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PAT test - leakage test


Craig A

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http://Not sure if I should post here or electrical but here goes!

I PAT test my equipment in our theatre but the school has asked me to do the rest of the IT equipment. I use a Seaward PT 100 and after the earth and insulation tests, it does a leakage test. Unfortunately, nearly all of the monitors are failing this test, which is normally a sign of user error as far as I'm concerned!

I've searched around for answers but can't seem to find much. However, I have recovered the text from my training and it appears to say that the leakage test, listed under 'Additional Tests', is pretty much the same as the insulation test and in the vast majority of cases it shouldn't be needed. Further on it explains testing IT equipment, and it mentions earth bond and insulation tests and nothing about the leakage test.

To me, this means that I don't need to do it and that would pass my monitors, as they pass both the earth bond and insulation tests.

Any advice?

Thanks in advance

Craig

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The leakage test uses the mains (a.c.) and the filter capacitors on the mains side of the power supplies will pass a significant current (up to a milliamp or so) at 50Hz. The insulation resistance uses 500V d.c. so once the caps are charged they don't pass any current. That's why one test passes and the other fails.

 

In my opinion you don't need to do a leakage test on the monitors. But I'm just a bloke on the internet!

 

Dave

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Can you set the pass level for the leakage test to something that suits what you're testing? A couple of milliamps is fairly standard leakage for IT gear as stated above.

 

Talking to a university IT tech several years ago, they had an RCD per bench of 8 PCs to avoid nuisance tripping, and he calculated total leakage for the building was something approaching 15 amps!

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Dave - I don't claim to be an electrician so you've lost me a bit there! But I think I kind of get what you're saying. I didn't think I needed to do it after my research but I just wanted to get a second opinion!

Adam - No I can't change the levels, but I've been told they're set low on my machine to make them 'idiot-proof'. I think they're set to 0.75 mA, most of the monitors are about 0.80-1.00 when I've tested them. Does that sound about right? (I've never tested IT before, only my theatre lights etc)

 

 

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It looks as if the PT100 has fixed pass values for all parameters, so you may have to use a more adjustable tester.

 

As an aside, it also appears that the earth bond resistance test current is only 200mA. I would not be happy with that test current for class I appliances such as theatre lights.

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Dave - I don't claim to be an electrician so you've lost me a bit there! But I think I kind of get what you're saying. I didn't think I needed to do it after my research but I just wanted to get a second opinion!

Adam - No I can't change the levels, but I've been told they're set low on my machine to make them 'idiot-proof'. I think they're set to 0.75 mA, most of the monitors are about 0.80-1.00 when I've tested them. Does that sound about right? (I've never tested IT before, only my theatre lights etc)

Sorry if I confused you. All I really meant was that you will always get some leakage with a.c. which you might not see with a d.c. test.

The values you measured sound ok. The limit is, I believe, 3.5mA for class I equipment. Less (0.75mA?) if it's hand held class I, and I think it's 0.25mA for class II.

You'll find the info in your copy of the Code of Practice for In Service Testing which I'm sure you have to hand ;-)

Dave

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I've got a Primetest 100 too and it also fails a lot of items that are perfectly fine due to its little extra AC leakage test.

 

I'm trying to think of a failure mode of an appliance that would somehow result in a higher capacitive coupling to ground while not affecting the insulation resistance to ground..... Nope. Can't think of one.

 

In the meantime the unit occasionally does its BEEP-BEEP-BEEP FAIL! When used on anything with beefy suppression, and it makes me wonder how many perfectly functional appliances had their plugs cut off by a non-technical PAT tester.

 

On a plus note I bought this tester second hand at a price that probably reflected the fact the previous owner couldn't actually interpret PAT tests beyond the unit saying pass or fail, and this little extra test was probably causing technical consternation. :rolleyes:

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Good point about some test people not fully understanding and cutting plugs off, Clive. Just to reiterate the utterly pedantic point that HSE states, more or less, "If you gotta ask, don't!" when it comes to PAT.

The person doing testing work needs to competent to do it. In many low-risk environments, a sensible (competent) member of staff can undertake visual inspections if they have enough knowledge and training. However, when undertaking combined inspection and testing, a greater level of knowledge and experience is needed, and the person will need:

the right equipment to do the tests

the ability to use this test equipment properly

the ability to properly understand the test results

 

In fact Jive has just crossposted saying much the same thing. The tester needs to understand the test results.

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Thanks Dave - that makes more sense!

 

I've had training about a year and a half ago but I haven't done it an awful lot since then, and when I have it's been more straight forward. I understand but not the most expert knowledge so I'm trying to refresh and get more experience as I'm going along.

 

Thanks for all of your help

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Also a weakness of a simple pass/fail tester is that it does not demonstrate changes over time. Actual values should be recorded on the equipment record. Then you can see if the equipment is deteriorating slowly over time; even if it passes the test now you can identify if it is likely to fail (possibly catastrophically) before the next test.

 

 

 

 

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The values you measured sound ok. The limit is, I believe, 3.5mA for class I equipment

 

I agree, this is what I was taught. So the pat threshold setting should be this for class 1.

 

I think they set this limit as a safeguard for the amount of danger present if a class 1 appliance became un-earthed for some reason.

 

Conversely I've seen leakage tests pass and insulation tests fail due to some more modern mains filtering that uses MOV's (varistors) between Live and earth. Yes..you can get class-Y varistors or TVS diodes that 'switch on' below 500V. :rolleyes:

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The trouble is that most courses teach you to believe the instruments and do what they tell you. In reality sometimes you fly by the seat of your pants and actually "interpret" this fail reading as a Pass because it's only failed on leakage and it's full of interference suppression.

 

I remember a post on here many years ago about a "generator city" somewhere dropping 16A as earth leakage due to all the smpsus in all the movers. Rf SMPAUs tend to lose RF that would be radiated by dumping it to earth through filter caps.

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