Looking for some clarity around Reg 717.411.4.  Outside broadcast trucks (eg Fig 717.3) or food trucks routinely pull up to venues around the country and take a temporary feed from a TNC-S electrical supply. Unfortunately websites and books only seem to parrot the above cited reg verbatim. But what does it actually mean in practice?? Mostly on temporary events point (i) of this reg is met as there will almost always be skilled supervision present.

So looking at (ii) 'the suitability and effectiveness of the means of earthing has been confirmed before the connection is made" ..... What does this actually mean? Lets say I test this TNC-S supply and I am happy with the readings, is that now good to hand over? What about an earth rod in case of of PEN failure? Or can this be omitted because points (i) and (ii) have been met? 

Likewise clarity on Reg 717.411.1 'For a supply in accordance with 717.313(ii), automatic disconnection of supply shall be provided by means of an RCD......not exceeding 30mA". Again what does this actually mean? The NIC in an article in 'professional electrician' say that this means the supply to (Fig717.3) must have a 30mA RCD. That is ludicrous as there would be no discrimination with the mobile units on board RCD's. Furthermore you cannot possibly run a 125A three phase supply to an outside broadcast truck on a 30mA supply without seriously risking a major event through nuisance tripping.

I am aware that the 3rd amendment is bringing in changes around mobile units, however I am curious to know how things stand with the current regs quoted. Really appreciate any competent responses