SCCR rating of 3-phase input, 24VDC output power supply?

[rupej]

Hello all,

I searched far and wide on the internet, but haven't found any conclusive answer- how do you calculate/estimate the SCCR rating for a 24VDC power supply that is fed from 3 phase, 480V? Like the one on this page: http://ab.rockwellautomation.com/Power-Supplies/Standard-Switched-Mode-Power-Supplies

Is it treated like a control transformer, where you ignore it, or do you assume a low rating such as 5kA for the device? I've not been able to find any documentation listing the SCCR ratings on any of the power supplies I looked up.

Thanks!

 

[petersonra]

The AB web site says they are exempt according to UL508a.

SB3.2.1 For control circuits tapped from the feeder circuit, the overcurrent protection for the common
control circuit or for the primary of a control transformer or power supply shall be provided with branch
circuit protective devices having a short circuit current rating not less than the overall panel short circuit
current rating, see SB4.4.4.

While this is worded in a confusing way it appears that the AIC rating of the power supply OCPD determines the SCCR rating of the power supply.

 

[Jraef]

Power supplies fall under the component list as a "power transformer" because that is the first component, so are not included in the SCCR calculations, per SB4.2 exception 1. It is not switching or passing POWER through it, other than as a consumer (load). But as mentioned, the PSU must have a properly sized OCPD feeding it and that device DOES fall into the calculations, be it fuses or a circuit breaker.