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Friday, 21 June 2013

Earth Continuity testing - 20 Amps or 200 mAmps

Following a recent Twitter campaign, we had a training organisation challenging the use of 200 mA for testing Earth Continuity. As it is impossible to respond comprehensively using Twitter, here is my view on the use of 200 mA for testing not only IT equipment but all equipment.


1.     The IET Code of Practice (Edition 4) outlines a Soft and Hard test for Earth Continuity on Page 92. It does NOT say that one is better than the other.



2.      The COP also says in para 4 of page 92: "It is very important that these non-safety earthed metal parts are not subjected to the "hard" test, otherwise damage may result". Now, even the most competent PAT testing professional cannot be expected to know which of the metal parts on an appliance is a safety earth and which one is a functional earth.

By the time a tester finds out that he has inadvertently connected to a functional earth, it is too late as the equipment might already be damaged.

3.      More than 90% of problems with appliances can be picked up by a Formal Visual Inspection (FVI). The testing is important, but arguments about which PAT tester is better for the job is quite counter productive and moves the discussion away form what is really important - a good understanding of how to carry out FVI properly.

On many occasions, I have seen PAT Testing carried out with testers that cost more than £1000 where the plug was not opened up and the fuse checked.

4.     With modern equipment, the definition of "IT equipment" is a bit vague. In our experience, a "hard test" for Earth Continuity carried out on PDQ machines, toasters and Hi-Fi equipment has resulted in permanent damage. (The toaster was an intelligent one with micro-controller built in).



Summary

For all the above reasons, I recommend the use of the 200 mA soft test for all Earth Continuity tests. 



2 comments:

  1. In response to your comments...

    (1) I agree. The CoP doesn’t say that one test is better than the other, but why publish two versions of a test if they are not both valid? The validity and use of the tests come down to the individual appliance under test and the knowledge/experience of the test operative.

    (2) + (4) Using a 200mA 'soft test' is perfect for IT equipment and is completely in accordance with the IET Code of Practice. We have no issue with that whatsoever.

    With regards to defining what is 'IT equipment', it is an area which causes much discussion. The reason for using a lower test current is to avoid damaging delicate electronic circuitry within the appliance, and as many appliances now contain electronics, no longer can the term ‘IT equipment’ be confined to business equipment such as computers, monitors and printers etc.

    So how does the test operative decide whether to use a hard test or a soft test? It is good practice to use a soft test where damage may inadvertently be caused to the appliance under test. As we cannot establish whether exposed metal parts are earthed for function or safety, I would agree that a soft test could be used to establish Earth Continuity on Class I appliances.

    But what about where we know that electronics are not being used? There is no reason why an IEC Lead, a basic extension lead or a 110V lead cannot be tested using a hard test, so why not give the engineer the choice?

    (3) I agree, although the User Check is equally as important, as an appliance which fails a Formal Visual Inspection has normally failed due to physical damage. When did that damage occur? Was it on the same day that the FVI was carried out, or many months earlier? This is impossible to determine, but regular User Checks by the equipment user should identify appliance damage and deterioration as it occurs. Relying on the FVI to spot the problems can lead to a significant time delay, thus putting the user at risk.

    A good Formal Visual Inspection is invaluable, and I agree it will find the majority of appliance faults. We have yet to come across PAT Test equipment which can determine the rating of the plug fuse, or the presence of a British Standard number on the plugtop, or whether the terminal screws are tight, or the presence of damage on the plugtop, flex or appliance – need I go on? A thorough FVI involves in excess of 30 basic checks on the appliance from the plug to the appliance – none of which can be checked by the test equipment, irrespective of how expensive it was. Indeed, some of the more expensive test equipment is often the worst when we look at functionality, so we never recommend this type of test equipment to our clients.


    In summary, the successful application of the PAT Testing process as defined by the Code of Practice, should be firmly in the hands of the people carrying out the User Check, the Formal Visual Inspection and the Electrical Tests. Any test equipment the Test Operative may use is simply a tool provided to help them to make an informed decision. Sometimes, more tools in the tool-bag will help them with the decision making process.

    With regards to my other question, “Is the BattPAT suitable for testing IT equipment in accordance with the IET Code of Practice?”, I don’t see your response.

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    Replies
    1. Hi Chris, so to summarise the first 5 paragraphs - you advise using the 20A test for "IEC Lead, a basic extension lead or a 110V lead". I can agree with that.

      Regarding your question in your last paragraph, I don't understand the premise of your question and why this is important.

      The IET Code of Practice is not a legal requirement. It is merely an opinion by a committee of how in-service testing can be carried out.

      I have many disagreements with this COP specially Edition 4 - please see my earlier posts for some of these. Also the COP has many internal contradictions. It is also NOT a practical "how to guide". Much of it's advise is open to different interpretations - for example our discussions above would not be necessary if the Code of Practice was more explicit in its recommendations.

      For these reasons, we have deliberately chosen to disregard some of the recommendations in the IET COP when we designed the BattPAT. These decisions were based on mine and others' experience over more than 30 years of electrical safety when designing products. We stand by our decisions.

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