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Connection of back-upgenerator to TN-C-S domestic installation

Kendo51

Hi All. I'm a medically retired electrician with additional strings to the bow throughout my career. From contracting on commercial electrical installations to building/designing large industrial containerised generators to managing landfill gas generator installations and finishing at building maintenance engineer covering alot of variable skills with HVAC and BMS.

I try to keep the mind busy and do what I can. 

Appreciate your viewing my question I'll try to be as descriptive as possible. 

My home is TN-C-S supply with no extraneous parts ( water supply is plastic pipe and no gas supply).

Approximately 5 years ago I installed a large metal garage/workshop 7.5m by 4.5m(concrete floor with metal box section frame bolted to the concrete pad) 

I supplied the garage with a 6mm armoured  via 30ma RCD and 40amp MCB ,armour being connected to house distribution board and isolated at the garage termination so as not to export the earth as per regulation,I installed 2m deep electrode and additional 1m deep electrode both wired back to garage DB. So in effect garage is on a TT system.

The house distribution boards (8 way mem bs88 and 4 way mem2000 RCD mcb) I want to change out for 1 large regulation DB , one in looking at is MK dual 100amp 30ma isolator with type 2 SPD rest populated with mcb's.

To the nub of my question. I want to install back up generator (7.5kva peak 6.25kva constant)which will be sited out the back of the metal garage( outside garage building not inside)some approx 12m from house. I'll give you my thoughts on what I'm thinking of installation to ensure complete separation from incoming mains supply to remove chance of any back feed especially under fault condition.

1. Install a single phase 32amp incomer socket ( will be supplied from generator in event of outage)to the exterior of house.

2.Wired back to a 20amp double pole rcbo enclosure in distribution cupboard.

3.From rcbo into a manual 3 pole  transfer switch.

4.The network supply wired the manual 3 pole transfer switch then from transfer switch to new distribution board (dB).

5. The generator chassis will be connected to garage earth electrodes when in use.

6.. 3 pole manual transfer switch? My thinking is to switch L. N. and the earth. 

My thinking on this: when mains is in use garage supply has isolated earth DB side connected only  as previously discussed to protect cable/circuit going to garage rest of house still using supplier earth path . 

When generator supply is used the whole installation house including garage would be effectively TT system this way no chance of back feed to the supplier network including under fault conditions from my/consumer end giving earthing is also switched at the manual transfer switch, i

Q.1.is this 3 pole manual transfer idea (switching earths) idiotic/unnecessary/ not advisable ?

Q. 2. Should I include a type 1 SPD to the 20amp 30ma generator supply enclosure inside distribution cupboard given large metal shed and lighting strikes ? 

Sorry for long winded pre log just trying to ensure you've all the information needed. Would be grateful for your thoughts and indeed advice.

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    1. Install a single phase 32amp incomer socket ( will be supplied from generator in event of outage)to the exterior of house.

    You're probably thinking correctly, but just to be absolutely clear (in formal terms and for others reading this) the thing on the outside of the house should be a plug (inlet), rather than a socket - the trailing part (from the generator) should have shrouded socket tubes, rather than exposed pins.

    When generator supply is used the whole installation house including garage would be effectively TT system

    The generator's system would almost certainly be TN-S - the local electrode acting as the "source" earthing (the first T) and c.p.c.s connected to N via the generator's N-PE link (so TN). To create a local TT system, you'd need two separate electrodes, for for the generator and another for the installation.

    The usual approach is you use a DP change-over switch (for single phase) and have Earth solidly connected to both a local electrode and the DNO's earth. In Grid mode it's then TN-C-S as before, and TN-S in generator mode.

    You local TT sub-installation for the workshop makes things more interesting. You could leave that as TT, even in generator mode, but you'd need at least two electrodes then, one for the generator and another separate one for the garage/workshop. You just need to make sure that the sum of the two electrode resistances is still gives a low enough loop impedance for ADS etc for your TT system (as your generator electrode is likely to have a higher resistance to Earth than the DNO's).

    One problem with that approach though is the proximity of the generator to the workshop - if it's within reach (2.5m?) they need to be on the same earthing system - which they won't be if the workshop is TT and the house TN. One option then is to bring the workshop onto the house TN system during island mode - perhaps a 3rd pole in your change-over switch could achieve that - but you'd have to be very careful how things were arranged both in grid-connected and island mode - as you'd want to ensure the workshop wasn't connected to the DNO earth, or within reach of the DNO earth, at any time, including when in island mode. Rating a change-over switch contacts to switch between a DNO Earthing facility and extraneous-conductive-parts (e.g. the workshop, presuming it's sat on the ground) can get "interesting" as the currents involved may include distribution network diverted neutral currents, which aren't related to the generator's output rating (consider why you might need 10mm2 main bonding even for tiny installations).

    Maybe one option to consider is to make both the house and workshop one common earthing system - TT in grid mode and TN-S in island mode.

    Diagrams often help. For starters, here's one I prepared for earlier threads, for a simple change-over arrangement:

       - Andy.

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  • 0

    1. Install a single phase 32amp incomer socket ( will be supplied from generator in event of outage)to the exterior of house.

    You're probably thinking correctly, but just to be absolutely clear (in formal terms and for others reading this) the thing on the outside of the house should be a plug (inlet), rather than a socket - the trailing part (from the generator) should have shrouded socket tubes, rather than exposed pins.

    When generator supply is used the whole installation house including garage would be effectively TT system

    The generator's system would almost certainly be TN-S - the local electrode acting as the "source" earthing (the first T) and c.p.c.s connected to N via the generator's N-PE link (so TN). To create a local TT system, you'd need two separate electrodes, for for the generator and another for the installation.

    The usual approach is you use a DP change-over switch (for single phase) and have Earth solidly connected to both a local electrode and the DNO's earth. In Grid mode it's then TN-C-S as before, and TN-S in generator mode.

    You local TT sub-installation for the workshop makes things more interesting. You could leave that as TT, even in generator mode, but you'd need at least two electrodes then, one for the generator and another separate one for the garage/workshop. You just need to make sure that the sum of the two electrode resistances is still gives a low enough loop impedance for ADS etc for your TT system (as your generator electrode is likely to have a higher resistance to Earth than the DNO's).

    One problem with that approach though is the proximity of the generator to the workshop - if it's within reach (2.5m?) they need to be on the same earthing system - which they won't be if the workshop is TT and the house TN. One option then is to bring the workshop onto the house TN system during island mode - perhaps a 3rd pole in your change-over switch could achieve that - but you'd have to be very careful how things were arranged both in grid-connected and island mode - as you'd want to ensure the workshop wasn't connected to the DNO earth, or within reach of the DNO earth, at any time, including when in island mode. Rating a change-over switch contacts to switch between a DNO Earthing facility and extraneous-conductive-parts (e.g. the workshop, presuming it's sat on the ground) can get "interesting" as the currents involved may include distribution network diverted neutral currents, which aren't related to the generator's output rating (consider why you might need 10mm2 main bonding even for tiny installations).

    Maybe one option to consider is to make both the house and workshop one common earthing system - TT in grid mode and TN-S in island mode.

    Diagrams often help. For starters, here's one I prepared for earlier threads, for a simple change-over arrangement:

       - Andy.

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