Generator earthing....Again!

Hi All,

We've just installed a 400A manual changeover switch for a client that requires a backup generator (Not permanently installed - as It's being supplied by a generator hire company as and when there are power outages) It's a semi rural location and they seem to get power cuts several times a year.

The generator company aren't being particularly forthcoming with information regarding the sets they will be supplying (Other than saying they all have in built earth fault Leakage protection) 

The existing supply is TNCS & I know we can't rely on the DNO earthing during a power cut. With this in mind and little more information to go on from the generator company,  Should we be installing a Rod(s) and just ensuring we have a resistance lower than 20 Ohms.  Is there anything else I need to consider, Obviously my concern is ensuring that any existing protective devices will still operate under fault conditions whilst supplied by the generator.

Given that  411.4.2 now recommends an electrode at the point of supply, I assume we have no real issue with a combined  TT / TNCS arrangement!

All thoughts are more than welcome,



  • Yes, you'll need your own local electrode. That won't make it TT though - in island mode you'd normally have a TN-S arrangement (check the generator has a N-PE link, introduce one if not) and in grid mode the electrode is just another bonded extraneous-conductive-part. Obviously the change-over switch should switch all live conductors including N, so that the generator's N-PE link isn't in-circuit when running off the grid.

    And yes, you'll have to check overcurrent protective devices given that the generator will likely have a much lower ability to supply fault currents than the grid (effectively a might higher Ze, although loop testers won't usually give you a sensible reading as the generator's AVR etc can fool the meter's mechanism). At worst (and if poor discrimination can be tolerated) a suitable RCD close to the generator might do.

       - Andy.

  • If it helps, a couple of diagrams we've used before:

    basic layout:

    Swicthed alternative generator, TN

    and more specifically for TN-C-S grid supply (you may or may not need the RCD):
    Generator, switched alternative, TN-C-S

       - Andy.

  • Tim

    This generator supply is a switched alternative to a DNO supply so requires a separate earth to the DNO supply See BS 7671 551.

    BS 7430 requires the earth resistance to be 20 ohms or less.

    Your generator may no be able to supply enough fault current for ADS so you may need earth fault protection on the generator output.

    We cover this subject on the IET presentations at the Elex shows. The next one is at Sandown Park in November.

    Hope this helps?


  • Just to add, you will need to check the earth resistance from time to time. The earth resistance can vary because of many things, such as how wet or dry the soil is, how hot or cold it is, how rusty the earth electrode is, how much plants grow around it, and what people do near it. As you already likely know there are different methods to do this. fall-of-potential method, the clamp-on method, the selective method, and the stakeless method .

  • BS 7430 requires the earth resistance to be 20 ohms or less.

    Indeed it does. The reason for that particular figure seems rather 'mysterious' and may be difficult (expensive) to achieve in some situations though. Interestingly the DPC for the COP for EESS suggests that 200Ω will be adequate for a wholly TN-S islanded generator systems, and where a TN-S system feeds a TT sub-installation, providing required Zs values within the TT system are still achieved, the source electrode impedance may still be higher than BS 7430's 20Ω - depending on the largest size RCD involved - e.g. 200Ω for up to 100mA devices and 50Ω for up to 300mA devices.

       - Andy.

  • Hi All,

    Many thanks for the informed responses. That's good to know about Sandown Park - I may be able to attend that one!!

    Possibly I'm over thinking this, but my concern is that without knowing what the generator hire company are supplying (& I can imagine it won't be an identical generator each time)  & without having the information on what RCD they will have on their generator (Again I assume it will be some form of adjustable RCD module in built to the Main MCCB of the generator) How can I ensure that we are meeting our obligations for fault protection of the existing final circuits of the installation when under generator control.  The general arrangement is a standard panel board with MCCBs and a few distribution boards with MCB / RCBO protection to various final circuits.

    I've tried to push back on the generator company - but keep getting the same response of ' All our generators meet regulations and have appropriate earth fault leakage protection' 

    My only option I feel I'm left with to mitigate our risk is to install a Rod(s) to get as low a reading as possible  & let the generator company know what the reading is (At the time of testing) & tell  them that their RCD need to be set at XX or lower?

    Thanks again,


  • My only option I feel I'm left with to mitigate our risk is to install a Rod(s) to get as low a reading as possible  & let the generator company know what the reading is (At the time of testing) & tell  them that their RCD need to be set at XX or lower?

    Provided your entire installation shares the same earthing system as the generator (i.e. it's one TN-S system) then the earth fault loop will be entirely though metallic conductors and the electrode resistance isn't a factor in normal ADS. The relationship between electrode resistance and RCD rating is more of a TT issue.

    How is the generator to be connected? - at 400A I take it's not intended to be some ordinary person with a commando plug - but rather a competent person would be involve with a bit of hard wiring each time? (I know there are 400A power link connectors, but do the generators come with those? I suspect most come with just a terminal box to connect to) If so I think it's reasonable to leave all the final checks/decisions to them (as at that point they'll know what they need to know about the generator and will have to check things like the N-PE link is in place etc). As long as you provide sufficient information about the installation side (e.g. absence of universal RCD protection - presume RCD within generator) and earthing electrode details, I think you could reasonably leave the rest of the worrying to them.

    Only other thought was if you had fault-only protection for live conductors somewhere in the installation (typically industrial motor circuits where the OPDs at the DB are oversized compared to the cables, to allow discrimination with overloads at the motor end) - the reduced fault current for L-L and L-N faults might mean that the protective devices doesn't open "instantly" (e.g. fault current too low for magnetic operation of MCB) - and then the slower thermal action isn't fast enough to provide overload style protection as it's rated higher than Iz of the cables. Ordinary ring final circuits (e.g. 32A MCB and 20A cable) might fall into that category too (especially unfused spurs), for smaller generators.

        - Andy.

  • assuming the earth fault relay on the genset supports it, you may well not want to set it to be 30mA instant, but something coarser and slower, so that the RCD protection for final circuits works as intended, rather than "one out all out".

    Certainly gensets from the larger hire outfits (Agrekko, Templant etc) all offer this programmable earth fault relay facility, and I'd expect it to be available on request if not standard fit.

    The exception is gensets configured for DNO use to inject power into street mains during transformer replacement - as most are now PME or at least combined NE somewhere, those gensets cannot have earth fault protection from any kind RCD or EFR  (or at least if they do it must be bypassed for DNO use)

    I have in the past used a nested arrangement, where the EFR at the generator was set to 300mA 1/3 seconds  fed submains of 63 or 100A protected by delay RCDs of  100mA and 1/10 seconds, and final circuits are 30mA instant.

    Such a set up is very robust, and still gives fault protection at all levels, and shock protection for sockets etc.


  • Agrred, earth fault relay on the genset should be set to a higher and slower value than the RCD protection for final circuits, to prevent unnecessary tripping and ensure selectivity. I also like the example that you have shared of a nested arrangement, where the EFR at the generator was set to 300mA 1/3 seconds, the submains were protected by delay RCDs of 100mA and 1/10 seconds, and the final circuits were protected by 30mA instant RCDs. This seems like a very strong and dependable setup, that provides fault protection at all levels, and shock protection for sockets and other equipment. Searching the web (As I like to do) and found some information that support your comment. According to [Electrical Engineering Portal], “the earth fault relay must be set above the maximum earth fault current that can flow through any of the RCDs in order to avoid nuisance tripping”. The article also recommends that “the earth fault relay should be set at least 1.5 times higher than the highest rated RCD in the installation”. According to [Electrical Installation Guide], “the time delay of the earth fault relay must be longer than the operating time of any RCD in order to ensure selectivity”. The article also gives some examples of time delay settings for different types of RCDs.

  • We were lucky in that we only needed 3 levels so we could open up and use a spacing factor of 3. 

    The other rule of thumb is the 1/1000 - a 30 or 32 A circuit is OK on a 30mA RCD, a 100A sub-main is OK on a 100mA RCD, and a 300 or 400A genset is OK on a 300mA RCD.

    - The gensets we used were similar to the original posters - largest on site was a megawatt unit but mostly we used half and third.

    We did have over 30 of them spread out over a large encampment and a dedicated tractor trailer unit to keep them topped up with diesel,, so its not quite the usual installation.