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

Question

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.

Dave Miller · Accepted Answer

yes, there are some parts of the supply regs which apply to legal entities who are "generators" as defined in the regs. however, there are other requirements (including regs 21 & 22 of the supply regs and regs 22 & 23 of the statutory rules for Northern Ireland) which apply to any person: 
 
 reg 21/22: Where a person operates a source of energy as a switched alternative to a distributor&rsquo;s network, he shall ensure that that source of energy cannot operate in parallel with that network and where the source of energy is part of a low voltage consumer&rsquo;s installation, that installation shall comply with British Standard Requirements. 
 reg 22/23: (1) Without prejudice to regulation 21[/22], no person shall install or operate a source of energy which may be connected in parallel with a distributor&rsquo;s network unless he&mdash; (a) has the necessary and appropriate equipment to prevent danger or interference with that network or with the supply to consumers so far as is reasonably practicable; (b) has the necessary and appropriate personnel and procedures to prevent danger so far as is reasonably practicable; (c) where the source of energy is part of a low voltage consumer&rsquo;s installation, complies with British Standard Requirements; and (d) agrees specific requirements with the distributor who owns or operates the network. (2) Sub-paragraphs (b) and (d) of paragraph (1) shall not apply to a person who installs or operates a source of energy which may be connected in parallel with a distributor&rsquo;s network provided that sub-paragraphs (a) and (c) of paragraph (1) are complied with; and (a) the source of energy does not produce an electrical output exceeding 16 amperes per phase at low voltage; (b) the source of energy is configured to disconnect itself electrically from the parallel connection when the distributor&rsquo;s equipment disconnects the supply of electricity to the person&rsquo;s installation; and (c) the person installing the source of energy ensures that the distributor is advised of the intention to use the source of energy in parallel with the network before, or at the time of, commissioning the source 
 
 you will observe that these key requirements (including preventing danger and complying with the wiring regs when in scope) apply to any person with a source of energy. reg 22/23 is so important that it applies retrospectively, which isn't generally the case when new rules are introduced 
 I agree that what matters most is that the installation is safe, but I think that understanding the legislation does matter too. I'm sure that there's something in UK-SPEC about understanding relevant legislative and regulatory frameworks, so you don't have to take my word for it. 
 There seems to me to have been some confusion in this discussion about which rules apply where, so I want to avoid people thinking that certain rules don't apply to their installations because they've seen what appears to be a loophole but the parliamentary draughtsman has been smarter than they may have been given credit for. the key rules apply to any person who operates a source of energy

mapj1 · Answer

I'm not for one moment suggesting we should not consider the intention of the legislation, but the presence of the Double Pole change over switch means this example is NOT source of energy (what we call a 'generator' ;-) ) which may be connected in parallel with a distributor&rsquo;s network (what we call 'the mains' ). Unless you think 'connected in parallel' does not mean electrically in parallel. However, failing that first test does not mean that it should not be wired up without due diligence. Things that can be configured to synchronise and export power back out and onto the grid are quite a different matter, as in fault states, they have potential to damage both the network and other users equipment, and quite rightly are treated with more rigour. Mike.

AJJewsbury · Answer

Maybe easier with a diagram. I think what you're suggesting is something like this: 
 
 Which I think is OK generally, but my worry would be if the generator (with it's frame connected to the supplier's PME earthing facility) is within reach of the TT'd workshop (with it's frame/cladding bonded to the TT earth and/or accessible class I items e.g. outdoor lights). 
 (Technically I suppose the wording of BS 7671 only demands that simultaneously accessible exposed -conductive-parts are connected to the same earthing system, so perhaps and exposed-conductive-part and a (bonded) extraneous -conductive-part on different systems aren't prohibited, but the laws of physics aren't so discriminating, so maybe good engineering practice would want to treat them the same as far a possible.) 
 - Andy.

gkenyon · Answer

Kendo51 said: Hi gkenyon, could i ask for your thoughts . Much appreciated. 
 Sure ... and apologies for not having time to reply for a few days. 
 So, from item 1 and items 1 and 3, if I've understood correctly what you are describing, with the generator plugged in, the garage is connected to the PME earthing arrangement through its earth electrode ... which we don't want? Also, I guess this would require the generator cable cpc, and the link to the earth electrode, to be sized as cpc for the PME system? Which is why, a good few posts back, I was assuming the generator would be connected TT back to the house via the garage? I'm not seeing a way round that at the moment.

gkenyon · Answer

Kendo51 said: This was the reason from the outset i wanted/thought towards switching the earths when generator was running via 3 pole break before make , DNO supply would be in fault, and when generator all earthing house and outside would be TT (inno TN-S given neutral earth link in generator winding but you get what i mean) and any fault currents on my side would be ref to earthing electrodes. Do you not think earth switching is permitted within regs from what was reviewed?, seems simplest answer to me and maintains integrity with both supplies while providing correct earthing. 
 The issue you can't get around with the arrangement you describe, if using the metal outbuilding's earth electrode, is that when the generator is in use, you will be connecting it to PME (even if it's only through extraneous-conductive-parts in the main building) without suitably-sized main bonding conductors. 
 Conflicting requirements I'm afraid. 
 Kendo51 said: o you not think earth switching is permitted within regs from what was reviewed?, 
 A multipole linked switch is permitted to switch protective conductors, but this won't change the fact that the DNO's neutral (often outer layer or, sometimes, copper covering of the cable, and the DNO's earthing conductor, will not be connected to the building. This can present a danger in itself. Regulation 411.3.1.1 requires that 'Simultaneously-accessible exposed-conductive-parts are connected to the same earthing system' because of the possible danger. 
 The published guidance on 'island mode operation' is clear in this regard, and DNO requirements (G99 for this aspect, Figure 8.9) clearly show DNO means of earthing for TN systems unswitched and remaining connected to MET during island mode ... G99 effectively describes the 'distributor's requirements' for Regulation 21 (22 in NI) of ESQCR for 'switched alternative supplies' and G98 and G99 (whichever applies based on current rating and type test of generators) covers the technical requirements for Regulation 22 (23 in NI) of ESQCR. I'm not saying your proposed arrangement will not work, just that I'm not sure it would be considered safe (or compliant) by the DNO under all circumstances.

mapj1 · Answer

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.

gkenyon · Answer

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.

Kendo51 · Answer

Hi mapj1 and Thankyou for your advice and assistance. 
 That was my reasoning and thoughts on for a 3 pole manual transfer (L. N. E) switch (make before break as you'd mentioned) was to ensure the generator set up and indeed it's earthing arrangement can be fully isolated from the DNO supply. Earth switching seems bit unnatural but to me was best means of ensuring complete separation as you'd said fault could occur further up DNO network and supply N being lost and thus earth connection to my property. Equally when running genset (DNO power failure )and fault occured on my side if not separated would result in fault current on the network. 
 The short circuit protection on genset is only breakers by look of it so would definitely be installing either additional protection on genset or at genset supply to house prior to manual bypass. 
 From what you have advised it would be prudent for me to check from earth rods at garage to the DNO earthing after neutral earth link to ensure that they are separate and not linked with an unknown extraneous part I haven't thought of. 
 Again Thankyou for assistance. Hopefully more will respond to see if my earth switching thought is valid.

AJJewsbury · Answer

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.

gkenyon · Answer

Kendo51 said: I will be installing a type 2 SPD in the replacement main db in the house as explained as existing boards are old, as the garage is effectively TT it has a Type ! locally as required due to increased likelihood of lightening strikes given its a steel structure. 
 The Type 1 would be required in the garage ... but also because of the MET in the house, one might be required on entry of the feed from the garage at the house. 
 Kendo51 said: I don't see how it would be classed as "prosumer electrical installation" this from my understanding is for renewables 
 'Prosumer' is defined as combining a consumer installation with generation ... yes, original intention of including Chapter 82 was to facilitate renewables, but the definitions are what they are, and the 'modes of operation' are identical ... In fact, a local generator is shown (2nd item from top-right) in Fig 82.1. Any combination of items in Fig 82.1 and it's effectively a PEI. Have a look at Regulation 823. 
 Kendo51 said: you cant use the generator to supply garage in "island mode" a generator in island mode (floating neutral) 
 'island mode' does not mean 'floating neutral'. Please see Definition in Part 2 of BS 7671, and usage in Chapter 82. (Regulation 824.2) 
 Kendo51 said: With ref to the earthing arrangement my thoughts for switching earth within the manual bypass is exactly because of 7.4.1.4 and to comply with BS7671 maintaining cpc continuity and earthing protection/main and supplementary bonding for the installation 
 My additional thought is if DNO has a power outage with fault on N conductor my house supply being TN-C-S id have no earth if had my generator earth connection tied to DNO and I'm running the generator to supply the house and it developed a fault the resulting fault currant would be back feeding onto DNO network . 
 The guidance is to keep DNO earth connected even though you aren't using it as a means of earthing directly (you connect an earth electrode to the MET yourself, but keep DNO earth connected also). You may well be connected anyway if you share extraneous-conductive-parts such as metallic pipes with other installations. If you disconnect DNO earth, there may (in some installations) be problems with Regulations 411.3.1.1. 
 Safer overall to keep DNO earth connected. 
 Kendo51 said: My additional thought is if DNO has a power outage with fault on N conductor my house supply being TN-C-S id have no earth if had my generator earth connection tied to DNO and I'm running the generator to supply the house and it developed a fault the resulting fault currant would be back feeding onto DNO network . 
 The IET guidance and Chapter 82 has been reviewed by DNO for safety. This is why the Neutral from DNO is disconnected, but the PE is not. See Regulation 826.1.1.2.2. 
 Kendo51 said: Would you mind referencing the section within it that related to use of a back up generator which is isolated by way of manual bypass from the DNO? 
 Regulation 826.

AJJewsbury · Answer

im use to referring to island mode = floating neutral at generator 
 
 Ah, OK. I was using island mode in the sense of your installation being separated from the grid (power wise). 
 
 This is my concern if I'm using generator the DNO supply will be off line likely due to a fault and if I've all earths tied to DNO earth and ive a fault at my end will this not sent fault current through the DNO Earth/neutral onto to network? 
 
 An earth fault in your installation will (presuming the generator don't just stall or its output voltage collapse) may well create voltage differences around your own earth loop - but your earth loop will be referenced to Earth at some point - if your connection to the DNO PE is at the same point, you probably won't be exporting any voltage differences to them. 
 - Andy.

gkenyon · Answer

Kendo51 said: Is it the "generator supply" your referring to buddy? you think would do no harm a type 1 SPD in the rcbo housing prior to manual bypass ? . 
 Yes, but also back out to the garage ... if the garage or generator gets hit by lightning, the lightning can back-feed to the house through either the generator cable, or the sub-main to the garage.

Dave Miller · Answer

thanks 
 yes, there are some excellent points about G99 and ESQCR that have been made by more informed commentators than me 
 my minor point doesn't affect any of that, it just creates the link from your installation through the supply regs to the wiring regs. specifically, reg 22 of the supply regs covers " Where a person operates a source of energy" and reg 23 covers "a person who installs or operates a source of energy". you'll see that regs 22 and 23 don't apply to "generators", they apply to "person[s]"

gkenyon · Answer

Dave Miller said: "generators", they apply to "person[s]" 
 For the purposes of ESQCR, 'generators' are 'legal persons' not pieces of equipment.

gkenyon · Answer

gkenyon said: For the purposes of ESQCR, 'generators' are 'legal persons' not pieces of equipment. 
 Specifically: 
 &ldquo;generator&rdquo; means a person who generates electricity at high voltage for the purpose of supplying consumer&rsquo;s installations via a network; 
 (See Regulation 1 Citation, commencement and interpretation )

mapj1 · Answer

the OP's generator isn't a generator. And the OP isn't a generator either, because their generator (that isn't a generator),,, 
 And that, ladies and gentlemen, is the sort of thing that happens when you let the legal chaps draft anything technical, and then let politicians edit it. Too many holes. 
 What for example if I had an LV generator and stepped it up to HV to run the length of a large farm and then stepped it back down to LV before it was used to supply any loads ? (I am aware of at least one of these in the UK ) 
 So it is far more important that this installation is safe for its users, and can not damage the DNO network, or rely on it when it may not be there for earthing, than agonising over which bit of legislation is or is possibly not appropriate, though it is quite fun. Unless the OP refers himself to the police, who probably won't be interested anyway, we'll never know. The only time such laws are tested in earnest is after there has been a serious accident and then some sort of inquiry, and the trick is to avoid that under all circumstances. Mike.

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

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