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

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.

Hi Mike thankyou. could you possibly explain out of loop, all earths appear to connect at MET in andys drawing

you think any earth fault would stay / dissipate with in my earthing system?, the earth rod link to the house MET would be via supply cable in event of power outage with gen in operation and linked to MET then just dp bypass, then run 10mm cable from genset frame to earth pit ive the garage earthing connected from.

Thanks again appreciate the discussion/advice

Would this not mean in grid mode having the DNO supply N-E link disconnected?

If you TT'd the main house (together with the workshop) in normal (on-grid) operation, you'd not use the DNO earthing facility at all - so the N-PE link would be irrelevant - still there in your cut-out, but not connected to your earthing system.

could you possibly explain out of loop

(and in answer to a couple of other points too). Consider an earth fault in the installation when running on generator. Something like this perhaps (I'm no artist, but blue flash represents the fault and the red smudge the earth fault loop):

Then plug a few values into Ohm's Law... How much current can the generator provide into a short circuit? The manufacturer should be able to provide an answer, but for simplicity shall we say a simple 100A for the sake of argument? Then the loop impedance - again may vary but for illustration say 0.5Ω - so we have 100x0.5 = 50V difference around the whole loop (the rest of the 230V will be lost internally in the generator). In terms of what's imposed on the DNO's earth terminal - what's the resistance of the loop between the MET (where the DNO connection is) and where the electrode is connected? - say 0.05Ω - so our 100A fault current generates 100x0.05 = 5V difference between the MET and our local true Earth. (Try again with other figures by all means). 5V is likely totally insignificant to the DNOs from a shock point of view, even if all their electrodes were disconnected from your installation and they see the whole of your 5V - they'd often get more than that from other installations whose bonding shares metallic pipework with installations connected to a different substation, and if your RCD is any cop, it'll disappear within a heartbeat or two anyway.  If you moved the connection to your local electrode from the generator's N-PE link to the MET you could reduce or even eliminate even that 5V (or whatever it happens to be).

In the general case, attempts to "switch off" the DNO's earth often don't work - you'll still need main bonding connected and often enough that means bonding to metallic supply pipework that's shared with  neighbours whose bonding is still connected to the DNO system... so the convention is to leave everything PE solidly connected together. That also removes a possible issue with moving switch contacts being less reliable than a solid joint - you really don't want to loose earthing to an entire installation because of one dodgy switch contact. Your independent earthing facility is independent in the sense that it keeps working perfectly well even if the DNO's facility completely disappears (e.g. because they're disconnected your supply cable) - the general consensus is that there's no need to provide isolation between them (and in the general case you couldn't anyway because your generator earthing would need to be main bonded to your supply pipework which, if shared with neighbours, would connect it directly back to the DNO's earth...)

 As above, another option might be to TT both the house and workshop together (one possible way of designing out the problem of having two different earthing systems within reach of each other):


Where your electrode serves as the consumer's electrode in a TT system when on-grid, and the source electrode in a TN-S system when running on generator. (There would of course be either extra RCDs or careful use of double/reinforced insulation to cover earth fault protection upstream of the first RCD in TT mode, not shown in the above diagram).

Food for thought perhaps anyway.

  - Andy.

Sorry just seen the 'out of loop' query, and yes, the loop I refer to is the loop of L>fault> CPC>NE bond> back to source, it is the only fault path to earth with a lot of amps.

And as Andy has so well drawn, that fault loop, neither requires nor includes the DNO NE bond when on genset, and if you switch over and redraw it does not include or require  the genset NE bond when switched to mains.

Casually  speaking the you might say that the unused NE bond is out of (the fault current) loop.
Apologies for the initial sloppy choice of words, hope the fog is clearing.

In terms of shocking the DNO you can probably hit it with anything less than 25V forever and the sort of events you would permit normally while ADS operates - so up to perhaps half mains for 0,4 secs or less, that sort of thing. It is not good, and should not occur by design, but in a rare transient fault condition, it is unlikley to kill anyone.

Mike.

Oooh, the earthing 'System'. A real hotch potch of misunderstanding disinformation and confusion.

We almost always show the live (power), and neutral ('returning power') neatly at the top and middle of our diagrams, and then mark the earthy things as an after thought at the bottom among the notes and other incidentals. A few symbols here and there, but without an real feel for what's going on (until push comes to shove and someone else causes a problem;-)

It's worth trying to draw out the circuits with the Earth System at the top (core mass of earth at the very top) and then see how well one can represent the earthing and its resistance/impedances at the various places (all those magic values the DNOs claim, and earth rod minimums, etc). The circuit should include representations of the physical current flows so those faulty street lights, neighbours and adjacent sub-station issues can be 'considered'.

It all gets very interesting once we realise just how 'bad' (as a consistent connection) the general earthy stuff is, and what different folks are expecting from it.  As an aside/excercise; consider if there is any effect from say, deliberately adding an extra earth rod by the house (adjacent to any existing earth bond) and a similar additional earth rod at the garage, and connect the two of them directly together (but independent of any other circuit) with a hefty insulated cable. It's just extraneous, non-contributing, surely? The question is, does it carry any current, whose current, when, who cares?

It's a rabbit hole, and all the specs/regs don't do a great job of explaining.

[Now to catch up with the rest of the discussions  ]

Hi Andy, Thankyou for you help , I've had a lot to mull over last few days .

If you could indulge me and hopefully offer a little clarity on my way forward.

1. Install generator behind metal garage outside of the building, with neutral earth link in windings and chassis/engine/alt connected to earth pit electrode (one of the two pits of electrodes linked together currently to garage db for metal garage TT set up from house TN-C-S supply, supply cable earthed at house db isolated at garage).

2. Install 32amp inlet socket to corner of house to an individual (separate to what will be new db) 20amp rcbo /spd encloser in distribution cupboard.

3. Then into 2 pole "break before make isolator/bypass" along with DNO supply connecting all earths to MET.

4. From "break before make isolator/bypass" to new bs7671 compliant db.

5. supply cable to house inlet will obviously be a temp affair if and when generator is to be used.

So in normal DNO power usage garage installation is still effectively TT.

During mains fail and gen hook up (inno over kill for 20amp from generator but was going to use 6mm hi-tuff or the like temp cable about 14m run to house inlet). Meaning in this scenario all earths are linked at MET in house and any fault current would be linked via temp lead to earth electrodes. 

Do you think this sounds better than my original switching earths at bypass so as to isolate from DNO earthing in cases of network fault/mains failure?

I'm still of the thought that all relevant regulations (reviewed and highlighted) that do apply would allow for switched earth, but appreciate I may be interpreting different to others and if consensus is not to ill go with that.

Appreciate all your input and advice, p.s don't know how these forum things work will everyone that's contributed see this or do i need to send to them individually . Thanks again. 

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.

p.s don't know how these forum things work will everyone that's contributed see this or do i need to send to them individually

It's a public forum - everyone sees everything! If you mention someone in particular they likely get a notification as well.

  - Andy.

lol i get that just didn't know cos they where on thread they'd get notification individually. Thanks 

Hi Andy, handy illustration thank you.

Maybe just one small point, what i was suggesting is that during mains fail and generator is to be used, the generator earthing connection  is linked to the existing garage earth pit electrode and removed after generator use.

Kendo51 said:

1. Install generator behind metal garage outside of the building, with neutral earth link in windings and chassis/engine/alt connected to earth pit electrode (one of the two pits of electrodes linked together currently to garage db for metal garage TT set up from house TN-C-S supply, supply cable earthed at house db isolated at garage).

with this in mind all should be equipotential, and all earths inside and out have physical cabling connection during generator use. 

Hi Andy, handy illustration thank you.

Maybe just one small point, what i was suggesting is that during mains fail and generator is to be used, the generator earthing connection  is linked to the existing garage earth pit electrode and removed after generator use.

Kendo51 said:

1. Install generator behind metal garage outside of the building, with neutral earth link in windings and chassis/engine/alt connected to earth pit electrode (one of the two pits of electrodes linked together currently to garage db for metal garage TT set up from house TN-C-S supply, supply cable earthed at house db isolated at garage).

with this in mind all should be equipotential, and all earths inside and out have physical cabling connection during generator use. 

Maybe just one small point, what i was suggesting is that during mains fail and generator is to be used, the generator earthing connection  is linked to the existing garage earth pit electrode and removed after generator use.

OK, so with the generator plugged in, would the workshop's earthing system be connected to the PME Earth? (defeating the TT'ing of it)

  - Andy.

Yes ,that was my thoughts buddy so in generation mode all earth's are linked externally and internally by earth link from existing earth pit electrode to genset frame, chassis etc then via earth in generator supply cable to house inlet ,house inlet internally connected to MET. When on mains supply again , generator supply cable removed garage is back on TT.

While running on genset, the system is TN-S, with a connection to a DNO earth that is only acting as either as an additional electrode or perhaps an extraneous part in that mode.

To the DNO it is PME/TNCs but the neutral the DNO earth is combined with is not the  one that is in use for neutral current when on genset.

It also may or may not be disconnected in whole or in part, if the DNO side of things is off and being worked on, so it can't be considered as the main means of earthing.

Mike.

Hi Mike I do agree in your summation in principle, there are some things to note.

mapj1 said:

While running on genset, the system is TN-S, with a connection to a DNO earth that is only acting as either as an additional electrode or perhaps an extraneous part in that mode.

The earth connection from the generator "when generator is running"(no mains supply) is only connected to the DNO via the earth withing the power supply coming into house from said generator.  If you see in the detail above the generator/chassis /engine itself when running is connected via10mm earth cable connected to 1 earth pit electrode (of which there are 2 pits, them selves linked together for the Garage supply.

What you highlight ref DNO connection is why originally i wanted to switch the earth within the "break before make bypass" so in generator mode all earthing inside and out would be only referenced back to earthing electrodes.

mapj1 said:

It also may or may not be disconnected in whole or in part, if the DNO side of things is off and being worked on, so it can't be considered as the main means of earthing.

I totally agree, again why i wanted to switch the earth so in generator mode my home is completely as an "island" with no ref to DNO because if there is a mains failure 99% fault is on their line/network.

From my reading and understanding switching the earth would be within regulations but opened my question to forum for advise and thoughts, alot of very helpful and knowledgeable people on here .

OK, so more like this then?

I presume you've TT'd the workshop to avoid exporting PME earth to the steel structure outdoors (PME earths not always being close to true Earth potential, especially during network problems, a broken PEN being the extreme example). But with this setup at the at the very time the PME earth is most likely to be hazardous (when the distribution network is playing up), you're deliberately connecting it to the workshop? That doesn't feel ideal somehow.

Also in that setup the workshop would be an extraneous-conductive-part w.r.t. the PME earth - so the connection would have to meet the requirements of a main bond - i.e. min 10mm² all the way (which means quite a chunky flex for the generator and suitable sized plug & inlet).

If you wanted to have the workshop TT and at have the house/generator in the same equipotential zone as the workshop, one solution might be to TT the house as well (and don't have any connection to the PME Earth). Often that's not practical as bonds to metallic services can defeat the isolation, but in your case with plastic water/gas that might work.

Alternatively keep PME for the house, but arrange things so the genny and workshop are out of reach of each other - e.g. locate the generator further away or interpose and insulating barrier (wooden fence) or similar.

  - Andy.