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Topic Title: Earth leakage rules, PAT, COP 4th Edition
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Created On: 10 July 2018 04:01 PM
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 10 July 2018 04:01 PM
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tenbob

Posts: 5
Joined: 25 July 2008

Can someone please help me to understand the rules for the protective conductor / touch current test. I'm looking at the notes to Table 15.3 in Section 15.6 of the 4th Edition Code of Practice. The table gives the maximum allowed values of the protective conductor current or touch current. But the notes underneath say this ...

"1. The values for the maximum protective conductor or touch current given above are to be doubled if:
i the appliance has no control device other than a thermal cutout, a thermostat without an 'off' position or an energy regulator without an 'off' position, or
ii all control devices have an 'off' position with a contact opening of at least 3 mm and disconnection in each pole."

There are two problems. The first is that I can barely understand what it means. The second is that I can't see why this should apply.

Clause i seems to refer to a device that has no on-off switch. Clause ii refers to a device that has a double-pole on-off switch. That leaves the third, undefined category which is, presumably, a device with a single-pole on-off switch.

So the rules seem to say "if the device has a single-pole switch you should use the values from the table. But if the device has a double-pole switch or it has no switch at all you should use double the values from the table.

That makes no sense to me. Can anyone please explain if my understanding is correct and the reason for this rule.
 10 July 2018 05:35 PM
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mapj1

Posts: 11436
Joined: 22 July 2004

It is a concession to the real behaviour of things with kettle-like or immersion-like mineral insulated metal elements.
The leakage from such devices after a period of disuse, is often higher than we would like, and some types of devices that use them are being given an exception. (and it would be better if it said it clearly)
Against this, you don't normally have hand held large areas of contact for anything that gets boiling hot, and so long as the element is properly isolated when off, either by DP switch, or because it has no switch and you must unplug it, the risk is felt to be acceptable. (when its off, its safe to touch, when its on, a shock from touching it is not the main worry)

-------------------------
regards Mike
 11 July 2018 07:44 AM
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aligarjon

Posts: 3997
Joined: 09 September 2005

You want to try reading the regs book. They seem to like writing around things instead of just stating what they mean.

Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 11 July 2018 03:25 PM
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tenbob

Posts: 5
Joined: 25 July 2008

Thanks, mapj1. So, in summary, with SP switching you should use the values from the table. With DP or no switching it's permissible to have double the leakage current.

I understand that real world devices may not easily meet the spec. But the rationale still seems a bit awry. When you first switch on a metal kettle it's quite likely your hand will touch the outside.

According to the COP, the max leakage current for, say, a 3kW kettle would be 2 x 3 x 0.75 mA. That's 4.5 mA. Provided the earth wire remains intact, it all goes to earth and the user won't know anything about it. If the earth wire breaks, the first thing the user will notice is up to 4.5 mA going through them to their other hand which may be resting on the cooker. It sounds quite a lot to me. Have I understood this correctly?
 11 July 2018 04:46 PM
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mapj1

Posts: 11436
Joined: 22 July 2004

I think you have it. Note that this advice is quite arbitrary and the limits are not unique, other organisations have said 3.5mA max overall, no exceptions, some other countries specify 5mA overall, and in the UK, by fitting a blue round pin plug or hard wiring, magically the limit rises to 10mA !

Yes the rationale is a bit off, you may think either 5mA is dangerous, or it is not, but it has to be balanced against the danger in the likely place of use and also the problems of false fails and nuisance tripping of RCDs as well.
You can just hope that the fails are by a large margin so there is no discussion

-------------------------
regards Mike


Edited: 11 July 2018 at 05:41 PM by mapj1
 11 July 2018 05:30 PM
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tenbob

Posts: 5
Joined: 25 July 2008

Hi, mapj1. Thanks for pointing out the arbitrariness of the whole scheme.

My Seaward Primetest 100 indicates a fail at 0.75 mA for Class I and 0.25 mA for Class II. So it's working to the tighter limits by default. Of course you can always re-interpret and override the results if you wish.

Thanks for your help.
 12 July 2018 09:57 AM
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lyledunn

Posts: 1209
Joined: 13 August 2003

Perhaps we ought not to seek engineering sense in such documents. Likely some of the content is the outworking of committee compromise, which, in turn, is sometimes moulded by commercial considerations.
The values in the table are touch current values which are based, I think, on a body model resistance of 1Kohms, thus actual earth leakage current would, inevitably, be higher.

-------------------------
Regards,

Lyle Dunn
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