Compliance strategy
A number of solutions were identified by Network Rail to comply
with the Improvement Notice. These are linked and need to be
combined in addressing the risks to the signalling power distribution
problem.
A suite of Class II-based power system equipment and designs
were developed and approved. Signal power network insulation
monitoring and fault location equipment have been introduced,
based on risk. The design and installation of signal power network
earthing arrangements have been revised, together with a much-
improved signalling power system inspection and maintenance
regime.
Competences and training have been reviewed and greatly
improved, both for signalling power system inspection, testing and
maintenance, together with improved safe working practices for
work on or near signalling power equipment.
The inspection and maintenance regimes have also been
improved. Responsibility and accountabilities are clearly defined
and include a requirement to inspect and take action based on risk.
A consistent national method of classifying defects by codes and
the required response, and by whom, has been implemented.
Additional resources, where required for the inspection and
implementation of identified control measures, have been
authorised, which includes the required competencies. Where
a hazard is identified, a feasibility design, based on risk and the
ground resistance, may require a Class II system to be installed
retrospectively, should an improved earth electrode not be
sufficient.
Traditionally, feeder insulation monitoring equipment can identify
that a fault has occurred, but not where the fault is located. The fault
could be anywhere on the power network, which can total more
than 30km in a large signal box area, requiring time-consuming
inspection and testing to locate the fault. Network Rail is currently
trialling ‘smarter’ insulation monitoring equipment, which is able to
trace the location of any fault better. The equipment is based on
designs used in the offshore oil and gas industries, and the results
are encouraging. Full approval is expected soon.
The Class I collective earthing system (which complies with
the regulations) was not specifically identified as part of the
improvement notice response, but such systems had been used
selectively around the network for a number of years, such as
on the Mickle Trafford resignalling in 2006 and Crewe-Winsford
remodelling in 2008. With the Class l collective earthed system,
a three-core armoured cable is used rather than a two-core 650V
supply cable. The third core and armour are used together in
parallel as a circuit protective conductor (CPC) to bond all the
equipment together equipotentially. The bond ensures that, in a
second fault situation, there is a low impedance path present.
The problem with this arrangement is a financial and
environmental one, as it requires 50 per cent more conductor than
the Class 1 individually earthed arrangement, increasing not only
cost but also the risk of theft.Class II power supply design
Class II was selected, not just as a means of eliminating safety risk
arising from second earth faults, but also as a means of eliminating
harmful voltage on accessible conductive parts with first earth
faults. An effective local electrical safety earth electrode is no longer
required at an FSP on a Class II power distribution network.
Using Class II for railway signalling power distribution satisfies
the requirements of BS 7671 regulation 410.3.3 by utilising
double or reinforced insulation instead of the traditional automatic
disconnection of the supply as protection against electrical shock.
This also has the benefit of lower capital cost. However, effective
continuous monitoring of the distribution system insulation remains
an essential safety feature of any Class II power distribution network.
A Class II-based design solution has been identified for new-build
signalling power distribution systems and the renewal of legacy
signalling power distribution systems, using two-core unarmoured
cables. A number of resignalling schemes have already beenRelevant Network Rail standards
» NR/L2/SIGELP/27416 Alternations to Signalling Power
Supplies
» NR/L2/SIGELP/27418 Design, Installation and Testing of
Earthing in Signalling Power Systems
» NR/L2/ELP/27408 Product Specification for Signalling Power
Distribution Cables
» NR/L2/ELP/27409 Product Specification for Functional
Supply Points (FSP)
» NR/L2/SIG/30007 [Issue: 2], Product Specification for Power
Transformers for Signalling Systems
» NR/L2/ELP/27410 Specification for Class II Based Signalling
Power Distribution Systems.
The key standards are issued as both signalling and
electrification/plant e.g. SIGELP standards and are designed to
be user friendly and to inform the right person what they need
to do to control the risk.FE ATURE 61