V2L

[JohnW2]

Hmm I didn't word that well did I? .... Working with audio/video/lighting/RF I find myself with some horrible earth loops hwen linking different areas together and running mixed services, .... from experience I know I really need to isolate such kit as much as possible from the very noisy mains and simply install isolation transformer, link secondary 'N' to the steel frame building local earth (which the dc grounded aerials are also bonded to).

Sure, but that's really an entirely different subject. I have relatively little experience of audio, but plenty in relation to RF, and I'm well familiar with the issue of 'earth loops'.

However, that's really not what we're talking about. Rather, we are talking about 'isolation' in relation to electrical safety - which, in practice, means isolation from the earth-referencing of the supply. In that context, as I've said (i.e. in relation to 'electrical safety') if one installs an 'isolating transformer' and then connects one side (or some tap) of the secondary to earth, then one has 'bypassed the (important part of the) isolation' and might as well have not bothered with the transformer in the first place (for 'isolation', although it may be needed for voltage-changing, as in the yellow brick, or for other reasons, such as you've just mentioned).

Kind Regards, John

 

[plugwash]

I disagree, an isolating transformer isolates you from certain supply problems even if you ground it's output.

Lets consider a hypothetical non-isolated version of of the yellow brick. We still want a 55-0-55 output, so we have an autotransformer wound with a 55-0-55-230 configuration. We connect the supply neutral to the "0" tap, the supply live to the "230" tap and the output to the "55" taps.

Now lets consider what happens if the supply neutral conductor breaks. With a traditional yellow brick our output just turns off, with out non-isolated yellow brick both sides of our output rise up to 230V.

Even worse, lets say the supply live and neutral are reversed. Out traditional yellow brick doesn't care, but our non-isolated yellow brick now presents 175V on one side of the output and 285V on the other!

 

[SUNRAY]

Sure, but that's really an entirely different subject. I have relatively little experience of audio, but plenty in relation to RF, and I'm well familiar with the issue of 'earth loops'.

However, that's really not what we're talking about. Rather, we are talking about 'isolation' in relation to electrical safety - which, in practice, means isolation from the earth-referencing of the supply. In that context, as I've said (i.e. in relation to 'electrical safety') if one installs an 'isolating transformer' and then connects one side (or some tap) of the secondary to earth, then one has 'bypassed the (important part of the) isolation' and might as well have not bothered with the transformer in the first place (for 'isolation', although it may be needed for voltage-changing, as in the yellow brick, or for other reasons, such as you've just mentioned).

Kind Regards, John

1. Working in environments where the imported mains earth and exposed earth/metalwork is at different potential is a safety issue and easily remedied with an isolating transformer and due to the 'standard' supply being referenced to ground I'd automatically repeat that... unless there is a specific reason not to.

2. I'd do the same thing if I find the N & E are at different potentials too.

 

[JohnW2]

1. Working in environments where the imported mains earth and exposed earth/metalwork is at different potential is a safety issue and easily remedied with an isolating transformer ....

Fair enough, and you're now talking about isolating one 'earth from another 'earth';.

However, as I suggested before, if you have two 'earths' between which (in the absence of faults) there is a high enough PD to represent an appreciable safety issue (say, more than 30-40V) then I would suggest that there surely must be something very wrong with at least one of those 'earths' - and something which probably needs to be addressed urgently!

2. I'd do the same thing if I find the N & E are at different potentials too.

I'm not sure what you mean. In a TN-C-S installation, N & 'E' cannot be significantly different, and if it's anything other than TN-C-S then there will almost always be a PD between N and 'E'.

Kind Regards, John

 

[SimonH2]

With a 'floating supply', one can only get a serious shock by simultaneously touching two 'poles' of the supply. However, if one 'pole' is earthed, then one can get a shock by touching the non-earthed 'pole' simultaneous with being in contact with 'earth' (including 'the earth') - which poses far more opportunities for shocks.
It doesn't. However, as above,what it does do is create an opportunity for someone in contact with 'the ground' to complete a circuit if they also touch the non-grounded side of the (now earth-referenced) supply.

And that touch voltage is now no higher than 55V - whereas with multiple faults (or combinations of fault) the touch voltage could easily be 110V

eric argues that by earthing a point on the secondary, one is facilitating RCD protection of circuits/loads connected to the secondary. Whilst that uis certainly true, as you know,, my counter-arguments are (a) why bother with the transformer at all (for 'isolation', rather than voltage change), if one is going to 'bypass' that isolation? and (b) as far as I can see, if the secondary truly is floating (no connection to earth) then RCD protection is not really required (unless one postulates multiple independent simultaneous faults) and only becomes required IF and because one earths a point on the secondary?

Bear in mind that the yellow bricks are mainly aimed at (e.g.) building sites where the people working with electrical stuff are not renowned for their careful adherence to safe working practices and molly coddling the tools. Rather than postulating IF a fault would be allowed to continue for long enough for a second one to occur and cause a hazard, I'd suggest it's more a case of the expectation that "if it's still working it won't get fixed" and the question is merely how long till the second fault happens and the full 110V L1 or L2 to earth that's been there for days, weeks, months, ... gets exposed.
Given the expectation that faults will be common, limiting touch voltages to 55V and hence making the hazard very much lower would seem a sensible option. AIUI, it's the limiting touch voltages to 55V that's the primary aim of the "yellow bricks".

As an aside, I have a supply sat in the garage that came out of a school lab. It has an RCD of the type with a hole to pass the cables through - but in this case, it's the centre tap earth wire that goes through the hole (3 times to reduce the nominal trip current to 10mA). I thought it was a rather neat arrangement.

 

[ericmark]

I think @plugwash is missing the point between isolating lives and isolating earths or bonding.

I have found 110 - 0 - 110 supplies, and 56 - 0 - 169 volt, but in both cases generators not transformers. I have seen an IT 110 volt supply from a transformer with an earth monitoring device which actually ends up connecting the supply to earth in order to monitor but through a resistor.

I have also replaced ELCB-v with ELCB-c and the old ELCB-v did have a resistor in the earth.

Reading the old Wiring matters supply to detached outbuilding autumn 2005

An exposed-conductive-part connected to one means of earthing must not be simultaneously accessible with an exposed-conductive-part connected to another means of earthing (Regulation 413-02-03 refers)

413-02-02 In each installation main equipotential bonding conductors complying with Section 547 shall connect to the main earthing terminal for that installation extraneous-conductive-parts of that installation including the following:
(i) water service pipes
(ii) gas installation pipes
(iii) other service pipes and ducting
(iv) central heating and air conditioning systems
(v) exposed metallic structural parts of the building
(vi) the lightning protective system.
Where an installation serves more than one building the above requirement shall be applied to each building.
To comply with the Regulations it is also necessary to apply equipotential bonding to any metallic sheath of a telecommunication cable. However the consent of the owner or operator of the cable shall be obtained.

So we have "Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in
groups or collectively."

This is the problem bit, with a caravan fire regulations require from memory 2.7 meters, so you can't touch the caravan and main building. It seems to allow EV charging points with 207 to 253 volt range over and under causing disconnection we have permitted the touch voltage to go to 70 volt? But it goes against the grain to have an auto disconnection device in the earth. OK lives are disconnected first (live being both line and neutral) but it seems wrong.

Also if the disconnection is due to loss of PEN then why not disconnect whole installation, why only the EV charge point?

OK can see problems in loss of freezer and not being auto reconnected, but a timed reconnection if voltage within limits for x time could get around that, as to loss of lights it would be better to loose them for ½ hour than until all bulbs replaced. However we would loose supply for ½ hour with every brown out.

So we need loss of PEN detection using an earth rod not voltage, and we have the problem where can we insert an earth rod safely. OK that is a problem if going full TT, but a spade depth is enough for detection.

The real problem is an earth connected through a contactor. Can we be sure earth is connected? So what is required is some thing like the old ELCB-v not electrically held in, but electrically disconnected in the event of a fault.

But this device must be tested to show it can do the job safely, not some thing botched together by some one thinking he has thought of all the scenarios which could cause danger, some think awarded a British Standard as being designed and safe to do the job.

Not a knee jerk reaction, but a considered and tested device. As said I have seen the problems with yellow bricks, and also with TN-C-S supplies, lucky the battery powered drill etc has reduced the use of yellow bricks, and a larger 110 volt transformers don't have the problem as output has the overload.

 

[JohnW2]

And that touch voltage is now no higher than 55V - whereas with multiple faults (or combinations of fault) the touch voltage could easily be 110V .... AIUI, it's the limiting touch voltages to 55V that's the primary aim of the "yellow bricks".

I imagine that that is, indeed, 'their' thinking - although I don't know how much reassurance one is meant to derive from the fact that touch voltage is "no greater than 55V" (aka potentially 55mA through a person with a body resistance of 1kΩ).

It seems to all come down to swings and roundabouts, with them differing from me in judging which one wins ...

... with a truly isolated (from earth) 110V secondary, the potential shock is, indeed, in most situations double that with a 50-0-50 secondary (with centre earthed). However, a shock can then occur only if someone somehow manages to touch both sides of the secondary - which seems to me to be an extremely unlikely happening, even with multipole faults. It is also comparable with someone simultaneously touching L and N of a standard mains supply - something against which an RCD offers no protection.

On the other hand, with a 50-0-50 supply (centre earthed) one only needs a single fault to make one side of the secondary touchable, and then for a person to be at risk of a shock if they touch anything else which is earthed (or, indeed, 'the earth').

I therefore see it as a choice between a very small probability of a 110V shock against a much higher probability of a 55V shock - and, given the potential danger of a 55V shock, I would probably be inclined to favour the other option.

Kind Regards, John

 

[SUNRAY]

I therefore see it as a choice between a very small probability of a 110V shock against a much higher probability of a 55V shock - and, given the potential danger of a 55V shock, I would probably be inclined to favour the other option.

Kind Regards, John

Have you ever handled live 55V ac circuits, by that I mean stripping cables and terminating them live?

My experience of doing that tells me there is hardly a tingle to be felt, if as all. On that basis I'm quite happy with 55-0-55 as a working system.

I'm very much with Simon when he describes the way building site electrics are handled. It's certainly not a question of 'if' a fault occurs, it's frequently more like if a non faulty device can be found, or at least that's how it feels. Sadly it's not uncommon to see taped joins (or even joins without tape), cables bare to the copper over significant distances where it's been dragged over a sharp edge or simply been trampled over a long period of time.

 

[JohnW2]

I disagree, an isolating transformer isolates you from certain supply problems even if you ground it's output. ... Lets consider a hypothetical non-isolated version of of the yellow brick. We still want a 55-0-55 output, so we have an autotransformer wound with a 55-0-55-230 configuration. We connect the supply neutral to the "0" tap, the supply live to the "230" tap and the output to the "55" taps. .... Now lets consider what happens if the supply neutral conductor breaks. With a traditional yellow brick our output just turns off, with out non-isolated yellow brick both sides of our output rise up to 230V.

I have to say that I was not even dreaming of considering the alternative to true isolation as being something as contrived as your "hypothetical non-isolated version", which sounds like probably the worst of most worlds .

As I have recently written, my real point relates to "We still want a 55-0-55 output", since that implies a lack of isolation from earth. Asa I recently said, I would personally have thought that a truly floating 110V output would, on balance, be 'safer' than a 50-0-50 one - since, although the voltrage of a potential shock was doubled, the probability of any shock would be greatly reduced, given that very very few shocks result from simultaneous contact, directly, with 'both sides of the supply'.

Kind Regards, John

 

[JohnW2]

Have you ever handled live 55V ac circuits, by that I mean stripping cables and terminating them live?

Nope. In fact, not even non-live 55V AC circuits

My experience of doing that tells me there is hardly a tingle to be felt, if as all.

I can but presume that you must have had very dry hands when you did it ... and I don't think that building sites are known for always being bone dry. Probably 5mA (equivalent to 11kΩ body path resistance) and certainly 10mA (5.5kΩ body path resistance) result in far more than "hardly a tingle".

On that basis I'm quite happy with 55-0-55 as a working system.

Fair enough, and good luck - but do try to 'keep dry'

I'm very much with Eric when he describes the way building site electrics are handled. It's certainly not a question of 'if' a fault occurs, it's frequently more like if a non faulty device can be found, or at least that's how it feels.

I think youy probably mean Simon, not eric, but I don't doubt you are both right - and I also imagine that extends to no care being take to avoid 'fiddling with things electrical' when they, or one's hands, are not bone dry!

Kind Regards, John

 

[SUNRAY]

Thanks, yes Simon - now corrected.

For what it's worth I've just measured my hand to hand resistance with a multimeter, by squeezing the probes very hard I was able to get the resistance down to 14MΩ and with dripping wet hands 4MΩ. Holding both probes between thumb and forefinger (~¼" apart) 8MΩ or wet 8KΩ.
As it happens I do have dry skin and regularly struggle turning pages in a book, opening thin poly bags from a roll etc which other don't. On that basis I'd expect to not be as conductive as others.

I've fairly regularly encountered incorrect building site electrics and in one form or another and found myself across 55V without realising it's powered. 110V is very different and for that reason alone I'd much prefer the current system where one fault will not hurt but stands a chance of being found/detected/rectified before the second fault hurts as opposed to the first (earth) fault of a floating 110V supply is less likely to be discovered, making the second fault hurt.

 

[JohnW2]

For what it's worth I've just measured my hand to hand resistance with a multimeter, by squeezing the probes very hard I was able to get the resistance down to 14MΩ and with dripping wet hands 4MΩ.

Hmmm. Assuming that you are not a rhinoceros, I presume that you were attempting to measure at very low voltage!

For what it's worth, I've just tried with a cheapo meter (9V battery) and, after licking my fingers, get 50 - 100kΩ between hands.

If it were really 4MΩ, or anything remotely that high, (at the voltages concerned) you could obviously grasp the L and N of a 230V supply, or probably even a 1,000V one, without probably even feeling a thing - even 4,000V would only result it 1mA, which you might not feel, and almost certainly would do you no harm (unless you have an extremely iffy heart).

For most safety purposes, a figure of 1kΩ (which assumes wet skin) is assumed.

I've fairly regularly encountered incorrect building site electrics and in one form or another and found myself across 55V without realising it's powered. 110V is very different ...

There's something odd there, given that you're only talking of a 2:1 difference in voltages. I find it difficult to see how 110V can be 'dangerous', yet 55V 'not felt at all'.

Kind Regards, John

 

[SimonH2]

However, a shock can then occur only if someone somehow manages to touch both sides of the secondary - which seems to me to be an extremely unlikely happening, even with multipole faults.

Really ? Extremely unlikely ?
I think that's why we disagree on the benefits of earthing the centre tap.
Make it a floating supply, and many common faults cold make one side or the other "earthed" to some degree. I think we can imagine plenty of faults that could do it - as Sunray says, cables with bare copper showing and then dragged over scaffolding, badly fitted connections with bare copper touching something else, something dropped in water, cable pulled part way out of the equipment and a core contacts the case, ... Without active earth fault monitoring, such a fault would go undetected with a fully floating system - and hence would be likely to persist for some time.
So then you are into a situation where the supply isn't floating, it's earthed at some point. So now it's a "single fault creates a hazard" situation - and the touch voltage is 110V. User picks up a drill that's got a bit damp, has other hand on scaffolding, zap

But if you make it so that the first fault causes a problem for the user - ideally power goes off - then it'll get investigated and hopefully fixed. But even if it doesn't cause anyone a problem and get fixed, the touch voltage is only 55V and so much less likely to cause injury. Note that it's not just the risk from shock - it's the risk to others if (for example) someone is up scaffolding, gets a belt, and drops his tools on the people below.

The thing is that most of us here understand the risks and the signs of damage. But the number of times I've (for example) seen equipment with the mains cord pulled out of its strain relief and the user is oblivious both to the fact of it having happened and the dangers makes it clear that most people really don't know and don't care. And casual observation on building sites shows, as Sunray points out, a common disregard for what we might think of as basic self preservation

 

[JohnW2]

Really ? Extremely unlikely ?

Well, just as unlikley as someone simultaneously touching the two sides (usually L & N) of any supply - something which (even with an earth-referenced N) we have to accept that no (electrical) technological device can protect anyone against.

Kind Regards, John

 

[scousespark]

Good to hear from you. I hope all is well with you and yours.

With respect, I think it's probably really you who is 'over-interpreting' - in the same way that some people seem to think that, for example, one member of the police force committing a horrendous crime means that 'police officers cannot be trusted'. There are 'black sheep' in every walk of life, every profession and every trade, but that is certainly not a brush with which one should tar everyone in the walk of life, profession, trade or whatever. However, by the same token, it would not be appropriate to pretend that these 'black sheep' do not exist.

Whether in relation to an electrician, police officer, politician, doctor, nurse, teacher, clergyman, mechanic or whatever, it is usually the case that the vast majority of (competent, conscientious and 'decent') members of the trade/profession concerned are as appalled by the very small number of 'black sheep' in their trade/profession as are everyone else, and I would have expected you to feel the same about the small minority of 'rogue' electricians.

I've come in contact with a substantial number of electricians over the years, and there has only been a very small minority whose (apparent lack of) 'competence' has given me cause for concern. Even in that small minority, it is often their training (particularly in relation to 'basic principles'), not themselves, that really deserves the criticism.

Kind Regards, John

Hi John. I think 'black sheep' is a bit strong, implying that there is malice in play when a mistake is made. I never stated, nor implied, that everybody involved in the electrical installation industry was perfect. My post came after seeing the following posts:

#26 Sunray
Sadly far too many 'know nothings' H&S's don't understand that. In my audio etc role I've frequently used inverters, usually in a situation where the whole installation is very much floating and I've often been instructed by on site safety people to insert RCD.

#29 You
That doesn't surprise me, but is surely means that the individuals and/or their training and/or their entire 'system' is not ';fit for purpose'.

If I, who have never had any sort of formal training in the field, can fully understand these things, then it's surely ridiculous that someone who is meant to be policing the 'Health and Safety' aspects does not understand,

#32 Bernardgreen
You and I and a few others have the ability to use the basic laws of science to evaluate a situation and produce the most appropriate resolution for a problem or a conflict.

Many of those policing the 'Health and Safety' aspects do not know the basic laws of science well enough to be able to use them.


Maybe I have been lucky when I've met members of the IET, HSE, NICEIC but I have found they actually know a bit about electrical installation. I have come across electricians who are not as clued up as they could be.