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

[cheater00]

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 :-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:

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.

- - - Updated - - -

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.

 

[chuckey]

This is a sledge hammer to ***** a nut. The best way is to find out what the landlords breaker is rated at, and install your own one which is much more sensitive. i.e. if the land lords breaker is 15 A, use a 2A. This way your one will trip before his one. Or use the lightbulb idea, install a switch across it so once you have ascertained that there is no problem, short the light bulb out. Buy your self an insulation tester?
Frank

 

[cheater00]

Hi Frank,

This is a sledge hammer to ***** a nut. The best way is to find out what the landlords breaker is rated at, and install your own one which is much more sensitive. i.e. if the land lords breaker is 15 A, use a 2A. This way your one will trip before his one. Or use the lightbulb idea, install a switch across it so once you have ascertained that there is no problem, short the light bulb out. Buy your self an insulation tester?
Frank

Unfortunately I don't have any access at all to the breakers.

What would the insulation tester accomplish? I am assuming you mean a megger type unit?

 

[chuckey]

Given that the piece of kit your are testing can run normally within the rating of your landlords breaker. Then the only way it would trip the breaker is if it took excessive current, which it could do if a switch mode power supply had short circuit transistors or such like. If you tested these high voltage points with your insulation tester then you could make sure that there is not a direct short on the mains. FWIW, the most likely trip would be via an RCD, i.e. too much inbalance between the line and neutral currents , such as a earth to neutral short or a bad filter capacitor.
Frank

 

[cheater00]

Hi Frank,

This is a sledge hammer to ***** a nut. The best way is to find out what the landlords breaker is rated at, and install your own one which is much more sensitive. i.e. if the land lords breaker is 15 A, use a 2A. This way your one will trip before his one. Or use the lightbulb idea, install a switch across it so once you have ascertained that there is no problem, short the light bulb out. Buy your self an insulation tester?
Frank

Unfortunately I don't have any access at all to the breakers.

What would the insulation tester accomplish? I am assuming you mean a megger type unit?

Thanks!

 

[avrilcory]

Power supply is a device that supplies electric power to an electrical load and UPS system fault protection devices protecting lighting circuit wiring from accidental short-circuits and overloads. It's properly coordinated with the branch circuit rating and will protect the load during overload. Every power supply must obtain the energy it supplies to its load, as well as any energy it consumes while performing that task from an energy source.


Thanking you !!!
Riello-ups