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

[cheater]

 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.

A 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. Plus I haven't been able to find a 1000W light bulb yet.

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.

[8086]

I think perhaps the root of the problem would be the best fix. i.e. question your landlord as to why you can't access your own breakers. 'Weird' is no excuse imo.

[codeboy2k]

Don't buy a UPS for this purpose.  Buy a UPS if you want to use it as a UPS.

My suggestion to you is to spend your money on a beefy ISOLATION transformer.  Make sure it is a tech version, not the consumer version.. that means that the neutral is not bonded to ground inside the unit.  i.e. it is truly floating.

Buy one with a breaker on the output side, or put a breaker on the output if it doesn't have one. Now you have a breaker protected isolated line to work from. And now it's even safer than the apartment's AC line.

And also, I suggest you read up on isolation transformer safety, if you're not already familiar with the proper precautions to take when using one with a device under repair and your test equipment (which might be non-isolated)

Also, see Todd Harrison's excellent video on isolation transformers here:
http://www.toddfun.com/2011/04/30/isolation_transformers/

[ovnr]

No, a UPS is not the right choice. I've got a few APC UPSes, and they've got simple resettable fuses on the outputs, plus an overload shutdown mode (but that is more for excessive consumption, not dead shorts).

Grab a lower-rated breaker and use that. I'd also consider some beefy NTC resistors to deal with inrush currents.

[cheater]

Hi guys,
I've spoken with the landlord already, that's not going to progress in any way, and there's no legal method to force him, so let's stop considering this.

Regarding using a transformer:

I've got a couple isolation transformers. I don't see how they'll stop the main fuse from tripping. Could you explain your idea to me?

No, a UPS is not the right choice. I've got a few APC UPSes, and they've got simple resettable fuses on the outputs, plus an overload shutdown mode (but that is more for excessive consumption, not dead shorts).

Why is an UPS not the right choice?

[M. András]

its not designed to be a lab power supply, to withstand continous short circuits. get a low 5-10amp fast circuit breaker and use that on the dut

[cheater]

Hi! Thanks again for this reply and all others. It's great one can get so much helpful replies here!

its not designed to be a lab power supply, to withstand continous short circuits.

I'm not sure if there is anything else that is designed to be a compact source of mains AC, disconnected from wall mains. Do you have any ideas? I've searched around but there's not much that I've found.

I see the UPS as something similar to a generator, just that it's smaller and doesn't require fuel.

get a low 5-10amp fast circuit breaker and use that on the dut

I'm afraid that the fast breaker might sometimes not be fast enough.

[codeboy2k]

Regarding using a transformer:

I've got a couple isolation transformers. I don't see how they'll stop the main fuse from tripping. Could you explain your idea to me?

An isolation transformer alone may or may not not stop the main fuse from tripping, because the primary side will still "see" the short circuit on the secondary.  What you want is for your secondary side to have a lower rated, fast acting breaker.

I said may or may not because it may be the case that the shorted secondary of a 1000VA or 1500VA transformer, when shorted, will instantaneously deliver it's maximum rated short circuit current, and may settle down to it VA rated current. These are limited by the resistance and inductance of the secondary windings.  Very quickly it may heat up the secondaries and smoke, catch fire, burn up, without ever tripping the primary side circuit, which could be a 15A or 20A circuit.  Certainly the smaller and more common 250VA and 500VA isolation transformers will not be able to blow the main breaker. The shorted secondary just cannot pull the current from the primary side. They'll just smoke and burn out the windings.   That's why you want to have a secondary side with a fast circuit breaker rated for less than the maximum short circuit current of your transformer's secondary, i.e. less than or equal to the VA rating divided by the line voltage.

( and I am neglecting any instantaneous current drawn from the primary as a result of the secondary short, and perhaps that may be enough to blow the main breaker -- more reason to protect it with a fast breaker on the secondary side )

[cheater]

Interesting. What happens to the primary side when the secondary is shorted: the flux produced by the primary has to go somewhere, right? What happens with it?

What about such a set up: UPS -> isolation transformer -> DUT

I'm thinking UPS might be able to tough out any instantaneous current, and the isolation transformer should limit total current, making sure the load the UPS sees will be limited.

[JohnnyGringo]

Also, see Todd Harrison's excellent video on isolation transformers here:
http://www.toddfun.com/2011/04/30/isolation_transformers/

I couldn't find the video. Can you please post a direct link for it?

Never mind found it: [/url]
EDIT: Change HTTPS: to  HTTP:

[andtfoot]

Also, see Todd Harrison's excellent video on isolation transformers here:
http://www.toddfun.com/2011/04/30/isolation_transformers/

I couldn't find the video. Can you please post a direct link for it?

Works fine for me, but here is the YouTube link anyway:

[JohnnyGringo]

Also, see Todd Harrison's excellent video on isolation transformers here:
http://www.toddfun.com/2011/04/30/isolation_transformers/

I couldn't find the video. Can you please post a direct link for it?

Works fine for me, but here is the YouTube link anyway:

Thanks. I couldn't find / see  the link on his web page.  Where was it?

[andtfoot]

Also, see Todd Harrison's excellent video on isolation transformers here:
http://www.toddfun.com/2011/04/30/isolation_transformers/

I couldn't find the video. Can you please post a direct link for it?

Works fine for me, but here is the YouTube link anyway:

Thanks. I couldn't find / see  the link on his web page.  Where was it?

Tutorial using a Tripp-lite IS1000 isolation transformer.

• What I bought for my tech bench.
• Some models and prices of isolation transformers.
• How to alter a public version into a tech version.
• Helpful tips on using your oscilloscope safely with and without an isolation transformer.
• How to be a little safer when working with high voltage live mains

--- VIDEO IS EMBEDDED HERE ---

These would be good tech isolation transformers that don’t need to be modified:
4.30A isolation transformer, 115/115VAC, 500VA
2.50A isolation transformer, 120VAC/120VAC, 300VA
1.25A isolation transformer, BK Precision BK1604A
0.43A isolation transformer, 115VAC, 50VA

[ResR]

Thanks. I couldn't find / see  the link on his web page.  Where was it?

He probably looked "todd harrison isolation transformer" as I did and second result was the video. Using key words in search is a good starting point.
Edit: Andtfoot made it before me...

[JohnnyGringo]

--- VIDEO IS EMBEDDED HERE ---

These would be good tech isolation transformers that don’t need to be modified:

Hmmm... interesting. It doesn't show up in my Firefox, even with adblock turned off.  But does with IE.

EDIT: And it's a rather good video.  Thanks for the help.

EDIT2:  In his video, he plugs the scope into the mains.  Why not also plug your scope into the tech-isolating transformer?

[kolbep]

In South Africa, you can get legal recourse to force him to provide access to the breakers.
The regs in South Africa say that in residences you shall not have to go into an adjacent residence to access the d.b. board. The db however may be in a common area such as a passageway.

The main switch shall always be easily identified, and easily operable for any able bodied person.
If you do not have easy access  to switch off power to the whole, or part, of the installation, for emergency or maintenance, then I would say that no matter what country you are in, it is unsafe and illegal.

Maybe give a qualified electrician a quick call, he can clarify the regs for your area.

[codeboy2k]

Interesting. What happens to the primary side when the secondary is shorted: the flux produced by the primary has to go somewhere, right? What happens with it?

I'm not by any means an expert on magnetics, but my thinking is this: At the one extreme, in open circuit, the primary current causes a rising magnetic field and thus an increasing magnetic flux density in the core, and the field lines of that flux passes through the secondary windings; since the secondary is open circuit, no current flows in the secondary, but instead a voltage is induced. There is no current, and thus no secondary magnetic field as a result of that current in the secondary windings. When the secondary is shorted, if we assume it to be a perfect conductor (which it's not) and there are no electrical nor magnetic losses (which is not true) then the short circuit current in the secondary windings would causes a rising secondary magnetic field in opposition to the primary magnetic field, trying to cancel it out. Note that the short circuit voltage is 0.  The rising secondary current and secondary magnetic field in opposition to the primary will cause a decrease in flux density of the core, as the secondary cancels the primary.

The primary side (assume an ideal source) will then try to supply more current.. but it can't supply infinite current, because it's current limited and there is a breaker in the primary circuit. so it blows the breaker;... But even this may not happen at low enough currents on the secondary side, because the increasing current in the secondary causes heating, and this increases the winding resistance, causing a decrease in current, so the secondary cannot infinitely oppose the primary, and the primary wins, and brings the magnetic field and flux density back up to whatever is needed to supply the current in the shorted secondary... until it burns out.

So.. again.. protect your secondary with a lower rated, fast acting circuit breaker, and it will trip first, protecting the secondary from damage and protecting your main breaker too.

For more technical details, look up Maxwell, Faraday, Lenz, and Lorentz

edit: added link to MIT's Prof. Lewin
http://videolectures.net/mit802s02_lewin_lec16/

[alm]

EDIT2:  In his video, he plugs the scope in to mains.  Why not also plug your scope into the tech-isolating transformer?

This called floating your scope, and generally considered a safety hazard. The problem is that many components like connectors, including the front panel USB and BNC connectors, are connected to ground. If you disconnect this ground and clip your ground lead to a dangerous voltage, say the 320 VDC rail in a switching power supply, and touched one of these connectors, you get shocked and possibly electrocuted (high current 320 VDC can easily be lethal).

[Stonent]

Also, see Todd Harrison's excellent video on isolation transformers here:
http://www.toddfun.com/2011/04/30/isolation_transformers/

I couldn't find the video. Can you please post a direct link for it?

Never mind found it:

The forum doesn't process https links just http. So if you take the "S" off the end it will work.

[JohnnyGringo]

Also, see Todd Harrison's excellent video on isolation transformers here:
http://www.toddfun.com/2011/04/30/isolation_transformers/

I couldn't find the video. Can you please post a direct link for it?

Never mind found it:

The forum doesn't process https links just http. So if you take the "S" off the end it will work.

I've changed it, but my Foxfire browser/configuration(?)  does some really bizarre things.  The "Preview" looks fine, but after I save the modification, the URL is reformatted.  The HTTPS version looked fine, worked and doesn't create all that white space (about 3/4 of a screen full) before the link.

[codeboy2k]

Also, see Todd Harrison's excellent video on isolation transformers here:
http://www.toddfun.com/2011/04/30/isolation_transformers/

I couldn't find the video. Can you please post a direct link for it?

Never mind found it:

The forum doesn't process https links just http. So if you take the "S" off the end it will work.

My original link was http, the other poster's link to youtube directly was an https, but I don't know why the OP couldn't see the video embedded in Todd's blog page. It is right there in the middle.  I made a link to his original blog post about it rather than just the youtube video because I wanted to give him credit in entirety, not just embed the video. Furthermore, his page has some Q&A and links to various other resources for isolation transformers.

Still weird to me why the OP couldn't see the video in Todd's blog posting.  BTW, is Todd on the forum?

[JohnnyGringo]

I tried it again from a different computer, with firefox, same result; couldn't see the embedded video.  Go figure.  Anybody else have a problem seeing it with foxfire? 

[codeboy2k]

I tried it again from a different computer, with firefox, same result; couldn't see the embedded video.  Go figure.  Anybody else have a problem seeing it with foxfire? 

I also use Firefox, on Linux. I see the embedded video on Todd's blog.  It's possible that an extension you have installed is removing it before you see it.  (AdBlock? NoScript? Curiyo? )

[cheater]

Hi guys,

I would appreciate if off-topic chat could be moved to a place more appropriate.

Thanks!

[djococaud]

Hi !
What about a good old light bulb in series (100W) ?
Lights up if there is a short, but only usable on small loads though...