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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.

  • 0

    **Needless to say neutral earth link at generator windings.

  • 0 in reply to Kendo51

    You need to make sure that when running on genset at least 3 things are true.
    Firstly it is adequately earthed, even if the suppliers earth is missing - as the cause of the power cut may also mean loss of the supply neutral/earth external to the building. 

    Secondly, that there is no condition when the NE bond in the service head from your DNO, and the one from your genset are both connected at the same time - or the one you are not using will look like a neutral earth fault to the supply you are using, and the RCDs wont like it, and by the way arguably its an ESCQR non compliance.

    Thirdly the ADS may not operate smartly with the reduced PSSC from the genset, so you are more reliant on the RCD/RCBOs for your fault protection.

    Its not normally necessary to switch the earth, and if you do, you need to be certain that no parts on either side of the break can be touched simultaneously, but double pole change-over of L and N is essential, and in a way that is break first make second, but I think you do have that.
    as regards SPD, it will do no harm, its not clear if it will add much, it needs to be the sort suitable for TT and protected by a level of  ADS that the genset can operate . 

    Apart from the switched earths where I am sucking my teeth,  I don't see anything to worry about. Wait a bit and see what others say though.
    Mike.

  • 0
    Kendo51 said:
    Q.1.is this 3 pole manual transfer idea (switching earths) idiotic/unnecessary/ not advisable ?

    This type of installation is covered by legislation - Regulation 21 of the Electricity Safety, Quality and Continuity Regulations (ESQCR) ... or Regulation 22 if you are in Northern Ireland. 

    A 3-pole transfer switch is unlikely to meet the requirements of G99 (or G99/NI) and hence legal requirements (ESQCR).

    The transfer switch should disconnect only live conductors from the DNO supply (L&N for single phase, L1-L2-L3&N for three-phase 4-wire) but retain connection of DNO's means of earthing.

    Kendo51 said:
    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 ? 

    Do you have Type 2's already in main house and garage?

    Other issues ...

    1. some of the protective devices will need to be suitable for bi-directional operation ... particularly RCDs/RCBOs.
    2. Is the garage TT (you say you have earth electrodes) ? If so, then perhaps another electrode connected to MET is required in the house ... in this case, in "island mode", the garage is likely to be considered TN-S and the house TT (so check RCD requirements for final circuits).
    3. Does the generator have an internal N-PE link? If not, a system referencing switch should be used ... or tcan he additional contact in the transfer switch used to make that connection when operating "off grid" ("island mode")?
    4. just to note you might well be removing selectivity by having 30 mA RCBO for generator.
    5. Current ratings of consumer units needs to be checked ... see Regulation 551.7.2 (v) of BS 7671:2018+A2:2022+A3:2024


    This type of installation is really now a "prosumer's electrical installation" and in addition to Section 551, you will need to apply Chapter 82 of BS 7671.

    Use of BS 7671 is actually mandated by ESQCR where you have a switched-alternative (backup generator) supply.

  • 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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