SCCR without fusing

[MSaeger]

I have been asked to calculate the SCCR on an existing piece of equipment. The circuit includes a 24Vdc power supply ( 100 watt, UL 508 rating). The input to the power supply, is tied directly to the disconnect ( 460Vac 3 phase), no fusing, no circuit breaker.

1. Do I give it the 5kA rating as a transformer?
2. Which codes do I quote to try and express to management that this design is a bad thing?

Thanks in advance.

 

[petersonra]

The problem is that this design is probably not compliant with UL508a, and thus you cannot use the supplement in UL508a to calculate any short circuit rating for this panel at all.

 

[MSaeger]

Bob,

Thanks for the quick response.

I am suspicious of the design, but I need to document which codes are explicitly violated. Equipment is already in production.

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Regards
Matt

 

[dkidd]

You can start with this

http://advanceelectricaltraining.com/iaei/nec-requirements-for-short-circuit-current-ratings/

The Bussmann website has a lot of information. So do other fuse manufacturers.

 

[MSaeger]

I think I found the answer: NEC 70, 725.127

Conductors and equipment on the supply side of the power
source shall be installed in accordance with the appropriate
requirements of Chapters 1 through 4. Transformers or other
devices supplied from electric light or power circuits shall be
protected by an overcurrent device rated not over 20 amperes.

Bottom line there must be a protective device in front of power supply.

Thanks for all the responses.

 

[Jraef]

But don't forget, the NEC is about INSTALLATION of equipment, not listing of equipment.

Most likely, the UL listing of the power supply will require that it be protected with fuses or a CB, so if there is nothing, you cannot list the assembly. But if not, then the rule in UL508A is that if the primary current is less than 2A, the upstream device can be as much as 500%, so 10A max. You didn't say if the Main is a CB or fused, or what size it is.