EARTHING AND EQUI-POTENTIAL BONDING

By Geoff Rowlands

Why Earth?

One side of the electricity supply (the neutral) is firmly connected to earth at the substation to prevent the supply 'floating' relative to earth for safety reasons.

Many electrically operated devices (e.g. washing machines, heaters and some lighting fittings) have exposed metalwork which could become live if a fault occurred. Anyone touching it could then receive a shock or even be killed depending on the current flowing through them to earth. To prevent this, an earthing conductor should be provided to all socket outlets, lighting circuits and any fixed appliances to which exposed metal parts are then connected. The earth connection limits the voltage which can appear on the exposed metal parts under fault conditions to a safe value until the fuse blows or the MCB or RCD trips. Note that earthing does not necessarily prevent anyone receiving a shock, but together with the time/current characteristics of the protective device (fuse, MCB or RCD) it should ensure that it is not lethal. It is desirable to make the impedance (resistance) of the earth wiring a low as practicable. (1000A flowing through 0.1 ohm drops 100V!)

Note that exposed metalwork cannot be protected by connection to the neutral because current flowing will cause a voltage drop between the metalwork and true earth. Also, if the neutral connection breaks or the appliance is plugged into a socket with line and neutral reversed (!), the metalwork will be at full mains voltage.

Appliances with an earth connection are called Class I (one): Class II or 'double insulated' appliances incorporate additional insulation to prevent exposed metalwork becoming live, and do not require an earth connection. This means that a 2-core mains lead can be used and internal earth connections are not needed.

A fundamental principle of electrical safety is that no single fault condition should cause a hazardous situation. This is why some of the regulations may appear to be rather stringent: it is better to be safe than sorry.

Who Supplies the Earth?

The earth connection will usually be supplied by one of the following methods:

a). By the electricity company. Either through the armouring of the supply cable or through a combined neutral and earth conductor. The latter method is termed PME (protective multiple earthing) and requires some special attention (see below). There will usually be a label near the meter indicating a PME system.

b). Through an earth electrode; usually a rod or plate driven into the ground. This method is found where the electricity company cannot easily supply or guarantee an adequate earth conductor; for example, where the supply comes on a pair of overhead wires. The user is generally responsible for the adequacy of the earth electrode.

The method of earthing can normally be found out by tracing the wiring from the meter/consumer unit. It is usually fairly obvious. IMPORTANT! - It is no longer permitted to use a water or gas pipe for the main or only earthing connection. There may, however be earth bonding wires connected onto the water and gas pipes for 'equi-potential bonding' (see below). If there is no electricity company earth or dedicated separate earth electrode, then one must be provided. Contact the electricity company if in any doubt.

How Good Should the Earth Be?

This is a difficult question to answer; in general the impedance of the earth connection must be low enough to ensure that sufficient current can flow through the protective device so that it disconnects the supply quickly (< 0.4 second) and that voltage on the earth connection does not rise more than 50V. RCDs operate at much lower fault currents than fuses and so can provide much better protection against shock. RCDs are therefore recommended whatever the method of earthing but where the electricity company cannot supply an effective earth and a local earth electrode is used, an RCD (30 mA trip) must be used. Measuring the resistance of an earth electrode is not easy and is really outside the scope of most d-i-y'ers.

Earthing of Electrical Installation

Each circuit requires an earth conductor to accompany (but kept separate from) the line and neutral conductors throughout the distribution. Where the distribution is in the form of a ring, the earth connection must also complete the ring.

The bare tails of earth conductors must be insulated with green/yellow sleeving from the exit from the cable sheath to the earth terminal.

All metal boxes should be connected to the earth; either through a short tail covered with green/yellow sleeving to the socket earth terminal or directly by the earth conductor for a switch box.

Equi-potential Bonding

As mentioned elsewhere, a fault current flowing in the earth wiring will cause the voltage on that wiring to rise relative to true earth potential. This could cause a shock to someone touching, for instance, the case of a faulty washing machine and a water tap at the same time. In order to minimise this risk, an 'equi-potential zone' is created by connecting the services to the main earthing point. Such services are:

  • Water Pipes
  • Gas Pipes
  • Oil Pipes
  • Central Heating
  • Metallic Ventilation Trunking
  • Exposed Parts of Building Structure
  • Lightning Conductor
  • Any other Metallic Service

The equi-potential bonding reduces the voltage difference which could exist between the metalwork of these services if an earth fault occurred to any one of them. It does not necessarily reduce the voltage to true earth. For this reason, metal window frames or patio doors should not be included in the bonding system - it could lead, for instance, to a window cleaner receiving a shock if an earth fault occurred inside the building.

The equi-potential bonding connections for incoming services should be made close to where the service enters the building on the consumer's side of the meter, stop cock etc. It is convenient to use purpose-made bonding clips (obtainable from most d-i-y stores) which include a label "SAFETY ELECTRICAL CONNECTION - DO NOT REMOVE". The connections must, of course, be made to metal pipes - not plastic. The bonding conductors back to the main earthing block should be 6 sq mm minimum with green/yellow insulation (but see 'PME.' if applicable).

Bathrooms require special attention: The aim is to create a local equi-potential zone, so all extraneous metalwork should be bonded together. This could include:

  • Hot & Cold water pipes to sink, bath, shower
  • Waste pipes (metal)
  • Central heating pipes, radiator, towel rail
  • Electric towel rail, radiant or convector heater

Shaver sockets incorporate special isolating transformers which provide an earth-free output. The primary (input) side requires an earth which is connected internally to the transformer core.

Protective Multiple Earthing (PME.)

With PME. the neutral and earth conductors of the supply are combined. The supply company connects the neutral solidly to earth frequently throughout the distribution network. At the customer's connection point the company supplies an 'earth' (which is actually connected to the neutral) to which all the installation earths and equi-potential bonding are connected. Note that within the installation, the earth and equi-potential bonding are kept separate from the neutral in the usual way.

With PME. there is a potential danger in that if the combined neutral/earth conductor of the supply became broken (very unlikely but nevertheless possible), the voltage on the earth conductors could rise towards the full supply voltage. It is most important therefore that equi-potential bonding is rigorously applied in installations supplied by PME. The minimum size of main bonding conductor is 10 sq mm but may need to be up to 25 sq mm depending on the size of the incoming neutral/earth conductor: the supply company will advise you.

Electricity System Earthing Arrangements

By Andrew Gabriel 27/4/1998

Mains electricity systems are categorised in the UK according to how the earthing is implemented. The common ones are TN-S, TN-C-S and TT. You will sometimes see these referred to in questions and answers about mains wiring.

Note that in these descriptions, 'system' includes both the supply and the installation, and 'live parts' includes the neutral conductor.

First letter:

T The live parts in the system have one or more direct connections to earth.

I The live parts in the system have no connection to earth, or are connected only through a high impedance.

Second letter:

T All exposed conductive parts are connected via your earth conductors to a local ground connection.

N All exposed conductive parts are connected via your earth conductors to the earth provided by the supplier.

Remaining letter(s):

C Combined neutral and protective earth functions (same conductor).

S Separate neutral and protective earth functions (separate conductors).

 

Valid system types in the 16th Edition IEE regulations:

TN-C No separate earth conductors anywhere - neutral used as earth throughout supply and installation (never seen this).

TN-S Probably most common, with supplier providing a separate earth conductor back to the substation.

TN-C-S [Protective Multiple Earthing] Supply combines neutral and earth, but they are separated out in the installation.

TT No earth provided by supplier; installation requires own earth rod (common with overhead supply lines).

IT Supply is e.g. portable generator with no earth connection, installation supplies own earth rod.

 

Inside or nearby your consumer unit (fuse box) will be your main earthing terminal where all the earth conductors from your final sub-circuits and service bonding are joined. This is then connected via the 'earthing conductor' to a real earth somehow...

TN-S The earthing conductor is connected to separate earth provided by the electricity supplier. This is most commonly done by having an earthing clamp connected to the sheath of the supply cable.

TN-C-S The earthing conductor is connected to the supplier's neutral. This shows up as the earthing conductor going onto the connection block with the neutral conductor of the supplier's meter tails. Often you will see a label warning about "Protective Multiple Earthing Installation - Do Not Interfere with Earth Connections" but this is not always present.

TT The earthing conductor goes to (one or more) earth rods, one of them possibly via an old Voltage Operated ELCB (which are no longer used on new supplies).

There are probably other arrangements for these systems too. Also, a system may have been converted, e.g. an old TT system might have been converted to TN-S or TN-C-S but the old earth rod was not disconnected.

IEE WIRING REGULATIONS 16TH EDITION

By Andrew Gabriel 2/3/1998

Amendment 2 edition (1997) has a yellow cover (original 16th edition has a red cover; amendment 1 (1994) has a green cover). Also known as "Requirements for Electrical Installations" and "BS 7671:1992 Incorporating second amendment"

http://www.iee.org.uk/publish/books/wireregs.html#wireregs

290pp., 297 x 210mm, Soft covers version, ISBN 0 85296 927 9, 1997 UKP 42 / US$ 84
Ring-bound version, ISBN 0 85296 842 6, 1994 UKP 55 / US$ 110

Updates are available to bring a first amendment version up to the second amendment:
For soft covers version, ISBN 0 85296 926 0, 1997 UKP 12 / US$ 24
For ring-bound version, ISBN 0 85296 938 4, 1997 UKP 12 / US$ 24

Any bookshop should be able to order these if they don't stock them, or you can order from the IEE's web page above.

INSTALLING CABLE IN CONDUIT

By Richard Gethin 25/11/1996

Tie a small piece of cloth to a piece of string and use a vacuum to suck this through the pipe. If the pipe is small you won't need the cloth tied to the end. Then use the string to pull the cable through.

DROPPING WIRES THROUGH CAVITIES

(Can't remember author of next paragraph: 
Please identify yourself to claim credit! Matthew Marks 7/5/1996)

 It can be very difficult to drop a cable down a cavity wall and retrieve it through a small hole into the cavity. Make the hole large enough to feed a loop of old steel measuring tape through it. As you push the tape into the hole, it will expand into a loop along the sides of the cavity. When you dangle the cable approximately above the hole, it will go through the loop, and pulling the tape out will then retrieve the cable. 

By John Stumbles 13/5/1996

I'd add that you may be better off going for something easier to manipulate than the cable you want pulled through eventually. I find the sort of plastic-covered springy net curtain rail stuff quite useful for this. Once you've got that through you can use it to pull the cable itself or, if the cable is heavy and/or there's a lot of resistance in the run, pull some strong string or light rope through first. I got some 4 or 5mm polypropylene rope from a camping and climbing shop, and it's quite smooth, strong and moderately stiff. It can help when pulling electrical cable through to lubricate it with something like KY jelly or Vaseline. Other tricks for getting cables through various spaces in buildings (e.g. under floorboards) include 'fishing' with lengths of electrical conduit: this is about 3/4" diameter, comes in 3 metre lengths and is quite flexible. I have a variety of lengths I use for this sort of work. You can feed it into the space under a floor where you have removed one board, and use it to get a string or light rope (e.g. your 4mm polywotsit rope) to another access point. Where you don't have large holes at each end of the run - I recently had to wire under a chipboard floor, and had to cut out one piece of board to get any access at all and didn't want to do that all over the place - then you can get away with one main hole, ideally large enough to stick your head into to see what you are doing, and holes just 6 or 10 mm dia at the other ends. Thread some thin wire or string through the 'fishing' pole with a loop of thin wire at the end:

                     fishing pole (conduit)                 __
wire      ------------------------------------------------ /  \
or -------------------------------------------------------+    |
string    ------------------------------------------------ \  /
                                                            --
                                                         wire loop

Dangle some wire (or your 4mm poly cord stuff) down the hole at the other end of the run and fish for it with the pole. A good torch to see under the floor, a light above the small hole, and an assistant to dangle the cord are useful/essential. When you've got the cord through the loop then pull the string/wire at your end of the fishing pole to snare it and you should be able to draw back the pole and cord - then you have a draw-wire to pull cables though with. If keep some thin wire or string tacked up at either end of the run then you'll be able to pull further cables through in future: when you want to pull a new wire through use the keeper string to pull through a stronger drawstring (yer 4mm stuff again) and use that to pull through the new cable and at the same time pull the keeper string back for another day. 

When, in a previous job, we had to install wires in the voids above suspended ceilings, over runs many metres long, we sometimes used a crossbow to fire a nylon line down the void, to save having to lift umpteen ceiling tiles along the way. Finding someone to catch the bolt at the other end wasn't easy though ;-)

By Jim Mortimer 16/5/1997

I find that curtain wire, of the type used to hang net curtains on is ideal for this kind of job, as well as a drawstring for running cables or hoses through any complicated route as it's so bendy.

 

By Richard Gethin 19/11/1998

 The lid from 16mm mini trunking is very good for pushing along voids/hooking cables in voids.

 

By Mungo Henning 16/5/1997

Buy a small length of ferrous chain (like the stuff which would be used in the dog-lead of a small mutt) of about three or four inches long (or long enough to be 'sufficiently' weighty: depends on the quality of the links) and tie it to a bobbin of twine. Drop this down the cavity, having inserted a magnet at the place where you wish to retrieve the cable. With a strong enough magnet, the chain will stick and you will be able to pull the twine through. 

I now have an old 35mm-film container which contains the chain connected to the twine, and a separate magnet attached to an old telescopic aerial (neat for the tool-case). Works a treat so long as you don't drop the chain onto Live and Earth!

Another idea is to connect a Lilliput bulb via bulb holder onto some co-ax cable (of the same diameter). Connect the other end to a suitable battery. This helps when you need to see into a cavity hole. The light has been worth its weight in gold over the last few years.

 

LOW-ENERGY LAMPS AND TIMER/DUSK-DAWN/PIR SWITCHES

By Matthew Marks 20/1/1997

There is often confusion as to whether low-energy light bulbs (fluorescent lamps designed to replace ordinary incandescent lamps) are compatible with devices such as timer switches, dusk-dawn switches or passive infrared (PIR) movement detectors. The problem occurs because these types of switches require a small operating current even when they are not activating the lamp. Ideally, there is a neutral connection so that the operating current can flow from live to neutral. However, some of these devices are designed to replace a conventional light switch, where there is no neutral present. They thus have to pass their operating current through the lamp.

A normal light bulb is quite happy to pass very small currents without protestation. However, low-energy light bulbs (both electronic and non- electronic, and old-style fluorescent lamps too) are highly non-linear, and will not pass any current at all unless a certain voltage is across them. This will result either in the switch malfunctioning, and/or (in the case of electronic lamps) the lamp periodically flickering when it is supposed to be off, as its internal smoothing capacitor is charged by the small current.

If the switch explicitly requires a neutral connection, or will operate without a load (plug-in timer switches, both electronic and electromechanical), it should be happy to run a low-energy lamp. However, devices which have a built- in load (such as PIR switches with lamps) may possibly still pass the operating current through the load, but gain no advantage from this, so are badly designed!

If you wire a conventional bulb in parallel with an energy-saving one, the switch should operate, but of course this compromises the energy efficiency of the set-up. A resistor could also be used, but it is difficult to give advice on this, because its value and power rating would depend on the switch.

WHY DO LIGHT BULBS ALWAYS BLOW WHEN YOU SWITCH THEM ON, AND WHY DO THEY BLOW FUSES?

By Matthew Marks and Andrew Gabriel 12/1/1998

An ordinary incandescent "light bulb" consists of a thin tungsten filament in a glass envelope containing an inert gas. The filament has a relatively high resistance, and thus gets hot - hot enough to give out useful amounts of light as well as lots of heat - when current is flowing through it. The inert gas prevents the hot tungsten rapidly oxidising, as it would in air, or rapidly evaporating, as it would in a vacuum. It does, however, reduce efficiency, by conducting heat away from the filament. (Different gases and pressures are selected for different applications: for example, krypton and xenon are advantageous because they convect less and prevent evaporation better than argon/nitrogen, and therefore allow a hotter, more efficient, filament to be used while maintaining lamp life. Note that quartz halogen bulbs are different again: here, evaporated tungsten is re-deposited on the filament, thus allowing it to be hotter still while maintaining its life.)

Tungsten, being a metal, has a resistivity which increases as its temperature rises. Therefore, when you switch on a lamp, it presents a much lower resistance than normal to the passage of electricity, and so your beefy electricity supply will drive through a great deal more current than normal while the filament heats up, putting it under thermal stress as it expands. This on its own encourages the filament to give up and break, but it is exacerbated by the fact that any thinned section will incur extra stress, as it will heat up more quickly than the rest of the filament (being thinner), present a higher resistance, and thus dissipate even more than its fair share of the (increased) power. This will tend to thin it further, rapidly, and hence lead to a point of failure.

How do you deal with it? Well, using a rotary on/off dimmer, where you always have to switch on the lamp at its lowest brightness, will help a lot. A dimmer will reduce the maximum available light output slightly. You can also fit negative temperature coefficient thermistors in series with the bulb. These have a resistance/temperature characteristic with the opposite slope to that of the filament, so give a "soft start" until they themselves warm up. Again, you will lose a little brightness, and waste a little energy in the hot thermistors. I am not aware of any "off the shelf" products containing thermistors, probably because they need to be selected for the wattage of lamp required.

It should be noted, however, that it is probably counterproductive to try to keep a light bulb alive for too long. This is because the thinned filament will be taking less current, so the light output will be reduced, and the tungsten that has evaporated from it will be deposited on the inside of the glass, reducing efficiency by blocking some of the light.

As regards blowing the fuse, this is never directly due to a broken filament falling onto the lead-out wires, and thus presenting a much lower resistance, but is due to the gas or vaporised filament in the bulb becoming ionised. The high temperature and large electric field (full mains voltage across a very small gap) which occurs when the filament breaks can cause the gas to go into a conducting state, and the plasma will "spread" until it shorts out the lead-out wires, because it presents a much lower resistance than the filament. This causes a "pop" due to rapid heating, and has been known to cause the envelope to explode. Light bulbs usually have built-in fuses to deal with this, but as they are built down to a price, they aren't always effective.

If you plug in a new light bulb and it only lasts a few seconds, leaving a white pattern on the glass, this is because it has cracked at some point, letting air in. When energised, the filament has oxidised to white tungsten oxide, which condenses on the glass in a pattern corresponding to the flow of air inside as the lamp is switched on.

Oh, by the way, "extra-long life" bulbs seem to be a con. They just run at a lower temperature than normal bulbs, thus lasting longer, but being a lot less efficient. There is no justification for the extortionate prices charged for them.

TWO-WAY SWITCHING

By Matthew Marks 15/9/1998

Many people have learnt the wiring of a two-way switch at school. How it is done in practice, though, is slightly different, to avoid having to put a connector block inside a switch box. This is the layout:

Line -----------------o------------------------------------
                      |                                    |
                      O        (triple and earth cable)    O
          Switch A      \________________________________/     Switch B
                      O                                    O
                      |                                    |
Lamp -----------------o------------------------------------

All connections between cables are made at switch contacts. The colour coding of the triple and earth is arbitrary, although I connect red to line (as the incoming core will be red), yellow to the switch commons (as it is the "middle" colour) and blue to the lamp (as it is nearest in colour to the outgoing black core).

Adding more switches is easily done: use a special "intermediate switch" which has four terminals. This breaks into the red and blue cores (if following the suggested colour code) and either connects them as normal, or connects incoming red to outgoing blue, and vice versa. With care, the yellow core can be left untouched, so a connector block does not need to be used here either.

As many intermediate switches as you desire can be fitted, although as far as I am aware, they are not available as doubles or triples on the same wall-plate.

A few tips:

  • Arrange the wiring so that the lamp is off if all switches are up. This makes it easy to be sure that the lampholder is not live when changing bulbs
  • As with all wiring, use green and yellow sleeving around bare earth conductors
  • As with all wiring, put a ring of red tape around exposed cable cores which are not red, but are nevertheless live: yellow and blue cores of triple and earth cable, and black cores of twin and earth switch feeds. (Twin and earth with two red cores is also available, but with a price premium.)

IS THE UK ON 240V OR 230V?

By Andrew Gabriel & Matthew Marks 30/9/1998

Traditionally, the UK has had a 240V (+/-6%) electrical supply since the 1960s, when the various local supply voltages (ranging from 200V to 250V) were all brought into line with each other. Continental Europe had a 220V, and Ireland a 230V, supply.

As part of European harmonisation effort to ensure electrical appliances manufactured for use in the EU can be used in any of the countries, a common nominal voltage for the whole EU has been set at 230V. The transition is a two stage process:

* On 1 Jan 1995: UK became 230V +10% -6%, and Continental Europe became 230V +6% -10%;

* On 1 Jan 2003: the whole EU becomes 230V +/-10%.

For most consumers, their measured mains voltage has and will not actually change, because it already falls into these ranges: this was intentional.

The transitions shift the burden of responsibility from the electricity suppliers to the appliance manufacturers, to increase the tolerance to supply variation of their products. However, they will benefit by only needing to supply one model (apart from the type of plug fitted, but that is another story!) for all countries. Generally speaking, modern technology allows devices to remain affordable while being more tolerant to supply variations anyway. The heavy engineering of electricity supply is less amenable to tightening up on performance.

So, you may well still find that your supply is 240V, but it is now magically 230V compatible!

Cables in contact with polystyrene

By Andrew Gabriel

Do not let electrical cables come into contact with polystyrene. It slowly leaches the plasticiser out of the PVC, so that it becomes stiff and brittle. Sometimes it looks like the PVC has melted and run a little.

Final testing of a ring circuit

By Andy Wade

A fairly rigorous method for testing the continuity of ring final circuits is given in the IEE On-Site Guide. It's really aimed at situations where the testing is done by someone other than the installer, and will catch out attempts at 'cheating' -- e.g. a small ring with an excessive number of spurs, or a patched-up broken ring. Arguably, this is OTT for DIY, when you've put the cables in yourself, but anyway, here goes:

The ring should be complete, except for the connection of all six wire ends at the consumer unit or distribution board. You need a low resistance ohm-meter with good resolution, say a range of 0 - 2 ohms with divisions of 0.05 ohm. At the board, start by measuring the loop resistance of each conductor (L1-L2, N1-N2, E1-E2, where the 1's and 2's represent the two ends of the ring) and record the values. The L-L and N-N readings should be the same and, with 2.5mm^2 twin & earth cable the E-E value should be a factor of 1.67 times higher. Now for a cunning stunt, cross-connect the L and N ends -- L1 to N2 and L2 to N1. This makes a 2-turn loop right round the ring. Now, with meter connected to the L and N pins of a 13A plug, work round every socket on the circuit. Lo and behold, every socket on the ring will (should) give the same resistance reading, equal to half of the L1-L2 reading. Any sockets on spurs will read higher by the combined L+N resistance of the spur cable, and so are easily identified (except for very short spurs). Now repeat the previous step with L and E cross-connected (L1 to E2, L2 to E1) and with the meter between the L and E pins of the plug. The readings now will not be constant (unless the CPC is the same size as the live conductors) but will increase toward the middle of the ring, and on spurs. This step effectively checks that the earth is connected at each socket. The highest reading obtained represents the contribution of the circuit to the overall earth fault loop impedance (used for ensuring compliance with 0.4s disconnection time in the event of an earth fault). The highest non-spur reading obtained should equal one-quarter of the sum of the initial L1-L2 and E1-E2 readings. If all is well, remove cross-connections and connect the ends to the supply.

Earthing Plastic Pipes

There is an excellent article about earthing and plastic plumbing on the Hepworth Plumbing Products website, written by Paul Cook of the Institute of Electrical Engineers. 

Quoting from the opening paragraphs:

"You do not have to earth plastic pipes. Plastic pipes make for a safer electrical installation and reduce the need for earthing. Festooning an installation that has been plumbed in plastic pipe with green and yellow earth wire is not necessary and is likely to reduce the level of electrical safety of the property, not increase it."