What are the dangers of poor insulation resistance results between neutral and earth?

[HappyHippyDad]

What are the dangers of poor insulation resistance results (i.e <1Mohm) between neutral and earth?

If all other tests are ok, what are the dangers of the above?

My thoughts are it may indicate a breakdown of the insulation somewhere. However, what are the dangers of this breakdown between neutral and earth? They are both at the same potential already.

What are the other dangers? Is there a risk of fire somehow with poor N-E readings? I can't see how (if all other tests are ok). Although, perhaps the poor IR results are due to a poor connection which is heating up the cable, causing the insulation to breakdown.... so perhaps fire is a possibility?

 

[sinewove]

What are the dangers of poor insulation resistance results (i.e <1Mohm) between neutral and earth?

If all other tests are ok, what are the dangers of the above?

My thoughts are it may indicate a breakdown of the insulation somewhere. However, what are the dangers of this breakdown between neutral and earth? They are both at the same potential already.

What are the other dangers? Is there a risk of fire somehow with poor N-E readings? I can't see how (if all other tests are ok). Although, perhaps the poor IR results are due to a poor connection which is heating up the cable, causing the insulation to breakdown.... so perhaps fire is a possibility?

Had a situation where neutral had failed at meter position so current was tracking back thro cpc because of low ir between n and e.Could have caused a fire.
S

 

[brianmoooore]

As above. The danger is a failed neutral, then the cpc starts carrying the current, and, if the cpc is open somewhere, parts of the circuit and connected equipment that should be at earth potential may be raised to a dangerous voltage.
The other 'danger' is now that most circuits are RCD protected, a N-E leak will cause nuisance tripping.

 

[HappyHippyDad]

Had a situation where neutral had failed at meter position so current was tracking back thro cpc because of low ir between n and e.Could have caused a fire.
S

I can see (with your example), how that could potentially cause an electric shock, but not a fire?

 

[HappyHippyDad]

As above. The danger is a failed neutral, then the cpc starts carrying the current, and, if the cpc is open somewhere, parts of the circuit and connected equipment that should be at earth potential may be raised to a dangerous voltage.
The other 'danger' is now that most circuits are RCD protected, a N-E leak will cause nuisance tripping.

So apart from nuisance tripping, you would have to have 3 things for 'danger to be present'?

1. Poor IR results
2. Broken neutral.
3. Open CPC.

 

[Risteard]

I attended a call-out a number of years ago where the neutral meter tail was broken somewhere and the entire return current for the installation was going back through a neutral/Earth fault on a lighting circuit. The cable sheath was literally bubbling from the heat generated. It's a very real fire risk.

 

[westward10]

It depends on what the actual resistance is as zero to 1 meg is a large range. Zero meg or an effective connection between the two conductors may result in the dangers mentioned above but 1 meg is going to have little or no effect.

 

[LastManOnline]

1 meg is going to have little or no effect.

1 Meg is actually allowable. The problem is that it is highly unusual to get a reading like that in the fixed wiring of a standard installation. It's highly likely that there is something connected in circuit that's dragging down the reading. And until you find out what it is, it can be vexing

 

[Lucien Nunes]

The risks arising directly from low (E.g. 0.1-1MΩ) insulation N-E are low to nil. The importance of such a reading, as hinted at above but not explicitly stated, is that it indicates that the wiring is damaged in some way and other hazards might exist that need to be investigated, more serious than the low IR itself. For example, a rodent might have stripped both the line and neutral to bare copper but at the present time only the neutral is in contact with masonry. In which case the low reading alerts and leads you to an unspecified problem, which when investigated, turns out to be high risk.

As mentioned above, hard N-E shorts are different and can cause other more serious problems quite apart from inevitable RCD tripping. With the general use of single-pole OCPD, neutral conductors are not generally protected against overcurrent other than as part of their circuit paired with a line conductor of identical rating. As soon as a main neutral connection or device (e.g. main switch N pole) other than a PEN goes open-circuit, small neutral conductors anywhere in the installation can be seriously overloaded if faulted to earth.