RCD's and SCCR

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ShortCircuited140

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Electrical Engineer
Hi,

I've designed power distribution panels in the past, and just slapped the SCCR of my panel onto it and let the facility installing the panel deal with getting the available short circuit current below the SCCR. I'd like to however start with increasing the SCCR on my panels so they can be installed in more locations, however I'm hitting a snag with one particular one that utilizes a 30mA RCD.

I have a design where I have an available fault current of 40kA @480VAC and I have a control panel where I'm distributing this power via some current limiting fuses in the feeder circuit through power distribution blocks down to some UL489 MCB's to a 30mA RCD's that have outlets downstream.

After doing the SCCR calc according to UL508A, I realize this design is flawed since, even though the let thru current of the feeder fuse is less than 10kA, there is a scenario where the RCD or the UL489 MCB will trip before the CL fuse does, which would destroy my RCD and or MCB. After thinking about it a bit more, in case of a phase to ground fault, the RCD would always trip first, since that's set to 30mA, way lower than the 15A for the MCB. This makes me realize that this 10kA RCD can never be used on a circuit where the available fault current on the line side of the feeder exceeds 10kA. Am I missing something here? Any advice would be appreciated. Even if I were to swap out the MCB for a CL fuse, the RCD is still the bottleneck, how do I solve this?
 
Are you talking AIC rating of the various breakers, or SCCR rating of the other components in the panel?

Current limiting fuses can not be used to increase the interruption rating of a breaker unless the breaker and fuse have actually been tested as a 'series rated' combination. This is precisely because the dynamic interaction of the breaker tripping and the fuse opening is not well defined.

The example of the ground fault trip is a particularly obvious case, but this is an issue for _all_ breakers in series with CL fuses.

Jon
 
Yes the AIC of my breakers.

Does this mean that any 10kA interrupting device mcb or gfci can never be placed on a circuit that has an available fault current higher than 10kA?
 
You need to check with the manufacturer of the RCD and breaker to see if they havd any listed series ratings.

Do not mix up AIC with SCCR they are not the same one deals with the ability to interrupt/clear a fault while the other is for tolerating a fault until it is cleared.

For faults above roughly 150A I would not be surprised to find your breaker clears before your RCD.
 
ShortCircuited140 said:
Does this mean that any 10kA interrupting device mcb or gfci can never be placed on a circuit that has an available fault current higher than 10kA?
Click to expand...

No. It simply means that you cannot use the 'let through' value of a current limiting fuse combined with the normal AIC rating of the breaker. You need information on an allowed combination from the manufacturer.

-Jon
 
Thanks for the inputs so far.

The RCD that I plan on using is already listed to be 10kA in combination with a CL fuse or UL489 breaker in series.
Does this mean I can never put this RCD in a circuit that has a fault current higher than 10kA?

Which leaves me with one option, List the panel as 10kA SCCR and somehow reduce the available short circuit current at the feed of the panel to be <10kA, using a transformer?

Or find an RCD that has a higher interrupt rating than 10kA, do these even exist?
 
ShortCircuited140 said:
Thanks for the inputs so far.

The RCD that I plan on using is already listed to be 10kA in combination with a CL fuse or UL489 breaker in series.
Does this mean I can never put this RCD in a circuit that has a fault current higher than 10kA?

Which leaves me with one option, List the panel as 10kA SCCR and somehow reduce the available short circuit current at the feed of the panel to be <10kA, using a transformer?

Or find an RCD that has a higher interrupt rating than 10kA, do these even exist?
Click to expand...
Your options are correct.

RCD is a term used in IEC devices. Here in N. America we would use the term GFEP (Ground Fault Equipment Protection), you can get lots of molded case circuit breakers with GFEP and higher SCCRs. They are just not going to be the cheap little DIN rail mounting type you have been using. They will be the full sized MCCBs.
 
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