Isolation Transformer - A discussion

[JohnW2]

And I know that. The reason I write earth/bond is either may be required depending on the situation.

Fair enough - but it is crucial to understand what situations require one and/or the other - in p[articular, as I keep saying, that if "the situation" is that one has a floating supply, then earthing will achieve nothing (but will increase some other hazards).

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I really don't understand what makes you think that I have changed my mind. Putting together the two bits you have highlighted in red, the meaning of what I have written several times is that "The whole point of having earthed exposed-c-ps in a Class I item is to facilitate [i.e. 'make possible'] the operation of a protective device if a 'live' (L not N) conductor comes into contact with the exposed-c-p." - which, as I've said, is only possible with an earth-referenced supply, since it simply doesn't 'work' with a floating supply.

You seem to have a rather worrying degree of faith/trust in what is achieved by 'earthing' (as opposed to bonding, which doesn't, per se, need to be earthed) exposed-c-ps. Consider a Class I item supplied by 2.5/1.5mm² T+E. In the event of a (negligible impedance', aka 'short') fault between L and the CPC/exposed-c-p in that item, the potential of its exposed-c-p will rise to about 62.5% of supply voltage (about 144V with a 230V supply) above MET potential. If there is another nearby Class I item on a different circuit, or even just a nearby earthed pipe/whatever, that exposed-c-p/whatever will still be roughly at MET potential. A 'touch voltage' of about 144V will therefore exist between those two 'simultaneously touchable' items, even though both are 'earthed' (connected to MET via {different} CPCs). The only thing that can minimise that PD is bonding between the two (both 'earthed') items.

Kind Regards, John

 

[bernardgreen]

as I keep saying, that if "the situation" is that one has a floating supply, then earthing will achieve nothing

If the transformer's insulation between primary winding(s) and secondary winding(s) breaks down then the floating output will become connected in some way to the incoming supply. This single fault means the output is no longer floating and thus a person in contact with Ground and the output will be at risk of an electric shock.

 

[SUNRAY]

Fair enough - but it is crucial to understand what situations require one and/or the other - in p[articular, as I keep saying, that if "the situation" is that one has a floating supply, then earthing will achieve nothing (but will increase some other hazards).

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I really don't understand what makes you think that I have changed my mind. Putting together the two bits you have highlighted in red, the meaning of what I have written several times is that "The whole point of having earthed exposed-c-ps in a Class I item is to facilitate [i.e. 'make possible'] the operation of a protective device if a 'live' (L not N) conductor comes into contact with the exposed-c-p." - which, as I've said, is only possible with an earth-referenced supply, since it simply doesn't 'work' with a floating supply.

You seem to have a rather worrying degree of faith/trust in what is achieved by 'earthing' (as opposed to bonding, which doesn't, per se, need to be earthed) exposed-c-ps. Consider a Class I item supplied by 2.5/1.5mm² T+E. In the event of a (negligible impedance', aka 'short') fault between L and the CPC/exposed-c-p in that item, the potential of its exposed-c-p will rise to about 62.5% of supply voltage (about 144V with a 230V supply) above MET potential. If there is another nearby Class I item on a different circuit, or even just a nearby earthed pipe/whatever, that exposed-c-p/whatever will still be roughly at MET potential. A 'touch voltage' of about 144V will therefore exist between those two 'simultaneously touchable' items, even though both are 'earthed' (connected to MET via {different} CPCs). The only thing that can minimise that PD is bonding between the two (both 'earthed') items.

Kind Regards, John

The whole point about highlighting your comments was purely to show your repeated insistance that the whole reason for any form of earthing is to trip a device to remove power whereas others believe it is more to do with protection of animals safety... a slightly different thing. Then your later comment seems to have softened a little.

And yes I know a lot about earth potential differences (which you initially seem to have recognised in this reply but not later), hence some of my earlier comments about installing (isolation)transformer and earthing the secondary windings and my repeated use of "earth/bond".
Much of my work has been within the public/entertainment environment where my work is inspected (sometimes by several layers of personel) before a license is issued for an event and many thousands of people are allowed to enjoy themselves. On that point I'll even repeat my earlier statement that inspection includes checking my emergency PA system is bonded to the stage earth system... and (for EFLs repeated question) that is an amplifier whose only power source is a 24V battery!

 

[EFLImpudence]

On that point I'll even repeat my earlier statement that inspection includes checking my emergency PA system is bonded to the stage earth system... and (for EFLs repeated question) that is an amplifier whose only power source is a 24V battery!

Why? Meaning what do you and the inspectors think it achieves?

How can you 'bond' 24V to 240V? Surely it is not a 'bond'; merely a pointless and hazardous 'earth' connection to a 240V system.

Would you 'bond' a lorry parked nearby? I say lorry because they are 24V but what about a car or food van parked next to the stage?
What about a car show? Are all the cars 'bonded' to the electrical supply?


Why are shavers 'sockets' allowed in zone 2 and within 3m. as long as they are isolating transformers?

 

[EFLImpudence]

Post #4. Eight pages ago.

This all seems so 'obvious' to me that I get a bit concerned when I see 'discussion' about it

 

[JohnW2]

In my haste in the middle of the night,I omitted the crucial 'punch line'of the point I was making. In response to Sunray, I initially wrote:

I really don't understand what makes you think that I have changed my mind. Putting together the two bits you have highlighted in red, the meaning of what I have written several times is that "The whole point of having earthed exposed-c-ps in a Class I item is to facilitate [i.e. 'make possible'] the operation of a protective device if a 'live' (L not N) conductor comes into contact with the exposed-c-p." - which, as I've said, is only possible with an earth-referenced supply, since it simply doesn't 'work' with a floating supply.

... and then went on to add ...

You seem to have a rather worrying degree of faith/trust in what is achieved by 'earthing' (as opposed to bonding, which doesn't, per se, need to be earthed) exposed-c-ps. Consider a Class I item supplied by 2.5/1.5mm² T+E. In the event of a (negligible impedance', aka 'short') fault between L and the CPC/exposed-c-p in that item, the potential of its exposed-c-p will rise to about 62.5% of supply voltage (about 144V with a 230V supply) above MET potential. If there is another nearby Class I item on a different circuit, or even just a nearby earthed pipe/whatever, that exposed-c-p/whatever will still be roughly at MET potential. A 'touch voltage' of about 144V will therefore exist between those two 'simultaneously touchable' items, even though both are 'earthed' (connected to MET via {different} CPCs). The only thing that can minimise that PD is bonding between the two (both 'earthed') items.

The 'punch line' which I forgot to type was that, in a situation such as I described. simply 'earthing' (connecting exposed-c-ps to the CPC of each item's supply), per se, does not necessarily prevent dangerous PDs ('touch voltages') 'existing (and persisting) between simultaneously-touchable exposed-c-ps (as I said, only bonding, separate from CPCs, can minimise that 'touch voltage) - so that, in situations such as I described, the protection afforded by 'earthing' all exposed-c-ps (to their CPCs) derives primarily from the fact that it 'facilitates' (causes to happen) clearing of the fault by operation of an OPD. If it were not for that (which some have called a 'side effect' of 'earthing'), the dangerous touch voltage would persist indefinitely - or, at least, until some protective device, somewhere, operated or some cable 'melted'.

So, in summary, one cannot rely on 'earthing' all exposed-c-ps (to their circuit's CPC) to eliminate dangerous 'touch voltages', but what is always true (if the circuit is designed properly) is that, with that 'earthing', a protective device will limit the duration of the dangerous 'touch voltage' to a pretty brief duration.

Kind Regards, John

 

[bernardgreen]

A few years ago.....

Pouring with rain, front door bell button sopping wet. Visitor gets a mild shock when touching the button.

Fault was in the 230 to 5-0-8 transformer. It was layer wound and "buzzy" in the bobbin. Nothing loose in the laminations.

 

[JohnW2]

If the transformer's insulation between primary winding(s) and secondary winding(s) breaks down then the floating output will become connected in some way to the incoming supply. This single fault means the output is no longer floating and thus a person in contact with Ground and the output will be at risk of an electric shock.

Sure, anything which negates the 'isolation' provided by a transformer will obviously remove the benefits of having a floating supply and, as you say, may create hazards -whether that "negation of the 'isolation' " arises because of a fault or because someone deliberately earths some point on the secondary of the transformer. What you describe is akin to the risk which always exists in the event of certain types of fault in a mains-powered ELV PSU.

However, I think that some people are still missing my point. For those concerned about fairly rare happenings such as you mention, I have no problem (other than the general undesirability of increasing the amount of unnecessarily earthed touchable metal around) with the exposed-c-ps of items being 'earthed'- and that will address your concerns.

What I do 'have a problem with' is the unnecessary deliberate earthing of some point on the secondary of an 'isolation' transformer, since that totally negates most of the ('safety') advantages of having a floating supply.

Kind Regards, John

 

[SUNRAY]

Post #4. Eight pages ago.

This all seems so 'obvious' to me that I get a bit concerned when I see 'discussion' about it

Kind Regards, John

I totally agree; This all seems so 'obvious' to me that I get a bit concerned when I see 'discussion' about whether we should operate class I equipment contrary to manufacturers instructions and BS7671 wiring regulations.

 

[EFLImpudence]

What about all my questions?


Not M.I.s but Definitions:

"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)."

Do you think that applies if the equipment is not part of the fixed wiring of the installation?


Not exactly comparable but similar in application; would you earth a metal part of a Class II item?

 

[JohnW2]

I totally agree; This all seems so 'obvious' to me that I get a bit concerned when I see 'discussion' about whether we should operate class I equipment contrary to manufacturers instructions and BS7671 wiring regulations.

The manufacturer's instructions (and probably also the spirit of the relevant parts of BS7671) undoubtedly assume that the supply will be earth-refenced - as is the case for 'virtually all' electrical installations in the UK - so they may well not even think to include a statement that earthing of exposed-c-ps of Class I is not required (since it would serve virtually no useful purpose) in the case of a floating supply.

In any event, as I recently wrote in response to bernard I think that some people may have been misunderstanding me and/or missing my point. To repeat, for those concerned about pretty improbable fault scenarios, I have NO problem (other than my general dislike of invoking the risk of 'unnecessarily earthing' anything) with the earthing of exposed-c-ps of Class I items supplied by floating supplies (even though, other than in the presence of very rare types of fault, it will achieve nothing).

However, as I also recently wrote, I do 'have a problem' with unnecessarily 'earthing' any pared of an isolated ('floating') supply (hence rendering it 'non-floating/'isolated') , since that defeats most of the point of the isolation, thereby resulting in a net increase in risks.

Kind Regards,John

 

[JohnW2]

Do you think that applies if the equipment is not part of the fixed wiring of the installation?

Quite. As far as I am aware, nothing in BS7671 applies to anything which is not "part of the fixed wiring".

Having said that, one has to accept that nearly all of the items of (Class I and Class II) equipment we are talking/thinking about do not come within the scope of BS7671, but are the subject of other regulations and/or Standards.

Kind Regards, John

 

[SimonH2]

I think you have yourself illustrated the way in which the two concepts are getting muddled. The purpose of bonding in to prevent dangerous PDs existing between two simultaneously touchable parts (whether parts of the electrical installation {exposed-c-ps} or otherwise), and/but the purpose of earthing is to facilitate the operation of a protective device if an exposed-c-p becomes 'live' (even if any other simultaneoudlsy-touchable parts are, incidentally or by virtue of bonding, equipotential with that live part).

No muddling in my mind - I am talking only of bonding. "Earthing" is a special case of bonding where the mass of earth is one of the conductive parts.
But as I recall (without wading back through the thread, the discussion was around having two faults - such that one cp was connected to one line/phase, and another was connected to a different line/phase, with the result that the two conductive parts now have a dangerous pd between them. If the supply is still floating, then it still doesn't matter about earthing provided the cps are bonded together.
If there is any doubt (as there usually would be) about the supply being truly floating (especially given the amount of capacitance there can be in filters these days) then the equipotential bonding would need to include "anything else" the user could be touching (directly or indirectly) - i.e. the bonding would need to include "to earth and any earthed items".

See the subtle difference ? We only use the term "earthing" because it's such a high proportion (i.e. most of them) where the equipotential bonding does need to include the earth and things in contact with it.
As an example that just came to mind. Consider a shipping container being used as a workshop - it doesn't actually matter if the supply is floating or earth referenced. We could stand the container on insulating mounts - and just bond everything together inside. From the PoV of anyone inside, it's all bonded and safe - but not earthed. In practical terms we only earth the container shell, and hence indirectly everything within it, for the safety of anyone OUTSIDE of the container and who might touch it while in contact with earth.

I find it hard to see why you don't, so maybe I was not clear enough - so let me try again ...

... in the name of "protection against electric shock" a Class II item is required to have "double or reinforced insulation. If, as very often is the case, the item has no exposed-c-ps (i.e. only plastic is touchable), then the requirement is clearly nothing to do with 'touch voltages'.

OK, now I see where you are coming from - and you've made an incorrect (IMO) premise from which to start. The entire point of di Class II items is touch voltages (there is no other reason) - i.e. there's two levels of insulation or one level of reinforced insulation between a user and any live part. It does not mean there has to be an exposed conductive part that a user can touch.

Taking the case of (e.g.) a drill. There's insulation between the windings and the rotor, and there's a second level of insulation between the rotor and the shaft that sticks out the front. So there's a clear demarcation of the double levels of insulation.

Now take the case of some self contained plug in item - perhaps the indoor part of a wireless doorbell. There is still the requirement for two levels of insulation between the user and any live parts - but now there is no exposed conductive part. BUT, if the user is touching the unit, then the expectation is the same - that there needs to be the failure of two levels of insulation in order for the user to get a shock. So the plastic case is one level of insulation - protecting against dangerous touch voltages.

Going back to the drill. As well as di between windings and the metal shaft, there also needs to be di between the stator windings and the user. One level is the winding insulation, the second level is the non-conductive plastic case.

The only rational (and reasonable) reason for the requirement for "double or reinforced insulation) is as an adequately robust (or 'with redundancy') means of preventing someone coming into contact with live parts within the (plastic) enclosure, presumably in the event that 'inadequately robust' insulation may become damaged/break thereby allowing one to touch live parts.

You've missed the obvious situation of the enclosure ceasing to be insulating - perhaps through getting damp. But I suspect the main consideration is for the case to form a second line of defence should the primary insulation fail.

Clearly nothing to do with anything being 'equipotential' with the plastic-enclosed item.
As above, that is, indeed, precisely what I was talking about in the part of the discussion you have picked up on - but not in terms of people "removing covers" but, rather, in the event that inadequately 'reinforced' single insulation became damaged. As above, what else could be the reason for the requirement for "double or reinforced insulation" in a plastic-enclosed item?

See above. The enclosure can be one of the levels of insulation - what is inside can be single insulated.

 

[SimonH2]

I really don't understand what makes you think that I have changed my mind. Putting together the two bits you have highlighted in red, the meaning of what I have written several times is that "The whole point of having earthed exposed-c-ps in a Class I item is to facilitate [i.e. 'make possible'] the operation of a protective device if a 'live' (L not N) conductor comes into contact with the exposed-c-p." - which, as I've said, is only possible with an earth-referenced supply, since it simply doesn't 'work' with a floating supply.

With a floating supply, the supply does not need to be disconnected to avoid a dangerous situation. That makes this point void.
As I've said before, the point of the bonding is to ensure that a dangerous touch voltage cannot occur.

You seem to have a rather worrying degree of faith/trust in what is achieved by 'earthing' (as opposed to bonding, which doesn't, per se, need to be earthed) exposed-c-ps. Consider a Class I item supplied by 2.5/1.5mm² T+E. In the event of a (negligible impedance', aka 'short') fault between L and the CPC/exposed-c-p in that item, the potential of its exposed-c-p will rise to about 62.5% of supply voltage (about 144V with a 230V supply) above MET potential. If there is another nearby Class I item on a different circuit, or even just a nearby earthed pipe/whatever, that exposed-c-p/whatever will still be roughly at MET potential. A 'touch voltage' of about 144V will therefore exist between those two 'simultaneously touchable' items, even though both are 'earthed' (connected to MET via {different} CPCs). The only thing that can minimise that PD is bonding between the two (both 'earthed') items.

Again, I suggest you are letting the tail wag the dog. What you have described is a situation where the bonding is inadequate. If (say) the CPC was 10mm² then the potential of the exposed cp would be about 20% of the supply voltage - i.e. less than 50V.
It would be "undesirable" to allow such a fault current to continue - hence we have OCP to protect the circuit by disconnecting the supply from the fault. IF, and only IF, we have arranged for the supply to automatically disconnect within a very short time, then we are allowed to relax the bonding requirement and tolerate a time limited higher touch voltage.
EDIT: In effect, the regs considerably muddy the waters by closely tying two different functions together - effectively using a combination to allow one of the requirements to be considerably relaxed.
If the ADS was not sufficient to ensure this short time, then the bonding requirements could not be relaxed - so it would certainly be possible to design a circuit such that the ADS took longer than the normal times provided that we increased the "strength" of the bonding to limit the touch voltage to a safe level AND the ADS does protect the cable (adiabatic equation time). That would be a somewhat non-standard setup to say the least.

 

[JohnW2]

No muddling in my mind - I am talking only of bonding. "Earthing" is a special case of bonding where the mass of earth is one of the conductive parts.

I though I had adequately illustrated with my examp0le that what we conventionally call 'earthing' (connecting exposed-c-ps to the CPC of the circuit concerned) does not ('in the real world') go anywhere near providing a safe level of 'bonding' between the exposed-c-ps of items on different circuits (which may be simultaneously touchable - there are certainly plenty of exaples in my house alone). As you go on to say, if the CPC were considerably larger than the live conductors (say a 10mm² or 16mm² CPC in 2.5 mm² T+E), then one could achieve a reasonable degree of bonding between such exposed-c-ps. A "2.5 mm² cable" with a 16mm² CPC would, indeed, provide an acceptably safe level of bonding (a 10mm CPC would be fairly marginal).

However, that is obviously not 'the real world'. Other than for 1mm², there is no size of T+E for which the CPC is even as large as the live conductors, so, in the event of a (L to exposed-c-p) fault, the potential of the affected exposed-c-p will rise to more than 50% of supply voltage above MET potential, whilst exposed-c-ps of items on other circuits will remain roughly at MET potential - hence dangerous 'touch voltages' between the two items.

OK, now I see where you are coming from - and you've made an incorrect (IMO) premise from which to start. The entire point of di Class II items is touch voltages (there is no other reason) - i.e. there's two levels of insulation or one level of reinforced insulation between a user and any live part. It does not mean there has to be an exposed conductive part that a user can touch.

That's what I said. When there are no exposed-c-ps (as is the case with many/most of my Class II items), the requirement for 'double or reinforced insulation' surely must relate to an adequately robust (and/or duplicated) mechanically means of preventing contact with live parts. What else could be its purpose/meaning?

.... So there's a clear demarcation of the double levels of insulation. ..... then the expectation is the same - that there needs to be the failure of two levels of insulation in order for the user to get a shock.

Just to be clear, many (quite probably most) of the Class II items I have (at least, those I have seen inside ) do not have "two levels of insulation" ("double insulation"), but seem to rely on the one 'level of insulation' qualifying as "reinforced" (something which is hard to believe/understand in some cases!).

You've missed the obvious situation of the enclosure ceasing to be insulating - perhaps through getting damp. But I suspect the main consideration is for the case to form a second line of defence should the primary insulation fail. .... See above. The enclosure can be one of the levels of insulation - what is inside can be single insulated.

I didn't 'miss' that, since it was what I was talking about - i.e. the breaking/breakdown in some sense of the insulation, allowing physical or electrical contact with live parts. However, as above the insulation is very commonly not "double" but, rather, single and allegedly "reinforced".

Kind Regards, John