Is 20 ohm earth electrode resistance required for back up supply?

Question

A landfill site supplied from private transformer has asked us to connect a back up generator to a changeover switch in the weighbridge. I&rsquo;ve had a look at BS7430 7.2.1 and it says an earth electrode with resistance of 20ohms or less is required. This is potentially quite difficult to achieve. It is a lot lower than the BS7671 requirement to be considered reliable. What is the reason for such a low figure and does it apply equally to private supplies as public supplies?

AJJewsbury · Accepted Answer

First thought - doe the transformer have its own LV electrode? If so, it's presumably already got an electrode with less than 20 Ohms resistance and as it's a private transformer, that electrode is presumably yours rather than the DNOs. So could you rely on that for the backup generator? OK there might be some limitations - e.g. you couldn't run the generator while the connection to the transformer electrode is broken (as might be the case during maintenance such as the replacement of the transformer or its connections) but that's all under your control rather than being some unknown as with normal DNO earthing facilities during a power cut. 
 But back to the original question... Why 20 Ohms? - that indeed has been asked many times and no-one has come up with a theory as to how it was calculated - it's not related to the voltage or size of the supply or load, or potential leakage or fault currents - but rather seems to be the outcome of historical custom and practice. The chosen value does have some implications though - e.g. if the system supplies a TT'd subsystem, increasing the source electrode's resistance will result in higher loop impedances for the TT part (e.g the 21Ohms rule of thumb for the supply side of the loop no longer works), and higher step voltage around the source electrode. But things could easily be re-calculated for some other resistance. Certainly for some smaller systems (e.g. home generators/energy storage systems) higher electrode resistances have been deemed OK in guidance (I can't remember the exact values recommended and I think it differs in different circumstances) 
 - Andy.

mapj1 · Answer

In terms of danger, it all rather depends on what current is going to be required to pass through the electrode in the largest fault you might not be able to detect. If for example everything is behind a 100mA or 300mA RCD, as well it might be in a modern set-up you may consider 100mA or 300mA as the largest un-detected fault that won't ever trip, and say that you want no more than 50V of earth potential rise (tingle voltage to true ground) if that happens. This leads to nearer 150 ohms for the 300mA case and 400 ohms for the 100mA. But, those would be an absolute upper bound, and some allowance must be made for degradation with dry weather, corrosion etc, so advice then ends up being to aim for 100 ohms or less. The 20 ohm figure goes back to non-RCD protected distribution, and js very much a compromise value - it is used for small pole-pig transformers as it is achievable with electrodes that are spikes or laid in the local trench, but in reality the current available could keep the electrode live all day with a good enough fault. Big substations aim for 1 ohm or less, where possible, but again there is a limit to the achievable . 
 How big is the generator ? - there is a sort of scale consideration here, a wheelbarrow genset of few kW supplying one site hut with a single spike at 100 ohms is probably credible, but to put in say a dedicated 100kVA for a largish area with many buildings, without at the same time getting the digger drive to punch in a few scaffold poles in the corners of the concrete pad of the generator shed is a bit remiss ! 
 Generally the larger the area served, the better the earth needs to be or odd things can happen, See https://engx.theiet.org/f/wiring-and-regulations/23375/rcd-failure-causes-shock-in-neighbours-house?pifragment-2370=3 for an example of the sort of thing. 
 Mike.

gkenyon · Answer

Damo said: It is a lot lower than the BS7671 requirement to be considered reliable. 
 That statement of reliability actually comes from BS 7430 ... note that BS 7671 doesn't really cover any specific requirements for earthing a 'source of supply', the correct standard for this is BS 7430. 
 Damo said: I’ve had a look at BS7430 7.2.1 and it says an earth electrode with resistance of 20ohms or less is required. 
 Indeed it does at the moment. 
 You might be interested in the alternatives, under certain circumstances, that are proposed in the draft for public comment of the next edition of BS 7430. If the relevant conditions are met, you might be able to get away with more than 20 ohms ... BUT ... this is a draft. See https://standardsdevelopment.bsigroup.com/projects/2023-01701 
 
 You haven't provided some important information for us to be able to help further; 
 1. What is the rating of the backup generator? 
 2. What is the earthing arrangement intended to be in 'island' mode (where the backup generator supplies all, or part of, the installation when the grid supply is lost)?

Is 20 ohm earth electrode resistance required for back up supply?

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