A landfill site supplied from private transformer has asked us to connect a back up generator to a changeover switch in the weighbridge. I’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?
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.
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)?
The back up generator they have on site and wish to use is a Hyundai DHY6000SE. 5.2kw 6kvaThe manual seems to show overload protection of some sort but no indication of any earth leakage device. I am told the star point is connected to the chassis and the earths of the socket outlets and obviously is to be connected to the earth electrode although originally it may have been floating. It is proposed that it is connected to the changeover switch by an appliance inlet and a short trailing lead plugged into the generator.
The supply from the transformer is tns. It supplies a large section of the site but the back up generator is only proposed to be used for the weighbridge portacabin which has a small CU protected by 30mA rcd main switch. The loads are sockets lights, heaters, the bridge itself, some small floodlights and traffic lights. The main reason for the supply being off would be issues on the generation side (the site generates electricity from landfill gas). So mostly the earth connection at the supply transformer would remain in place but it is possible that maintenance work on the transformer would mean it being disconnected so would not want to rely on it.
II was reading 7.2.5 as meaning all generators acting as a back up changeover supply have to be earthed generators and come under section 7.2 . Does it actually specify the requirements where an earthed generator is being used but not preclude using an unearthed generator as a back up changeover supply where it is below 10kw, meeting the requirements of section 7.1 ?
It’s a single phase generator, obviously no star point.
Using a separated (unearthed) system for small generators is indeed a possibility, and often done with small portable setups - e.g. a genny and one power tool, but it gets tricker where you're supplying multiple appliances (you fall into BS 7671 section 418.3 so is limited to where the installation is under the control or supervision of skilled/instructed persons. It also gets dubious with multiple Class I appliances with filters that can allow currents to flow from exposed conductive parts back into system.
- Andy.
The back up generator they have on site and wish to use is a Hyundai DHY6000SE. 5.2kw 6kvaT
So, at that fault level, what is providing automatic disconnection of supply for protection against electric shock? Usually, we'd revert to RCD protection i.e. all circuits that are energised in 'island mode' are supplied by RCD or RCBO, but still form TN-S for the generator?
if that is the case, there's an approach in the IET Code of Practice for Electrical Energy Storage Systems (or the IET Electrician's Guide to Domestic Electrical Energy Storage Systems), which permits higher earth electrode resistances, in some cases up to 200 ohms ... although note that some RCBO's require an earth resistance of 100 ohms maximum.
If this approach is used, the recommended maximum earth electrode resistance depends on the largest residual current rating of all RCDs protecting circuits in island mode. This approach is proposed in the latest draft for public comment (2025) for the next edition of BS 7430 ... but it's not in BS 7430 yet.
The back up generator they have on site and wish to use is a Hyundai DHY6000SE. 5.2kw 6kvaT
So, at that fault level, what is providing automatic disconnection of supply for protection against electric shock? Usually, we'd revert to RCD protection i.e. all circuits that are energised in 'island mode' are supplied by RCD or RCBO, but still form TN-S for the generator?
if that is the case, there's an approach in the IET Code of Practice for Electrical Energy Storage Systems (or the IET Electrician's Guide to Domestic Electrical Energy Storage Systems), which permits higher earth electrode resistances, in some cases up to 200 ohms ... although note that some RCBO's require an earth resistance of 100 ohms maximum.
If this approach is used, the recommended maximum earth electrode resistance depends on the largest residual current rating of all RCDs protecting circuits in island mode. This approach is proposed in the latest draft for public comment (2025) for the next edition of BS 7430 ... but it's not in BS 7430 yet.
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