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it is then a faulty isolation transformer and the protection device(s) should operate
Even if primary and secondary are wound on the same bobbin, is it not possible to incorporate an earthed 'foil/whatever (as well as 'insulating tape') between them, such that it is effectively impossible for the primary-secondary isolation to fail without a protective device operating,even if the secondary is not earthed?
Yes it is provided the foil does not ( and cannot ) create a shorted turn.
That could more-or-less be achieved, couldn't it?
Yes Bernard it is the age old question, Can we guarantee floating will not be compromised? If yes then it is safer to float. If No then safer to Earth (Ground)
Indeed - in the case of an LV->ELV power supply. However, one presumably has to trust, or not trust, the isolation - in which case one would presumably never, or always, earth the ELV side?
It's different with LV-LV isolating transformers (e.g. 'shaver sockets'), since to earth the secondary would then ('dangerously') defeat the whole point of the isolation - so in that case one really has no choice but to 'trust the isolation'.
Kind Regards, John
Yes and the fact that each isolation transformer runs only one item of equipment is a great feature of this.
If you tried a similar set up but on a larger scale say the beefburger van and trailer running a few sockets and perhaps extension leads the the risks rapidly increase and therin lies the problem.
Our installations domestic and others contain more than one outlet, the risk of it becoming referenced to Earthlike increases greatly, so we deliberately earth/bond accordingly and mitigate risks from that stance.
If my home had two conductors with 230/240V across them and no voltage to earthy stuff and I could 99.99% guarantee it stays that way it could be a lot safer. But it is almost impossible, so Earthing and Bonding it is then.
If you tried a similar set up but on a larger scale say the beefburger van and trailer running a few sockets and perhaps extension leads the the risks rapidly increase and therin lies the problem.
Our installations domestic and others contain more than one outlet, the risk of it becoming referenced to Earthlike increases greatly, so we deliberately earth/bond accordingly and mitigate risks from that stance.
If my home had two conductors with 230/240V across them and no voltage to earthy stuff and I could 99.99% guarantee it stays that way it could be a lot safer. But it is almost impossible, so Earthing and Bonding it is then.
Indeed so. Given that earth 'is all around us', electrical installations would be safer if they were not earth-referenced at all - since one would then not get shock if one simultaneously touched something earthed and one of the 'live' conductors.
However, as you say, that ideal is very rarely attainable (except in small, circumscribed, environments), so we have no real choice but to invoke "if you can't beat them, join them" by deliberately earthing and bonding.
Kindest Regards, John
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Not easy, as far as I was aware, you must not earth SELV, with PELV you must earth, and with FELV you can decide what is best. Which asked of why
for an accessory which is SELV? There seems to be no exception for ELV with either earthing/bonding or RCD protection, common sense tells us under 50 volts we don't need to worry about it, but it does not say that.
So what ever we think should be done, or it means, the book does tell us.
50 volts AC is enough to cause an involuntary muscle reaction which could result in an accident.
I'm not sure that there is much clarity in relation to that definition of FELV.
... would seem to be vague in the extreme. Which of the "measures" are not applied? On the face of it, that would allow some pretty iffy situations to be described as "FELV", wouldn't it?
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As said, we need to use common sense, in the snow, I have had quite a belt, enough to make me jump, from a gritter wagon's 24 volt battery. I remember some years ago arguing over if RCD protection was required for reduced low voltage supplies (110 volts) BS 7671:2008 was in force at the time, in the end did find
but it took some finding. (411.3.3)
And in real terms most 110 transformers are designed to run from 230 volts, and most of our socket outlets are over 230 volts, so the RMS valve does exceed 110 volts in many cases.
Indeed - and, as I often say, not only that, but there are plenty of people who (often unknowingly) are walking about with hearts which are so 'electrically unstable' that they are at risk of electrocution with shocks of appreciably less than 50 V.
However, for example, being frighting by, say, a sudden noise or movement of a cat or dog whilst one is up a ladder could also 'result in an accident'
Risks need to be considered 'in perspective'!
The problem with keeping things floating that it`s difficult to confirm achievement 100% of the time, is some conductor is by design or accident referenced to earth and we are unaware of it then the safety we enjoy in a bathroom and a proper isolated SELV safety socket is easily defeated.
PS SELF in a bathroom would not be considered SELV at 50V either.
Yup one side gets strapped to earth and your float idea disappears so back to square one and earthing/bonding.
Pity the earth is not a massive flat piece of conductor like copper or aluminium too, otherwise it would get more reliable if we need to earth things,
two earths at a distance can be at differing potentials until we get farther away.
Those early Daleks that ran on metal plates just like our 1960s and 1970s Dodgem Bumper Cars had a good idea
PS SELF in a bathroom would not be considered SELV at 50V either.
Yup one side gets strapped to earth and your float idea disappears so back to square one and earthing/bonding.
Pity the earth is not a massive flat piece of conductor like copper or aluminium too, otherwise it would get more reliable if we need to earth things,
two earths at a distance can be at differing potentials until we get farther away.
Those early Daleks that ran on metal plates just like our 1960s and 1970s Dodgem Bumper Cars had a good idea