isolation transformer needed
in [Repair]
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From: Dave Plowman (News) on 30 Dec 2009 06:00 In article <hher8n$7cu$1(a)news.eternal-september.org>, mike <spamme0(a)go.com> wrote: > > If your mains supply has one side grounded, then touching the 'live' > > side causes a shock. If you use an isolating transformer, you can > > touch either leg safely. That plus an RCD feeding it provides the best > > degree of safety in the workshop. I'm surprised you don't see this. > > > You amaze me. You're so bent on proving me wrong that you are not open > to the possibility that applying logic to misguided advice might yield > useful information. > Normally, I'd not waste my time trying to train > internet denizens. But in cases where a plethora > of misguided advice presented vehemently and with > AUTHORITY puts people at risk of DEATH, > I feel compelled to speak up. This stuff hangs > around for years. And you still haven't learnt anything, obviously. -- *There are two kinds of pedestrians... the quick and the dead. Dave Plowman dave(a)davenoise.co.uk London SW To e-mail, change noise into sound.
From: Phil Allison on 31 Dec 2009 20:22 "mike" > Normally, I'd not waste my time trying to train > internet denizens. ** Or teaching pigs to sing. > The primary reason to want an isolation transformer to > troubleshoot a power supply is to work on the primary side. > Now, the KEY word is TROUBLESHOOT. What that means is that > the power supply has a FAULT in the primary circuit. > What's the fault? You don't know, or you'd just fix it. > How safe is it to troubleshoot that particular fault? > You don't know, 'cause you don't know what it is. > > You're gonna RISK ELECTROCUTION based on the misguided > assumption that an isolation transformer keeps you safe. > YOU'RE NOT SAFE. YOU DON'T HAVE ANY IDEA WHAT THE PRIMARY > CIRCUIT IS, BECAUSE IT'S NOT AS DESIGNED. IT HAS A FAULT!!!!!!! > The node that the designer called common may not be common > at all. IT HAS A FAULT!!!! You should not arbitrarily ground > ANY node. It has a fault!!! (I'm skipping over the obvious > question, "what is ground anyway?") > > An isolation transformer is not inherently bad. It can > provide a layer of protection. What is bad is the FALSE > sense of security > that the transformer makes it safe to poke around inside > a supply WITH A PRIMARY FAULT. ** Standard procedure for servicing any unit with an unknown fault is to FIRST see what happens when plugged into a normal AC outlet - ie one with neutral and ground linked at the service box. That outlet MUST have an RCD fitted as a standard safety precaution for ANYONE working on mains powered equipment while it is energised. The RCD will trip if the unit has a fault that causes even 10mA of leakage from the AC supply to ground - which includes a short from internal neutral conductors to safety ground. If the unit is of class 2 ( double insulated ) construction, the external metalwork should be linked to safety ground in order to test the integrity of that insulation and render the unit safe to handle. Obviously, if the RCD trips under these initial tests the cause MUST be tracked down and dealt with before going any further. The ONLY time a service tech needs to use a mains isolation transformer is when the unit has a "live chassis" as some TV sets do OR it contains an off-line SMPS that needs detailed analysis with a scope in order to effect repairs. The tech must be VERY aware that the electric shock protection afforded by the RCD is LOST when the isolation tranny is in use and so use it as sparingly as possible. It a damn good idea to have the isolation tranny VERY visible on the bench to constantly remind the tech it is in use. When work is completed, the unit should be again connected to a normal, RCD protected, outlet and checked. If the unit it uses class 2 insulation, the AC leakage current to safety ground should be measured with a suitable load and meter. The leakage current should not exceed 0.5 to 1 mA, depending on the item and the actual AC voltage in use. There are a host of other checks and inspections a service tech needs to do on and around the AC wiring, fusing devices and any AC power leads and plugs attached to a unit. Any parts that are damaged, worn out or otherwise seem unsafe must be repaired or replaced. I go through a LOT of AC plugs, fuses and fuse holders in my work. ...... Phil
From: Jeff Liebermann on 31 Dec 2009 20:45 On Wed, 30 Dec 2009 00:23:40 +0000 (GMT), "Dave Plowman (News)" <dave(a)davenoise.co.uk> wrote: >If your mains supply has one side grounded, then touching the 'live' side >causes a shock. If you use an isolating transformer, you can touch either >leg safely. That plus an RCD feeding it provides the best degree of safety >in the workshop. I'm surprised you don't see this. What's an RCD? We call it an GFCI on the other side of the pond: <http://en.wikipedia.org/wiki/Residual-current_device> If you put a transformer before or after the GFCI, any leakage current between either side of the xformer secondary will have no path to ground. The GFCI won't trip. You can prove it to yourself. Find a GFCI. Plug in a common 5, 9, 12, or whatever AC output wall wart into the GFCI. Touch one of the output pins (xformer secondary) to the AC ground somewhere. The GFCI won't trip. Neither will a faulty power supply, sitting on the output of the isolation transformer. I found this out the hard way. At the advice of a former friend, I built a test box consisting of a variac, isolation transformer, circuit breakers, voltmeter, ammeter, and GFCI. A power cord line input and several wall jacks for the outputs. After getting zapped, shocked, and fried several times without the GFCI ever tripping, I realized that this wasn't going to work. The only real advantage to this derrangement was that I could use my grounded oscilloscope without fear of destroying the scope probe lead. -- # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 # 831-336-2558 # http://802.11junk.com jeffl(a)cruzio.com # http://www.LearnByDestroying.com AE6KS
From: GregS on 4 Jan 2010 10:19 In article <3dhu1a.srn.17.9(a)news.alt.net>, Meat Plow <meat(a)petitmorte.net> wrote: >On Fri, 1 Jan 2010 12:22:41 +1100, "Phil Allison" ><phil_a(a)tpg.com.au>wrote: > >>** Standard procedure for servicing any unit with an unknown fault is to >>FIRST see what happens when plugged into a normal AC outlet - ie one with >>neutral and ground linked at the service box. > >Funny that was never on any service literature or diagnostic procedure >documentation I ever read. Nor would I have ever made it my own >practice as a first step thinking back 30 through 30 years. > >But then again this is an internationally mirrored forum so I don't >really try to pawn off my particular service procedures as the gospel >where the diagnostics flow chart may be much different in certain >regions of the globe. > I have hooked things up with isolation and later found things like a motor short. Works fine with the transformer. I didn't always first check the customers explanation and plug something in right away before doing anything, but I do it now. greg
From: GregS on 4 Jan 2010 10:29
In article <24c28c02-731b-450f-a0bc-98b975cec73d(a)26g2000yqo.googlegroups.com>, Andy <andrewkgentile(a)gmail.com> wrote: >On Dec 2, 12:03 pm, zekfr...(a)zekfrivolous.com (GregS) wrote: >> In article <50c36de762d...(a)davenoise.co.uk>, "Dave Plowman (News)" > <d...(a)davenoise.co.uk> wrote: >> >> >> >> >In article <hf5svh$5g...(a)usenet01.srv.cis.pitt.edu>, >> > GregS <zekfr...(a)zekfrivolous.com> wrote: >> >> In article <%llRm.94141$gg6.27...(a)newsfe25.iad>, sbnjhfty >> > <fgsdf...(a)asfffwer.com> wrote: >> >> >I'm looking for an isolation transformer for repairing small (less >> >> >than 1000W) power supplies. I see loads of them on ebay and some are >> >> >medical grade and inexpensive. Are these usable for what I want? >> >> >Other than that, any tips on where to get a good deal on one? I >> >> >don't want to spend a load as this is hobby work only. >> >> >> Just an 'isolation transformer" means NOTHING for what you want. Most >> >> all isolation transformers sold have the secondary tied to ground. You >> >> need a service isolation transformer. You can usually get inside and >> >> remove a secondary "neutral" connection. >> >> >That's interesting. I've never seen any transformer with the secondary >> >tied to ground - unless as part of an installation. The word 'isolation' >> >means just that. Sure you're not thinking of an auto transformer? >> >> I have been thinking about it. The MAIN purpose of an isolation transformer >> is to make it non isolated by making a NEW neutral close to the device >> being powered. The main purpose of an isolation transformer is noise control. >> >> Now we have us tecks who all our lives have learned isolation and that stuff. >> I taliked to the people at Tripplite and verified ALL their isolation >> transformers ARE grounded. Read the paragraph, and yet it says above, > "Complete >> line > Isolation"http://www.tripplite.com/en/products/model.cfm?txtSeriesID=325&EID=1 >3... >> Also look > throughhttp://www.tripplite.com/en/keyword-search.cfm?q=isolation%20transforme >r >> The isolator you buy from MCM is very unlikely to have the secondary > grounded, but >> I really can't verify that. >> >> In reading PC Power Protection by Mark Waller, he >> pointed out their is a NEC requirment for this grounding. >> I have not found an exact description in the code. There is about 75 > references >> in that book, but no index marks. Man, I should sell that book !!!! Big > Bucks. >> >> greg > >On an isolation transformer (single phase), the primary side is always >grounded because the neutral leg is always bonded in the distribution >panel. This is a code requirement. If the secondary is also grounded >(I am pretty sure this is also a requirement), the two sides are still >electrically isolated. The current leaving the secondary hot leg must >return to the neutral (grounded) leg of the secondary. There is no >path from the secondary hot to the primary neutral. The secondary has >to leave the hot leg and return on the secondary neutral leg. The >secondary current cannot go any where except through secondary loop. >Any noise on the high side (from nearby VFDs or other) must pass >through the primary winding in order to induce a current onto the >secondary. The winding itself is a low pass filter, so much of the >noise on the primary is dissipated as heat. I would agree with this except you also state then, there is no path from secondary hot to primary neutral, which makes no sense in your explanation. In electronic servicing you don't want to ground the secondary making a new neutral. You got to get away from the code to do it. greg |