Fuseing neutral

[JohnW2]

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.

Indeed, as I wrote ....

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, having said that, I don't think that any measures involving fuses will be of much benefit to people at increased risk from electric shocks.

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 should either have unplugged it or know how to work safely. That's little different to equipment with no internal fusing.

Are you still talking about medical equipment, or generally? If the former, then this discussion has gone even further off-topic relative to the OP, which was about neutral (only) fusing. If the latter, are you suggesting that all electrical equipment should have internal fusing (to provide fault protection)?

Kind Regards, John

 

[SimonH2]

However, having said that, I don't think that any measures involving fuses will be of much benefit to people at increased risk from electric shocks.

I was thinking of the case where someone uses a cord with a 13A fuse fitted, and an IEC extension lead as well, so Zs at the inlet to the equipment could be higher than envisaged. So there's an argument for fusing at the inlet to the minimum level required for the equipment - and thus minimise the fault current/duration for an internal fault, and thus minimise the touch voltage/duration on the equipment case & terminals (and electrodes that might be attached to a vulnerable patient though I hope things like that would be class II at least). Given that the equipment may be sold in places like Europe where supply polarity is variable, and you need to fuse the neutral as well.
I dare say you can't assume RCD protection in every market you sell to either.

 

[JohnW2]

I was thinking of the case where someone uses a cord with a 13A fuse fitted, and an IEC extension lead as well, so Zs at the inlet to the equipment could be higher than envisaged. So there's an argument for fusing at the inlet to the minimum level required for the equipment - and thus minimise the fault current/duration for an internal fault.....

Well, yes, but that argument can be generalised to virtually any electrical equipment. In practice, the vast majority of exposed-c-ps (which need fault protection) are in connected equipment, rather than the fixed wiring (the main exceptions being metal electrical accessories), yet the regs only require Zs to be low enough for required disconnection times at the point of connection to the fixed wiring (socket or whatever). Hence it is always theoretically possible that, although Zs is low enough in the fixed wiring, it will theoretically not be low enough within the equipment - due either to the cabling (between fixed wiring and equipment) or (I would think very rarely) a situation such as plugwash mentioned in which there are components within the equipment which increase the impedance of the earth fault path.

However, such considerations would really only be relevant if the Zs of the (fixed wiring of the) circuit was already pretty marginal and, even then, any credible amount of addition to loop impedance by cabling and internal components would probably be unlikely to make a material difference.

One could, of course, use internal fuses of much lower rating that the circuit's OPD. However, the most that could do would be to reduce the duration of a 'touch voltage' to some extent - it would not affect the magnitude of the fault current or touch voltage prior to operation of the fuse. That's really the basis of my point about fuses not being very relevant to the protection of people with increased susceptibility to electric shock.

(and electrodes that might be attached to a vulnerable patient though I hope things like that would be class II at least)

Wherever possible, such things would usually be optically isolated.

Kind Regards, John