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UPS Output Cable Circuit Protection Requirements

Spencer Walburn

Hi,

Hoping that somebody can help me with my understanding on circuit protection requirements for UPS systems specifically for the output cable from the UPS to the downstream switchboard.

I have 40kVA double conversion UPS (three phase 400VAC input/output) with a bypass.

The UPS Vendor has not provided circuit protection at the output of the UPS for the outgoing cable. This cable cable supplies the downstream Essential switchboard in the same room approximately 10 metres away. The Essential switchboard consists of an incoming Isolator and various 40A BS88 fuse feeder circuits. The cable that feeds this essential switchboard does not have circuit protection, however my understanding is that the Inverter in the UPS will restrict the short circuit current to approximately 200% of full load current for 200mS which will be a short circuit current of approximately 115A.

In bypass mode the upstream fuse would clear a fault on the UPS output cable, but in the the event that the UPS is in operation and using the batteries then a fault on that cable would be reliant on the Inverter of the UPS switching off to restrict and clear the fault. Is this acceptable and the norm? The UPS output cable will be sized to withstand a fault in the bypass mode (Circa 2kA of fault current) and therefore would be easily sufficient to withstand an inverter short circuit current of 115A.

Thanks to anybody that can offer some advice.

Regards

Spencer

  • 0

    To give you some pointers to this.....

    You've stated that the UPS will give 200% for 200mS in the event of a short circuit on its output - ie. around 115A

    Its normal max output current would be around 58A per phase.

    So lets consider that you fit a 60A MCB on the output..... looking at the tripping curve for a 60A MCB ... how long would it take to trip at 115A?. Would it trip with 200mS?

    Repeat this same exercise for your 40A BS88 sub-circuits. 

    I suspect that you'll soon see why the UPS manufacturer has omitted outgoing circuit protection other than electronic shutdown.

  • 0

    the electronics tripping  will probably protect the fuses... expensive semiconductors do that  ;-)
    Mike

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