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

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 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:

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

Desktop:
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.

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.

The UPS will work for you.

However, I would use a metered Variac with a suitable fast blow fuse. And you could place the Variac between the UPS and the DUT.

You can make any 120 VAC lamp wattage you like by paralleling lamps. And by unscrewing them from their sockets (or using switches) you can quickly configure it for any wattage.

David, thanks a lot for your reply. Have you ever used an UPS in this capacity?

Can you suggest a suitable fuse type for 0.5A, 1A, 2A, 5A?

I have used (and designed) many switching power supplies and that is what a UPS is.

The fuses must be rated for your AC line voltage and most devices that you will repair already have a fuse. I would use whatever fuse type is recommended. It's my impression that in modern electronic gear the fuse rating is much higher than the normal operating current. The fuse is sized to blow during major failure and exhibit no nuisance blowing.

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Thanks !!!