Isolation Transformer - A discussion

[JohnW2]

Just to be sure: is there a typo?

Yep - "to" should be "no". Now corrected. Thanks.

Kind Regards, John

 

[EFLImpudence]

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

Surely, the fact that it is deemed Class I is only relevant when the supply is such that the Class I precautions need to be considered.

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.

You might as well say the same if the so-called Class I item were supplied by a 12V supply.

The exposed metal-work is irrelevant.

 

[JohnW2]

Surely, the fact that it is deemed Class I is only relevant when the supply is such that the Class I precautions need to be considered.

Well, as I've said, the "Class I precautions", per se, appear to be useless when the supply is floiating.

However, the fact that something is classified as Class I presumably means that it is not considered that the insulation (basic, plus any 'additional' which exists) does not afford adequate protection against touching live parts within the item (i.e. it doesn't qualify as Class II). If that is true when the supply is earth-referenced, I presume that the insulation is also regarded as providing inadequate protection against touching (both of) the live parts when the supply is floating?

This really brings me back to a question that I must have asked dozens of times, without getting an answer - if an item does not have "double or reinforced insulation" (hence cannot be Class II) and also has no exposed-c-ps (hence cannot be Class I)m, then "What is it?" (maybe 'not allowed'?).

You might as well say the same if the so-called Class I item were supplied by a 12V supply.

I haven't looked recently, but seem to recall that the regs are a little silly in not making exceptions for ELV in relation to these matters.

Remember that most of these recent exchanges resulted from Sunray expressing 'shock' when I suggested that if a supply is (and remains) 'fully floating', that nothing is then achieved by earthing the exposed-c-p of a Class I item connected to that supply. Do you disagree with that?

Kind Regards, John

 

[plugwash]

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

Yes, generally the idea is that an appliance should be safe under "single fault conditions". Failure of basic insulation would be considered a "single fault".

But a floating supply (of sufficiently low leakage) is single-fault safe by design.

(b) is irrelevant

bonding all the metalwork in the vicinity of a supply together with bonds that have sufficiently low impedance (ideally significantly less than the source impedance of the supply) will limit the touch voltages that can develop between those parts.

With a floating supply this only really becomes relavent when you consider multiple fault conditions.

 

[EFLImpudence]

This really brings me back to a question that I must have asked dozens of times, without getting an answer - if an item does not have "double or reinforced insulation" (hence cannot be Class II) and also has no exposed-c-ps (hence cannot be Class I)m, then "What is it?" (maybe 'not allowed'?).

No one knows; it is like a plastic switch or socket - nothing?

Remember that most of these recent exchanges resulted from Sunray expressing 'shock' when I suggested that if a supply is (and remains) 'fully floating', that nothing is then achieved by earthing the exposed-c-p of a Class I item connected to that supply. Do you disagree with that?

I do not disagree. On the contrary, I do not think it should be 'earthed'

Hence, in reply to others, I do not think the item's metal parts should be 'bonded'.

 

[ericmark]

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

Same reason why yellow sockets are marked line and neutral not line 1 and line 2.

 

[plugwash]

This really brings me back to a question that I must have asked dozens of times, without getting an answer - if an item does not have "double or reinforced insulation" (hence cannot be Class II) and also has no exposed-c-ps (hence cannot be Class I)m, then "What is it?" (maybe 'not allowed'?).

I haven't read the standards for commercial grade appliances (I have read the medical standard IEC 60601 but it was quite some time ago) but my understanding is that the basic idea is.

Class 0: protection by basic insulation only, not single fault safe, generally not allowed anymore.
Class 1: protection by basic insulation in combination with metalwork connected to a protective conductor.
Class 2: protected by double (basic plus supplementary) or reinforced (a single layer of insulation that provides protection equivalent protection to basic and supplementary insulation combined)

I don't think it matters whether the basic insulation is directly exposed to touch, is covered by non-earthed metalwork or is covered by a layer of insulating material that is electrically or mechanically inadequate to count as supplementary insulation it would still be class 0.

Also double or reinforced insulation isn't the be-all and end-all of class 2, there is still a need to assess the risk of faults. For example a wire coming out of a terminal would reasonably be considered a single fault condition. This is why when you look at a class 2 metal light fitting you will see cord grips on the terminal box.

An earthed metal part can be part of the protection strategy even if it's not directly exposed to touch. There are many DC power bricks out there that have floating outputs and plastic cases but are nevertheless class 1.

Electrical accessories are not designed to IEC appliance standards and hence won't be assigned an appliance class. In practice though I would say a plastic accessory competently installed does provide similar protection to a class 2 appliance.

 

[JohnW2]

Yes, generally the idea is that an appliance should be safe under "single fault conditions". Failure of basic insulation would be considered a "single fault".

Indeed.

But a floating supply (of sufficiently low leakage) is single-fault safe by design.

Are you suggesting that a failure of the insulation which allows ones to touch both sides of a a floating supply (e.g. both terminals of a lampholder) constitutes more than a single fault?

bonding all the metalwork in the vicinity of a supply together with bonds that have sufficiently low impedance (ideally significantly less than the source impedance of the supply) will limit the touch voltages that can develop between those parts.

Indeed, but we're not talking about bonding different bits of metalwork to one another but, rather, of bonding the exposed-c-p of one (Class I) item to earth. That will therefore only limit the touch voltage between that exposed-c-p and 'something earthed' - which really cannot arise (with low enough path impedance to be important) when the supply is, and remains, floating.

With a floating supply this only really becomes relavent when you consider multiple fault conditions.

Quite so - and, as I have said, I think a problem could only arise if one of the multiple faults had the effect of rendering the supply no longer floating.

Kind Regards, John

 

[JohnW2]

I haven't read the standards for commercial grade appliances (I have read the medical standard IEC 60601 but it was quite some time ago) ...

Fair enough, but I was thiunkiung primarily about domestic / consumer products, anyway.

but my understanding is that the basic idea is.
Class 0: protection by basic insulation only, not single fault safe, generally not allowed anymore.
Class 1: protection by basic insulation in combination with metalwork connected to a protective conductor.
Class 2: protected by double (basic plus supplementary) or reinforced (a single layer of insulation that provides protection equivalent protection to basic and supplementary insulation combined)

That is also my understanding.

So, as I said, if Class 0 is generally not allowed and an item has no metalwork that can be connected to CPCs (so can't be Class I) and is not protected by double/reinforced insulation (so can't be Class II), then "What is it?" Indeed, is it 'allowed'?

An earthed metal part can be part of the protection strategy even if it's not directly exposed to touch.

Indeed - but, as above, what if there is no metal (to earth) at all - it surely can't them be regarded as Class I?

Electrical accessories are not designed to IEC appliance standards and hence won't be assigned an appliance class. In practice though I would say a plastic accessory competently installed does provide similar protection to a class 2 appliance.

You pre-empted a comment I was about to make - namely that plastic accessories obviously cannot be Class I and do not bear a Class II marking, again leading to the "What are they?" question. However, as you say, one assumes that they do have what would, in other contexts, be regarded as 'reinforced insulation' so are conceptually 'Class II', even if the bureaucracy is such that they are no so marked.

Kind Regards, John

 

[JohnW2]

I do not disagree. On the contrary, I do not think it should be 'earthed' ... Hence, in reply to others, I do not think the item's metal parts should be 'bonded'.

Nor do I, but it is that suggestion that led Sunray to be shocked that I was suggesting one should ignore the MIs which said that the Class I item "must be earthed".

However, as you will have seen, I then thought a little more, and wondered whether this Class I item (which I had not earthed), which presumably did not have adequate 'double/reinforced' insulation to qualify it as Class II would theoretically be regarded as 'safe' to use with a floating supply at all.

Kind Regards, John

 

[EFLImpudence]

Nor do I, but it is that suggestion that led Sunray to be shocked that I was suggesting one should ignore the MIs which said that the Class I item "must be earthed".

Yes, but surely, for the isolated supply there is no earth.
That is, no path back to the supply either through the wiring or the planet.
Isn't that the point and purpose of isolated supplies?

However, as you will have seen, I then thought a little more, and wondered whether this Class I item (which I had not earthed), which presumably did not have adequate 'double/reinforced' insulation to qualify it as Class II would theoretically be regarded as 'safe' to use with a floating supply at all.

Well, as you have said, this supposed 'earth' would not have achieve the function of a CPC.

 

[JohnW2]

Yes, but surely, for the isolated supply there is no earth. That is, no path back to the supply either through the wiring or the planet. Isn't that the point and purpose of isolated supplies?

Quite so - why not see if you can alleviate some of Sunray's 'shock' at the suggestion that one might ignore the "must be earthed" instruction in MIs when the supply is floating

Kind Regards, John

 

[EFLImpudence]

If one had a 240V battery (or generator I suppose) running a single appliance then surely connecting the appliance metal case to the house supply earth would only introduce hazards to the appliance and user?

 

[SUNRAY]

If one had a 240V battery (or generator I suppose) running a single appliance then surely connecting the appliance metal case to the house supply earth would only introduce hazards to the appliance and user?

What hazards?

 

[EFLImpudence]

The same hazards as earthing any other part which does not have to be earthed.