One can free down-load the Part P regulation and one can try to find loop holes. However I would say you should be looking more to the general health and safety and less to Part P.
To sit down and decide what is or is not a domestic supply, or any of the other definitions required to work out when Part P applies. Like if there are any food preparation areas is not really helpful. I have heard it said when the sockets were installed there was no food preparation areas therefore it was not a kitchen. I don't think that is helpful in any way. One would be better being honest as saying to hell with Part P I'm doing what I like.
In real terms what you need to look at is the IET and BSI regulations. These are more to do with safety than trying to put up another tax. And here we see the problems with generators and that is where more than one item is powered installing a safe earth system.
Using a generator with one item is like using a shaver socket and likely any earth electrode could introduce as much danger as it removes. But once you power a number of items then the danger lies where multi faults remain undetected and as a result one is placed in danger. So we must install an earth electrode. However now we have a second problem how do we test the electrode. With a supply from a supply authority we can use our earth loop impedance meter to compare our earth to the supply earth. But with a generator we don't have that option. So we have to use a very special meter which compares our earth with two test electrodes normally at 15 meters and 30 meters and we move the centre electrode 3 meters either way to ensure the distance is enough. These meters that use this system are not cheap. Last one I used cost over £1000 compared with £250 for an earth loop impedance meter.
In the main we hire the earth rod meter which is a sort of wheatstone bridge. Of course you also need to select and install an earth leakage device. In the main we now use 30ma as it protects personnel. In theory you will have a TN-S system so there is no need for a RCD other than the 30ma where required for sockets under 20A etc. But in real terms since the area bonded is so small it is easy for a supply to extend beyond the safe zone. So I have always used a RCD to ensure the supply will auto disconnect when required.
Although often one does not follow all the rules and regulations as they are written to cover all situations they are a good guide to safety. And in the past I have advised some one "Oh you don't need that." only to find the generator was powering a huge marque filled with general public and the risks were far far higher than I had ever considered.
It must be therefore up to the installer to consider all risks and to adjust what is required in view of that.
The regulation book is not law, but it can be used in a court of law. Which means you don't have to follow all it says, but if you don't and some one is injured your in deep trouble.
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