I f***** up! WHY? - Page 1
Forum section: electronics / beginners

author: target7500
author_rank: Newbie
timestamp: January 04, 2015, 12:20:51 pm
content: I'm the kind of guy who likes to tinker around and, by doing things that way, messes a lot of things up. My father found a UPS which we couldn't for some reason connect to an self-made battery expansion pack. There is almost no documentation that we could find on the internet so we tried to do it ourselves but it wouldn't work. For the expansion pack there were three connection, positive. negative and ground. The voltage between positive and negative is 40V and the voltage between positive and ground is 40V. So when the current flowing through from positive to negative was almost nothing, i tried to connect positive to ground for a split second. WHAT THE F**** DID I DO? My father told me that i burned it out, but he probably can't explain me why. He fixes machines for work, but isn't good at explaining and I like things explained. So what happens when you connect positive to ground?

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2015, 12:30:51 pm
content: Probably because the negative was connected to ground?This is quite common. It's done to stop the power supply from floating at any voltage. If the negative wasn't connected to ground, it's possible that under certain circumstances, it could float at a high voltage relative to ground and cause a safety issue.It isn't always the negative which is earthed, sometimes it's the positive which is very common in telecommunication systems.

author: wraper
author_rank: Unknown
timestamp: January 04, 2015, 12:37:47 pm
content: My guess is that this UPS needs split 40V+40V battery power, 80V in total. (+40V)---GND---(-40V). Probably UPS is not damaged, does it work with internal battery?

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2015, 12:44:28 pm
content: Quote from: wraper on January 04, 2015, 12:37:47 pmI doubt it's a bipolar supply because he said the voltage between negative and positive is the same as the voltage between positive and ground:Quote from: target7500 on January 04, 2015, 12:20:51 pmWhat did you do?You shorted the positive to ground which is also connected to the negative, causing a very high current to flow which could've damaged the batteries and wiring.

author: wraper
author_rank: Unknown
timestamp: January 04, 2015, 12:48:45 pm
content: Better to post actual UPS model.

author: target7500
author_rank: Newbie
timestamp: January 04, 2015, 12:54:38 pm
content: So the UPS kindda works but my father complains that there is current flowing through the housing. So could i have done a lot of damage and is it possible to fix?Quote from: wraper on January 04, 2015, 12:48:45 pmIt's a SOCOMEC UPS ENTERPRISE tower 800VA UPS and, since I couldn't find anything on the manufacturers website I guess the model number is ENT-TW08 from other websites I visited.

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2015, 01:03:11 pm
content: Quote from: target7500 on January 04, 2015, 12:54:38 pmA wiring problem could be causing current to flow through the housing.It's possible you've completely ruined it but it's equally likely no damage has been done. Does it still read 40V between the positive and negative? If not, are there any fuses or circuit breakers which have blown and can be replaced or reset?

author: Thilo78
author_rank: Regular Contributor
timestamp: January 04, 2015, 01:18:37 pm
content: Quote from: target7500 on January 04, 2015, 12:54:38 pmHow do you know that current is flowing via the housing (or case, I presume?)Also take a look at the documentation:http://www.effekta.se/doc/manualer/socomec/2dt00r02_enterprise_manual_engelska.pdfIt looks like the expansion connector takes Plus, Minus, and Ground to the expansion battery unit.I'd assume, that the DC circuit is grounded on the minus side. So in fact you may have actually shorted plus and minus from the battery and the rectifier.Depending on the design (which I'd expect to be pretty robust with Socomec brand UPS), there might be no damage to the rectifier. But as you assume that current is flowing via the case, there might be damage to internal wiring.But it would be safer to have the unit checked by Socomec service.

author: target7500
author_rank: Newbie
timestamp: January 04, 2015, 01:40:25 pm
content: Since I am pretty much a total newbie to this s*** and have little knowledge about wiring habits in the industry I still do not understand what that "the negative is grounded" stuff means. (from my perspective: if the negative was grounded why should there be a ground lead at all, and can someone show me a schematic or something like that of a grounded negative lead?)

author: Thilo78
author_rank: Regular Contributor
timestamp: January 04, 2015, 02:00:19 pm
content: Quote from: target7500 on January 04, 2015, 01:40:25 pmAh, I see. No problem there.Grounded negative just means, that the negative lead is referenced to the earth potential. Thus, Minus is really zero volts compared to the case. This is necessary as the DC circuit might be isolated and thus floating if not reference properly.Practically, there is one (and only one!) point, where the minus is connected to earth, usually halfway between rectifier and inverter.Earth means "PE" or protective earth, which is mandatory for this kind of appliance.Quote from: target7500 on January 04, 2015, 01:40:25 pmPE must not be part of the function, but is a separate safety measure to prevent electrical shocks. It's intended to keep the voltage zero at exposed parts and has to be designed to carry failure currents. (usually 6 or 10 kA for domestic installation, until the circuit breaker kicks in or the fuse blows)So it is okay to use PE for the reference potential, but it must not replace any functional connection. (Except for special kinds of network in power distribution.But that's another story. Once you have advanced more into electrical connections, try googling for TT networks using PEN conductors)So, to have the battery feed into the Inverter (#4 on the diagram), you have to connect the DC circuit, both plus and minus.The PE runs in parallel to ground the case and protect the user. This connection has to be kept continuous between the mains connection (i.e. the power plug) and the load (which are the connectors where you plug in the computer or whatever load)Basically, it's the same principle as with mains voltage:Plus and Minus work like Live and Neutral. PE is PE.Edit: I just sketched the typical layout of a UPS, neglecting the bypass or separate charger and other components.Maybe that's easier.Edit 2: Forgot to pencil in the PE to the battery case/module...

author: target7500
author_rank: Newbie
timestamp: January 04, 2015, 02:18:43 pm
content: I think I get it now. Thanks a lot.

author: Thilo78
author_rank: Regular Contributor
timestamp: January 04, 2015, 02:22:50 pm
content: You're welcome!If in doubt, try to get in touch with the people at Socomec. From my experience, they're really nice and will be happy to assist you with the UPS and some learning. You just have to ask nicely

author: German_EE
author_rank: Super Contributor
timestamp: January 04, 2015, 08:35:13 pm
content: I cannot remember the exact details of the tale but it goes something like this:An engineer makes an error and destroys $2000 worth of equipment. The Managing Director of the company wants to fire the guy when he hears this but the guy's manager refuses to do so. Why? He's just received $2000 worth of training.In other words, learn from your mistakes and don't F*** up again