Fuseing neutral

Surely if such products are made with no guarantee which country they will be used in then they must be the same products which are available in the UK and people would have seen them - with their two fuses -

or such things do not exist and there is no internal fusing for the live to externally touchable metal fault therfore it doesn't matter which way the plug is inserted (and vice versa). As in boilers, washing machines etc., for example, the internal fuses ony protect the circuit boards.

EFLImpudence said:

Surely if such products are made with no guarantee which country they will be used in then they must be the same products which are available in the UK and people would have seen them - with their two fuses -

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Quite so. That's why I expressed surprise at my not recalling ever having seen such 2-pole fusing in anything.

EFLImpudence said:

or such things do not exist and there is no internal fusing for the live to externally touchable metal fault therfore it doesn't matter which way the plug is inserted (and vice versa). As in boilers, washing machines etc., for example, the internal fuses ony protect the circuit boards.

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Well, I've certainly seen (single, L) fuses in some things, and I'm sure that must include some with externally touchable (earthed) metal - and such a fuse presumably would operate in the case of those touchable parts becoming live, but one surely could not argue that, despite that, such was not the intended purpose of the fuse, since it was only there to protect circuit boards?

Having said that, at least in the UK (and I imagine also elsewhere) an internal fuse with that 'intended purpose' really does not need to exist, since the circuit itself should have adequate fault protection to provide such functionality - so, as you suggest, maybe items of equipment containing fuses "designed to have that purpose" simply don't exist?

Kind Regards, John

JohnW2 said:

Well, I've certainly seen (single, L) fuses in some things, and I'm sure that must include some with externally touchable (earthed) metal - and such a fuse presumably would operate in the case of those touchable parts becoming live, but one surely could not argue that, despite that, such was not the intended purpose of the fuse, since it was only there to protect circuit boards?

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I've just looked at the manual for the boiler I had in the UK and there are two fuses on the circuit board: a 1.6A and a 630mA in different locations so presumably they protect different parts on the board.

It seems odd to me that these would provide normal earth fault protection for the whole boiler.
If they do then there is definitely no reason for the ubiquitous 3A fuse on the supply cable.

I can't make up my mind if that is more or less likely bearing in mind that it might only have a 16A MCB in Europe and reversed polarity.

EFLImpudence said:

If they do then there is definitely no reason for the ubiquitous 3A fuse on the supply cable.

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The 3 amp fuse in the supply cable "protects" the whole unit. The fuses on the PCB may not protect all the items in the the unit.

In some countries where boilers are allowed to be connected with random polarity into 16 amp fused sockets the "regulations" seem to rely on any electrical fire in the boiler being contained sufficiently to prevent the building catching fire.

The fuse in the plug is one that a user is more capable of changing, many users would happily change one but would not dare open up a unit to change a fuse on the pcb.

I thought euro stuff had double pole appliance switches and that’s all.

How often does a fuse blow anyway? Isn’t it to protect the cable?

Fuses on boiler pcbs are for fans and pumps to try and prevent the PCB tracks being damaged under fault conditions.

EFLImpudence said:

I've just looked at the manual for the boiler I had in the UK and there are two fuses on the circuit board: a 1.6A and a 630mA in different locations so presumably they protect different parts on the board.

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Are either (or both) of those fuses actually in the 'mains' supply path, rather than in ELV circuitry?

EFLImpudence said:

It seems odd to me that these would provide normal earth fault protection for the whole boiler.

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Per my question above, the answer to that presumably depends entirely upon where the fuses 'are' within the circuitry.

Kind Regards, John

JohnW2 said:

Are either (or both) of those fuses actually in the 'mains' supply path, rather than in ELV circuitry?

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No.

Per my question above, the answer to that presumably depends entirely upon where the fuses 'are' within the circuitry.

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Others appear to agree with me.

To try to answer a few questions... We used to manufacture a product which we only ever imagined would be sold in the UK, and we hadn't even considered the consequences of selling it elsewhere.
It was a small piece of laboratory (mostly home laboratory) equipment for computer enthusiasts. In those days it was more normal to use the mains as a supply and to provide within the equipment any safety features which seemed appropriate to cover one's backside. So as our equipment had a metal case, we decided to earth the metal case, and to use an internal fuse in the live from the mains to eliminate the possibility of overheating etc in the unlikely event that the live conductor from the mains should break off internally and move over and touch the casing insides.
After a while H&S began to become more prominent, and we joined the BSI and bought the British Standards which seemed appropriate to our product. It was a long time ago and I don't remember which standards these were. However things like 4mm gaps, and protection against issues involving a single fault were involved.
Around this time we started to sell our product in other countries; Norway and Germany being the first.
It dawned on us one day that in Germany we had no guarantee that the red or brown wire of the mains cable would be the live conductor, so we reasoned that if we considered it necessary to protect the live against the unlikely possibility of the wire coming free, then we had exactly the same responsibility with the neutral (as it may actually be the live). This was simply logic and not specifically mentioned in the Standards we had bought. However the Standards did have a section on double pole fusing, simply saying that if this was the case, there had to be a visible warning for service engineers, which we duly provided.
I do not believe that any of either of these fuses ever operated, and it could be considered that they were purely precautionary so we could sleep easy. So the logic was that if you had one fuse in the mains input then it was totally illogical not to have two.

I am not sure that saying that one has never seen double pole fusing is very meaningful. I would doubt that any of our customers ever knew this was the case as they would not usually have needed to open the box. Fuses at any other location will be there for other designer purposes, so not related to the double pole fusing issue.

These days, of course, we do not use mains in any of our equipment. We simply buy in power adapters which already have the necessary certification.

EFLImpudence said:

No.

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EFLImpudence said:

Others appear to agree with me.

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Well, given that your first answer is 'no' they are obviously right to agree with you. An internal fuse (in the L) could only "... provide normal earth fault protection for the whole boiler ..." if the fuse was not only in the 'mains' (LV) circuitry but also as close as practicable to where that 'mains' entered the item.

In any other situation, the internal fuse would (if in LV circuitry), at best, be providing only 'partial' earth fault protection and, if it were in ELV circuitry, no such protection at all.

Kind Regards, John

Isn't that what I have been saying?

EFLImpudence said:

Isn't that what I have been saying?

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I think it is - and don't believe that I have disagreed.

I think that, as I've previously suggested, it probably comes down to the fact that, although any such equipment (if it existed) presumably would need 2-pole internal fusing if the supply could be reversed, I suspect that, in this day and age, an item "... which is considered to require internal fusing designed to operate if a live accidentally connects to the externally touchable earthed metal of the item" (as mentioned by Detlef) probably does effectively not exist, since most/all countries in which it was used would probably already require that the supply circuit afforded that protection (so no need for additional internal fusing).

Kind Regards, John

DetlefSchmitz said:

To try to answer a few questions...

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Thanks for the detailed clarification.

DetlefSchmitz said:

... In those days it was more normal to use the mains as a supply and to provide within the equipment any safety features which seemed appropriate to cover one's backside. So ...

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Is this not what has changed? The situation now is surely that the expectation (and requirement) is that the 'mains supply' itself should afford adequate earth fault protection to Class I items, such that internal fusing withing the equipment to achieve the same thing is not required ("not considered to be required").

In other words, to repeat what I've just written to EFLI ... it probably comes down to the fact that, although any such equipment (if it existed) presumably would need 2-pole internal fusing if the supply could be reversed, I suspect that, in this day and age, an item "... which is considered to require internal fusing designed to operate if a live accidentally connects to the externally touchable earthed metal of the item" (as mentioned by Detlef) probably effectively does not exist.

DetlefSchmitz said:

I am not sure that saying that one has never seen double pole fusing is very meaningful. I would doubt that any of our customers ever knew this was the case as they would not usually have needed to open the box.

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Although it obviously proves nothing, I was talking about my personal experiences, not your customers' - and, for what it's worth, my personal experience, having 'opened the boxes' of an awful lot of electrical and electronic items over the years, is that I cannot recall ever having seen DP fusing - whereas, had there been times when that DP fusing had been a "must" for products which might be sold (or merely used) outside of the UK, I would have expected to have seen it at least occasionally.

Kind Regards, John

JohnW2 said:

I think that, as I've previously suggested, it probably comes down to the fact that, although any such equipment (if it existed) presumably would need 2-pole internal fusing if the supply could be reversed, I suspect that, in this day and age, an item "... which is considered to require internal fusing designed to operate if a live accidentally connects to the externally touchable earthed metal of the item" (as mentioned by Detlef) probably does effectively not exist, since most/all countries in which it was used would probably already require that the supply circuit afforded that protection (so no need for additional internal fusing).

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I recall a post (can't remember where, may have been piclist or electronics stack exchange) about design of a product (I think it was a medical device, not 100% sure) where they were having conducted EMC susceptibility issues with the capacitive touchscreen. To fix this they installed a common mode choke on all the supply conductors (including earth). However this added substantial extra resistance to the earth path, so they installed DP fusing to ensure adequate disconnection of earth faults.

I think some manufacturers, particularly of medical equipment also install DP fusing as part of a "belt and braces" attitude. On the other hand manufacturers of low-margin consumer gear are going to install the minimum amount of fusing they think they can get away with.

DP fusing is certainly common enough that DP fused IEC inlets are an off the shelf item.

didn't know you could get DP Fused IEC inlets