PROTECTIVE CONDUCTORS

The earthing conductor is commonly called the earthing lead. It joins the installation earthing terminal to
the earth electrode or to the earth terminal provided by the Electricity Supply Company. It is a vital link in
the protective system, so care must be taken to see that its integrity will be preserved at all times.
Aluminium conductors and cables may now be used for earthing and bonding, but great care must be
taken when doing so to ensure that there will be no problems with corrosion or with electrolytic action
where they come into contact with other metals.
Where the final connection to the earth electrode or earthing terminal is made there must be a clear and
permanent label Safety Electrical Connection - Do not remove . Where a buried
earthing conductor is not protected against mechanical damage but is protected against corrosion by a
sheath, its minimum size must be 16 mm² whether made of copper or coated steel. If it has no corrosion
protection, minimum sizes for mechanically unprotected earthing conductors are 25 mm² for copper and
50 mm² for coated steel.
If not protected against corrosion the latter sizes again apply, whether protected from mechanical damage
or not.
Earthing conductors, as well as protective and bonding conductors, must be protected against corrosion.
Probably the most common type of corrosion is electrolytic, which is an electro-chemical effect between
two different metals when a current passes between them whilst they are in contact with each other and
with a weak acid. The acid is likely to be any moisture which has become contaminated with chemicals
carried in the air or in the ground. The effect is small on ac supplies because any metal removed whilst
current flows in one direction is replaced as it reverses in the next half cycle. For dc Systems, however, it
will be necessary to ensure that the system remains perfectly dry (a very difficult task) or to use the
'sacrificial anode' principle.
A main earth terminal or bar must be provided for each installation to collect and connect together all
protective and bonding conductors. It must be possible to disconnect the earthing conductor from this
terminal for test purposes, but only by the use of a tool. This requirement is intended to prevent
unauthorised or unknowing removal of protection.

Protective conductor types

The circuit protective conductor (increasingly called the 'c.p.c.') is a system of conductors joining together
all exposed conductive parts and connecting them to the main earthing terminal. Strictly speaking, the
term includes the earthing conductor as well as the equipotential bonding conductors.
The circuit protective conductor can take many forms, such as:
1. - a separate conductor which must be green/yellow insulated if equal to or less than 10 mm2crosssectional
area.
2. - a conductor included in a sheathed cable with other conductors
3. - the metal sheath and/or armouring of a cable
4. - conducting cable enclosures such as conduit or trunking
5. - exposed conductive parts, such as the conducting cases of equipment
This list is by no means exhaustive and there may be many other items forming parts of the circuit
protective conductor as indicated in. Note that gas or oil pipes must not be used for the

purpose, because of the possible future change to plastic (non-conducting) pipes.

Protective connection for socket outlet in conduit system

It is, of course, very important that the protective conductor remains effective throughout the life of the installation. Thus, great care is needed to ensure that steel conduit used for the purpose is tightly jointed and unlikely to corrode. The difficulty of ensuring this point is leading to the increasing use of a c.p.c. run inside the conduit with the phase conductors. Such a c.p.c. will, of course, always be necessary where plastic conduits are used. Where an accessory is connected to a system (for example, by means of a socket outlet) which uses conduit as its c.p.c., the appliance (or socket outlet) earthing terminal must be connected by a separate conductor to the earth terminal of the conduit box. This
connection will ensure that the accessory remains properly earthed even if the screws holding it into the box become loose, damaged or corroded.

Bonding conductors
The purpose of the protective conductors is to provide a path for earth fault current so that the protective
device will operate to remove dangerous potential differences, which are unavoidable under fault
conditions, before a dangerous shock can be delivered. Equipotential bonding serves the purpose of
ensuring that the earthed metalwork (exposed conductive parts) of the installation is connected to other
metalwork (extraneous conductive parts) to ensure that no dangerous potential differences can occur.
The resistance of such a bonding conductor must be low enough to ensure that its volt drop when

carrying the operating current of the protective device never exceeds 50 V.

Thus R < 50
Ia
where R is the resistance of the bonding conductor
Ia is the operating current of the protective device.
Two types of equipotential bonding conductor are specified.
1. - Main equipotentiol bonding conductors
These conductors connect together the installation earthing system and the metalwork of other services
such as gas and water. This bonding of service pipes must be effected as close as possible to their point
of entry to the building, as shown  Metallic sheaths of telecommunication cables must be
bonded, but the consent of the owner of the cable must he obtained before doing so. The minimum size
of bonding conductors is related to the size of the main supply conductors (the tails) and is given in table below.
2. - Supplementary bonding conductors
These conductors connect together extraneous conductive parts - that is, metalwork which is not
associated with the electrical installation but which may provide a conducting path giving rise to shock.
The object is to ensure that potential differences in excess of 50 V between accessible metalwork cannot
occur; this means that the resistance of the bonding conductors must be low.

Supplementary bonding in a bathroom

The cross-sectional areas required for supplementary bonding conductors are shown in .
Where connections are between extraneous parts only, the conductors may be 2.5 mm² if mechanically
protected or 4 mm²if not protected. If the circuit protective conductor is larger than 10 mm², the
supplementary bonding conductor must have have at least half this cross-sectional area. Supplementary
bonding conductors of less than 16 mm² cross sectional area must not be aluminium.  shows
the application of a supplementary bonding conductor to prevent the severe shock which could otherwise
occur between the live case of a faulty electric kettle and an adjacent water tap.
There will sometimes be doubt if a particular piece of metalwork should be bonded. The answer must
always be that bonding will be necessary if there is a danger of severe shock when contact is made
between a live system and the metal work in question. Thus if the resistance between the metalwork and
the general mass of earth is low enough to permit the passage of a dangerous shock current, then the
metalwork must be bonded.
The question can be resolved by measuring the resistance (Rx) from the metalwork concerned to the

main earthing terminal. Using this value in the formula:

will allow calculation of the maximum current likely to pass through the human body where :
Ib - is the shock current through the body (A)
Uo - Is the voltage of the supply (V)
RP -is the resistance of the human body (Ohms) and
Rx - is the measured resistance from the metalwork concerned
to the main earthing terminal (Ohms)
The resistance of the human body, RP can in most cases be taken as 1000 Ohms although 200 Ohms
would be a safer value if the metalwork in question can be touched by a person in a bath. Although no
hard and fast rules are possible for the value of a safe shock current, Ib, it is probable that 10 mA is
seldom likely to prove fatal. Using this value with 240 V for the supply voltage, uo, and 1000 Ohms as the
human body resistance, RP, the minimum safe value of RP calculates to 23 kOhms. If the safer values of

5 mA for Ib and 200 Ohms for RP are used, the value of Rx would be 47.8 kOhms for a 240 V supply.

To sum up when in doubt about the need to bond metalwork, measure its resistance to the main earthing terminal. If this value is 50 kOhms or greater, no bonding is necessary. In a situation where a person isnot wet, bonding could be ignored where the resistance to the main earthing terminal is as low as 25 kOhms. To reduce the possibility of bonding conductors being disconnected by those who do not appreciate their importance, every bonding connection should be provided with a label.