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How does overload protection in an UPS work during a short-circuit?

Discussion in 'General Electronics Discussion' started by cheater, Nov 2, 2013.

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  1. cheater


    Nov 2, 2013
    Hi guys!
    First of all, I wanted to say hi! This is my first post on this board. I've been looking for a nice place to chat about electronics, this might be it! :D

    I was wondering if someone could help me by explaining how overload protection in an UPS (specifically an APC Smart UPS) worked.

    I'm looking to set up a hobby work bench for analog design and simple repairs. Naturally some devices I have will create a mains short when first brought in. Due to a rather weird landlord I don't have access to the breakers, so if I short my wall mains, I'm kind of hosed until I can locate him (that can take several days). The idea is to use an UPS, running the DUT off its battery, and see if that gets shorted - if it doesn't, the DUT should be safe to connect to wall mains.

    In theory, an UPS has overload protection. Having spoken to the APC support line, I have heard the following from them:

    1. when the UPS output gets shorted, overload protection kicks in, and after the short is cleared the UPS can be reset and is ready to go
    2. doing this repeatedly is not something the UPS is meant to do, and they say it might damage the UPS.

    I recognize (2) is just defensive thinking, and was wondering if anyone had any experience or insight into how the overload protection works.

    Asked whether adding a fuse to the output of the UPS would work, they weren't able to say if that would help at all. What do you guys think?

    Here are the UPSes that I'm currently considering:

    APC Smart UPS SU2200RMI3U
    APC Smart UPS SUA1500RMI2U
    APC Smart UPS SU1400RMI2U
    APC Smart UPS SC 1500

    APC Smart UPS SUA1500

    I would appreciate any insight. I have been unable to find schematics for any of those, so I'm hoping someone here has worked with one of those devices or a similar one and can tell what's going on inside. Will the circuit really become damaged if it is shorted "repeatedly"? This shouldn't happen too often. I'll have time to sit at the work bench once a week or two, so if I have a new device which I bought second-hand, it might have a short in it. As I'm trying to clear up the short inside the device under test, I'll be trying things and checking with my multimeter if there's a short. Then if the DMM indicates no short I would try with the UPS again. So in fact overload or output shorting should not happen too often, but I'd be testing with it as a precaution. Sometimes the DMM can lie, especially with larger power supplies, or if the short is intermittent.
  2. cheater


    Nov 2, 2013
    BTW, before someone thinks I'm over-engineering this :)

    It's often recommended to put a light bulb in series. The light bulb series trick doesn't work perfectly - there's a noticeable voltage drop. I could make it up with a Variac, but then this draws even more current, so I get even more voltage drop, and the whole thing ends up being less than perfect with a dynamic load. Plus I haven't been able to find a 1000W light bulb and fixture so far.
  3. duke37


    Jan 9, 2011
    Tjhe light bulb method is good for limbering up old valve radios. If you want 1kW for bigger things, then go for a one bar electric fire.

    You could also provide your workbench with low current high speed trips which are faster than fuses and perhaps faster than the landlords tackle.
  4. cheater


    Nov 2, 2013
    I'll do that too - but as you say it's not certain the trips will be fast enough. So I'm going to use the UPS to test first.
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