5.6.4 - Special requirements PME-fed installations
An installation connected to a protective
multiple earth supply is subject to special requirements
concerning the size of earthing and bonding leads, which
are generally larger in cross-section than those for installations
fed by supplies with other types of earthing. Full discussions
with the Electricity Supply Company are necessary before
commencing such an installation to ensure that their needs
will be satisfied. The cross-sectional area of the equipotential
bonding conductor is related to that of the neutral conductor
as shown in {Table 5.9}
Table
5.9 - Minimum cross-sectional area of main equipotential
----------------- bonding
conductor for PME-fed installations |
|
Neutral conductor c.s.a. (mm²)
|
Main equipotential bonding conductor
c.s.a (mm²)
|
| 35 or less |
10
|
| Over 35 and up to 50 |
16
|
| Over 50 and up to 95 |
25
|
Danger can arise when the non-current
carrying metalwork of an installation is connected to the
neutral, as is the case with a PME-fed system. The earth
system is effectively in parallel with the neutral, and
will thus share the normal neutral current. This current
will not only be that drawn by the installation itself,
but may also be part of the neutral current of neighbouring
installations.
It follows that the earth system for an
installation may carry significant current (of the order
of tens of amperes) even when the main supply to that installation
is switched off. This could clearly cause a hazard if a
potentially explosive part of an installation, such as a
petrol storage tank, were the effective earth electrode
for part of the neutral current of a number of installations.
For this reason, the Health and Safety Executive has banned
the use of PME in supplies for petrol filling stations.
Such installations must be fed from TN-S supply systems
(RSE booklet RS(C)41
- 'Petrol
Filling Stations: Construction and Operation').
|