I have heard of anything 5A and above being reportable to the DNO which seems reasonable, however what if you were to get a reading of 4.5A, due to those variations this could be higher taking it beyond the 5A at other points in the day etc.

Diverted N currents can easily be an order of magnitude higher than than in some "healthy" circumstances - it's unavoidable where other metalwork (e.g. water or gas pipes) are in effect connected in parallel with a length of neutral (PEN) conductor. It's less down to whether the installation is connected or not, rather the size of the loads that happen to be running at the time (and their imbalance if polyphase) - the larger the neutral current, the larger the amount that'll flow through parallel paths. That's why for PME installations we end up with such massive main bonding  conductors. In some circumstances, DNO try to mitigate the problem - e.g. in blocks of flats or where there are multiple occupiers in a steel framed building, they'd often these days have a single N-PE link for the entire building and run separate N and PE conductors to each intake - but there are probably still millions of installations out there with parallel paths that offer resistances not significantly higher than the supply N.

There are two sides to this coin though. In normal healthy circumstances, the diverted N current is a bit of a nuisance rather than an immediate danger. A few tens of amps flowing along a low impedance with likely only generate a potential difference of a handful of volts. The much more significant problem comes when the supply PEN conductor goes open-circuit - then that potential few amps (or fractions of an amp) can result in exposed metalwork being held a very hazardous voltages (even when there aren't any parallel paths) - that's really the issue that plagues EV installations.

As for detection of partially failed PEN conductors - measuring the standing current isn't really a direct metric - as the reading can be influences both ways by the size and nature of the loads (in other installations as well as your own) and the presence/absence of parallel paths (e.g. metallic pipework or structural steel)  - a much more direct method would be simply to measure the L-PEN loop impedance - i.e. the normal Ze test. I'm not sure of the nature of the majority of PEN fault - whether it's typically some gradual degradation, or some short event that causes the break - broken overheads and gaping holes being blow in underground cables by short circuits probably fall under the latter, loose connections the former.

   - Andy.

I have heard of anything 5A and above being reportable to the DNO which seems reasonable, however what if you were to get a reading of 4.5A, due to those variations this could be higher taking it beyond the 5A at other points in the day etc.

Diverted N currents can easily be an order of magnitude higher than than in some "healthy" circumstances - it's unavoidable where other metalwork (e.g. water or gas pipes) are in effect connected in parallel with a length of neutral (PEN) conductor. It's less down to whether the installation is connected or not, rather the size of the loads that happen to be running at the time (and their imbalance if polyphase) - the larger the neutral current, the larger the amount that'll flow through parallel paths. That's why for PME installations we end up with such massive main bonding  conductors. In some circumstances, DNO try to mitigate the problem - e.g. in blocks of flats or where there are multiple occupiers in a steel framed building, they'd often these days have a single N-PE link for the entire building and run separate N and PE conductors to each intake - but there are probably still millions of installations out there with parallel paths that offer resistances not significantly higher than the supply N.

There are two sides to this coin though. In normal healthy circumstances, the diverted N current is a bit of a nuisance rather than an immediate danger. A few tens of amps flowing along a low impedance with likely only generate a potential difference of a handful of volts. The much more significant problem comes when the supply PEN conductor goes open-circuit - then that potential few amps (or fractions of an amp) can result in exposed metalwork being held a very hazardous voltages (even when there aren't any parallel paths) - that's really the issue that plagues EV installations.

As for detection of partially failed PEN conductors - measuring the standing current isn't really a direct metric - as the reading can be influences both ways by the size and nature of the loads (in other installations as well as your own) and the presence/absence of parallel paths (e.g. metallic pipework or structural steel)  - a much more direct method would be simply to measure the L-PEN loop impedance - i.e. the normal Ze test. I'm not sure of the nature of the majority of PEN fault - whether it's typically some gradual degradation, or some short event that causes the break - broken overheads and gaping holes being blow in underground cables by short circuits probably fall under the latter, loose connections the former.

   - Andy.