuit of the UPS under battery or normal operation. So that I can then verify that the maximum current available at the downstream panel is less than 10,000 Amps so that panel can have 10,000 aic breakers.

Correct me if that is wrong also, please.

 

[GoldDigger]

I am not sure just where to find a spec, but I would be very surprised if the instantaneous peak output current of the UPS is anything greater than ten times the rated output current. Unlike the grid, the inverter in a UPS has a relatively low value of peak output current based on the components and circuitry it contains.

If nothing else, you could approximate an upper limit on that current by measuring the voltage drop at the output terminals from no load to full rated load and use that to develop a percent impedance value.

 

[__dan]

The inverter output is natively current limiting. There is a phyiscal limit on charge carriers available at either at the battery or through the semiconductor(s). The IGBT output is probably a fast switching PWM through an inductor / reactor / transformer at a much faster carrier frequency. If the gate driver has control of the IGBT, it probably knows what's happening. The manufacturer must have a spec for fault condition at the inverter.

For large UPS I've seen specs for a fault clearing mode rating of 200% for three seconds (irrc). Semiconductors are fragile devices outside their rating. They can fail shorted and take out the fuses (fast fuses used with semiconductors). For large power and if the breaker is slow to open, the IGBT can fail or blow open (in pieces).

 

[Jraef]

“AIC” stands for Amps Interrupting Capacity. The term only applies to things that INTERRUPT a Fault (i.e. a circuit breaker or fuse). A UPS is not such a thing, so you will not find an “AIC rating” for any UPS. So if it is not an Interrupting device but it still handles/passes power, then it has what used to be called a “withstand rating”, but is now called the “Short Circuit Current Rating” (SCCR) or sometimes called the SCCA (SSC Amps). So when your data sheet says it has an SCCA of 30kA, what it is telling you is that you can connect it to a circuit that is capable of delivering up to 30,000 Amps during a short circuit event, but not more than that.

On the output side, if it is a “double conversion” UPS, the maximum output cannot be higher than the output amp capacity of the inverter. But if there is a bypass, then technically you must consider that in bypass, it will pass ALL current through it. So if you have, for example, 29,900A of available fault current, in bypass the downstream devices (panel) will still see 29,900 A (actually a little less based on circuit impedance). But because your UPS is rated for 30kA, series listed panels at values higher than that become irrelevant because that 30kA is your limit for the circuit.

 

[zbang]

But if there is a bypass, then technically you must consider that in bypass, it will pass ALL current through it. So if you have, for example, 29,900A of available fault current, in bypass the downstream devices (panel) will still see 29,900 A (actually a little less based on circuit impedance). But because your UPS is rated for 30kA, series listed panels at values higher than that become irrelevant because that 30kA is your limit for the circuit.

To slightly muddy the waters, some UPS installations have both an electronic (static) bypass which uses semiconductors to switch and a mechanical bypass switch, which could be either inside or outside the main UPS cabinet.

Generally the supply to each stage outlet is limited by either a fuse or a very-fast blowing electronic component.
--charles shope

 

[ron]

No, the SCCA is the maximum allowed amperage on the input side. I think I am looking for the maximum current that can be produced by the output circuit of the UPS under battery or normal operation. So that I can then verify that the maximum current available at the downstream panel is less than 10,000 Amps so that panel can have 10,000 aic breakers.

Correct me if that is wrong also, please.

SCCA = short circuit current amp rating

There is no UPS that you are going to deal with that takes in 30,000A on a regular basis. the 30,000A IS the short circuit current rating.

Most UPS manufacturers will have a SCCA rating for the associated static bypass switch, which is a withstand rating or an interrupting capacity of a breaker /fuse in the circuit.

The output from the inverter will be low when feeding a fault, which is why they need to be way oversized when trying to feed something with high inrush.

The downstream short circuit ratings are almost always dictated by the bypass current, not the inverter.

 

[Strathead]

The downstream short circuit ratings are almost always dictated by the bypass current, not the inverter.

I think this touches on my question and perhaps I am just complicating it too much. My specifications call for series rating my panels. I have a panel feeding the UPS and the UPS feeds a sub panel. I believe that would mean that under normal operation the UPS panel will need to be rated for the maximum current available from the UPS during a bolted fault. That is the value I am looking for. The panel will have to be fully rated for this amperage. When or if in bypass, the panel will be series rated with the panel above which basically means it will be allowed to have 10KAIC breakers in it.