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Tricky PAT question

Hello everybody! I could use a bit of sage advice regarding PAT testing unusual items.

To give you a bit of background, I'm in a bit of a tricky spot because I just took over a role as a PAT tester and it came with two very smart testing devices which save the identification number, create a bar code and save the tests required for whatever item it is. Subsequently everyone in my company seems to think that all I have to do is type in a number and the test is automatic.

Unfortunately, the more items I test, the more I notice that the tests that have been inputted, are not anything I want to put my name/initials to. I mean, a few days ago I was prompted to complete a touch current test on a bog standard class I toaster. I've found; tests on leads that do not test for polarity, a test that checks IR at 500V and also checks leakage but nothing else, and it seems to have been standard to cobble together 4 and 5 pin plug and socket adapters for testing 35/63/125A cables using 3 pin plugs and sockets which connect to the PAT tester... d'oh! So it's taking me a lot longer to test things than my company expects. It's a bit of a nightmare if I'm honest as almost every test needs checking and most need reprogramming.

To make the problem worse, it seems my bosses really trusted the person who was doing this before me and so every time I run into a problem or a test that doesn't work properly, sometimes in front of them, it's sort of taken as me being incompetent rather than the test being substandard in the first place. So, I can't really ask my colleagues for advice because it kind of works against me. So that's why I joined here and I'm asking you. Thank you for reading this far by the way. I'll get down to the real question now...

I test everything from museum exhibits and set for TV shows to your average kettles, toasters, microwaves and so on. But occasionally something comes up with a lot of exposed painted metal which is earthed using an earth nut. In this case it's a booth for a TV game show. How would one decide what a suitable resistance might be for such items during an earth bond test? Also what are the implications for Insulation resistance? You'd think it would fail because of the large amount of metal with no real insulation on it connected to the circuit. This has me beat! I have no idea what to do to test it properly.

If anybody out there has any advice I would really appreciate it.

Thanks

Trig

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0 mapj1 over 5 years ago

There are I think two problems here - the first is that the ' all singing all dancing' PAT machine sounds like it has been set up as a bit of a lottery, so devices that probably need identical tests and limits , are set up quite differently.

The only tests that need doing to mitigate immediate danger are insulation at 500V L+N to E, and here we expect megohhms, unless it is a mineral insulated heater, where it may be lower, and on class 1 equipment, the continuity of the earth to the case, which needs to be low enough to blow the plug fuse, ideally in less than half a heartbeat.

The touch current is a second tier test, but is most useful on electronic kit where there are capacitive filters between L and E, and these may pass the insulation test at DC, but still if the CPC were to fail would make the case dangerous to touch.

The second query is about what happens with coated cases, and there is a judgement call, as the idea is that the parts that may shock someone need to be earthed.

Thin paint may be scratched and really should be earthed, but if the coating is thick, or the mounting naturally insulating, then arguably it is not likely to either become live, if a wire brushes the inside, nor to shock someone touching it even if it is. There are plenty of devices that are really a mixed class I class II and may have parts such as metal lamp shades that do not need earthing, because in the eyes of the designers no credible fault may make it become live.
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0 mapj1 over 5 years ago

There are I think two problems here - the first is that the ' all singing all dancing' PAT machine sounds like it has been set up as a bit of a lottery, so devices that probably need identical tests and limits , are set up quite differently.

The only tests that need doing to mitigate immediate danger are insulation at 500V L+N to E, and here we expect megohhms, unless it is a mineral insulated heater, where it may be lower, and on class 1 equipment, the continuity of the earth to the case, which needs to be low enough to blow the plug fuse, ideally in less than half a heartbeat.

The touch current is a second tier test, but is most useful on electronic kit where there are capacitive filters between L and E, and these may pass the insulation test at DC, but still if the CPC were to fail would make the case dangerous to touch.

The second query is about what happens with coated cases, and there is a judgement call, as the idea is that the parts that may shock someone need to be earthed.

Thin paint may be scratched and really should be earthed, but if the coating is thick, or the mounting naturally insulating, then arguably it is not likely to either become live, if a wire brushes the inside, nor to shock someone touching it even if it is. There are plenty of devices that are really a mixed class I class II and may have parts such as metal lamp shades that do not need earthing, because in the eyes of the designers no credible fault may make it become live.
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Tricky PAT question

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