KevinE Posted March 30, 2011 Share Posted March 30, 2011 I'm just investigating a PAT test fail on an LCD screen attached to an instrument. It's Class 1 and fails on insulation test, the tester digital readout shows 160K. Another simpler pat tester shows it as 'fail - insulation'. An independent test with a 500V megger connected from L&N strapped and then to earth also shows a similar earth fault. The LCD screen has a universal 110-240 ac supply as part of the main pcb. Opening up the monitor shows 2 MOV's (varistors) rated 431VDC connected between L and earth and N and earth. I have removed them and it clears the fault...they are obviously conducting under the 500V DC test. I just wondered if anyone else has seen MOV'S between line and earth, I was really under the impression that it was ok for Y-class filter caps (they're designed for this!) but any other components were a no-no. :huh: Link to comment Share on other sites More sharing options...
Shez Posted March 30, 2011 Share Posted March 30, 2011 Virtually anything with surge protection components will fail the 500v insulation test. By definition. Link to comment Share on other sites More sharing options...
Andy Wynne Posted March 30, 2011 Share Posted March 30, 2011 Isn't it advisable not to run the 500v Insulation test on anything containing what you might call sophisticated electronics? Off the top of my head, the IEE Code of Practice recommends skipping an Insulation Resistance test for "IT Equipment" and running a "softer" Earth Leakage test instead. This might be too general, but I tend not to run the 500v test through anything that has in-built electronics (PCBs etc...) I appreciate you know much more than me about electronics, and that's just the way I do things. I'd be interested in hearing from better-informed others on what can and can't take a 500v test. Link to comment Share on other sites More sharing options...
KevinE Posted March 30, 2011 Author Share Posted March 30, 2011 Virtually anything with surge protection components will fail the 500v insulation test. By definition. Not necessarily, most MOVs go from live to neutral (which isnt subject to the 500VDC) and the filters are capacitors which block the 500V DC test, and only register on the AC 'powered up' earth leakage test in microamps. What has irked me is that the PAT contractors have failed this £20,000 unit on insulation. The management of the company won't allow it back into service without a completed pass certificate. How can we differentiate between an MOV to earth and a genuine insulation failure!? Link to comment Share on other sites More sharing options...
Jivemaster Posted March 30, 2011 Share Posted March 30, 2011 I've never tested a "surge protected " extension lead that passes the insulation test, due simply to the surge protectors. Tested at a lower voltage they sail through. Link to comment Share on other sites More sharing options...
Brian Posted March 31, 2011 Share Posted March 31, 2011 I just wondered if anyone else has seen MOV'S between line and earth, I was really under the impression that it was ok for Y-class filter caps (they're designed for this!) but any other components were a no-no.VDRs are certainly permitted on a primary circuit. They need to comply with IEC 61051-2 and should be rated at least 120% of the upper rated voltage of the equipment. Link to comment Share on other sites More sharing options...
grizzly Posted March 31, 2011 Share Posted March 31, 2011 What has irked me is that the PAT contractors have failed this £20,000 unit on insulation. The management of the company won't allow it back into service without a completed pass certificate. How can we differentiate between an MOV to earth and a genuine insulation failure!? This is what the reduced voltage (250V) insulation test is for.This is available on many modern PAT instruments, and was introduced for the insulation testing of appliances and extensions that contain surge protection and similar.If the tester(s) that the PAT contractors are using cannot perform this test, then they are not using the appropriate test equipment for the appliances they are being paid to test. Point both the contractors and the management towards the IEE Code of Practice chapter 15.5 (page 75), first paragraph, and also this Seaward press release may be useful to point them in the right direction. There's no reason why the appliance should be failed just because it's got surge protection fitted, and unfortunately this says a fair bit about the competence of the PAT contractors. Link to comment Share on other sites More sharing options...
kerry davies Posted March 31, 2011 Share Posted March 31, 2011 Thank you, Sir Grizzly, a gentleman. I have been looking for something to back up my complaints about a testing company that is taking one of my clients for thousands and sticking labels on pre-formed IEC leads without testing any equipment associated with it. I knew they were wrong but needed to be able to quote some authentic reference and you've done it for me. Thanks again. Link to comment Share on other sites More sharing options...
sleah Posted March 31, 2011 Share Posted March 31, 2011 We had a similar - in fact I suspect the same - problem with some 25inch CRT Panasonic TVs. A call to Panasonic confirmed that there was no 'fault' as such. Our PAT tester took this onboard and passed the TVs (if the rest of test was OK of course!).However, he did change all the fuses to 3A, which we had to go round and change back to 10A due to the surge at switch on..... Link to comment Share on other sites More sharing options...
KevinE Posted March 31, 2011 Author Share Posted March 31, 2011 Mm, the IEC standard for the use of VDRs bridging the insulation barrier in earthed appliances in the name of surge suppression seems dubious to me. Ive seen hundreds of failed l-n VDRs in my career, though never a short-circuit Y filter cap! But seriously, these common-mode MOV's /vdr's were causing more trouble than they were worth, threatening to bring a business to a halt. They are now in the WPB. The varistor between L-N is of course still there as the SMPSU transistor requires it to prevent punch-through due to mains switching spikes. Link to comment Share on other sites More sharing options...
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