Note though that in this instance, if leads are run outside the unit, and damaged such that a line conductor becomes connected to Earth (the ground - or something metal in good contact with the ground), then the frame/chassis/container of the unit becomes LIVE, and no protective device would operate

You can get a very similar effect with a normal (e.g. DNO) supply too - a downed overhead line conductor in a puddle or just an uncleared L-PE fault in a TT installation - current flows to true Earth and returns through the generator/transformer's electrode - the current * electrode resistance producing a voltage difference between the general mass of the earth and the system's PE reference ... which is then exported to all connected TN installations.

   - Andy.

So in this case the 20 ohms, 200 ohms, or 2000 ohms path back to the genny is irrelevant surely? only a good contact with earth? it'd give an earth reference, its not a fault path.....

It's not a fault path for a conventional ADS style L-PE fault, correct. In an additional protection kind of situation (e.g. picking up the end of a severed flex while stood outdoors) it does though. Likewise for the (rare) situations where a line conductor come into contact with true Earth without an PE conductor intervening. On larger systems you start to see stray capacitance and general leakage currents due to imperfect insulation having similar effects.

    - Andy.

AJJewsbury said:

which is then exported to all connected TN installations.

Yes, however in general those installations have equipotential bonding and fortuitous local earthing, so the effects are generally mitigated or limited until the condition is addressed (but OK there is a small chance of an issue).

The big difference with the mobile/transportable unit fault discussed, is that it's immediately a potentially fatal shock risk, which will go undetected (and won't operate any protective devices) until the moment someone gets a shock ... and even then unless you have properly co-ordinated RCD protection, a protective device won't operate.

Likewise for the (rare) situations where a line conductor come into contact with true Earth without an PE conductor intervening.

Not so rare. That is your expensive Ho7 style  rubber 3 phase 100A extension lead  being thrown over a metal fence and snagging on a sharp edge, and striking a phase core first, or the free socket on the far end going 'splosh' as it lands on the other side .

While in a perfect world this would not occur, it can and does happen when things are being deployed in a hurry and / or  in bad light.

Something like SY or CY cable that actually had a low enough resistance braid to operate a large MCB would help with the snagging, but the flooding has no easy fix. If your genset has sockets for leads for unknown loads to plug into, it needs an electrode, an  NE bond and an RCD.

Yes, however in general those installations have equipotential bonding and fortuitous local earthing, so the effects are generally mitigated or limited

Until you get to the outside tap or EV situation...

   - Andy.

Thank you for the replies all..... appreciate it.

AJJewsbury said:

Until you get to the outside tap or EV situation...

At the moment, we only have evidence that this is genuinely problematic on a widespread basis for open-PEN situations, not particularly the situation you describe (it could be related to both frequency of occurrence and also the fact that effects of one are more readily noticeable than the other).

Yes, there may be an elevated voltage in those situations, but there don't seem to be the same number or severity of instances as open-PEN. And even then I'm not aware of any deaths.

However, compared with the "floating system with protective conductors connected to neutral point" ... whilst the damaged cable might appear to be rare, the consequences are very serious (and have happened) ... definitely a real problem with the possibility of instant death in a muddy field or on a wet construction site.

AJJewsbury said:

Obviously the change-over switch should switch all live conductors including N, so that the generator's N-PE link isn't in-circuit when running off the grid.

Andy
Could you direct me to the relevant legislation regarding the 'obviously' bit.
I see many 2 pole single phase COS's, switched line and neutral but for 3 phase we seem to have what is described as 3 pole and neutral type, presumably a bolted neutral terminal and 4 pole type where we would be switching the neutral as a matter of course

Could you direct me to the relevant legislation regarding the 'obviously' bit.

Generally BS 7671 reg 444.4.6 - the system shall be earthed at one point only (i.e. only on N-PE link can be in-circuit at a time).

Legislation wise (in the UK), it's the Electricity Safety Quality and Continuity Regulations - regulation 8(4) - "A consumer shall not combine the neutral and protective functions in a single conductor in his consumer’s installation." as the arrangement effectively turns the linked N and PEs into PEN conductors.

3-pole change-over switches would be fine when switching between alternative loads, but usually not between alternative sources.

   - Andy.

Many thanks. That's not quite the information I received via the NICEIC. It unfortunately seems to depend on who you speak to nowadays on the helpline.