Isolation Transformer - A discussion

[Harry Bloomfield]

Near enough, or actually stated?

It was perfectly clear to me, what was meant, even if words of more than one syllable were used.

 

[JohnW2]

This seems to be contradiction, first you say using a cpc on the classI appliance does not introduce a hazard then you say it is not safe. In fact you have stated it is not safe several times unless I'm totally missing your points.

I think you need to read more carefully what I have written, several times. What I have said is:

1... Using a Class I item when the supply is floating is, in itself, not safe in the first place, even if its exposed-c-ps are earthed. The whole point of having earthed exposed-c-ps in a Class I item is so that a protective device will operate if a 'live' (L not N) conductor comes into contact with the exposed-c-p. With a floating supply that will not happen. As I said, presumably because floating supplies are rare, MIs of Class I items don't seem to point out that they cannot be used safely with floating supplies.

2... If one has a Class I item (with earthed exposed-c-ps) connected to a floating supply, and if one side of that floating supply somehow comes into contact with the exposed-c-ps, there will be no immediate hazard. However, that fault will have 'converted' the floating supply to an earth-referenced one so, in terms of any subsequent other fault, it will have lost the safety advantages of being floating.

Kind Regards, John

 

[ericmark]

As I said at the beginning, this isn't a request-for-advice thread. Its more of a Sunday afternoon discussion thats now run over into Monday. I understand the pros & cons of running an earth free system. Most of the suspects I expected to join-in have, with the exception of Eric who I'm sure will be along in due course.....

I have been reading, and also considering the one and only time I have worked with an IT system. There was a step down transformer 660 volt to 220 volt delta it supplied a tunnel boring machine (TBM) made by Robin, in Hong Kong very wet iron laden water so very good conductor, it was very shocking, and a nightmare to work on, but no one died or had a belt bad enough to say anything more than ouch.

OK all adults and we all used neon screwdrivers before touching anything.

I also worked in Heathrow T5 on a batching plant, this time single phase 220 volt for control but with a earth monitoring system, the worry was stop a batching plant full and it is a shovel job to empty it quick before the concrete sets, so last thing we wanted was a RCD, when the red light comes on then no new batch is made.

As to why not 110 volt control ask the Germans who made it. We found out it was an export model, it should have been exported from the EU as did not comply with EU rules, but some one felt UK was not part of EU so we got it. It was in a way responsible for my accident, as safety relays should be red or yellow, but the Germans had used black, so it was not considered as safety relay and was removed (no disconnected) to assist in maintenance as it fouled the lifting of the recycling unit lid, so I have been recycled.

However here we are looking at bulbs, and I have taken a bulb and looked for and non of my BA22d bulbs have that symbol on them, but BS 7671:2008 411:3:1 "except a lampholder having no exposed-conductive-parts and suspended from such a point." So the lamps even if three connections do not require an earth, maybe a kick back from before 1966, but lamp holders when suspended do not require an earth. This also means if not suspended they do, so must run an earth to GU10 even when nothing to connect it to.

I remember the problem with the two pins system, as still used with 24 volt ASii PLC's and two cables look nearly the same, but one is self healing and is some sort of rubber, but does not like oil, the other does not mind oil but not self healing, so seem to remember the system was banned for reduced low voltage use.

But festoon lights are still two wire with no earth. Where I work we have loads, the building was part stripped of all electrical gear with the idea it would be turned into a workshop and completely rewired, however it was realised the building was too low, and to get rails into the building would be a problem, so scheme was abandoned so we have half a dozen 16 amp outlets and all lighting is temporary, using yellow brick transformers.

The problem is not electric shock, it is fire, the yellow brick has a 10 amp overload on its supply, 10 x 230 / 55 = 41.8 amp and 1.5 mm² will not take 41.8 amp without melting.

So a risk assessment, which is higher risk, shock from 230 volt, or fire from 110 volt? If up to me I would ban yellow bricks unless overload fitted on outlet.

And this is the whole point of the thread, what are the risks, and are they worth it? So we look at this account of Harvey Tyrrel's death and I ask myself could I have made the same mistakes as Bearman's brother-in-law Colin Naylor, the electrician, and being honest I could have been caught out, I trust my brother-in-law and if he said he had done some thing I would likely believe him.

The BBC account does not really help in working out exactly what went wrong, but at this time of year we tend to use temporary outside lights without too much thought to safety.

To my mind the largest problem today is the TN-C-S supply. If we did not have a TN-C-S supply the RCD would in most cases protect us. Using class II items outside we have removed most of the danger to people with the TN-C-S, but it seems some EV's are not class II, my e-bike uses a fig of 8 2.5 amp connector and has no earth even to the charge module, wife's has clover leaf connector so does have an earth, but the 48 volt to my e-bike or 36 volt to wife's e-bike is not really a problem. Same with larger lead acid battery EV's, milk floats and fork lifts have the charger static, and only a DC connection under the low voltage thresh hold, they are all extra low voltage.

There are items which are of concern, patio heaters, flood lights, and electric cars. i.e. those items where we earth them.

But the scenario here is lighting, and I would say in the main class II and RCD protected and no real problem.

OK now I stand back and see how you all pick holes in my thoughts.

 

[SUNRAY]

I think you need to read more carefully what I have written, several times. What I have said is:

1... Using a Class I item when the supply is floating is, in itself, not safe in the first place, even if its exposed-c-ps are earthed. The whole point of having earthed exposed-c-ps in a Class I item is so that a protective device will operate if a 'live' (L not N) conductor comes into contact with the exposed-c-p. With a floating supply that will not happen. As I said, presumably because floating supplies are rare, MIs of Class I items don't seem to point out that they cannot be used safely with floating supplies.

If this is the latest definition of classI then I have not kept up to date and neither have the first 3 definitions that have come up on our friend, none of them have come up with anything stating: 'The whole point of ... Class I item is so that a protective device will operate ...'. They, and my early learnings, indicate the reason for the earthed barrier is to prevent it becoming a electric shock safety hazard and AFAIC preventing harm to an operator is still paramount, whether that trips a protective device or not.

I have not yet seen what I would describe as an official definition later than my 2004 brown book:
"Class I equipment. Equipment in which protection against electric shock does not rely on basic insulation only, but which includes means for the connection of exposed-conductive-parts to a protective conductor in the fixed wiring of the installation (see BS2754)"
Sites seem to indicate BS2754 as withdrawn/inactive.

If that info is not up to date then I put my hands up and accept it may not be safe.
If that info is up to date then how is it not safe?

 

[ericmark]

"Class I equipment. Equipment in which protection against electric shock does not rely on basic insulation only, but which includes means for the connection of exposed-conductive-parts to a protective conductor in the fixed wiring of the installation (see BS2754)"

Class I equipment. Equipment in which protection against electric shock does not rely on basic insulation only, but which includes means for the connection of exposed-conductive-parts to a protective conductor in the fixed wiring of the installation (see BS EN 61140).

Seems nearly the same.

Protective conductor (PE). A conductor used for some measures of protection against electric shock and intended for connecting together any of the following parts:
(i) Exposed-conductive-parts
(ii) Extraneous-conductive-parts
(iii) The main earthing terminal
(iv) Earth electrode(s)
(v) The earthed point of the source. or an artificial neutral.

Oddly never bothered reading that, wonder what an artificial neutral is?

 

[SUNRAY]

I have been reading, and also considering the one and only time I have worked with an IT system. There was a step down transformer 660 volt to 220 volt delta it supplied a tunnel boring machine (TBM) made by Robin, in Hong Kong very wet iron laden water so very good conductor, it was very shocking, and a nightmare to work on, but no one died or had a belt bad enough to say anything more than ouch.

OK all adults and we all used neon screwdrivers before touching anything.

I've worked in a number situations with floating systems and totally agree we each carried neon & LED version screwdrivers and habitually tested everything before touching, even if it had been tested 5 minutes before.

...using yellow brick transformers.

The problem is not electric shock, it is fire, the yellow brick has a 10 amp overload on its supply, 10 x 230 / 55 = 41.8 amp and 1.5 mm² will not take 41.8 amp without melting.

So a risk assessment, which is higher risk, shock from 230 volt, or fire from 110 volt? If up to me I would ban yellow bricks unless overload fitted on outlet.

My similar thoughts have been documented on here before.
One of my yellow bricks is 3.3KVA and has a primary 15A overload and a pair of 16A sockets, I haven't been inside it but others I've repaired are wired with 1.5mm² and assume min is too, adequate for the 16A sockets but not for the full rated power of 30A and certainly not for the potential >63A at 55V.

OK now I stand back and see how you all pick holes in my thoughts.

Are there any?

 

[SUNRAY]

Seems nearly the same.

Oddly never bothered reading that, wonder what an artificial neutral is?

The only times I've ever really read them, other than during 16th college course, is when put to task, and I must add it's usually been in relation to a consultants error.
Artificial neutral I was told at college in 70's is the star point in a floating 3ph system but I don't understand that myself. However a while back I was working on a 3ph Δ panel and wanting to make some 'relative' measurements I used 3x 1KW heating elements to create a star point and somebody described it as such.

 

[SUNRAY]

Another interesting question. Why are the 2 live conductors in an IT system called 'L' & 'N' ?

 

[plugwash]

The whole point of having earthed exposed-c-ps in a Class I item is so that a protective device will operate if a 'live' (L not N) conductor comes into contact with the exposed-c-p. With a floating supply that will not happen.

The whole point of earthing the exposed conductive parts is to provide protection against electric shock in the event of a failure of "basic insulation". That can be achieved either by tripping a protective device, or by limiting the touch voltage to an acceptable level.

If one has a Class I item (with earthed exposed-c-ps) connected to a floating supply, and if one side of that floating supply somehow comes into contact with the exposed-c-ps, there will be no immediate hazard. However, that fault will have 'converted' the floating supply to an earth-referenced one so, in terms of any subsequent other fault, it will have lost the safety advantages of being floating.

Indeed, which is why floating supplies work best on a small scale.

As a system gets larger, the probability of their being a fault somewhere in the system, and/or an accumulation of leakage currents to hazardous levels starts to approach 1. So the safety advantages of floating systems are lost and it is better to have a system with a well-defined relationship to earth.

Another alternative is to have a supply that is only weakly tied to earth, then implement fault monitoring by measuring the voltages between the supply conductors and earth. This is arguably the safest system for a small scale supply and I belive is used in some medical situations, but again it doesn't scale up well.

 

[SUNRAY]

The whole point of earthing the exposed conductive parts is to provide protection against electric shock in the event of a failure of "basic insulation". That can be achieved either by tripping a protective device, or by limiting the touch voltage to an acceptable level.


Indeed, which is why floating supplies work best on a small scale.

As a system gets larger, the probability of their being a fault somewhere in the system, and/or an accumulation of leakage currents to hazardous levels starts to approach 1. So the safety advantages of floating systems are lost and it is better to have a system with a well-defined relationship to earth.

Another alternative is to have a supply that is only weakly tied to earth, then implement fault monitoring by measuring the voltages between the supply conductors and earth. This is arguably the safest system for a small scale supply and I belive is used in some medical situations, but again it doesn't scale up well.

What a brilliant summary

 

[JohnW2]

The whole point of having earthed exposed-c-ps in a Class I item is so that a protective device will operate if a 'live' (L not N) conductor comes into contact with the exposed-c-p. With a floating supply that will not happen.

The whole point of earthing the exposed conductive parts is to provide protection against electric shock in the event of a failure of "basic insulation". That can be achieved either by tripping a protective device, or by limiting the touch voltage to an acceptable level.

That's not materially different from what I wrote, is it? In the context we are discussing (a fully-floating supply), having earthed exposed-c-ps will not facilitate the operation of any protective device (OPD or RCD). As for "limiting touch voltage" (which I would have thought was technically bonding, rather than earthing), the only touch voltage it will limit will be that between the exposed-c-p and something else earthed and, again, in the context of a fully-floating supply a hazardous touch voltage cannot really arise (since any pd between the exposed-c-p and earth will be via stray capacitance etc., hence too high impedance a path to be hazardous).

The point I was making to Sunray was that if an item is designated as Class I, the implication is surely that the "basic insulation" (plus "additional insulation" if present) is not considered adequate to provide the required degree of protection against electric shock (i.e. does not qualify as "Class II") . If that is the case, and if (as above) earthing the exposed-c-p does not really afford any additional protection against electric shock, then it surely follows that Class I items cannot be 'safely' used when the supply is floating, doesn't it? [Since the basic+additional insulation does not provide adequate protection , and the earthing of exposed-c-ps does not increase protection, IF the supply is fully floating].

Indeed, which is why floating supplies work best on a small scale. As a system gets larger, the probability of their being a fault somewhere in the system, and/or an accumulation of leakage currents to hazardous levels starts to approach 1. So the safety advantages of floating systems are lost and it is better to have a system with a well-defined relationship to earth.

Undoubtedly true, but I would think that all the scenarios we've been discussing here count as 'small scale'.

The entire electricity generation/.distribution network and all electrical installations presumably would be safer if everything could be 'earth-free', but since that would be impractical/impossible, the lesser of the evils is, as you say, "to have a system with a well-defined relationship to earth"

Another alternative is to have a supply that is only weakly tied to earth, then implement fault monitoring by measuring the voltages between the supply conductors and earth. This is arguably the safest system for a small scale supply and I belive is used in some medical situations, but again it doesn't scale up well.

Indeed, and I've seen that system used in some "medicakl situations". However, again, that goes way beyond what is being discussed in this thread.

Kind Regards, John

 

[SUNRAY]

I believe you are now really confusing the issue

The point I was making to Sunray was that if an item is designated as Class I, the implication is surely that the "basic insulation" (plus "additional insulation" if present) is not considered adequate to provide the required degree of protection against electric shock (i.e. does not qualify as "Class II") .
Kind Regards, John

You seem to be describing class II but then saying it doesn't qualify... Class I does not require an "additional insulation" so why introduce it into the description?

If that is the case, and if (as above) earthing the exposed-c-p does not really afford any additional protection against electric shock,
Kind Regards, John

Why does it not offer electric shock protection when the multiple faults, that have been described so many times, occur?

then it surely follows that Class I items cannot be 'safely' used when the supply is floating, doesn't it? [Since the basic+additional insulation does not provide adequate protection , and the earthing of exposed-c-ps does not increase protection, IF the supply is fully floating].
Kind Regards, John

If it cannot be safely used as you insist on saying what makes it dangerous when there are no faults?
If it cannot be safely used as you insist on saying what makes it dangerous when there are faults?

 

[JohnW2]

.... They, and my early learnings, indicate the reason for the earthed barrier is to prevent it becoming a electric shock safety hazard and AFAIC preventing harm to an operator is still paramount, whether that trips a protective device or not.

That's the bottom line of the reason for having the 'earthed barrier', but, as I've just written, it only achieves that with an earth-referenced supply.

As plugwash and myself have written, that bottom line can only be achieved by (a) facilitating operation of a protective device (OPD or RCD) and/or (b) 'bonding' exposed-c-ps to earth, so as to limit 'touch voltages' between the exposed-c-p and earth.

That works with an earth referenced supply. However, with a fully-floating supply, (a) cannot happen and (b) is irrelevant - so earthing the exposed-c-p essentially provides no protection against electric shock.

Hence, as I've said, if a Class I item is not considered safe (with an earth-referenced supply) if the exposed-c-p is not earthed, then this implies that the same item cannot be safe with a fully-floating supply, even if the exposed-c-ps are earthed - since that connection to earth affords to no additional protection when the supply is floating.

Kind Regards, John
Edit: typo corrected

 

[EFLImpudence]

Hence, as I've said, if a Class I item is not considered safe (with an earth-referenced supply) if the exposed-c-p is not earthed, then this implies that the same item cannot be safe with a fully-floating supply, even if the exposed-c-ps are earthed - since that connection to earth affords to additional protection when the supply is floating.

Just to be sure: is there a typo?

 

[JohnW2]

I believe you are now really confusing the issueYou seem to be describing class II but then saying it doesn't qualify... Class I does not require an "additional insulation" so why introduce it into the description?

I am saying that it is CXlass I (and hence only considered safe if its exposed-c-ps are earthed) BECAUSE it does not qualify as Class II.

Since, with a floating supply, earthing the exposed-c-p does not provide any additional protection, if a Class I item with an earth-referenced supply is not regarded as 'safe' unless the exposed-c-p is earthed, then nor can it be 'safe' with a floating supply, even if the exposed-c-ps are earthed.

Why does it not offer electric shock protection when the multiple faults, that have been described so many times, occur?

If the first of the faults is such as to turn the floating supply into an earth-referenced one then, yes, one is then back to the situation which would have existed had the supply never been floating.

If it cannot be safely used as you insist on saying what makes it dangerous when there are no faults?

It's no different from Class I items in general. "Deemed to be 'safe' " relates to the situation if/when a fault does arise. A Class I item without its exposed-c-ps being earth is perfectly 'safe' so long as there is no fault, but that doesn't make it acceptable to omit the earthing.

If it cannot be safely used as you insist on saying what makes it dangerous when there are faults?

If it is not "deemed to be 'safe' " even when there are no faults, that question becomes moot.

Kind Regards, John