Melting fridge lead whilst shower is on

[JohnW2]

You would think so wouldn't you, but every case I have seen, the customer has said things linke "ah, that's why the lights have been flickering", and things like that.

Interesting. I would have thought that anything approaching 100V L-N drop when one switched on a shower would have a more profound impact than 'flickering lights' - but maybe people are more tolerant to such problems than I would have thought! Apart from anything else, if the shower itself were, say, 8.5kW at 240V, that would fall to around 2.9kW at 140V, which I would have thought would be noticed

Kind Regards, John

 

[Lectrician]

It really depends on the impedance of the N to E fault and the parallel earth path.

Sometimes these things can on un-noticed for years. I was ripping out an off peak cu once and had a sudden arcing then darkness as I cut the earth to the board.

 

[bernardgreen]

The presence of a neutral to earth fault and a low impedance path from fridge body to ground or earth could result in the earth conductor in the lead to the fridge over heating. If the fridge is one with a water supply to an ice maker and the supply is plumbed in copper then there will be a near zero impedance path to earth via the bonding of pipes to the MET. Even so the RCD should have tripped.

 

[JohnW2]

It really depends on the impedance of the N to E fault and the parallel earth path. Sometimes these things can on un-noticed for years.

If you're talking about just the N-E fault then, yes, in the absence of RCDs that can presumably go on unnoticed for years. Hence, as I said, if such a fault pre-dates the appearance of a high impedance supply neutral, the first one knows of it is likely to be when the combination of the two faults shows its face (as is being postulated in this thread).

I was questioning whether it would be likely to happen the other way around (i.e. the high impedance neutral pre-dating the N-E fault), and I still think that unlikley. As I've said, for fault current high enough to melt cables to arise, then return path via the high impedance neutral would have to be appreciably higher (probably at least 'many times higher) than the fault path through N-E fault and 'parallel paths' (or maybe a TN-S earth) to earth. Given the likely values of the latter, the impedance of the high impedance neutral would have to be pretty high (as I suggested, probably at least an ohm or three) - and, in view of the VD that would produce, I find it hard to believe that it would go unnoticed for all that long. ... but maybe I'm wrong!

Kind Regards, John

 

[JohnW2]

The presence of a neutral to earth fault and a low impedance path from fridge body to ground or earth could result in the earth conductor in the lead to the fridge over heating. If the fridge is one with a water supply to an ice maker and the supply is plumbed in copper then there will be a near zero impedance path to earth via the bonding of pipes to the MET. Even so the RCD should have tripped.

If you’re talking about the situation in which there is also a high impedance supply neutral, then that is essentially the scenario we have been discussing.

In the absence of a high-impedance supply neutral, the currents through that fault path will obviously be lower, and whether or not they result in appreciable overheating will depend on the relative impedances of the ‘proper’ neutral return path (from the load concerned) and the path via neutral wiring and hence the fault path.

Kind Regards, John

 

[Lectrician]

I have seen the results of dodgy neutrals a good few times in various different circumstances - none of them nice. ... Never seen an appliance cause this, but it could happen.

I would guess that the situation is only likely to arise if the N-E fault within the installation (wiring or appliance/equipment) pre-dates the appearance of the high supply neutral resistance ...

... to get (as is being postulated in this case) very high currents flowing from the installation's neutral (from loads) through an N-E fault and a low resistance path to earth (bonded extraneous-c-ps, or perhaps a TN-S earth) requires that the impedance of the faulty supply neutral has a considerably higher impedance than that path through the N-E fault. If the supply neutral had had as high a impedance as that prior to appearance of the N-E fault, I would imagine that the considerable voltage drop in the faulty supply neutral would have already brought that problem to the occupier's attention (dim lights and appliance/equipment malfunction etc.).

For example, I would guess that the neutral impedance would usually have to be at least an ohm or two to create the situation we're postulating - so that, in the OPs case, had that fault been present prior to the NE one, switching on the shower would probably have resulted in a drop of 40V - 100V is the installations L-N voltage - which would surely not have gone unnoticed. Is my thinking reasonable?

Kind Regards, John

Yes, but if the fault and parallel path are low impedance, the high impedance neutral is not going to cause any volt drop, as there is another path.

 

[JohnW2]

Yes, but if the fault and parallel path are low impedance, the high impedance neutral is not going to cause any volt drop, as there is another path.

I've acknowledged that such could be the case once both faults are present. What I said was that if the (very) high impedance neutral was present before the N-E fault arose, then I would have expected the large VD it produced (before the N-E fault arose) to have caused it to be 'noticed'.

Kind Regards, John

 

[Lectrician]

Yes, but that goes without saying doesn't it?!?! Which is why i said rare to see on an appliance, rather than fixed wiring. A fridge though is plugged in 24/7.

 

[JohnW2]

...What I said was that if the (very) high impedance neutral was present before the N-E fault arose, then I would have expected the large VD it produced (before the N-E fault arose) to have caused it to be 'noticed'.

Yes, but that goes without saying doesn't it?!?!

Indeed. I certainly would have thought that it ‘goes without saying’, but ....

I would guess that the situation is only likely to arise if the N-E fault within the installation (wiring or appliance/equipment) pre-dates the appearance of the high supply neutral resistance ... Is my thinking reasonable?

You would think so wouldn't you, but every case I have seen, the customer has said things linke "ah, that's why the lights have been flickering", and things like that.

I'm a little confused. I have been saying the same all along. Initially you appeared to be disagreeing, but are now saying that it 'goes without saying'! Am I missing something?

Kind Regards, John

 

[Lectrician]

A low impedance parallel path and N to E fault is unlikely to be reliable and will vary in impedance, causing flicky and dippy lights.

 

[Lectrician]

This seems pointless nit picking.

An installs neutral will not just jump to high resistance, it takes time. Lights will dip and flick before it gets to a stage where it is really high impedance. Sometime during this time the N to E fault could occur.

The N to E fault could also have an impedance causing flicking and dipping.

 

[JohnW2]

This seems pointless nit picking.

I agree. I merely asked whether I was being reasonable in assuming that the sort of problem we're discussing was much more likely if the high-resistance neutral pre-dated the appearance of the N-E fault ... and then the nits started appearing and being picked

Kind Regards, John