Is this possible.....?

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Is this possible.....?

Postby remedy234 » Fri May 04, 2012 10:13 am

I recently tested a PC at one of our clients site, 4AMP CLASS1, After testing it powered up but wouldnt boot, it turned out to be a faulty memory card. The customer is asking for us to pay for the new memory card. Is it possible that we might have damaged it or is he trying it on?
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Re: Is this possible.....?

Postby Grizzly » Fri May 04, 2012 1:53 pm

"4AMP CLASS1"? So do you mean you tested the PC with an earth bond current of 4A?
If so, wouldn't the 'soft' test option (20mA - 200mA) been a wiser choice?

I'm presuming that by memory card, you are meaning a RAM module?

If you don't use the soft test, as recommended by manufacturers and the IET (IEE CoP, chapter 15.4, Note 1: page 74), then it certainly is possible you may have damaged something...
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Re: Is this possible.....?

Postby remedy234 » Fri May 04, 2012 4:59 pm

super NAPIT strikes again then as we were told to do the 4amp (supposed soft test) as opposed to 25 amp on IT equipment.

Strange tho as we must have done over 250,000 of these tests on IT equipment and not had anything go wrong once
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Re: Is this possible.....?

Postby Grizzly » Fri May 04, 2012 8:33 pm

Seems NAPIT's idea of a soft test is very different to the IET's, then :(

It's pretty impossible to say if that was the cause of the failure, but as it's not in accordance with the generally accepted Code of Practice, you could have a hard time defending it.
Also, the manual for the Europa Plus (I'm assuming from past posts that this is the tester you're using?) shows that the 'DC Earth Bond Test' and the 'Earth Screen Test' both have low test currents (200mA & 100mA respectively), and does also state:
Earth Screen Test: This test is to check the earth screen connection using a current which will prevent damage that may be caused by testing using high currents. This is often required by sensitive electronics such as computers and other Information Technology (IT) equipment.

(page 88).
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Re: Is this possible.....?

Postby Alan Freeman » Fri May 04, 2012 8:49 pm

Remedy

It is the usual problem. You found the one weak RAM card, and the test fried it (assuming it was the test). You have just been very lucky so far.

It is like static discaharge can fry CMOS chips. I have handled hundreds with out anti static precautions. I then fried one one day - and I was using a wrist strap - but it was faulty!!.
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Re: Is this possible.....?

Postby remedy234 » Sat May 05, 2012 8:13 pm

Thanks for your replies, i will adjust the tests accordingly. To be fair the NAPIT course was beyond laughable, the "examiners" told you the answers as you were taking the test

AVOID NAPIT
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Re: Is this possible.....?

Postby remedy234 » Sat May 05, 2012 8:18 pm

Alan Freeman wrote:Remedy

It is the usual problem. You found the one weak RAM card, and the test fried it (assuming it was the test). You have just been very lucky so far.

It is like static discaharge can fry CMOS chips. I have handled hundreds with out anti static precautions. I then fried one one day - and I was using a wrist strap - but it was faulty!!.


I previously worked for another company tho. At the time there were 8 of us all doing 4amp class 1 tests on IT equipment and never had a problem. Thats around a million tests with no problems
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Re: Is this possible.....?

Postby remedy234 » Sun May 06, 2012 9:36 am

Hi ive just looked at changing the settings but unsure which one to use as there are a few 200mA ones

Theres, -200mA. +200mA and +_200mA
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Re: Is this possible.....?

Postby Jim Wallace » Sun May 06, 2012 1:41 pm

Hi Remedy

I'll cover the various 200mA tests first before I comment on the memory card failure....

When testing earth continuity using low test currents, it's possible for corroded joints in the earth to present a diode effect in that the resistance reading will vary with the direction of the test current i.e. you might get a different reading with +200mA that you would with -200mA. It's for this reason that it's common practise when testing the earth on a fixed installation to reverse the test leads (and therefore reverse the test current) and measure again. The same recomendation is not given for appliance testing (in the UK).

With regards to the failed memory card, the primary reason for using a low test current when testing IT equipment is that parts may be connected to protective earth for one of two reasons:

1. To prevent accessible parts becoming hazardous live under a fault condition i.e. single fault protection
2. For screening or EMC purposes.

Connections to earth which are there to provide fault protection must be able to carry high currents otherwise they will not provide protection. Lets say a live wire comes into contact with an earth metal part - in this case the earth path will have to carry a large current (possible hundreds of amps) until the fuse clears the fault. A high test currrent should not damage the protective earth path.

Connections to earth for screening purposes may only be capable of carry a low current e.g. the screening on a screened cable. If a high test current is applied to such parts it is possible that the appliance may be damaged by the earth continuity test.

It is difficult, if not impossible, to deterime by inspection why a part is connected to earth so we take the precaution of using a low test current when testing IT equipment.

Now the question of whether semi-conductors can be damaged by passing a high test current around the protective earth path or earth screen path. Semi-conductors can be damaged by excessive voltages as this punches a hole through the oxide layer inside the semi-conductor. This is the effect that Alan and Grizzly describe above. As an aside, one of the interesting things about static damage to semi-conductors is that it does not always show up immediately. It is possible to "wound" a device through static discharge where it continues to function but fails prematurely so always best to take precautions.

For an earth continuity test to cause a failure the test would have to somehow expose the memory device to a high voltage - you may do this is you cause an arc when performing the test i.e. a spark between the probe tip and the appliance.

It's one of those cases that it difficult to prove one way or another. Might be a case of taking it on the chin in the interests of customer service - it's probably cheaper in the long run to pay for a memory card.

Right - back to the MotoGP.....
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