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I haven't been able find a rewirable unearthed (as in 3-pin, one of which plastic) UK mains plug. (I didn't think before hand, but I suppose they're not allowed by BS1363 since you could then too easily fit one where earth should be connected.)

Is it 'allowed' to use an earthed one for a double insulated appliance? Should the earth pin be left NC, or tied to neutral?

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  • In North America(might be different in UK) most double insulated usually only needs two blade/pin plugs(no ground pin). Would think you could get a repair/replacement double pin plug.
    – crip659
    Commented Aug 3, 2021 at 18:21
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    Doesn't sound like double-pin plugs are a thing in the UK.
    – isherwood
    Commented Aug 3, 2021 at 18:25
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    They're not no, because the earth pin is slightly longer, and must enter the socket first in order to physically open the others. (A safety feature.) Double insulated appliances have a plastic 'earth' pin to perform this function without making an electrical connection. Apparently called an 'Insulated Shutter Opening Device' (ISOD): en.wikipedia.org/wiki/…
    – OJFord
    Commented Aug 3, 2021 at 18:41
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    In that case, if it is okay with UK electrical code, then a regular three pin replacement plug should be okay. Just leave ground pin not connected, do not connect to neutral.
    – crip659
    Commented Aug 3, 2021 at 19:08
  • Yeah, BS1363 plugs require a physical grounding pin to be present to function Commented Aug 4, 2021 at 1:35

2 Answers 2

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I haven't been able find a rewirable unearthed (as in 3-pin, one of which plastic) UK mains plug. (I didn't think before hand, but I suppose they're not allowed by BS1363 since you could then too easily fit one where earth should be connected.)

That is correct, "insulated shutter opening devices" as the standard calls them are only allowed for non-rewirable plugs. Presumably for the reason you suggest.

Is it 'allowed' to use an earthed one for a double insulated appliance?

Of course.

Should the earth pin be left NC,

Yes.

or tied to neutral?

NO!

Neutral and earth should never be interconnected in ordinary portable equipment, or ordinary UK domestic electrical installations for that matter. Neutral and Earth are normally connected somewhere, but in the UK it happens on the electricity supplier's side of the system (this differs from the USA where it happens in the customer's main panel) not within the customer's installation.

The only exception you might possibly run into as a DIYer is when you have a supply that is separately derived from a transformer, generator or inverter. In such cases it may be appropriate to tie neutral and earth of said supply together in exactly one place.

There are other exceptions in certain specialist non-domestic situations, but they are outside the scope of what a DIYer could reasonably be expected to encounter.

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  • Nice summation of all the "comments that shoulda been answers".
    – FreeMan
    Commented Nov 21, 2022 at 18:03
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[earth...] or tied to neutral?

No! NEVER tie earth to neutral in branch circuit wiring or appliances! Never, ever! (obviously they're connected somewhere, but usually outside the bailiwick of a UK consumer, and even in North America it's one specific place).

Tying neutral to earth defeats the system bonding that makes the earth wire safe in the first place. And the problem is, if you jumper them in the plug, that affects the whole system while it's plugged in - and the receptacle switch does not solve this!

So that leaves the other option. Use a grounded plug and leave the ground pin disconnected, if your codes support doing that.

By the way, for an object lesson in the perils of earthing an appliance, consider the American experience: from 1966 to 1996 dryers and ranges were exempted from the earthing requirements and told to "earth" their frames to neutral (think: TN-C-S to the appliance). Just as John Ward says, this energized the chassis when the PEN wire failed.

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  • As someone who uses UK mains plugs natively - leaving the ground plug disconnected is the correct answer. Commented Aug 4, 2021 at 8:53
  • "Never, ever" - it is only not in isolated systems, or those with a separate local 'unconnected' earth right? I assume you mean "never ever... in a plug/so close (low impedance) to appliance"?
    – OJFord
    Commented Aug 4, 2021 at 10:50
  • @OJFord. Common fallacy, assuming everything else is working perfectly. That's not how you design electrical systems. You assume wires are going to break. Or hardwire connections just go bad for reasons we haven't figured out yet. The main thing is, wires are not diodes, jumping the plug doesn't only affect the appliance, it affects the whole system. What happens if you lose neutral and earth on that circuit? What happens when your entire house loses the neutral wire from the utility, and neutral seeks through earth? Could be a lot of amps. Are there fuses on neutral and earth? Commented Aug 4, 2021 at 14:42
  • "Common fallacy" - what is, low impedance between N&E? "What happens if you lose N&E on that circuit?" - nothing different than without such a plug? "What happens when your entire house loses N" - that circuit trips, others are just out? I appreciate your answer, just aiming to understand/ensure we're on same page. AIUI N&E are "almost always" (not "never ever") connected. That coupled with thinking where earth conns present they must be connected, (which I believe applies for example if wiring a back box, and the existing cabling is twin & earth, it must be metal) made me consider it here.
    – OJFord
    Commented Aug 4, 2021 at 17:49
  • There is no N-E bond in a UK consumer unit, in the case of a TT supply there is no metallic connection at all, in a TN-S or TN-C-S supply they are interconnected but it's on the suppliers side of the demarcation. Commented Nov 21, 2022 at 13:50

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