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Generator earthing....Again!

gibb01

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,

Thanks,

Tim

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  • 0

    Tim

    This generator supply is a switched alternative to a DNO supply so requires a separate earth to the DNO supply See BS 7671 551.4.3.2.1.

    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?

    JP

  • 0 in reply to John Peckham

    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 .

  • 0 in reply to John Peckham

    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.

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  • 0 in reply to John Peckham

    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.

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