Earthing of standby generators

[Km1777]

Afternoon guys. Recently been asked to do a generator changeover switch for a friend it’s a 6.0kva standby generator . The earthing system is tnc-s . Just seeking a bit of advice with regards to earthing requirements as it’s a portable gen . Also it will be sitting on top of block paving . Can’t see an easy way to drive an eart rod in. The output wiring is neutral and earth are separate . I’m a fully qualified electrician have all up to date qualifications just haven’t had any dealings with standby generator equipment . Any advice would be appreciated.

 

[R-fur]

You could run a 10mm earth from the main earth terminal of the house. I would link the neutral and earth at the generator and make sure the changeover switch switches the neutral as well as phases.

 

[mattg4321]

I've been putting off asking the question on here as I know the can of worms it usually opens! I hope you don't mind me piggybacking onto your thread as it's essentially the same question.

I'm being strongarmed into wiring up a generator for my inlaws. I can't really get out of it! They've already bought the below generator.

Hyundai 6600W / 6.6kW Petrol Inverter Generator, Remote Keyfob & Electric Start, Wheels & Closed Case Design, DC & USB | HY6500SEi

The HY6500SEi inverter generator is ideal for powering large motorhomes or sports trucks, comes with remote control start/stop and produces a huge 6600w of clean pure sine wave electricity. Available now.

hyundaipowerequipment.co.uk

I think I know pretty much what needs doing, it's just the best/safest way of going about it.

Looks as though the earth is 'floating', so will presumably need to link neutral and earth somewhere close to the generator. Perhaps in the 32A commando appliance inlet?

My plan is to wire tails from meter to changeover switch. 1 side of changeover switch goes to consumer unit. Other side of changeover switch goes to the generator via a 30mA RCD and 32A MCB. Wiring from changeover switch to RCD/MCB (which will be adjacent) in 6mm T&E and wiring from RCD/MCB to commando appliance inlet to be 4mm HO7. Neutral to earth link made in commando appliance inlet. Then make up a short lead (plug to socket) from the commando inlet to the generator.

Will also be installing an earth rod, connected to MET as current supply is TN-C-S.

 

[Km1777]

You could run a 10mm earth from the main earth terminal of the house. I would link the neutral and earth at the generator and make sure the changeover switch switches the neutral as well as phases.

thanks for replies.
The changeover does switch neutral as well as the phase . Where would you link neutral and earth. On the outgoing sides of generator sockets

 

[mattg4321]

Not a good idea to link earth and neutral in output sockets on generator imo.

If it were used elsewhere then you would have the generator casing potentially at a different voltage to true earth as it would still be tied to 'neutral'

 

[R-fur]

I link the neutral and earth at the generator.
In my own house (TNS) I fitted a generator connection point on the outside of the house and a 13A double socked back to back as a dedicated generator socket. This is in the living room next the telly, it is a simple matter to carry the kettle or microwave through to the socket. The socket is earthed to and existing socket on the mains supply. My central heating is supplyed through a plug and socket so installing an extension from the generator socket to the boiler gets the heating running. I did it this way to avoid the need for a change over switch.

 

[Rockingit]

Section 7 of BS7430 is all you will ever need to know on this subject.

7.1.3 Unearthed generators (rating below 10kW) supplying a fixed installation
Where an unearthed generator is to supply a fixed installation it is recommended that ADS is adopted as follows:

a) One pole of a single phase generator should be connected to the installation MET
b) The MET should be connected to an earth electrode
c) The installation should conform to BS7671 with all exposed conductive parts and all extraneous conductive parts connected to MET
d) The installation should be protected by RCD's

The earth electrode should have a resistance to earth not exceeding 200Ω

The RCD will not provide protection for faults on the generator side of the RCD, and consequently precautions should be taken.

 

[Rockingit]

 

[Rockingit]

Also - a word of caution about in-built generator RCD's. Very common in smaller sets is that the RCD sense coil actually has the chassis bond running through it, not the outgoing CPC - so you actually need a full-throttle fault via actual earth and back through the chassis for it to trip. Often easily rewired / ask the supplier first! And yes, always a PEN link at source, it's just not worth leaving it to chance! Remember also that your electrode doesn't have to be immediately next to the MET or Generator, just as close as is practicable and if that's 20+m up the garden then so be it.

 

[Km1777]

Section 7 of BS7430 is all you will ever need to know on this subject.

7.1.3 Unearthed generators (rating below 10kW) supplying a fixed installation
Where an unearthed generator is to supply a fixed installation it is recommended that ADS is adopted as follows:

a) One pole of a single phase generator should be connected to the installation MET
b) The MET should be connected to an earth electrode
c) The installation should conform to BS7671 with all exposed conductive parts and all extraneous conductive parts connected to MET
d) The installation should be protected by RCD's

The earth electrode should have a resistance to earth not exceeding 200Ω

The RCD will not provide protection for faults on the generator side of the RCD, and consequently precautions should be taken.

The only problem is there isn’t anywhere to drive an earth rod in . It’s block paving/ tarmac

 

[Rockingit]

The only problem is there isn’t anywhere to drive an earth rod in . It’s block paving/ tarmac

Remember - doesn't have to be right next door. There's always somewhere

 

[R-fur]

As Rockingit says the definitive guide is BS7430, but if you cant get your hands on it, the IET book Temporary Power Systems is very good, it describes electrical systems for shows and events and covers generators and earthing in detail.

 

[pc1966]

The only problem is there isn’t anywhere to drive an earth rod in . It’s block paving/ tarmac

The problem is under loss of supply conditions you can't assume the supply earth is still present, the whole LV cable could have been sliced.

If they expect to use the generator for any length of time then use a SDS drill to put in a 20mm hole (carefully) and then put the rod down though that. However, try to find where any service pipes/cables go first! So water, gas, sewage, and obviously power cables.

Or inside the house f you can get under floor boards? Just needs a link back to the MET.

 

[mattg4321]

Anymore thoughts on the best place to make the N/E link?

As above I’m not convinced the generator itself is the best place. If it gets wheeled off and used for temporary power at a neighbours house or something its probably best to have a ‘floating’earth imo as there probably won’t be a proper rod used, if any!!

 

[Rockingit]

As Rockingit says the definitive guide is BS7430, but if you cant get your hands on it, the IET book Temporary Power Systems is very good, it describes electrical systems for shows and events and covers generators and earthing in detail.

The 'problem' with James Eade's stuff is that it's written for a particular audience and doesn't actually say much that isn't known to a competent electrician already.

 

[Rockingit]

Anymore thoughts on the best place to make the N/E link?

As above I’m not convinced the generator itself is the best place. If it gets wheeled off and used for temporary power at a neighbours house or something its probably best to have a ‘floating’earth imo as there probably won’t be a proper rod used, if any!!

You can only really make a PEN at the source. Normally just lug the two terminals together.

 

[Km1777]

Cheers guys For the advice. Looks like I’ll have to find a suitable place to drive in earth rods and link n-e at source. It’s a pretty straight forward job apart from the rods side of things

 

[mattg4321]

You can only really make a PEN at the source. Normally just lug the two terminals together.

Which terminals are you meaning if the generator has a 32a commando socket as it’s outlet?

The terminals in the socket that’s part of the generator itself? Or the terminals in the lead to the generator from the house?

 

[Rockingit]

Which terminals are you meaning if the generator has a 32a commando socket as it’s outlet?

The terminals in the socket that’s part of the generator itself? Or the terminals in the lead to the generator from the house?

In an ideal world as upstream as possible. Remember that ADS and RCD’s are both affected by this so if there’s access to a primary bus then that’s where to do it.

 

[mattg4321]

In an ideal world as upstream as possible. Remember that ADS and RCD’s are both affected by this so if there’s access to a primary bus then that’s where to do it.

That all makes sense to me, until the genny is wheeled away and used elsewhere with a single piece of equipment (such as temp power for tool or floodlight etc perhaps for an hour or two). Then you have one pole tied to the chassis of the generator and no earth rod...

To me it makes sense to do it on the customers installation, rather than the generator for the reason above. Also what happens when the generator packs up, customer buys new and just plugs it in?

Unless there's a downside to making the link at the origin of the customers installation that I'm missing? Is there any specific regulation that addresses this? What's accepted general practice?

Appreciate the replies. I'm not massively expereicned when it comes to generators, and most I've seen have been dangerous imo! Cheers