Yes,  the cable - which if you like can have an earth braid like SY, but equally if rubber covered does not need an earth at all, is protected against damage by its own construction and the PME earth,  including any supply side RCD. The incoming shore power PME earth must not connect to the chassis of the vehicle or the transformer winding or core however, but stops at that point.

The  transformer on board the vehicle, inside the insulating box either single phase or  3 , has its own neutral earth bond on the secondary side that is connected to the vehicle chassis, and from that point on it is TNS Note that a transformer primary insulation failure will not operate any ADS unless there is an electrode, so the transformer needs to have reinforced insulation or bobbins to the same standard as the enclosure - equivalent to class II.

Mike

As I read figs 717.4 and 717.6, the approach uses an isolation transformer within a Class II or equivalent insulating enclosure. That means that there's no demand made on the supply systems earthing system at all - it's simply not used - the unit could be fed using an insulated & sheathed flex that didn't even contain a c.p.c. and be none the worse from a safety point of view. (In practice you'd more likey use a flex with an insulated c.p,c. and leave it unconnected at the load end). Zs could be high, low or non-existant, RCD present, faulty or absent - it makes no difference, as there's no reliance on ADS for shock protection until after the transformer.

Baiscally you take the live conductors from a PME system but make no connection to the supply's PE conductor, your installation is not subject to PME conditions - after all that's how most TT installations are arranged.

  - Andy.

Thanks for your reply

Hi Andy, Thanks for your reply 

There is another point to consider Terry, and this is probably why GN7 14.4.2 is as it is. The double wound transformer is fine but it also means that no connected equipment should go anywhere near another mains supply. I don't know your purpose of the "unit" but typically broadcast vans will have cables going to remote positions which also have a local mains supply for the equipment, which must normally be considered to be PME. Connection of one of these with normal cables will immediately make the connection of the van to the PME Earth, unless a lot of effort is made to deliberately isolate it, such as fibre cable connections. The staff of these may well be considered to be skilled, again it all depends on your vans purpose. Do notice that the van needs a local Earth connection, as per the diagram.

How far away from the building earth does the unit earth need to be?

Far enough away to not be influenced by any local P.M.E. earthing from anywhere, and where a rod can be driven into the ground, so not on concrete I suppose. A flat earthing plate has been used under a vehicle wheel before. Perhaps the vehicle should be situated on a large rubber insulating mat! Or, perhaps the 4 stabiliser legs will act as earth electrodes on suitable ground.

Z.

You want to avoid someone straddling a PD with a human body, so usually assumed to be arms lengths - 2 yds, or 2metres. Indirect connection via soles of feet, paving, wet soil and other randoms are not really something that can be covered for all cases in regulation writing, so some gumption is needed, but in general metal bits either need to be solidly connected so no great voltage develops, or far enough apart to be inaccessible from one another when the normal activity of the place is happening.

Mike.

lyledunn said:

How far away from the building earth does the unit earth need to be?

There are two considerations

(a) Simultaneously-accessible exposed-conductive-parts

Regulation 411.3.1.1 states 'Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.'

Sadly, like EV, we can't simply bond the two earthing systems together not least because of the PME risk, that is unless one of the exceptions in 717.411.4 applies.

(b) Hazard caused by broken PEN

Similar to EV, it's not necessarily just building earth, but may include buried metalwork (e.g. extraneous-conductive-parts) connected to the PME earthing system.

Unlike EV, however, we are not given any clues as to the acceptable touch voltage to achieve for the separation. This would be up to the Designer, and might depend on the anticipated contact area and whether the persons at risk were anticipated to be wet, and the type of footwear (or not) they are wearing. So, 70 V is used for EV, but a designer may choose, say 50 V to be on the safe side ... they may also choose to take account of the composition of the ground itself (e.g. tarmac or crushed stones would be far more resistive than wet grass).

How far away from the building earth does the unit earth need to be?

From the point of view of someone stood on the ground touching the outside of the unit, you'd want the electrode to be as close as possible to the potential of that particular piece of ground. Going to extra effort to ensure the unit's electrode is at 'true Earth' potential, when the ground the unit stood on isn't, feels counterproductive to me.

Perhaps the question should be: how close to the unit does the electrode need to be....

That said, I'm not sure the unit necessarily needs an Earth electrode in all circumstances - the chassis as a reference can often be sufficient (the unit on its own little planet as it were). Even Class 1 equipment used outside the unit would have some protection against shock under single fault conditions as the supply is isolated from Earth.

   - Andy.