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Class 1/2 confusion on psu

PostPosted: Wed Aug 15, 2012 3:03 pm
by Matt Macdermott
Hello I'm a bit confused about testing some Unilab psu's at the school where I work. When I took the plug apart there was an earth wire i.e. its class 1; however I could not get any earth bond readings. I took the case apart to discover that the earth wire had been snipped off inside the case.; I then discovered this had been done by the previous tester. When I asked him about this he said to just treat the units as class 2 which I think is wrong as when you test for continuity between the transformer body and a screw head on the outside of the case there is continuity. Could you advise me please??

Re: Class 1/2 confusion on psu

PostPosted: Wed Aug 15, 2012 4:18 pm
by Alan Freeman
I would suggest you are right, and it should have earth continuity.

I would re-terminate the cable inside the unit including the earth to a metal tag round a screw/bolt.

You can not over earth the casing of an item. Even if it is class 2 by design, having the earth is belt and braces. Safer than before, but class 1 with no earth is a fail!

Re: Class 1/2 confusion on psu

PostPosted: Thu Aug 16, 2012 9:18 am
by Grizzly
By the look of it, Unilab make bench-top power supplies, so we're probably talking about something a bit more complicated than a wallwart.
Do these PSUs carry the Class II construction mark (square-in-a-square)?
If not, then they are Class I, and should have an earth connection, so you'd be wise to reattach the earth as Alan suggests. Is it obvious where the earth was snipped off from?

If it is the case that these appliances are Class I, you should report the previous tester to your school authorities. In fact, this issue should be reported as a 'near miss' on your accident/incident reporting system.
The previous tester doesn't appear to be competent to carry out in-service I&T, and has placed the users of this equipment in substantial danger.

Re: Class 1/2 confusion on psu

PostPosted: Tue Aug 21, 2012 4:27 pm
by Matt Macdermott
Thank you both for your replies; this confirms my suspicions. When I investigated inside the units I found that the earth wire had been snipped off (you could see the end of it in the cord) and there was a spade type terminal in the unit attached to the transformer where the earth should have been, which would earth the screw heads on the outside. Any way thanks again I may ask for further advice soon

Re: Class 1/2 confusion on psu

PostPosted: Tue Aug 21, 2012 5:18 pm
by Alan Freeman
Matt

That is the whole point of the forum. To learn by asking questions you are not sure about. We have all been there!!