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

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? 

  • I think you're correct - it's the number of N-PE links they're counting, rather than the number of electrodes/rods.

       - Andy.

  • Except in certain special cases (e.g. downstream of an open-PEN detection device, see Regulation 722.411.4.1), PE conductors in TN, TT, IT, FELV and PELV systems can be Earthed (via electrodes to the general mass of earth) as many times as you like - and even cpc's of different circuits bonded together (for example, Regulations 415.1.2, 701.415.2, 444.5.2).

    Note: This in no way detracts from the requirement that, where ADS is used, a cpc must be run to each point in wiring, and be run in the same wiring system as the live conductors or in its immediate vicinity - i.e. each circuit must have a cpc of its own suitably sized for the circuit protection for ADS to operate in the required disconnection time, and for fault current thermal effects relating to the circuit's overcurrent protective device.

    The Neutral or earthed Line conductor may be Earthed at one point only, however ... for the following reasons:

    1. Effectively makes the Neutral a PEN (or earthed line a PEL) conductor ... this arrangement is effectively prohibited by Regulation 8(4) of ESQCR.
    2. multiple N-E connections may cause spurious operation of RCDs
    3. definitely leads to EMC/circulating current issues
  • 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.


  • 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. 

  • Thank you for the referenced reply, very handy for additional context!  What you have put was certainly along the lines of thinking and has given me some more reading to support that argument. 

  • 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?

  • In that case there may be merit in having the common NE link at the power management module and 5 wires to each genset (LLLNE) In such a case one can have an earth leakage relay enabled on each genset, though it is not that useful, it will detect winding to chassis faults, and all 3 gensets are configured the same without  a link rather than link in one 'master genset' only. Once you are on the distribution side of that module you are in strict TNS country and are allowed as many rods, bonds to tent poles and water pipes as you like bridging between CPC and terra firma, but neutral is to be treated as if it may be alive.

    As it is a supervised system this may relax the rigour needed in terms of allowing the unusual, but it does need to be done in a way that the most likely configuration mistakes (such as avoiding having 2 master gensets or at all none if you have both types) are mitigated. Are the gensets connectorised or is it a case of  ring lugs and socket set at each new  location?

    (I have seen some errors with things that bolt together in the field that are toe-curling - the desire to get stuff up and running as the daylight fails can do funny things to judgement.)