The trackside cables are usually, but not always, two-core,
and each FSP is individually earthed using an electrode formed
of one or more buried earth rods. An appreciable earth fault
current may flow, but the length and resistance of the feeder
circuit conductor to the fault locations, as well as the resistance
of the Loc connection to earth, can result in a fault current that is
insufficient to cause automatic operation of the protective device
in an acceptable time.
The general mass of earth can be variable, which makes the
magnitude of earth fault current difficult to predict.
The problem is that this arrangement is not compliant with
the Electricity at Work Regulations 1989, nor BS 7671 (Wiring
Regulations). The highest permitted accessible voltage is 50V
(BS7671) or 60V (EN 50122-1). This may be exceeded on some
large legacy power distribution 650V networks.
An even bigger problem on a system with an IT Earthing
arrangement is with a second earth fault situation, as an
accessible harmful voltage is very likely to be present for an
unacceptable duration. The ratio of the two earth fault resistances
will determine how much of the 650V system voltage appears on
each faulty equipment case.
It is believed that the non-compliance with BS 7671 may have
arisen because there is an exemption for ‘railway signalling
equipment’ in the standard, which railways relied upon for many
years. However, the Office of Rail and Road (ORR) took the view
that the exemption to the regulations is only for the ‘signalling
equipment’ itself and not the electrical distribution networks
feeding signalling equipment. In any case, the Electricity at Work
Regulations 1989 are applicable, so non-compliance is not an
option.Improvement Notice
The legacy design, and, in particular, the hazard from exposed
conductive parts of signalling equipment in public areas, resulted
in the ORR issuing Network Rail with an Improvement Notice
on 19 November 2013. In the notice, the ORR stated: “650V
power distribution networks used to supply power to track-
side signalling equipment at various locations on Network Rail
managed infrastructure are not constructed to prevent, so far as
is reasonably practicable, danger. Signal location cases, to which
members of the public have access, are not adequately earthed
and bonded to prevent danger should exposed conductive parts
become charged at dangerous voltages”.
The scope of the notice applied to all of the 650V power
distribution networks across Network Rail managed infrastructure,
and the ORR considered that the situation contravened the
statutory provisions contained in Section 3(1) of the Health and
Safety at Work Act 1974 and regulations 4(1), 4(2) and 8 of the
Electricity at Work Regulations 1989.
Network Rail was required to undertake a review of 650V power
distribution networks to which members of the public have
access, to:
1) Identify assets with exposed conductive parts that are not
adequately earthed and bonded to prevent danger, should they
become charged at dangerous voltages;
2) Subject to the findings of 1) above, undertake suitable
remedial works, prioritised on the basis of risk, to ensure that
650V signal location cases which may reasonably foreseeably
become charged as a result of an electrical fault are earthed and
bonded;
3) Devise and implement inspection and maintenance
arrangements for ensuring that 650V signal location cases to
which members of the public have access are maintained to
prevent, so far as is reasonably practicable, danger;
4) Implement any other equally effective means of achieving
compliance with the notice.60 FE ATURE