Mobile or Transportable units: What is classed as a unit and the supply cable from an external supply to the unit?

Question

We have recently modified transportable units to the configuration described in Figure 17.6 of BS 7671. These units are connected at various third party locations around the country, with different types of supply systems. 
 IET Guidance Note 7: 18th: Amendment 2: 14.4.2 TN system states A PME earthing facility shall not be used as the means of earthing for mobile and transportable units falling within the scope of this Chapter, except where: (a) the installation is continuously under the supervision of an electrically instructed person competent in such work; skilled or (b) the suitability and effectiveness of the means of earthing has been confirmed before the connection is made; or (c) the unit is located within, or outdoors on an upper storey or roof of, a building or structure containing the electrical installation which supplies the unit. 
 The note goes onto state: There are many installations in premises that are TN-C-S in form and ordinary persons will be most unlikely to understand and fulfil the requirements set out in (a) and (b) above. It is therefore recommended that the arrangements of Figure 14.6 are followed wherever an ordinary person is required to use, operate or connect a unit to a supply with unknown or uncertain earthing arrangement. 
 As we have understood the unit to include the electrical installation within it, does this mean the cable used to supply the unit (up to the primary side of the isolating transformer) could be connected to a PME supply provided the earthing system at the supply point provide by the third party is suitable and effective (Zs is acceptable and the RCD trips)?

mapj1 · Accepted Answer

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

AJJewsbury · Answer

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.

Mobile or Transportable units: What is classed as a unit and the supply cable from an external supply to the unit?

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