For many years the Regulations required
that each l.v. system should be solidly connected to earth at only
one point, that being the neutral of the source transformer. Special
permission was necessary to earth at more than one point.
The Regulations also required that
cables buried in the highway must have a metallic sheath. Systems
earthed at only one point require the neutral conductor to be
electrically separate and are now known as SNE (separate neutral and
earth).
It was, and still is, the
responsibility of each consumer to provide the earth connection for
his own installation. This was commonly achieved by connection to a
metallic pipe water main.
The growing use of PVC water mains
makes this impossible for new installations and causes problems with
existing ones when water mains are replaced. Gradually, supply
companies developed a practice of providing consumers with an earth
terminal connected to the sheath of their service cable.
This is, of course, a very satisfactory
arrangement but it is not universally practical as many cables laid
in the 1920s or earlier are still in use and many of these are not
bonded across at joints. The arrangement is not practical on most
overhead systems.
In Germany and elsewhere in Europe an
earthing system known as ‘nulling’ grew up. This employed the
principle of earthing the neutral at as many points as possible.
It simplified the problem of earthing
in high resistance areas and by combining the sheath with the neutral
conductor permitted a cheaper cable construction. These benefits were
attractive and during the 1960s the official attitude in the UK
gradually changed to permit and then encourage a similar system known
as PME (protective multiple earthing).
Blanket approvals for the use of this
system, and the required conditions to be met, were finally given to
all area boards in 1974. In BS 7671 – the 16th edition of the IEE
Wiring Regulations this system is classified as TN-C-S.
Providing the consumer with an earth
terminal which is connected to the neutral conductor ensures that
there is a low impedance path for the return of fault currents, but
without additional safeguards there are possibilities of dangerous
situations arising under certain circumstances.
If the neutral conductor becomes
disconnected from the source of supply then the earthed metalwork in
the consumer’s premises would be connected via any load to the live
conductor and thus present an electric shock hazard from any
metalwork not bonded to it, but which has some connection with earth.
In order to eliminate this rare potential hazard the Secretary of
State, in his official Regulations, requires that all accessible
metalwork should be bonded together as specified in the IEEE Wiring Regulations and so render the
consumer’s premises a ‘Faraday cage’. This is the reason for
the more stringent bonding regulations associated with PME.
Under the extremely rare circumstances
of a broken service neutral and intact phase conductor, there may be
a danger of electric shock on the perimeter of the ‘cage’ to
someone using an earthed metal appliance in a garden, even though the
appliance may be protected by an RCD (residual current device) in
accordance with the IEE Wiring Regulations. For the same reason metal
external meter cabinets are undesirable.
In order to eliminate as far as
possible the chance of a completely separated neutral, a number of
precautions are taken. First, all cables must be of an approved type
with a concentric neutral, either solid or stranded, of sufficient
current carrying capacity.
Secondly the neutral conductor of a
spur end on the system is connected to an earth electrode if more
than four consumers’ installations are connected to the spur, or if
the length of the spur connection from the furthest connected
consumer to the distributing main exceeds 40 metres.
Where reasonably practicable, cable
neutrals are joined together to form duplicate earth connections. A
faulty or broken neutral will give an indication of its presence by
causing supply voltages to fluctuate, which, of course, should be
reported to the local DNO as soon as possible. All these measures
contribute to a system which is as safe as practicable and
self-monitoring.
It is the declared intention of the EI
in the UK to provide earth terminals wherever required and
practicable within the foreseeable future. The local DNO should be
contacted regarding their requirements for the use of PME earth
terminals for TN-C-S systems.
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