shorting an UPS?

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

  1. cheater

    Thread Starter New Member

    Nov 2, 2013
    Hi guys!
    First of all, Hi! It's my first post here. This looks like a really nice place to talk about electronics, so I'll be coming back hopefully :)

    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 will be faulty and 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.

    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.

    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.

    P.S. if one of the mods sees this. I wanted to set the topic to: "How does overload protection in an UPS work during a short-circuit?"

    However, it's not possible due to the bug mentioned here:

    If someone could change it, I'd be thankful.
  2. MrChips


    Oct 2, 2009
    I would not use a UPS. There are many other ways to skin a cat.

    • Use a fuse or circuit breaker.
    • Use a thermal resettable fuse.
    • Use a power bar with built-in fuse or circuit breaker.
    • Build your own power panel or power bar and install a circuit breaker.
    • Build your own electronic circuit breaker that consists of a current monitor and relay. I have done this for this explicit purpose.
  3. poopscoop


    Dec 12, 2012
    Start tripping breakers everyday until he gives you access to the panel. Heck, trip the breaker for the heater and make him jump through his butt to fix it.

    Take a tour of your local hardware store, they'll have breakers in all shapes and flavors that you can hook to the outlet yourself. Much easier and cheaper than a UPS will ever be.
  4. cheater

    Thread Starter New Member

    Nov 2, 2013
    Hi guys, thanks for the replies. The breakers seem like a good idea but sadly don't give me assurance that the main breaker won't trip. And if the breaker is tripped then I'm out of electricity for a few days... not the greatest way to be.
  5. adamclark


    Oct 4, 2013
    most main breakers these days are at least 150 amps, 200 is the norm though... in order to trip the main your house would have to have very serious electrical problems if you could take out the main before the secondary and lower amperage breaker for the circuit your plugged in to.. that would also be a very serious fire hazard.. on another note, you would have to be working on something outside the range of hobbiest to throw a 200 amp main... I think adding a 20/30 amp breaker before the wall outlet would suit your purpose nicely..
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    Install a slow blow fuse holder in the AC line. Have a wide range of fuses available for substitution into the holder. Select a fuse based on the stated amp draw of your DUT.

    Example: DUT draws 4 amps when performing correctly. Install 4 or 5 amp slo-blo in holder. power up DUT. If fuse blows you have isolated a fault in the DUT power input.

    House mains and breakers will not 'trip'. The cheap slo-blo fuse will prevent an overcurrent situation from affecting your inaccessible breaker panel.
  7. MrChips


    Oct 2, 2009
    You can go through a lot of fuses very quickly if you are attempting to trouble shoot a faulty system.

    I built a settable/resettable electronic circuit breaker that allows me to read the current draw as well as set the trip current that will disconnect the AC mains.
  8. cheater

    Thread Starter New Member

    Nov 2, 2013
    Hi guys, thanks for all the replies.


    Well, that's right, the main breaker for the building is something like 200A, however my unit has at least one more breaker, and that's through empiric evidence 30A. I don't have access to that one. So my new fuses would be the third or fourth stage.