Domestic electrical installation earthing and circuit protection – part 2

In part 1, I left the tale at the point where the DNO had provided a supply to the premises, most probably with a TN-C-S earthing scheme for a new build.  For the purposes of this article, I’ll assume you have been provided with a supply capable of running a TN-C-S earthing scheme, and that the incoming supply looks something like the diagram below, once the house is completed:

In the first part, I mentioned the breakdown of ownership and responsibilities within the meter box, and how they would be different if you had a TT earthing scheme.  Taking the ownership thing a bit further, you own the meter box and are responsible for ensuring that it is of the correct type, is located so that it is easily accessible by your electricity supplier and that it is is securely fitted into a structure, like a wall or other strong enclosure.  It’s worth noting that meter boxes are to a standard design, and frankly a lot of of them aren’t that weatherproof.  They are just good enough to keep the interior dry if mounted in a relatively sheltered location.  I’ve heard tales of them being located in exposed locations and rain being driven in through the door edges and pooling in the bottom  The door hinges are not that robust on some of the cheaper ones either, so my personal view is that it’s a good idea to get a good a quality box as you can and fit it in a fairly sheltered location.

The DNO will normally want to install the incoming cable through the hockey stick conduit you normally provide (sometimes they will provide it, but you can’t always rely on this), in the left lower corner of the box, with their sealed box, that terminates their cable and provides the location for their 100A fuse*.  This termination unit, often referred to as the “head”, or just the “company fuse”,  provides terminals at the top for the Line and Neutral tails to the meter and a terminal at the side for the earth conductor connection (except if your supply is TT).  The electricity supply company you’ve chosen (unlike the DNO, this can be any supply company that offers a supply in your area) will arrange to come and install their meter in the box, usually above and slightly to the right of the incoming cable termination.  Before they come, ideally you need to put in place the consumer-side equipment, so that they can also connect your Line and Neutral tails to the meter and seal the connections.

Some suppliers are now switching to using meters with an isolating switch built in (usually operated via a screwdriver slot) and an unsealed access cover to the consumer-side terminals.  This is a big advantage, as it allows your electrician (or yourself, if a “competent person”) to isolate the supply and connect the consumer-side up later, as only the supply side terminals have a sealed cover.  It goes without saying that only the DNO or the electricity supplier have the authority to break the seals and access these connections, neither you, nor your electrician, are allowed to break these seals.  This is, I have to say, a rule that is observed as much by the times it’s broken as by the times it’s observed, in practice, but nevertheless, if you find a broken seal on the DNO termination then you should report it to them as quickly as possible and the same with the electricity supplier if the meter seal is broken.  The reason for quick reporting is that, quite often, broken seals are taken as evidence of tampering, usually with intent to steal electricity from the un-metered side of the supply.  It’s also bloody dangerous working on live part of the installation unless you have the proven competence and equipment to so so, so these parts should be sealed so that they cannot be accidentally uncovered.

The tails (the short sections of heavy duty conductor that make the Line and Neutral connections within the meter box) are your responsibility (from the meter) and must be double insulated, with a blue inner sheath for the Neutral and a brown inner sheath for the Line.  The outer sheath will often be grey.  To show which conductor is which, the Line should have a brown or red sleeve or bit of tape around it, and the  Neutral should have a blue or black sleeve or bit of tape around it.  They can be tagged or labelled “N” and “L” instead of using colour coded tape or sleeves, but often they are left unmarked.  If in doubt, the normal sequence of connections at the meter is that the left outer connection is the Line in from the DNO fuse, the left centre is the Neutral from the DNO termination box, the right centre connection is the Neutral to the consumer side and the connection to the right is the Line to the consumer side.

The Line and Neutral tails should be 25mm² in cross sectional area, and must be no longer than 3m from the meter termination to the Consumer Unit or fused isolating switch.  The Earth conductor should have a green and yellow striped sheath and be 16mm² in cross sectional area, and should normally terminate either to the Consumer Unit, or to an earth block, to which all other earth connections can be made.

With a new supply for a self-build, then you have two initial choices.  You can ask (and pay) for a Temporary Site Supply to be installed.  You provide the cabinet, consumer side equipment for the site supply and the earth rod, as all temporary site supplies have to be TT earthed – it’s the DNO rules.  The consumer side of the temporary site supply needs to be signed off by a competent person, to keep your site workers safe, to comply with the regs and to keep your site insurer happy.

Alternatively, you can do as I, and others have done, and decide to have a permanent domestic supply installed on site, before the foundations or house is built, that happens to have an external socket as a site supply.  This way you avoid having to pay the DNO twice, as all future changes are on the consumer side, and none of their business.  To do this you have to erect a structure in which to fit a standard meter box.  In our case I needed a fence to screen the place where the wheelie bins were going to go, as originally we had to comply with the Code for Sustainable Homes, and that required a recycling bin storage area, and because we’re inside an AONB and adjacent to a Grade II listed building, the planners demand that all wheelie bin storage areas in new builds must be screened from the road.  By a mixture of luck and design, we managed to locate a thick, box-like, fence, over the top of a big three phase DNO underground cable that we had relocated as a part of the site preparation.  There was no recorded wayleave or easement for this cable in its original location (diagonally right across the site, see the second photo in this blog entry: ), so technically the DNO had no right to have run it under the site anyway.  When we wanted it moved we came to an arrangement with them to put it exactly where we wanted it to go, right along our boundary (well, about a metre inside it for safety) and right under where I planned to fit the meter box.

The reasons for wanting the meter box very close to this big supply cable were two fold.  Firstly, I wanted the possible option of being able to have a 3 phase supply in the future, as I have machine tools in the workshop and generally buying second hand 3 phase tools is a lot cheaper than buying single phase ones, and they tend to be a bit beefier.  As it happens I don’t think I’ll ever bother to do this now, I’ve had enough hassle with services!

The second reason was to do with our planned PV panel installation.  I wanted to exceed the 3.68 kWp limit that applies to an installation that doesn’t need DNO permission, and so wanted as good (in terms of low impedance) connection to the local distribution network as possible.  Being over the three phase cable also meant that if the DNO refused a request for more than 3.68 kWp, then I could have a second phase installed, and that would allow  me to fit two 3.68 kWp PV systems, each to a separate phase, without needing DNO permission.  In the end they granted permission for a single 6.25 kWp PV installation on a single phase supply, which was good news, as it was cheaper all-round.

Getting back to the initial installation, if you do as I did and fit a permanent domestic supply from the start, then there are several things you need to look out for.  Firstly, as mentioned above, even though this is a permanent domestic supply, the DNO rules say that you must not connect the Earth to their PEN terminal.  It doesn’t make any sense at all in this scenario, but they are adamant that, until the house is erected, you must use a TT earthing scheme.  Their rule makes perfect sense for a true temporary site supply, but none at all for a properly wired permanent supply that just happens to exist on a site without a house.

So you will need to put in an earth rod, as close as practical to the meter box (being aware of the location of any underground cables or pipes), connect a 16mm² green and yellow sheathed Earth wire to this, with the proper clamp, protect the termination at the top of the Earth rod with a green plastic protection box and protect any exposed length of Earth conductor by putting it within conduit, secured to the green plastic box.  This Earth conductor needs to be terminated in the meter cabinet to an earth block, where the other Earth conductors can also be connected.  This Earth also needs to be tested, to ensure that it is under the normal maximum allowable impedance of 0.8 ohms.

The next point to watch is one I made earlier.  Under the regs, you are only allowed to run three metres of conductor tails from the meter to the main switch in your consumer unit or other form of isolating switch.  In our case, or the case of anyone who chooses to locate their meter box away from their house as we have done, then the tails from the meter need to terminate in a fused DP isolating switch, ideally with a fuse that’s rated at a slightly lower value than the 100A* company fuse.  For example, I fitted a DP fused isolating switch with an 80A fuse in the upper right corner of our meter box, and that then connects to the three core length of 25mm² SWA (Steel Wire, Armoured) cable that runs in a duct under the house and up through the floor and wall service space to our services room, where it is terminated properly in a metal box, with 25mm² Line and Neutral tails and a 16mm² Earth tail.

As an aside, getting hold of three core SWA with the correct core colours isn’t that easy, as it normally comes with three phase core colours.  Having the wrong core colours isn’t a problem, but does mean that the sheaths around the internal cable ends need to be sleeved or taped with the correct colours.  I managed to find a supplier of three core 25mm² SWA that had brown, blue and green/yellow cores, for pretty much the same price as the more common three phase stuff (which has brown, black and grey cores), which made for a neater looking installation (not that you can see it under the covers!).

Because I wanted to keep the house supply, fed from the SWA cable mentioned above, separate from the supplies feeding the borehole pump, the sewage treatment plant, an external 16A Commando socket (as a site temporary power connection) and the garage, which was to be used as a workshop, I terminated the tails from the meter to a Henly block and then fed separate short tails to the fused isolating switch for the house and a small Consumer Unit that contains two 40A, C curve, DP MCBs.  This small Consumer Unit protects, and provides a means of isolating, the 6mm² SWA that runs inside the meter fence for a short distance to a waterproof (IP68) four way Consumer Unit, that houses four DP RCBOs.  The second 40A DP MCB feeds and protects the garage/workshop 6mm² SWA supply cable.

As an aside, I mentioned these were “C curve” MCBs.  This means they have a slower trip time when overloaded, more of which later when I get to describing the garage supply itself, but it’s useful to be able to provide adequate cable protection without over-sensitivity that might cause nuisance tripping.

This means that, from the meter cabinet the house supply can be isolated without affecting the external supplies, and if there is a need to work on the waterproof Consumer Unit, then it can be isolated using the DP MCB in the small consumer unit and also the garage supply can be isolated from the meter box using its DP MCB.

This diagram shows the consumer-side wiring inside the meter box, with the supply from the meter, in the TT configuration that’s required for the temporary site supply (this was changed later to a TN-C-S connection, after the house electrical installation is completed):

The important thing to note here is the way each outgoing cable and circuit is protected.  For example, the cable feeding the waterproof Consumer Unit, that supplies all the external power feeds, is protected by the 40A DP MCB, from current overload.  That cable is capable of safely carrying over 60A, so is adequately protected against an over-current fault by that MCB.  Having a DP MCB as an isolating switch allows the external electrical installations fed from the waterproof consumer unit to be isolated from a single point, without affecting the supply to the house.  It’s acceptable to use a DP MCB as an isolating switch in this application.

Similarly, the 6mm SWA cable feeding the garage is protected by another 40A DP MCB that allows the garage supply to be isolated and which protects the cable under slow overload conditions.  The latter point is significant, as it’s best to try and stage overload trips so that the most sensitive device is nearest the end user, in my view.  For this reason, C curve DP MCBs were used at the meter box end, as they are slower to respond than the more normal B curve units.  At the garage end, there is a B curve RCBO that should trip faster in the event of an overload than the cable protection DP MCB.

In the same way, the 80A fused isolating switch protects the 25mm² three core SWA cable that feed the house from an over-current fault, and also allows the house supply to be isolated whilst maintaining power to the external electrical circuits.  This isolating switch also has the capability to be locked, using a small metal device and a small padlock.  This was to ensure that, when the external electrical circuits were live, to provide water, sewage treatment and site temporary power, via the 16A Commando Socket, the house supply could be locked off, with the key to the isolating switch kept by the electrician working in the house.  Whilst not completely foolproof, this provided a safe enough method of working for those working in the house, whilst we still had live circuits elsewhere on site.

The diagram below shows the waterproof consumer unit, that has four, DP RCBOs.  The reason for fitting DP RCBOs in this box was to provide both over-current protection on the (mainly) underground cables and to provide protection from earth leakage faults as well.  This was particularly important during the time when the 16A Commando socket was being used as the temporary site supply, as it provided a reasonably high level of fault protection to any cable plugged into that socket.  It proved itself when one contractor tried to use a small submersible pump, to drain a trench, which tripped the RCBO.  I checked his pump and it clearly had an internal fault that was making the case live!  The chap had been using it for a fair time, and said this was the first time he’d had a problem, but added that he usually used it with his own generator, which probably didn’t have adequate protection on the output, and which he admitted he never earthed with a rod, either.

Two of the other circuits running from this Consumer Unit are running to wet areas, one from a 10A DP RCBO feed runs to the sewage treatment plant, the other, with a 20A DP RCBO runs to the borehole pump.  The third circuit is another 6A DP RCBO that feeds power to the energy metering and excess PV power diversion system, mounted in an IP68 waterproof box adjacent to the meter cabinet.  The fourth circuit in this box is a 16A DP RCBO that feeds the front panel mounted Commando socket, for site power.

Since completing the house, the Commando socket has been disconnected and this 16A feed now runs via a length of 2.5mm² SWA to one of my electric car charge points, at the end of the drive nearest the house.  This is only a 15A charge point, and so using 2.5mm² is over-kill – it happens to be a bit of cable I had left over from wiring the borehole pump.  Electric vehicle charge points must have double pole residual current/earth leakage protection, as defined in J1772 and the IET Code of Practice  for EVSE installations (as far as I know, they are not yet included in the wiring regs, BS7671, or they weren’t at the time of my installation).

The garage supply is slightly different, in that the length of 6mm² two core SWA that feeds the garage does not also supply the garage protective Earth.  The garage runs on its own TT earth system.    The reason for this has more to do with my personal view about Earth protection in what will often be used as a workshop, with machine tools, than anything else.  It would have been perfectly OK under the regs to export the earth from this Consumer Unit to the garage, using 3 core SWA, and that would have been simpler to install, too.  The Earth impedance at the garage would have been within limits and the installation would have been considered safe, in regulatory terms.

The concern I had was that I wanted to be near 100% certain that the exported Earth and the concrete floor of the garage were at the same potential, even under very short duration, high instantaneous fault current, conditions.  Running an Earth down a fairly long conductor increases its impedance slightly, and this tends to slow down the operation of RCDs and MCBs a little bit.  I decided that I’d feel more comfortable if I adopted a TT earthing scheme at the garage itself.  To overcome one of the main issues with a TT system, the vulnerability of the Earth rod and connection to damage, I fitted a double length rod (two sections screwed together) through a hole in the concrete slab that was lined with a short length of plastic pipe.  The rod was driven around 2m down into the gault clay subsoil, to a level below the local water table, in order to ensure a low resistance connection to the ground.  Clay is usually reasonably good at ensuring a good contact between the rod and the soil, because of it’s fine soil particles, as long as it’s moist.  In this case, because the lower part of the rod was into clay that was always moist, I was ensured of a good earth.  It turned out to be slightly “better” than the Earth provided by the DNO when tested, which just proved to me that, in this case, using a local Earth for the garage was a sensible move.

There are particular issues that have to be resolved when using a different earthing scheme at one end of a supply like this.  Firstly, the DNO provided Earth needs to be kept separate from the local Earth.  What I did, which was more because of the sequence of doing the external wiring and building the garage and house than anything else, was to connect the incoming SWA, with an external gland, to the base of a plastic IP68 junction box and secure this to the floor of the slab in a corner, right next to the plastic box with the earth rod terminal.  I could do this because the SWA cable coming up through the garage floor was in a 50mm flexible duct, cut flush with the floor.  This allowed the gland to be made off and then the cable pushed back down a few inches to secure the box, maintaining the Earth isolation.  The 6mm² cable feeding the small two way consumer unit in the garage was fed through fixed conduit, with a short length of conduit between the Earth rod box and the IP68 box for the earth conductor.  These conduit connections were made after the garage was erected, when the IP68 box no longer needed to be watertight.

The garage consumer unit has a 40A B curve DP RCBO in place of the main switch, followed by two MCBs to provide over-current protection, a 6A one for a lighting circuit and a 32A one for the garage power ring main.  The diagram below shows how the garage end is wired:

With this TT earth at the garage, it was important to ensure that the Earth rod was both effective, in terms of having a low enough impedance (the regs state it must be under 0.8 ohms) and that it was well-protected from damage or corrosion.  The main downside to having a local TT Earth like this is that you are responsible for ensuring it’s still OK over the years, but really that also applies to any electrical installation, it makes a great deal of sense to undertake regular safety checks, just to ensure everything remains OK.

There are ways of doing DIY  checks on things like the effectiveness of earthing system, the conductivity of earth rods when in the ground, etc, but they require a degree of knowledge to do, plus some basic test equipment.  I won’t describe them here, only because there are risks involved and I wouldn’t wish anyone to follow advice from here and get hurt.  Similarly, all of the information above is given for educational, not practical, purposes.  All of the wiring I’ve described either has to be installed and tested by a “competent person”, or has to be inspected and tested by one, and that inspection involves being able to physically see all of the cables and connections, which in many cases makes DIY wiring impractical.

Finally, as mentioned earlier, under the DNO rules, you have to connect your main supply Earth to an Earth rod (i.e. TT) whilst you are building the house, but as soon as the house is built you are allowed to move your Earth from the Earth rod to the connection provided on the head and have a TN-C-S system.  Why?  I’ve no idea.  Nothing has changed as far as the DNO or the level of risk is concerned, but they have rules, so you must follow them.  As someone with a technical background this rule is illogical under these specific circumstances, but I think it’s down to the “tick box” mentality of the DNOs.   As soon as the house electrical installation is complete and signed off, you (if you are a “competent person”) or your electrician can swap the main 16mm² Earth wire over from the TT earth rod to the TN-C-S Earth connection, usually provided under a small plastic plug or cover on the right hand side of the incoming supply, and marked as the Earth connection point.


*The company fuse should normally be 100A, as this is the standard domestic supply, but you may only be offered a restricted supply, because of local network capacity issues, and I have seen supplies limited to as low as 60A in some rural areas.  This is well worth asking about when you request a new supply from your DNO, as if there is a local capacity limit and you need more power than is being offered, then you maybe asked to pay for the local network upgrade to provide this – which is never cheap!

7 thoughts on “Domestic electrical installation earthing and circuit protection – part 2”

  1. Hi Jeremy.

    I guess you spotted this and wondered why you left it in or perhaps I have missed something but in your scheme it looks like you could blow the 100A DNO fuse if the 80A supply runs at 90% and either one of the 40A circuits does the same? (72A+36A, other combinations are possible)


    1. Hi Mike,

      No, it’s fine, as the diversity rules in the regs take account of the unlikely event of a max load being applied to all connections. There’s a way of calculating this for different connection schemes, a bit like the way you can have, say, many 13A outlets on a ring final that is protected by a 32A overload protection device (fuse, MCB or RCBO).

      The relevant bit in the regs gives ways to calculate diversity (not very clearly in some respects, though, IMHO). For example, on the forum there was a discussion about us needing to re-wire part of the kitchen supply in order to fit an oven of a different spec to the one we’d ordered. It’s a good example of the way diversity works, so I’ll use it here. The hob is rated at 18A max, one oven is rated at 10A max and the other is rated at 16A max, and all three are fed from a 40A RCBO. A simple sum of the currents shows that 18 + 16 + 10 = 44A, over the 40A rating of the RCBO.

      Using the diversity calculation that applies in this particular case, then the allowance for each “cooker device” should be 10A + 30% of the balance .

      So, the hob, at 18A, is rated at 10A + (30% x 8A) = 12.4A, the big oven is 16A, so becomes 10A + (30% x 6A) = 11.8A and other oven, at 10A, which stays at 10A. The total load, allowing for diversity, is now 12.4A + 11.8A + 10A = 34.2A.

      There are differing diversity rules, depending on the particular application, and in some cases the regs make no mention of a rule for a specific case, so some sensible judgement is needed. For example, there’s no diversity allowance for waters heaters, so if you had two or more instant water heaters on the same radial (not a particularly good idea, IMHO) then you have to allow 100% for each of them.

      Some of the diversity calculations, on the other hand, are dead easy. A lighting circuit is rated at 66% of it’s maximum current, the assumption being that you will never have more than 66% of the lights in any one circuit turned on at the same time. However, this may be OK in a house, but there are locations where you would apply judgement to a diversity rule like this. For example, my garage/workshop has a power outlet ring final plus a lighting ring. It’s very likely that ALL the lights in there will be on quite often, so it would make no sense to apply any diversity allowance to the lighting circuit rating, it needs to be 100%.

      In the case of the waterproof CU, the 40A DP MCB is only protecting the cable that feeds the CU, and the nature of the loads fed from that are that many are intermittent and a lower current than the devices protecting their supply cables. For example, the 6A RCBO supplies less than 1/50 A to the energy monitoring stuff, but 6A is about smallest DP RCBO you can get and it’s adequate to protect the 1mm² cable that supplies that equipment. Other loads are very intermittent and unlikely to be on at the same time; the 20A DP RCBO supplying the borehole pump is an example. the maximum the borehole pump can draw is around 4A continuous, with a starting surge for a fraction of a second of around 10 to 12 A. The 20A rating is to protect the 2.5mm² cable that supplies the pump system – that cable being rated at over 30A.

      Hope the above makes sense, it’s really based on some assumptions as to how many loads will be on at any one time in a circuit. The assumptions are that things like electric showers probably won’t be running at the same time that an oven and hob are drawing full power, for example, or that a hob will very rarely have all the rings running at maximum power.

  2. Generally, the Building Regulations covering domestic electrical installations will apply to all electrical work. Excellent post. I want to thank you for this informative read. I really appreciate sharing this great post. Keep up your work.

    1. Yes, you’re absolutely right, and I should have added that everything described here would be Notifiable Work to a Building Control body, and so subject to Part P installation, inspection and test requirements.

      We do have a small problem here with the regulations allowing DIY LV work like this by a competent person, as long as it is notified to Building Control and inspected and tested by a Part P accredited person, who holds accreditation allowing for the inspection, test and certification of third party work. Sadly, in many areas there are very few, if any, electricians with such a ticket, presumably because it’s not really in their interest to have an extra accreditation that allows competent DIY’ers to do work they might otherwise get! Even our Building Inspectors, who are required to offer a test and inspection service often can’t, as they don’t have anyone with the appropriate ticket.

  3. Very useful.
    Sadly, I’m not up for self build, just changing from TT to TCNS in a very old house. I have a modern (though plastic) CU with split rail each half protected by an RCD and each line/load protected by an MCB. You’ve answered my initial question, what size earthing conductor to use: 16mm2 (to connect the CU earth bar to the new PME connection on the incoming DNO fuse unit).
    Other questions remain.
    1. Between the DNO equipment and my CU I have an old ELCB, probably voltage operated. It connects to the the CU with single insulated cables. If I remove the old TT stake, then it can’t provide protection and so the single insulated cables to the CU and within the CU would make the installation non-compliant. Also, I gather it can be unwise to combine ELCB protection with RCD. What to do?
    2. I have a relatively recent unbonded gas supply and unbonded sink etc in a new utility room. With the latest regs, which I gather relaxed the bonding requirements, do I still need to fix these unbonded items, or are they now compliant?
    If you or anyone has time to respond I would be grateful.
    Thanks again.

    1. Hi Mike, Sorry for the delayed reply.

      In general, if the incoming supply has a low current (30 mA) RCD, then the requirements for earth bonding are reduced. The concept of bonding all exposed metalwork to earth wasn’t entirely safe in the first place – there were some fault cases where earth bonding all exposed metal work actually increased the risk.

      Your old ELCB on the TT rod is most probably a 100 mA RCD, as this was (and still is) the most common arrangement. In general, this is OK, but my personal view is that it’s better to protect individual circuits with either a 30 mA RCD on the feed to that group of circuits (as in the 17th Ed split consumer unit) or better with 30 mA RCBOs on every circuit (more of a convenience than an essential requirement).

      There’s now no need for equipotential bonding in most cases of new work, with a few exceptions, due, in part, to the introduction of so many plastic and non-conductive parts, along with better residual current protection. This pre-supposes that the circuit has RCD protection.

Leave a Reply

Your e-mail address will not be published. Required fields are marked *