Cable sizes

[Stuart981]

Hello chaps

A quick question here if I may about domestic power cables. The cables that run from the network providers fuse to my meter are 10mm diameter for the live and 6.5mm for the neutral as measured with a vernier calliper (not sure what that is in X sectional area). Does that seem normal with different sized cables?

Regards Stuart

 

[flameport]

They should be the same size.
Possibly one is single insulated and the other isn't.

 

[Stuart981]

They should be the same size.
Possibly one is single insulated and the other isn't.

Thanks for that. If they are of the same cable standard but just different size then is this something I need to be concerned about.

Regards Stuart

 

[JohnW2]

Thanks for that. If they are of the same cable standard but just different size then is this something I need to be concerned about.

Although they certainly should be the same size, if only in the interests of 'neatness', all that actually matters, electrically, is that the smaller of the two is at least large enough (to carry the current concerned, and to be adequately protected by the DNO's fuse) - but if the neutral only has an outside diameter of 6.5mm, I would not be at all sure that such is the case.

As has been said, the neutral may be smaller because it has only one (rather than the required two) layers of insulation - but that would also be a theoretical 'problem'.

Kind Regards, John

 

[plugwash]

that would also be a theoretical 'problem'.

It would be a problem if the wire was being judged according to BS7671, but afaict the electricity supply industry doesn't work to BS7671.

From a first principles safety perspective it appears to me that a single insulated neutral conductor would require a double fault (failure of the basic insulation, plus an open circuit allowing a dangerous voltage to develop on the neutral) to become dangersou.

 

[JohnW2]

It would be a problem if the wire was being judged according to BS7671, but afaict the electricity supply industry doesn't work to BS7671.

All true.

From a first principles safety perspective it appears to me that a single insulated neutral conductor would require a double fault (failure of the basic insulation, plus an open circuit allowing a dangerous voltage to develop on the neutral) to become dangersou.

Again, agreed. In fact, if we only wanted protection against a single-fault situation, I suppose that there would be no reason why a neutral had to have any insulation at all

In any event, even if BS7671 requires it, I'm not at all sure about the value of 'double insulation' (insulation and sheathing) of conductors in cables. The risks all really relate to mechanical (or thermal/chemical) damage to the insulating layer(s) surrounding the conductor, and it's probably not much less likely that the damage will extend through both layers as it is through one. In any event, if the inner layer of insulation is not considered 'adequate', why not allow the option of 'thicker' and/or 'stronger' insulation (a bit like the option of "reinforced", rather than "double" insulation for a Class II item) - I really don't see what's magic about it being split into 'two layers'.

Kind Regards, John

 

[plugwash]

Again, agreed. In fact, if we only wanted protection against a single-fault situation, I suppose that there would be no reason why a neutral had to have any insulation at all

A disconnected neutral will become live, so assuming you treat a disconnected conductor as a "single fault" then a bare neutral conductor would become dangerous to touch under single fault conditions

Of course that brings us to the whole discussion of PEN conductors and the associated risks..........

 

[plugwash]

In any event, even if BS7671 requires it, I'm not at all sure about the value of 'double insulation' (insulation and sheathing) of conductors in cables. The risks all really relate to mechanical (or thermal/chemical) damage to the insulating layer(s) surrounding the conductor, and it's probably not much less likely that the damage will extend through both layers as it is through one.

I see several ways insulation can fail.

1. Manufacturing defects. A misalignment in an extruder or similar. If the two layers are extruded in seperate processes, then these are less likely to affect both layers.
2. Hardening and then cracking due to age. This was an issue with old rubber cables (and i've seen someone who works in the rail industry comment that more modern synthetic rubber isn't necessarily immune), it seems to be much less of an issue with PVC. Having two layers means it's likely that the cracks will be in different places giving at least some protection.
3. Damage due to external influences, either mechanical or chemical. In terms of resisting the damage total thickness is probablly more important than number of layers, but having multiple layers of different colors does make the damage more visually obvious (I'm not a fan of the meter tails with blue and brown sheaths for this reason).

 

[JohnW2]

A disconnected neutral will become live, so assuming you treat a disconnected conductor as a "single fault" then a bare neutral conductor would become dangerous to touch under single fault conditions

Yes, that's true, and not something I had considered - but my comment obviously was totally 'tongue-in-cheek (complete with smilie)

Kind Regards, John

 

[JohnW2]

I see several ways insulation can fail. 1. Manufacturing defects. ....... 2. Hardening and then cracking due to age. .... Having two layers means it's likely that the cracks will be in different places giving at least some protection.

Yes, and those two are conceptually fairly similar, at least in the sense that defects, whether due to manufacturing or deterioration, are unlikley to be 'in the same place in both layers. However, (2) is unlikely to be an issue with PVC/PVC cables, particularly for the inner layer, which is not exposed to UV/'elements'.

3. Damage due to external influences, either mechanical or chemical. In terms of resisting the damage total thickness is probablly more important than number of layers,

That was obviously what I was thinking.

but having multiple layers of different colors does make the damage more visually obvious (I'm not a fan of the meter tails with blue and brown sheaths for this reason).

That's true, but 'meter tails' (usually 10mm² - 25mm²) are the only common manifestations of 'double insulated' singles and, as you imply, ones with both layers the same colour seem to be becoming more common. However, again, with PVC/PVC this is probably only relevant to your (1), since 'deterioration' is particularly not likely to affect the inner PVC layer and damage from external causes could be addressed equally with a thicker/'stronger' single layer.

Kind Regards, John

 

[Risteard]

Although they certainly should be the same size, if only in the interests of 'neatness', all that actually matters, electrically, is that the smaller of the two is at least large enough (to carry the current concerned, and to be adequately protected by the DNO's fuse) - but if the neutral only has an outside diameter of 6.5mm, I would not be at all sure that such is the case.

As has been said, the neutral may be smaller because it has only one (rather than the required two) layers of insulation - but that would also be a theoretical 'problem'.

Kind Regards, John

Not just in the "interests of neatness". The neutral conductor must be at least the same cross-sectional area as the phase conductor(s). Technically the phase(s) could be smaller though, assuming they are not undersized.

 

[EFLImpudence]

The neutral conductor must be at least the same cross-sectional area as the phase conductor(s).

Unless the Line conductor is bigger than required.

 

[Risteard]

Unless the Line conductor is bigger than required.

Not without the neutral conductor having overcurrent protection which disconnects the associated line (phase) conductor(s).

 

[JohnW2]

Not just in the "interests of neatness". The neutral conductor must be at least the same cross-sectional area as the phase conductor(s).

Why, if the L conductor is over-sized?

 

[JohnW2]

Not without the neutral conductor having overcurrent protection which disconnects the associated line (phase) conductor(s).

I think the assumption has to be that the supplier (or 'meter operator') who installed the cables between cutout and meter was satisfied that (per whatever regs or conventions they work to) both conductors were adequately protected by the DNO fuse (even if the L conductor was 'over-=protected').