V2L

[ericmark]

One of the problems is people incorrectly use the term 'isolating' when they really mean 'voltage changing'. Typically the yellow 110V transformers fall into this category and of course they are centre tapped to earth.

Had a machine burn out as a result, 110 volt control transformer changed and secondary had internal connection centre tapped to frame, and line 1 was fused but not line 2, so a short on line 2 resulted in complete re-wire being required.

Many yellow bricks only have overload on supply side, they are often rated 20 amp output rather than 16 amp, common to find 10 amp overload on supply so 10 x 230 = 2300 / 55 = 42 amp to earth, common to find long 1.5 mm² extension leads from the brick, often long enough so a short to earth does not draw 42 amp result in melted cables, one reason why always open both plug and socket to look for damage with PAT test, seen electricians cutting off visible damage in plug and returning to service, to my mind with 110 volt leads sign of melting whole lead cut up and binned.

I would ban yellow bricks if I could, the fire risk with them is far higher than the electric shock risk they remove. Overloads should be on the output not input.

 

[bernardgreen]

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,

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.

Personally I have no problem with earthing to real Earth the secondary of a "yellow brick" as it allows for an RCD to be used on the 55-Ground-55 supply to lights and tools.

That 55-0-55 midpoint ( 0 Volt ) is the same as the star point ( 0 Volt ) Neutral of the big grey brick's ( substation transformer ) output that is connected via the ground to all the other 0 Volts on all the other big grey bricks on the supply network.

One connection via ground across the isolation barrier does not create a circuit.

 

[JohnW2]

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.

Quite so, but if they don't understand the relevant 'bsic laws of science', in my opinion there is no way (and certainly no safe way that they should be 'policing' matters of electrical safety.

Personally I have no problem with earthing to real Earth the secondary of a "yellow brick" as it allows for an RCD to be used on the 55-Ground-55 supply to lights and tools.

It does - but, as well as "allowing for and RCD to be used" is it not also the cause of there being any need for the RCD? In other words, the earthing creates a hazard, which can then be partially mitigated by also adding an RCD - but why have either?

In any event, if you did earth some point on the secondary, I would hope that you would not then describe it as an 'isolation transformer', would you?

That 55-0-55 midpoint ( 0 Volt ) is the same as the star point ( 0 Volt ) Neutral of the big grey brick's ( substation transformer ) output that is connected via the ground to all the other 0 Volts on all the other big grey bricks on the supply network.

All true, but the world would be a safer place if it were possible (which, in practice, it isn't) for all supplies to NOT be earth-referenced, wouldn't it?

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.

One connection via ground across the isolation barrier does not create a circuit.

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.

Kind Regards, John

 

[ericmark]

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.

I think the idea is that things trip before it causes danger. The old idea of two bulbs on the supply centre connected to earth and if either leg goes to earth one bulb goes bright is rather basic.

Without the earth you would not know there was a fault until too late.

 

[scousespark]

I’m just curious whether you guys believe any electrician, IET member, CPS member/assessor, HSE inspector is competent enough to work in the industry. If we were as bad as made out here, the UK would be a death trap.

 

[JohnW2]

I’m just curious whether you guys believe any electrician, IET member, CPS member/assessor, HSE inspector is competent enough to work in the industry. If we were as bad as made out here, the UK would be a death trap.

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

 

[JohnW2]

I think the idea is that things trip before it causes danger. ... Without the earth you would not know there was a fault until too late.

I'm not sure what sort of fault ("which may cause danger") you have in mind.

In relation to an earth-referenced supply, the usual concern relates to someone touching something (an 'exposed-c-p') which has become 'live' (relative to earth) due to a fault between the 'high potential' (relative to earth) side of the supply and the exposed-c-p.

However, if the 'supply' (downstream of whatever is 'isolating' it) is truly isolated/floating (i.e. not earth-referenced), then it doesn't really matter if one side of that supply is connected (deliberately, or via a 'fault') to the exposed-c-p, would it? - by virtue of 'stray capacitances', one might possibly get 'tingle' from it, but it would not represent a danger.

In fact, I think my statement is probably self-fulfilling' - in the sense that if such a fault did create a true danger, it would mean that the supply was actually not 'truly isolated'.

Kind Regards, John

 

[SUNRAY]

One of the problems is people incorrectly use the term 'isolating' when they really mean 'voltage changing'. Typically the yellow 110V transformers fall into this category and of course they are centre tapped to earth.

Well, again, if that is the level of their knowledge/understanding, they really should not be trying to talk about such matters at all, no matter what terminology.

Any non-faulty transformer 'isolates' the secondary from the primary, give or take very small 'stray paths' that bernard might wish to talk about. Whether that isolation is 'bypassed' by a user's wiring is a totally different matter!

Kind Regards, John

Trouble is one only has to start looking at listings/descriptions
https://www.screwfix.com/p/carroll-...UZYqEi_oElUdH0gQvsIaAp-IEALw_wcB&gclsrc=aw.ds
Then read the description...

Or even better look on the side of the transformer

 

[SUNRAY]

Had a machine burn out as a result, 110 volt control transformer changed and secondary had internal connection centre tapped to frame, and line 1 was fused but not line 2, so a short on line 2 resulted in complete re-wire being required.

I did a spell working for a water controls company. Amongst other things we populated the green glass fibre huts seen on the side of roads etc, everything from a BT size up to 4m square or so.
The bigger units would have mains power but transformed down to 110V for everything. The first I worked in, in the workshop, had the power wired by the company's electrician consisting of: 32A or so 2 pole RCBO (in preparation of DNO installing C/O & meter)- transformer ~6KVA or so - domestic style CU (2 pole main switch, single pole MCB's) feeding the various circuits including 16A yellow sockets. Possibly more switching on the primary but no additional OCD.

I queried the lack of 2 pole MCB's in the CU and basically told I was a subby and to go away. Not satisfied I went to a manager and made same query, he had no clue and didn't understand so I demonstrated by inserting a plug with 'cold' to 'earth' short on the end of a short length of 1.5mm² yellow flex, I stood there watching wires overheat and start smoking as did the manager and the electrician.
The later said something like "Oops a daisy" and he was instructed to replace the damaged wiring and change the CU (needed to be bigger to take 2 pole MCB's) Additionally they had to do an upgrade program to a great many cabins.

 

[JohnW2]

Trouble is one only has to start looking at listings/descriptions
(link here) ... Then read the description... ... Or even better look on the side of the transformer

Quite. As I said (and you quoted) ...

Any non-faulty transformer 'isolates' the secondary from the primary, give or take very small 'stray paths' that bernard might wish to talk about. Whether that isolation is 'bypassed' by a user's wiring is a totally different matter!

Any (wirewound ) transformer, per se, will isolate, in the sense that there is only inductive (and a little capacitive) coupling between primary and secondary, without any 'direct connection'. As you said, whether or not it 'isolates' (the circuitry on primary and secondary sides) therefore depends entirely on what is connected to the windings (i.e. 'how it is wired').

I cannot see that anything at all would be achieved by putting a 1:1 transformer between a floating supply and its load. Only if the 'upstream' supply is earth-referenced does it serve a purpose, and that purpose is to 'isolate' the secondary (and associated circuitry/loads) from earth - so if one then connects some point of the secondary to earth one has removed the point in having an ('isolating') transformer in the first place.

Of course, transformers are often not 1:1 but, instead, are used to change voltage (as in your yellow bricks). However, the fact that the brick (or whatever) contains a transformer (between input and output) does not, in itself, mean that the output is necessarily 'isolated' - that depends upon whether or not copper (or other) conductors are connected between a point on the primary and a point on the secondary - if so, the 'brick' as a whole does not 'isolate'#, even if the transformer itself does.

Kind Regards, John

 

[SUNRAY]

Quite. As I said (and you quoted) ... Any (wirewound ) transformer, per se, will isolate, in the sense that there is only inductive (and a little capacitive) coupling between primary and secondary, without any 'direct connection'. As you said, whether or not it 'isolates' (the circuitry on primary and secondary sides) therefore depends entirely on what is connected to the windings (i.e. 'how it is wired').

I cannot see that anything at all would be achieved by putting a 1:1 transformer between a floating supply and its load. Only if the 'upstream' supply is earth-referenced does it serve a purpose, and that purpose is to 'isolate' the secondary (and associated circuitry/loads) from earth - so if one then connects some point of the secondary to earth one has removed the point in having an ('isolating') transformer in the first place.

Of course, transformers are often not 1:1 but, instead, are used to change voltage (as in your yellow bricks). However, the fact that the brick (or whatever) contains a transformer (between input and output) does not, in itself, mean that the output is necessarily 'isolated' - that depends upon whether or not copper (or other) conductors are connected between a point on the primary and a point on the secondary - if so, the 'brick' as a whole does not 'isolate'#, even if the transformer itself does.

Kind Regards, John

Did you see (and read) the pic before you penned your reply?

 

[JohnW2]

Did you see (and read) the pic before you penned your reply?

Do you mean the fact that it is described as an "Isolating Transformer"? If so, then, yes, I both saw and read it.

However, as I've been asking by implication, what on earth does that mean? - i.e. what is it isolating from what? In a literal sense, it is obviously true that there is no 'wire' connection between either L and N of the supply and the output, but by connecting a point on the secondary to 'earth' (close in potential to the primary N) one has surely done away with anything that could be described as 'isolating'/'isolation', hasn't one?

The voltage change and centre tap make the yellow brick slightly more difficult to think about. Consider the simpler and 'purer' case of a 1:1 transformer (with no tapped windings) between 230V mains and a load (something I've certainly seen done when everything downstream of the tranny was 'earth-free'). I don't think anyone would disagree that that was an 'isolating transformer', which was providing 'isolation'- and, indeed, that was the very type of 'isolating transformer' that I was brought up with many decades ago.

However, if you now connected one side of the secondary to earth (close in potential to the N connected to the primary), then one would surely have annihilated any 'isolation' and, indeed, rendered the transformer totally redundant, since the 'isolating' it would otherwise have provided had been bypassed by the earthing.

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?

Kind Regards, John

 

[SUNRAY]

The voltage change and centre tap make the yellow brick slightly more difficult to think about. Consider the simpler and 'purer' case of a 1:1 transformer (with no tapped windings) between 230V mains and a load (something I've certainly seen done when everything downstream of the tranny was 'earth-free'). I don't think anyone would disagree that that was an 'isolating transformer', which was providing 'isolation'- and, indeed, that was the very type of 'isolating transformer' that I was brought up with many decades ago.

However, if you now connected one side of the secondary to earth (close in potential to the N connected to the primary), then one would surely have annihilated any 'isolation' and, indeed, rendered the transformer totally redundant, since the 'isolating' it would otherwise have provided had been bypassed by the earthing.

Kind Regards, John

Again this is down to application. I have occasionally used an isolating transformer then linked one side to earth, however to a different earth to the incoming supply, one such situation had a difference of several tens of volts between local and imported earths.

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?

Kind Regards, John

I've already agreed in principle with (b) with regards to inverters.

 

[JohnW2]

Again this is down to application. I have occasionally used an isolating transformer then linked one side to earth ...

When you say 'one side', do you mean 'the secondary side'? For the purpose of this discussion, Neutral and 'earth' are much the same/similar things, and the incoming neutral will obviously be connected to the primary.

... however to a different earth to the incoming supply, one such situation had a difference of several tens of volts between local and imported earths.

I think that begs the question as to what you mean by 'earth' / 'earths' - and also, if there are "several tens of volts" between two of them (and one is close in potential to 'true earth') what (in the absence of faults) it the cause of the high pd between one of them and true earth?

I've already agreed in principle with (b) with regards to inverters.

Fair enough, but I would say that it is a general truth, applicable to anything.

Kind Regards, John

 

[SUNRAY]

When you say 'one side', do you mean 'the secondary side'? For the purpose of this discussion, Neutral and 'earth' are much the same/similar things, and the incoming neutral will obviously be connected to the primary.
I think that begs the question as to what you mean by 'earth' / 'earths' - and also, if there are "several tens of volts" between two of them (and one is close in potential to 'true earth') what (in the absence of faults) it the cause of the high pd between one of them and true earth?
Kind Regards, John

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, for example running a music link between radio studio and transmitter site, incoming music link for outside broadcasts and 2 way talk back. the 'transmitter rack' and aerials is frequently in a different part of the building to the studio or possibly several building over, an example being in hospital with the 'transmitter rack' in a roof plant room directly above the x-ray/MRI/CT department with the aerials above and the interconnecting cable is catII (4 pair telephone) possibly laying across roofs and hundreds of metres. 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).

Rural supplies can be horrible and exporting mains from a farm building for the PA system in one of the fields I automatically do the same using an earth spike and often see volts between the farm electrical earth and field mud.
Seeing the state of farm electrics often results in thesein no particular order.