Fuseing neutral

[JohnW2]

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 don't fully understand that.

I can understand that if things within the equipment increase the earth loop impedance (as seen from within the equipment) to a level which means that the circuit's OPD does not provide adequate fault protection, that one would need an internal OPD (fuse or whatever) within the equipment with a low enough rating to achieve the desired level of fault protection. However, I don't see why that, per se, would generate a need to fuse the neutral - so, provided that the polarity of the supply could not be reversed, I don't see why that would result in a need for DP fusing.

I think some manufacturers, particularly of medical equipment also install DP fusing as part of a "belt and braces" attitude.

Medical equipment is, of course, a fairly special case, in which one often sees safety-related measures which go beyond what one would see in other equipment. However (again assuming that the supply polarity can't be reversed), I can't see why DP fusing would represent a "belt and braces" approach to safety - in fact, under certain complicated fault conditions, possibly even the converse.

Kind Regards, John

 

[JohnW2]

didn't know you could get DP Fused IEC inlets

Nor did I - but that doesn't prove much

Kind Regards, John

 

[plugwash]

In single phase portable appliance design the assumption is the supply polarity CAN be reversed.

 

[bernardgreen]

I had a text book for electrics published in the 1940's which I recall mentioned fusing both sides of the supply as it could not be assured which wire would be live. Before the national grid came into being many power stations were operated by local councils or companies generator were (nominal) 240 volts and connected directly to the street mains, no transformers involved. I also recall mention of supplies where the generator ouput was centre tapped to ground, 120 to ground being safer than 240 to ground but required fuses in both supply wires.

I cannot find the book, fear it went in one of the clear outs..

 

[JohnW2]

In single phase portable appliance design the assumption is the supply polarity CAN be reversed.

Hmmm - OK. If one is going to make that assumption then, yes, in the sort of situation you have described, one would need DP fusing within the equipment.

However, I think that that situation is probably a pretty rare one - I would have thought that it will nearly always be the case that the circuit's fault protection would afford adequate fault protection for the equipment, hence no need (from the POV of fault protection) for internal fusing (SP or DP) of the equipment.

Kind Regards, John

 

[EFLImpudence]

Yes.

There seems to be a lot of confusion where possible reversed polarity of the equipment is taken to also mean reversed polarity of the supply.

In single phase portable appliance design the assumption is the supply polarity CAN be reversed.

Did you mean to say the supply polarity?

 

[plugwash]

I mean the polarity of the supply entering the equipment, whether that reversal is caused by a non-polarised plug, a wiring system that doesn't care about polarity or some other reason is irrelevent to the equipment designer.

 

[EFLImpudence]

I mean the polarity of the supply entering the equipment, whether that reversal is caused by a non-polarised plug, a wiring system that doesn't care about polarity or some other reason is irrelevent to the equipment designer.

Yes but that is not reversed polarity of the supply where the OPD will/should always be in the line conductor regardless of which way the plug is inserted.

Such equipment should/will be designed so that it can cope with the characteristics of the OPD.

 

[JohnW2]

Yes but that is not reversed polarity of the supply where the OPD will/should always be in the line conductor regardless of which way the plug is inserted.

Indeed. As I've said, the fault protection of the circuit will presumably nearly always be adequate to provide adequate fault protection for the equipment, but ...

.... Such equipment should/will be designed so that it can cope with the characteristics of the OPD.

Plugwash has given an (I imagine very rare) example of a situation in which the circuit's fault protection may not be adequate within the equipment - i.e. if the EFLI as measured within the equipment is greater than the EFLI of the supply entering the equipment to the extent that the circuit's OPD does not provide adequate fault protection for faults within the equipment.

Kind Regards, John

 

[John D v2.0]

We seem to have moved off the original post into double pole fusing. This change in topic negates the serious objection of having no protection against l-e faults, so the only remaining objection is that someone might assume the device is isolated when it's only off due to a blown fuse.
This does sound like an undesirable situation but still doesn't sound catastrophic as long as the neutral is correctly insulated until proved dead and all isolators including fuse pulling are designed to make it obvious all the poles are together.

 

[WhydidIstart]

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.

I'm aware of an industrial controller that has an IEC C14 inlet (standard 3-pin style) which uses DP fusing. The rationale is that the supply polarity cannot be guaranteed and the equipment may also be used with generator and /or UPS systems which sometimes operate with centre tapped Earthing and hence DP fusing is considered to provide additional protection.

Relevant standards are Low Voltage Directive and EN/IEC 61010, with which the equipment complies. I can’t say if these mandate DP fusing, but I can say they don’t rule it out.

 

[EFLImpudence]

John

Well, he did say that this was because of modification to the equipment, and

it was a medical situation which could have been installed to meet whatever precautions were required.



As for the EFLI, what has been said might be correct - in the worst case scenarion of the regulations.
I.e. EFLI at cable temperature of 70°, Cmin these days, Ia is 5x and not lower.
Anyway, as said, in a medical situation it can be put right.

 

[JohnW2]

We seem to have moved off the original post into double pole fusing.

We do.

This change in topic negates the serious objection of having no protection against l-e faults, so the only remaining objection is that someone might assume the device is isolated when it's only off due to a blown fuse. This does sound like an undesirable situation but still doesn't sound catastrophic as long as the neutral is correctly insulated until proved dead and all isolators including fuse pulling are designed to make it obvious all the poles are together.

Agreed.

It could, of course, be 'catastrophic' for someone who did not adopt the appropriate procedures and safety precautions, but I agree that there is a limit to how far one can/should go to address such eventualities.

Kind Regards, John

 

[JohnW2]

John ... Well, he did say that this was because of modification to the equipment ...

I read it as design, rather than 'modification' - but, whichever, it is components within the equipment which are giving rise to the issue.

As for the EFLI, what has been said might be correct - in the worst case scenarion of the regulations. I.e. EFLI at cable temperature of 70°, Cmin these days, Ia is 5x and not lower.

Sure, but we always have to consider the 'worst case' scenarios. If, as required, the fault protection of the circuit only just achieves adequate protection under 'worst case' conditions then, if there are additions to the EFLI within the equipment, then the EFLI from within the equipment would fail to achieve the requirements under those same 'worst case' conditions.

However, as I've said, I imagine that this (at least, to the extent of making any significant difference) is probably a very rare situation.

Kind Regards, John

 

[SimonH2]

In the case of medical equipment, you have to consider it's manner of use. likeky to be used in conditions where someone is more susceptible to shocks than normal.
And with IEC inlets, you have to assume that someone will eventually use a different cable with a 13A fuse in the plug. Also, I've observed that it seems to be common practice to be using long leads and/or IEC extension leads.
So it makes perfect sense to provide local fusing, and since in many markets (as already said) polarity is flexible, that means DP fusing. Internal to equipment I see little problems with that - anyone working inside it shoukd either have unplugged it or know how to work safely. That's little different to equipment with no internal fusing.