How does overload protection in an UPS work during a short-circuit? - Page 1
Forum section: electronics / projects

author: cheater
author_rank: Regular Contributor
timestamp: November 02, 2013, 10:00:11 am
content: 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 go2. 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 SU2200RMI3UAPC Smart UPS SUA1500RMI2UAPC Smart UPS SU1400RMI2UAPC Smart UPS SC 1500desktop:APC Smart UPS SUA1500I 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.

author: 8086
author_rank: Super Contributor
timestamp: November 02, 2013, 10:12:36 am
content: 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.

author: codeboy2k
author_rank: Super Contributor
timestamp: November 02, 2013, 10:38:34 am
content: 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/

author: ovnr
author_rank: Frequent Contributor
timestamp: November 02, 2013, 10:51:52 am
content: 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.

author: cheater
author_rank: Regular Contributor
timestamp: November 02, 2013, 11:19:41 am
content: 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?QuoteWhy is an UPS not the right choice?

author: M. András
author_rank: Super Contributor
timestamp: November 02, 2013, 11:25:25 am
content: 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

author: cheater
author_rank: Regular Contributor
timestamp: November 02, 2013, 11:36:04 am
content: Hi! Thanks again for this reply and all others. It's great one can get so much helpful replies here!Quote from: M. András on November 02, 2013, 11:25:25 amI'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.Quote from: M. András on November 02, 2013, 11:25:25 amI'm afraid that the fast breaker might sometimes not be fast enough.

author: codeboy2k
author_rank: Super Contributor
timestamp: November 02, 2013, 12:49:22 pm
content: Quote from: cheater on November 02, 2013, 11:19:41 amAn 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 )

author: cheater
author_rank: Regular Contributor
timestamp: November 02, 2013, 01:01:53 pm
content: 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 -> DUTI'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.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 02, 2013, 01:33:34 pm
content: Quote from: codeboy2k on November 02, 2013, 10:38:34 amI couldn't find the video. Can you please post a direct link for it?Never mindfound it:EDIT: Change HTTPS: to HTTP:

author: andtfoot
author_rank: Unknown
timestamp: November 02, 2013, 01:38:24 pm
content: Quote from: JohnnyGringo on November 02, 2013, 01:33:34 pmWorks fine for me, but here is the YouTube link anyway:

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 02, 2013, 01:42:41 pm
content: Quote from: andtfoot on November 02, 2013, 01:38:24 pmThanks. I couldn't find / see the link on his web page. Where was it?

author: andtfoot
author_rank: Unknown
timestamp: November 02, 2013, 02:05:40 pm
content: Quote from: JohnnyGringo on November 02, 2013, 01:42:41 pmQuote

author: ResR
author_rank: Regular Contributor
timestamp: November 02, 2013, 02:12:36 pm
content: Quote from: JohnnyGringo on November 02, 2013, 01:42:41 pmHe 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...

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 02, 2013, 03:14:03 pm
content: Quote from: andtfoot on November 02, 2013, 02:05:40 pmHmmm... 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?

author: kolbep
author_rank: Frequent Contributor
timestamp: November 02, 2013, 03:27:10 pm
content: 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.

author: codeboy2k
author_rank: Super Contributor
timestamp: November 02, 2013, 04:37:27 pm
content: Quote from: cheater on November 02, 2013, 01:01:53 pmI'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 Lorentzedit: added link to MIT's Prof. Lewinhttp://videolectures.net/mit802s02_lewin_lec16/

author: Unknown
author_rank: Unknown
timestamp: November 02, 2013, 04:41:38 pm
content: Quote from: JohnnyGringo on November 02, 2013, 03:14:03 pmThis 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).

author: Stonent
author_rank: Super Contributor
timestamp: November 02, 2013, 08:15:58 pm
content: Quote from: JohnnyGringo on November 02, 2013, 01:33:34 pmThe forum doesn't process https links just http. So if you take the "S" off the end it will work.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 02, 2013, 09:07:40 pm
content: Quote from: Stonent on November 02, 2013, 08:15:58 pmI'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.

author: codeboy2k
author_rank: Super Contributor
timestamp: November 03, 2013, 01:54:55 am
content: Quote from: Stonent on November 02, 2013, 08:15:58 pmMy 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?

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 03, 2013, 02:23:30 am
content: 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?

author: codeboy2k
author_rank: Super Contributor
timestamp: November 03, 2013, 04:36:01 am
content: Quote from: JohnnyGringo on November 03, 2013, 02:23:30 amI 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? )

author: cheater
author_rank: Regular Contributor
timestamp: November 03, 2013, 02:33:03 pm
content: Hi guys,I would appreciate if off-topic chat could be moved to a place more appropriate.Thanks!

author: djococaud
author_rank: Regular Contributor
timestamp: November 07, 2013, 08:26:21 am
content: 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...

author: cheater
author_rank: Regular Contributor
timestamp: November 07, 2013, 11:39:30 am
content: Hi djococaud,Quote from: djococaud on November 07, 2013, 08:26:21 amFor one thing, light bulbs introduce a voltage drop during limiting which can destroy the circuit. In specific they will lower the voltage anywhere form 20-50%. If a switching mode power supply is operated with a voltage drop this high, it will try to make up low voltage with higher current, which will drop more voltage because of the light bulb, and eventually it'll self-destruct. To prevent this, you have to make the light bulbs provide much more power margin than is safe for a power supply in fault condition. However, if the light bulb limiter can offer so much current, then the power supply will be able to self-destruct during short circuit. So you're really trading off one self-destruct scenario for another.A separated power supply with the right fuses will act better in that case. A lot of power supplies are smps nowadays.On the other hand fuses + bulbs might work as well, except you need a very high margin between the power you want to deliver to your device normally, and the maximum power that your mains can deliver before tripping the fuse. That's an issue here.

author: djococaud
author_rank: Regular Contributor
timestamp: November 08, 2013, 11:48:55 am
content: yeah, I agree...It was just an idea...You should use a small circuit beaker instead... (fuses would be expensive on the long term, and we are always run out of fuses when we need a new one !)

author: Strada916
author_rank: Frequent Contributor
timestamp: November 08, 2013, 01:16:20 pm
content: The light bulb idea is a great one. Used it on many occasion repairing VHS and CRT tv's Granted the load is light.(no pun intended) Nearly all TV I repaired had SMPS, they handled the lower voltage ok. (240mains here is AU)What's stopping you from having multiple bulbs in parallel? 4 x 250W would be just fine. Now SMPS rated at 1000kW is massive. Sound like you are repairing industrial equipment? Yes I know they create Vd but if the DUT is shorted then you can tell as the lights will be bright. The bulb idea is also ideal as it has very low resistance on start up. once on it rises.You could put switches in circuit so you can control how many light are in parallel.Anyway my two cents worth.

author: cheater
author_rank: Regular Contributor
timestamp: November 08, 2013, 01:47:59 pm
content: Strada, I'm glad you have had success repairing SMPS with this method. This means I'm likely just worrying too much. I'll try it out and tell you where I get. I'll need to come up with a lot of light bulbs! :-)