Tricky PAT question

" 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. "


I think the recommended minimum insulation value is 1M ohm (0.25mA max) for class 1 equipment. However, there maybe other considerations which might come into play for a person being enclosed in a metal box while under fault conditions.

So what sort of electrical circuit is being housed in the metal box with little or no insulation between the enclosure, electric circuit and human being?


Legh

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.

I also find myself on the end of a PAT machine these days Trig,  having obtained the C&G years back and ignored it as too dull.   I suddenly became the only qualified person on the block during an emergency to update some PAT so off I went to a remote beach in Wales for a week to test a load of our off site bits -computer racks and scopes, radiators, fridges, power tools and all sorts.  I took with me the new Fluke 6500-2 and an old Seaward SuperNova. Fortunately I also took the new Megger 1741.


What are they thinking?  25Amps on a class one appliance?  That's live working isn't it?  I was horrified.  On one or two bits the earth clamp had to be held to stop it falling off and I swear I can feel it vibrating as it tests.  If you use the auto setting the appliance even tests itself and, for example, the pump you are testing comes on.  Woah!


I ended up by doing a fair amount of the tests using the old fashioned method of testing the earth bonds and a gentle IR test wherever I though it was appropriate.  On some I used the Megger MFT just to confirm earthing etc.   The Fluke is brilliant for IEC leads but the tests are not intuitive so it will take some getting used to.  The Seaward is, well, a bully.


I came back and sat down with a colleague to say that I think PAT is absolutely brutal, should be classed as live working and that our regime needs modifying.


None of that helps if you are working for an organisation that want to see the downloaded results and that said brutal menu of test choices have been used.  Fortunately they took my views on and are happy with my 'limitations'.   I blew up one oil filled rad by the way.  Class one but the thermostat will never work again I reckon.


Zs

Some thoughts in defence of the 'brutal' Seaward Machine.

If the cpc and any internal bond to the case  cannot handle 25 amps for a few seconds, it should not be relied upon blow a 13A fuse, rather the CPC may blow clear, and the 13A fuse remain intact, and the case energised. The tester's intention is to emulate this, and although it is a 25 A test, it is derived from an ELV transformer, so the touch potential is limited.


Yes, you may well feel it buzzing in your hand if the contact is a croc clip, but unless the test lead is coiled it is unlikely to be magneto striction, but thermal oscillation at the points of contact being heated and cooled - similar to the operation ball-bearing "motor".  a digression, into how heat can move things.


I'd suggest that it is only truly live working, if the case is open and the parts that come live to mains voltage when it is powered  up are exposed. The preceeding visual inspection should reveal this.

Insulation testing even at the 500V, (and in some models also a flash test at 3500V) is carefully current limited, designed to simply deliver an agonsing but non-lethal shock to anyone fool enough to insulation test themselves as well as the device under investigation.

I would agree there is a case for not performing an on-load test for certain pieces of equipment that can be damaged if run up without other things in place - rotating machines with cutters and certain types of pump that do not like to be dry run come to mind.

To be any use the PAT machines have to emulate realistic fault conditions- which are by definition single fault to danger.

Hi Legh, Mapj1 and Zs.


Thanks so much for your replies.


Legh, the actual circuit attached to this gameshow set is just a light bulb, it has about 4m of maybe 1mm cable and it tested as 0.9 ohms on the earth. Which is the number I went with in the end. I guess it's slightly reassuring that there's a fuse at one end and a filament at the other!  


Mapj1 You're completely right, it is a lottery. Although I decided to start deleting and re-writing the programmes today and noticed the Seaward remembers regardless of whether tests have been edited or deleted so this could take a long time to sort out. I think whoever designed this gameshow booth was banking on the no credible fault could make it live thing, so you might be right there.  


Zs.. lol tell me about it. I was testing 125A cables today... I know the "portable" in PAT went out with the bath water recently but surely there is a point where what is needed is initial verification and regular inspection and testing. All things considered though it's quite a fun job. Always challenging, if somewhat scary/frustrating/boring sometimes.


 Appreciate the help!


Trig

I use an old Robin PAT 5500. its programable, heavy and very yellow. It produces various currents up to 26A at 3-4V when checking the earth bond. For most appliances that have resistive elements 26A is appropriate and for mains powered IT equipment I use the earth bond test current at 100mA . The earth bond maximum resistance value can be adjusted so that long extention leads can be checked without automatically failing.

" Insulation testing even at the 500V, (and in some models also a flash test at 3500V) is carefully current limited, designed to simply deliver an agonsing but non-lethal shock to anyone fool enough to insulation test themselves as well as the device under investigation." 


Lol .... do people really test themselves before testing the equipment?

Personally, I find the 5500 a very reliable, robust and accurate machine which can be tailored to cope with most types of electrical appliances


Legh

I was Pat testing over the weekend, usually I really hate this but thankfully for me for once I was in a nice clean environment, once I'd linked my phone to my Bluetooth speaker put on some old school streetsounds electro I was away and found the experience surprisingly enjoyable ?

Btw I use the Apollo 500. 

Regards TS