Transformer SCCR - short circuit rating

[akagie20]

I have a multitude of questions about the secondary short circuit current rating of a step-up transformer, but will try to ask them in separate posts for clarity.

Background: We had a custom transformer designed for us by a transformer manufacturer, which we now need to be tested by UL. UL wants to test the transformer first, and then our product with the transformer in it - we are actually just swapping out an existing custom transformer for a slightly different version of the same, but the transformer is not listed or recognized, hence the testing.

Transformer: It is a step-up transformer: 120VAC:3000VAC and 75 VA.

The problem: During the UL testing of the transformer, they have asked us what the SCCR (secondary short-circuit current rating) of the transformer is, because they have to test the transformer when the secondary is short circuited and measure the current at not more than 110% of the rated value (SCCR). I have asked our manufacturer for this information, but they do not know what it is.

Question: How does one determine the SCCR of a transformer?

I have found that one can calculate the secondary short-circuit current by determining the impedance percentage (Z%) and then use:

secondary short-circuit current = VA rating / (V-secondary x Z%), the transformer operates with single phase, 120 VAC, so no factor is needed.

In my case, the Z% has been measured at about 7.7% (I believe), the VA rating is 75 and the secondary voltage is 3000 V.

Therefore, my secondary short-circuit current = 75 / (3000 x 7.7%) = 325 mA.

At first I thought that this would also be the SCCR value, but I cannot find any information about how to determine the actual rating.

Thanks for any help you can provide.

 

[beanland]

AFC

AFC

What you have calculated is the maximum available fault current. If the wire in the transformer cannot successfully carry this current, the transformer will fail if shorted. You need to know what wire was used to calculate the permissible I^2t for the wire. If the unit is wound with #12 cu, it sould be shorted almost forever without damage. If it is wound with #30 AWG, not so.

I am not sure what "t" needs to be. In large transformers, 2 seconds is standard.

 

[akagie20]

Further question

Further question

So, presumably, the transformer manufacturer knows the wire that was used - I can find the UL file number and the materials for this "Magnet Wire" but I don't know at the moment what the gauge is or even the material of the wire. Does the length of the wire impact the SCCR determination? What else?

But, once I find this out, then what? I'm sorry for being ignorant, but how do I (or the xfrmr manuf.) figure out the current carrying load?

Also, you mentioned you don't know the time frame needed for this SCCR value, where would I find this information out?

It seems to me that UL should be testing the SCCR, not asking us for it - at least this manufacturer doesn't seem to even know what this SCCR is, let alone how to determine it.

Thanks again, for the confirmation that I was not determining the correct value for the short-circuit current - this was what I started to determine after awhile anyway, but I had found noone stating this precisely.

 

[RCinFL]

There is two possible solutions. One is if the transformer core saturates, the other is the limitation of the resistance of the windings. There is a third possible limiting factor and that is core coupling of windings (leakage inductance)

For a 3000 volt / 25 mA secondary transformer it is likely the resistance of the secondary winding will dominate the maximum short circuit current with some additional reduction due to leakage inductance.

You can probably measure it directly without damaging the transformer. Just don't leave the shorted secondary on for more time then needed to make measurement.