table 41.1 max. disconn. times and the extra notes

I'm not deliberately trying to be obtuse or incomprehensible, and neither are the reg writers. However at the risk of muddying the water, I'll try a different attack.


In general  such circuits as this applies to may need to be also be protected by RCD if they supply sockets but  this is for additional protection for high resistance events between live and earth  such as a person between a live wire and terra-firma.

This table 41.1 is not really about that, but about faults inside equipment from a live conductor to the CPC - such as a failure of a winding on a transformer or motor in class I kit, so the windings connect to the earthed core.


Let us only consider 230V AC supplies for now, and table 41.1 collapses to two figures 0.2 seconds for TT, where the maximum shock voltage  on earthed metalwork during fault is assumed to be 230, and 0.4 seconds for TN, where the peak voltage that might appear on exposed metal is perhaps half to two thirds of the supply (for reduced CPC T and E).

Here there is a relaxation on TT ciruts where the likley shock voltage is reduced.

Distribution circuits are something else, because unlike fixed low current or plug in equipment there is an assumption that no-one is holding the CPC during the time that the ADS operates and the fault is cleared.

The figures of 5 seconds or 1 second translate to being some way up the thermal part of the MCB curve,  and are more about protecting things being likely to overheat than shock protection, and at the same time not removing the supply to a bunch of circuits due to a fault at the far end of one of them.

You may wonder what happens to folk holding a portable appliance on a final circuit supplied by such a distribution circuit, but that is not considered. If it was you'd need 0.2 or 0. 4 seconds all the way back to the origin, and there would be no time discrimination possible,

So why 1 second for TT and not 5 - probably again due to the higher exposed voltage, and the greater exposure, in that assuming the electrode dominates, and not the wiring, all the metal work comes live

In contrast in a TN system, generally the cables get thinner towards the fault and therefore, much as the voltage on the live is sloping down towards the fault the voltage on the CPC is sloping upwards,

So would it be safe to relax the 1 second to 5 seconds for TT systems with a very low Zs and or a very light load ? not really, as there is still the larger area of exposed live stuff to consider.


Actually 5 seconds is a really long time to be standing against an almost unlimited fault current on any system and in practice you'd not want that very often, even if it is permitted.

I'm not deliberately trying to be obtuse or incomprehensible, and neither are the reg writers. However at the risk of muddying the water, I'll try a different attack.


In general  such circuits as this applies to may need to be also be protected by RCD if they supply sockets but  this is for additional protection for high resistance events between live and earth  such as a person between a live wire and terra-firma.

This table 41.1 is not really about that, but about faults inside equipment from a live conductor to the CPC - such as a failure of a winding on a transformer or motor in class I kit, so the windings connect to the earthed core.


Let us only consider 230V AC supplies for now, and table 41.1 collapses to two figures 0.2 seconds for TT, where the maximum shock voltage  on earthed metalwork during fault is assumed to be 230, and 0.4 seconds for TN, where the peak voltage that might appear on exposed metal is perhaps half to two thirds of the supply (for reduced CPC T and E).

Here there is a relaxation on TT ciruts where the likley shock voltage is reduced.

Distribution circuits are something else, because unlike fixed low current or plug in equipment there is an assumption that no-one is holding the CPC during the time that the ADS operates and the fault is cleared.

The figures of 5 seconds or 1 second translate to being some way up the thermal part of the MCB curve,  and are more about protecting things being likely to overheat than shock protection, and at the same time not removing the supply to a bunch of circuits due to a fault at the far end of one of them.

You may wonder what happens to folk holding a portable appliance on a final circuit supplied by such a distribution circuit, but that is not considered. If it was you'd need 0.2 or 0. 4 seconds all the way back to the origin, and there would be no time discrimination possible,

So why 1 second for TT and not 5 - probably again due to the higher exposed voltage, and the greater exposure, in that assuming the electrode dominates, and not the wiring, all the metal work comes live

In contrast in a TN system, generally the cables get thinner towards the fault and therefore, much as the voltage on the live is sloping down towards the fault the voltage on the CPC is sloping upwards,

So would it be safe to relax the 1 second to 5 seconds for TT systems with a very low Zs and or a very light load ? not really, as there is still the larger area of exposed live stuff to consider.


Actually 5 seconds is a really long time to be standing against an almost unlimited fault current on any system and in practice you'd not want that very often, even if it is permitted.