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Old 29th Apr 2021, 9:03 am   #21
John10b
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Default Re: PAT Testing.

Thank you pmmunro for the updated information. I think it will take a long time before the term “PAT” testing is replaced with ....What?
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Old 29th Apr 2021, 9:12 am   #22
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Default Re: PAT Testing.

Quote:
Rated for the cable surely.
No, although I know that the fuse is technically 'to protect the cable', where the appliance clearly does not need a 13A fuse I downrate the fuse based on the wattage of the item. I don't have time to look inside every item to see if it is correctly internally fuse protected.

Historically, we had a lot of items with fitted plugs which had come with 13A fuses fitted and had been used used as-was without any consideration for the maximum current likely to be drawn by the appliance.
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Old 29th Apr 2021, 10:16 am   #23
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Default Re: PAT Testing.

Quote:
Originally Posted by SiriusHardware View Post
Quote:
Rated for the cable surely.
No, although I know that the fuse is technically 'to protect the cable', where the appliance clearly does not need a 13A fuse I downrate the fuse based on the wattage of the item. I don't have time to look inside every item to see if it is correctly internally fuse protected.
Yes that's me too, within 10 feet of me is a GEC 5842 table woodie, no internal mains fuse. The (fixed) mains cable is rated for 3A but the plug has a 1A fuse in it, to protect the radio if the mains transformer ever develops shorted turns.

It's been PAT tested several times by our sub-contract tester. The underneath cover is actually missing, it's therefore possible to touch the chassis, and although isolated it isn't a Class II appliance. So in my book it ought to fail visual inspection, but I'm happy to see him put on a sticker and move on...
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Old 29th Apr 2021, 10:52 am   #24
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Default Re: PAT Testing.

It may not matter in many cases but, as has been mentioned before, the rating of BS1362 fuses corresponds to the current at which they're dissipating 1W in their ohmic resistance. Lower-current fuses have higher resistance. So if we fit a lower-current fuse it will run hotter. BS1363 plugs are supposed to be able to withstand 1W being dissipated in the fuse, but some cheap ones seem to struggle with this. So dropping the fuse rating will involve a trade-off between better overcurrent protection in the event of a fault versus certain long-term increased heat stress on the plug.

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Old 29th Apr 2021, 11:36 am   #25
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Default Re: PAT Testing.

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Originally Posted by GrimJosef View Post
as has been mentioned before, the rating of BS1362 fuses corresponds to the current at which they're dissipating 1W in their ohmic resistance
That's a new one on me, TBH! And it seems surprising.

I have just checked four fuses, all marked BS1362. Results as follows:

1A fuse:
Test current 1.002A. Resultant voltage across it 0.182V. So power = 0.182W

3A fuse:
Test current 3.02A. Resultant voltage across it 0.118V. So power = 0.356W

5A fuse:
Test current 5.06A. Resultant voltage across it 0.094V. So power = 0.475W

13A fuse:
Test current 13.03A. Resultant voltage across it 0.100V. So power = 1.3W

So a lot of variation, but the 13A fuse actually dissipates MORE than 1W at its rated current and the rest less than half a watt.

Certainly I believe that checking plug fuses are appropriate, should be part of the PAT test.
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Old 29th Apr 2021, 2:55 pm   #26
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Default Re: PAT Testing.

I've had the 1W dissipation fact in my head for so long now that I can't remember where it came from. I'm sure either BS1362 or BS1363 would be the definitive reference. A quick Google turns up quite a few fuse datasheets. The Bussmann one https://docs.rs-online.com/9819/0900766b8002b71e.pdf, for example, notes that their BS1362 fuses dissipate a maximum of 1W. There will be some physics behind how the dissipation scales with current rating. Your figures suggest it's a bit slower than linear, but not hugely.

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Old 29th Apr 2021, 3:38 pm   #27
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Default Re: PAT Testing.

Quote:
But what if the earth wire parts first? How do you know that one of your two lives has been used up? No fuse blows, nothing trips out. This is where the testing comes in.
Yes, the lives silently being used up is a concern.

The one that I think should be in an appliance test is to measure the value of any Y-class capacitors between live and earth.
Why?
Because the Y-class protection comes from being constructed like 2 capacitors in series. It one has failed the only thing you see is an increase in capacitance. So I measure it to check. (I'll bet it is just me doing this).
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Old 29th Apr 2021, 4:22 pm   #28
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Default Re: PAT Testing.

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But what if the earth wire parts first?
I've mentioned this before elsewhere. This is precisely why the BBC and British Steel (when I worked for them) insisted on making the earth wire longer than the live conductors so that in the unlikely event of a cable being yanked from a plug, the earth has a sporting chance of remaining intact until the supply had been removed by the disconnection of either or both of the live conductors.

I can see no possible disadvantage in leaving the earth wire longer, unless there are some sort of 13A plugs that have no room to accommodate it. But I've never seen any.
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Old 29th Apr 2021, 4:23 pm   #29
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Default Re: PAT Testing.

In-circuit capacitor measurments are difficult. In the case of the normal delta-capacitor mains filter circuit, if you measure from, say, L to E, you get the capacitor you expect in parallel with the series combination of the class X and the other class Y. Other circuitry connected to that circuit will confuse things even more.

So you either have to desolder the capacitors to check (which is impractical for a PAT-like test) or record the measurement each time and hope that any failure of 'half of a class Y' makes enough difference to that reading which it may or may not do. You certainly can't have a general pass/fail threshold as you can for insulation resistance and earth continuity.

[Just noticed I've reached a round number of posts here. 4096 = 2^12 of course]
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Old 29th Apr 2021, 5:18 pm   #30
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Default Re: PAT Testing.

Should really have been a post in Vintage Computing.

To be honest I am glad to have been relieved of the yearly humdrum chore of crawling around under benches trying to find out where extension leads are plugged in and so on - no-one is ever pleased to see the PAT tester, not if they are doing their job properly, and it is a thankless, repetitive job. The only thing which made it worthwhile was finding one or two things a year which might possibly have killed someone. Even then, we'd get a tirade of dismay while the offending item was sidelined for repair which, where possible, it was, unlike the outside contractors whose responsibility ends at putting a 'Failed' sticker on any item they don't like the look of.
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Old 29th Apr 2021, 5:23 pm   #31
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Default Re: PAT Testing.

Most annoying for me was the destruction of an expensive aerial distribution amplifier by
a contractor, who did not know a coax plug body was NOT a bonding terminal.
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Old 29th Apr 2021, 5:27 pm   #32
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Default Re: PAT Testing.

I once had a BBC micro to repair where some 'genius' had done a PAT test using the outer of the video output BNC socket as the earthing point. He did the earth test at 25A (!) which burned out PCB traces and blew ICs all over the mainboard. Yes I did fix it, but the bill for parts was considerable and my time even more.
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Old 29th Apr 2021, 5:34 pm   #33
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Default Re: PAT Testing.

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Originally Posted by Restoration73 View Post
Most annoying for me was the destruction of an expensive aerial distribution amplifier by
a contractor, who did not know a coax plug body was NOT a bonding terminal.
Did that coax body have a big label on it saying it was not a main earth, in all known languages?

No?

Then next year, at PAT time they'll be round to blow up the new dist amp, and the year after that. They have to maintain sustained performance for ISO 9000 accredition.

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Old 29th Apr 2021, 5:44 pm   #34
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Default Re: PAT Testing.

Quote:
So you either have to desolder the capacitors to check (which is impractical for a PAT-like test) or record the measurement each time and hope that any failure of 'half of a class Y' makes enough difference to that reading which it may or may not do.
Or have the equipment that can meaure a capacitor with other parts connected, e.g. a transformer bridge.
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Old 29th Apr 2021, 5:46 pm   #35
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Default Re: PAT Testing.

The relevance of fuse rating does depend on the age of the appliance. Historically, some reliance was placed on having appropriate protection upstream. More recently, anything CE marked for sale in the EU/UK should be safe with any fuse rating up to 16A, as this is typical of operation in all other countries with protection in the distribution board rather than the plug.

It is important to distinguish between overcurrent protection and short-circuit protection in this context. A 13A fuse will protect a 0.75 sq.mm flex (normal rating 6A) against short-circuit but not against overload. However, if the appliance protects itself against overload and the flex is correctly rated, the flex is protected downstream against overload and upstream against short-circuit, and all is good. Hence, a 13A fuse is now almost universally suitable. Clearly, a fuse with a smaller let-through will cause a smaller splash so there is a theoretical advantage in downsizing from 13A. But it is not mandatory to do so and a 13A fuse in a captive cable of lower rating on a low-power appliance is not an ISITEE fail.
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Old 29th Apr 2021, 5:46 pm   #36
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Interesting point David, is there an ideogram label for that case?
 
Old 29th Apr 2021, 6:56 pm   #37
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Default Re: PAT Testing.

For the future, I think we have to design things to survive the most inexpert forms of PAT testing. Everything that connects to earth having a seriously hunky connection!

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Old 29th Apr 2021, 7:12 pm   #38
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Default Re: PAT Testing.

some of the things i had come across pat testing brand new fridges and freezers in school canteens fitted with euro plugs 2pin and used with adaptor to convert to our 3 pin and measured no earth i chopped euro plug off and put our 3pin plugs on the adaptor worked and measured earth alright on older fridge with euro plug there but i chopped plug off that and put our 3pin plug on
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Old 29th Apr 2021, 8:08 pm   #39
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Default Re: PAT Testing.

Many years ago when I worked as a research officer at a Scottish university It fell to me to do the electrical testing of all equipment in our department at the annual shut down. This was well before proper PAT testing but all equipment was visually inspected (including fixed apparatus). I introduced the idea of engraving the fuse rating on the rear of the plug cap to assist non technical people. This was originally scoffed by the building service officer until they had a fire with a 13A fuse on an appliance with a a 100w loading and slim cable. He then started his own inspection and engraving and became an enthusiastic convert!

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Old 29th Apr 2021, 8:35 pm   #40
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Default Re: PAT Testing.

Quote:
Originally Posted by russell_w_b View Post

I've mentioned this before elsewhere. This is precisely why the BBC and British Steel (when I worked for them) insisted on making the earth wire longer than the live conductors so that in the unlikely event of a cable being yanked from a plug, the earth has a sporting chance of remaining intact until the supply had been removed by the disconnection of either or both of the live conductors.
I also came across this, though less formally. And it makes sense. It was while I was a trainee in an electronics lab and was more of a 'why doesn't everyone do this?' conversation but the logic of it has stuck with me.

On the other hand, my brother wired plugs 'interestingly' on two occasions I caught. L/N swapped and N/E swapped. I'm glad he never did the third possible given we lived at the time in a house that had only Wylex wire fusing and no other protection! PAT ing would have caught that in a relevant workplace at least. These day's he is a technical trainer for a massive telecoms company.

On the other hand, the last place I worked, there was a policy that you couldn't possibly use anything, even bought brand new from a reputable source (like RS) until it had been tested. TBF that was used to most effect by people who basically didn't agree with what you had sourced (because it wasn't their idea) and used PAT to stop it getting being put into service as they were the only ones who could arrange the testing. What was most stupid about this was that the item in question solved a problem that they spent most days whingeing about!

Finally, I was really irritated once, when a desk lamp (like a cheap anglepoise clone) I had taken to work to use on the bench was failed by the contract PAT er who came in. Until I saw they were exactly right because the flex part way up had started to have insulation failure. I could have replaced the two core flex but TBF it would have been too hard so it was scrapped.
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