Neutral grounding resistor

This is a function of expected fault type and knowing the fault impedance. The anticipated fault in a HV system is a line to ground fault (line to line is possible, but not so likely) limiting the fault current that can flow reduces the damage done and the otherwise dangerous rise of earth potential at the point the fault occurs.

In turn this means that the maximum current that things like expulsion fuses may have to break is better defined, and makes it very clear what earth impedance for such things of transformer cores on the HV LV transition can be accepted, so that an HV to core and LV secondary fault cannot impress a dangerous voltage on the LV side during the time it takes for the automatic disconnection to kick in and the supply to be cut.

However, it pushes the voltages relative to ground on the on the non-faulty phases. On an HV network,  the insulation is carefully designed such that for example on an 11kV network (lowest common HV in the UK) all the system is designed to be happy with the full 11kV phase to phase voltage from any pair of phases to ground - because if phase 1 is grounded, the NER sees most of the 6kV or so of phase to ground voltage, but the phase to ground voltages become the phase to phase voltages.

In the UK the HV network does not normally distribute the neutral *,  so there are only 3 phase wires and earth to think about, and at the supply end current transformers on the phases and the NE link detect a fault to earth and allow automatic disconnection.


* except trains which are 25KV AC overhead, and for these the tracks are the return, and near enough to ground to be considered neutral-like. But they have their own rule book !


On the LV network, the phase voltage pushing is very undesirable, as a lot of domestic equipment will fail if it sees 400V live to earth,  and in many places there are not separated neutral and earth conductors in the street network anyway, so even if it was desired, the resistor could not be at the substation, it would have to be at the consumer end, in the block under the company fuses where the combined earth neutral are split into two conductors. Given the requirement for massive power dissipation during fault , this would not be practical, and a solid link is preferred - the maximum fault current is usually only a few kA unless you are right adjacent to the substation.


Even on an HV network, NERs are not needed if the fault current can be known and limited in other ways, and in the UK at least it is far from universal practice.

There was a suggestion, in one of the "Cahier Technique" papers I think, to include a resistance between star point and Earth in LV systems - but with the consumer's earth solidly connected to the supply's earth electrode. The big advantage being that during an earth fault all the earthed parts remain at substantially at true earth potential (rather than some being pulled up to a substantial fraction of line voltage as happens with conventional TN and TT systems). The disadvantage is that the earth fault current is small so neither fuses nor (overcurrent type) circuit breakers can be used for providing automatic disconnection - so you need complete reliance on RCDs - which of course is incompatible with most existing consumers.


  - Andy.