Simple Redundant AC Input Circuit Issue 3

[Kepco]

Hi Guys,
The attached circuit is an AC redundant input circuit for a chassis that runs several SBC cards. The AC powers four PSUs. For some reason when the AC is turned ON, occasionally the fuses on both AC inputs (one fuse each on each side) burns. Switching the AC inputs individually the circuit works normal. The fuse is 15A and the load is 8A max. The fuse is fast blow type. The PSUs are inrush current limited at 10A a piece.

The circuit works like this. When the top AC input is present the relay turns ON and connects the Top AC input to the PSUs. Now when the top AC input is turned OFF the relay turn OFF and the lower AC input powers the PSUs.

The attached scope shot shows the AC waveforms when both AC inputs are killed at exact same time manually. CH1 and CH2 are AC inputs and CH3 (magenta)is current. This shot is taken at the AC input side before the filter. The shots shows the high current condition happens for 1.25ms which is also current limited since it is flat at the top. Not sure why this is happening and how a fuse can respond to such a short duration of surge current.

Any ideas regarding this failure?
Thanks

 

[Skogsgurra]

Kepco asks "can you shed some light on using relay interlocking with a work around to the arcing"

Well, if your PSU has a ride-through like 20 ms, it will probably be a problem.

I have a Kepco 72-6. Four quadrants of DC +/-72 V and +/-6 A output current. Big, noisy and just great.

Gunnar Englund

http://www.gke.org

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