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Understanding of regulation 444.4.6 when dealing with mobile site with multiple generators

Ian C D Smith

I have a mobile site with three 35KW generators.  The system is designed to power share at peak times across two of the three, with the third for maintenance purposes to allow continuity of supply. Each generator is able to have the Neutral line connected to the Earth line via a removable link, and the distribution system is such that, as long as the cabling remains physically connected, the Neutral and Earth lines remain connected at all times irrespective of status of breakers through the system. 

My understanding of the regulations will require one single generator to retain the N-E link, and the other two systems will have the link removed to avoid the possibility of harmonics and doubling up of Neutral return lines.

As the system is mobile, currently each generator is earthed via earthing rods, and the various elements across the mobile sites are also earthed using their own earthing rods and grids, with these earthing grids interconnected via separate earthing cables, as well as the obvious system earth through internal wiring.

Reading through Regulation 444.4.6, it mentions that for TN and TT systems, the system should be earthed at only one point.  My current understanding of this is tied into the next line in the regulations, which states that for TN systems the Neutral line should only be bonded once with the Earth line, and in fact number of interconnected physical earth rods isn't what that statement refers to. If this is the case and 'the system' refers to the N-E connection, would individually earthed generators with only one generator having the N-E link in place satisfy what the regulations state? 

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  • 0

    Although it probably does not apply, if the gensets may also be used islanded - i.e more than one turning, but not synchronous, then the earths need to remain linked, unless the loads are far enough apart that you cannot touch a class 1 appliance on two separate islands at the same time.

    Note that the one NE link is a UK specific thing, and applies to load side (consumer) wiring, and not distribution wiring - so you may have transformers or gensets in parallel, especially if they are DNO managed,  with a bolted NE link each, but only if they then combine immediately - which is actually quite common if they are side by side e.g. twin transformers are fitted as an upgrade as the load rises- before going on to a common panel or breaker and then out to loads as TN-S  If you ever encounter one of these, great care should be taken that the N-N link/ EE link is/are capable of taking the full load current and not to unbolt any of it unless both sources are dead.

    When the DNO patch in a generator at a substation to do back feeding of a small village or whatever, they disable the genset earth leakage trips, as there is no  sensible alternative - going round converting to TN-S  is not really an option.

    Mike.

  • 0 in reply to mapj1

    In this case the three generators all combine via a power management module for 'single' output (actually multiple outputs, but shared from whichever generators are running).

    The issue is this system has no transformers for supply / load separation, instead the outputs utilise live-neutral tap-offs (eventually) to provide single phase outputs for running of equipment.  In this instance I would presume that the regulations would require removal of any additional N-PE links to provide only one 'system ground / earth' and to prevent unwelcome harmonic outcomes. 

    Just for additional context, as this is a mobile facility there is no provision for connectivity into local power grid generation so the system will always operate independently, managed and maintained by the on-site engineers. 

  • 0 in reply to Ian C D Smith

    In that case, there may be further considerations, although a TN-S approach is still perhaps the most appropriate. We'd probably not recommend TN-C-S in most mobile/transportable set-ups in the UK, but as Mike says they exist elsewhere.


    Is there a possibility a generator could be unplugged, or cable severed and be running with Neutral disconnected from Earth - unless of course there's residual current protection and/or insulation monitoring provided directly at the generator output (because it would be effectively IT system under those conditions).

    Does BS 7909 apply to the use-case?

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  • 0 in reply to Ian C D Smith

    In that case, there may be further considerations, although a TN-S approach is still perhaps the most appropriate. We'd probably not recommend TN-C-S in most mobile/transportable set-ups in the UK, but as Mike says they exist elsewhere.


    Is there a possibility a generator could be unplugged, or cable severed and be running with Neutral disconnected from Earth - unless of course there's residual current protection and/or insulation monitoring provided directly at the generator output (because it would be effectively IT system under those conditions).

    Does BS 7909 apply to the use-case?

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