Railway Signalling Power – Economic and Performance

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RAILWAY SIGNALLING POWER – ECONOMIC AND
PERFORMANCE ENHANCEMENTS FOR TOMORROW’S
RAILWAY
Simon Hua MEng(Hons) MIET
Electrical Power Project Engineer
Network Rail, Suite 2 Floor 2, Waterloo General Offices, Waterloo Station, London SE1 8SW
[email protected]
29th June 2011
Keywords: Signalling power, legacy, existing, class II.
Abstract
Network Rail has been set challenging performance targets by
the Office of Railway Regulations (ORR). The aim of this
paper is to examine novel electrification and plant (E&P)
engineering solutions which will reduce the risks to today’s
signalling power supplies with regards to performance,
reliability, availability, maintainability and safety (PRAMS)
to as low as reasonably practicable (ALARP) to help meet
these challenging targets. The results show that PRAMS
improvement and cost savings can be achieved by the
implementation of class II. This is advantageous for the
railways in this ‘time of austerity’.
1
Introduction
The importance of reducing costs is emphasised by the fact
that in the previous year (2010/11 Period 1 to 13), signalling
and power supply failures attributed to approximately
700,000 delay minutes [1]. This incurred a significant
financial impact caused by train delays and poor punctuality.
A contributor to this is the loss of power supply to critical
signalling equipment such as aspect signals, points and track
circuits. In these situations the railways will literally come to
a stand still.
The PRAMS risk in the event of a single prospective earth
fault current (PEFC) on an IT (Insulation-Terra) supply shall
be investigated in section 2. Sections 3, 5 and 6 will describe
the effect of three types of IT signalling supplies in the event
of a double prospective earth fault with the resultant
consequence to PRAMS. These are the, Legacy, Existing and
Proposed IT Systems.
The paper shall also consider the potential for achieving
value for money. This is essential as Network Rail has been
set a savings target by the ORR of £1 billion a year until 2019
(encapsulating control period 4 and 5) [2].
2
IT Earthing System
IT earthing systems are used in the majority of the British
railways. The area and historical design practices generally
dictated whether a step-up (400V/650V) or a unity wound
(400V/440V) isolation transformer was utilised. For example,
the step-up transformer was typically used in the British Rail
Western Region (BR WR), whilst the British Rail Southern
Region (BR SR) employed unity wound transformers.
However, they all have one feature in common. They all
provide protection to electric shock caused by indirect contact
(fault protection) by insulating live parts from earth as
defined in BS7671 regulation 411.6.1. This is essential in an
IT earthing system. Typical IT signalling power supplies are
distributed line (L1)-to-line (L2) at 440V or 650V.
A characteristic of this system is the single PEFC. In the
event of an insulation fault on one of the line conductors, a
high impedance/ low current fault develops. On the railways
this is generally caused by insulation degradation, accidental
impact or rodent gnawing. When this happens, there is no
requirement to provide fault protection as long as touch
voltages are compliant to British and European standards.
This is classified in BS EN 50122-1 as a ‘permanent’
condition. The ability of the system to survive a single PEFC,
whilst other earthed systems would have disconnected, allows
for a high availability of supply.
The single PEFC is generally low. Therefore, the
possibility of a fire hazard due to arcing caused by a high
impedance such as a loose connection, is highly unlikely.
This highlights the high reliability and safety qualities of an
IT system. Figure 1 shows the simple circuit breakdown of a
single PEFC occurring in a location case (LOC) at the end of
a distributed signalling feeder.
Figure 1: Single PEFC as a result of a first line conductor
insulation breakdown
The touch voltage is generally low for an IT system. It can be
calculated by:
Z  R  jX , 1st PEFC 
U
and
Z
U C  1st PEFC  R E (see BS7671 regulation 411.6.2)
where R  RTx  R L1  R L 2  R E  RTx and X  X C ;
NB1: X C 
1
; NB2: C  C1  C 2 .
2fC
Z = Earth fault loop impedance (Ω); R = Resistance (Ω); X =
Reactance (Ω); 1st PEFC = Current of single PEFC (A); U =
Voltage between lines (V); UC = Touch voltage (V); Tx =
Isolation Transformer; L1 = line L1; L2 = line L2; E = Local
earth electrode; C = Earthing impedance capacitive
component.
A maximum touch voltage of 50V between all exposedconductive-parts of the LOC, is permitted prior to and in the
event of a single PEFC. This complies with BS7671,
regulation 411.6.2. The implementation of BS EN 50122-1 in
1998 allowed for a greater leniency in touch voltages of 60V
as a permanent condition, based on studies performed into the
effect of current passing through the human body (IEC –
Publication 479-1:1994) and the fact that protective footwear
should always be worn in a railway environment. These
assumptions are detailed in BS EN 50122-1 Annex D.
The single element which presents the greatest effect on
touch voltage in the event of a single PEFC is RE. Although
Network Rail standards demand RE must not exceed 10Ω [3],
in practice this is very difficult to achieve without installing
large earth farms at great expense. The value of RE is also
highly volatile and varies with the weather and ground
conditions.
3
Legacy IT Systems
Legacy IT systems were adopted by British Rail and Railtrack
until 1999. This type of signalling power system accounts for
approximately 90% of the signalling power systems used on
today’s railway infrastructure [4].
It uses high rupturing capacity (HRC) BS88 fuses or
double pole miniature circuit breakers (MCB) on both line
conductors for overcurrent protection for the signalling power
feeder cable. The sizing of protective devices on legacy IT
systems is predominantly based on the distribution/ final
signalling circuit load. This may have been compliant with
the electrical installation regulations at the time of
installation. However, the inherent high impedance double
PEFC path results in a non-compliant disconnection time with
regards to BS7671. This is discussed in section 3.3.
3.1
Basic Protection
Basic protection is the prevention of electric shock caused by
direct contact. In legacy IT systems this is provided by basic
insulation and Class I rated equipment. This is defined as
bonding all the exposed-conductive-parts together via the
main earthing terminal (MET) of the trackside enclosure
using the circuit-protective-conductor (CPC). The MET is
connected to an earth electrode in the ground, referred to as
RE in section 2, via the earthing conductor. This act of
bonding all metallic objects in a lineside enclosure is a form
of additional protection known as ‘localised’ supplementary
equipotential bonding (SEB).
The signalling distribution power cables are stranded
copper or aluminium cores with a single rubber or ethylenepropylene-rubber (EPR) sheath. This is either distributed in
troughing (with lids), directly buried in the ground or laid at
the trackside.
3.2
Double PEFC
Following the event of a single PEFC, the legacy earthing
system is converted from an IT system to a TT (Terra-Terra)
earth referenced system with localised earthing at the
trackside enclosures. This is due to the low impedance, short
circuit between one of the line conductors and earth,
effectively converting it from an earth-free line conductor to
an earth-referenced conductor. Line conductor L1 is used in
this example.
A double PEFC is then created as a consequence of this
when the insulation fails on the other line conductor, L2.
Figure 2 shows the simple circuit breakdown of a double
PEFC occurring at the power pillar/ Principal Supply Point
(PSP) and in a LOC at the end of a signalling feeder.
Figure 2: Legacy IT system – Double PEFC
This will present the worst case touch voltage as it is the
highest earth fault loop impedance (Z). This can be calculated
by:
Z  R  R L1  R E1  R E 2  R EA , 2 nd PEFC 
U
and
Z
U C  2 nd PEFC  RE (see BS7671 regulation 411.6.2)
RE1 = Resistance of local earth electrode at the power pillar;
RE2 = Resistance of local earth electrode at the location case;
REA = Resistance of the mass of earth; 2nd PEFC = Current of
double PEFC (A).
The reactive effects of cable capacitance (XC) has a
minimal effect on double PEFC as the overall fault current
flows through the line conductors and earth electrodes rather
than capacitances to earth.
3.3
Non-compliance
due
to
the
Evolution of Electrical Standards
The legacy IT system may have met electrical standards at the
time of installation. However, many of these installations predate both the Electricity at Work Regulations (EaWR) 1989
and BS7671:2008 17th Edition IEE Wiring Regulations. To
comply with modern British and European standards, a form
of protection against indirect contact is required in the event
of a double PEFC. The legacy IT system would be unable to
meet the touch voltage requirements (see section 3.2) as the
value of REA is so high. Therefore the touch voltage, as a rule
of thumb, would exceed 60V and requires a form of fault
protection using either automatic disconnection of supply
(ADS) or double or reinforced insulation under BS7671
regulation 410.3.3. ADS will be difficult to achieved, as the
fault current is so low and the let through current (I2t) of a
HRC fuse is very high. The use of ADS is also ruled out
further by BS7671 regulations 411.6.4 (ii) and table 41.1,
which requires a highly onerous maximum disconnection
time of 0.04 seconds (for a TT circuit). It is emphasised by
Network
Rail
in
the
project
advice
notice,
PAN/E/EP/PRO/0035. The use of Class I equipment also
rules out double or reinforced insulation as a solution to fault
protection.
An insulation monitoring device (IMD) is used to improve
safety by providing alerts and alarms warning of insulation
degradation. It is a requirement of BS7671 regulation
411.6.3.1 in an IT system where supply continuity is
essential. Early legacy IT systems did not implement these.
They were implemented in recent legacy IT system schemes
whilst resignalling schemes have retrospectively fitted these.
IMDs are explained in detail in section 5.4.
3.4
PRAMS
RELIABILITY = 1.5/3 (LOW/AVERAGE)
[P] The legacy IT system typically extends several kilometres
in length, which results in the REA being high and the
double PEFC being low. An implication of this is a low
risk to reliability and safety, as the likelihood of fires
caused by cables quickly overheating or current arcing
due to a loose connection is low. This also favours the
legacy IT system in relation to train performance.
[C] A compromise to reliability is the use of rubber/ EPR
sheathed cable. This has the lowest life span compared
with other cables mentioned in this paper.
[C] Without the use of IMDs, cable insulation failures are not
realised until a fault has occurred, which impacts on the
reliability of the signalling system. This has been
resolved by the retrospective installation of IMDs, which
is discussed in section 5.4.
AVAILABILITY = 3/3 (HIGH)
[P] In the event of a single and double PEFC, the supply does
not disconnect. This allows a high degree of availability,
which has the greatest effect of reducing train delays.
MAINTAINABILITY = 2/3 (AVERAGE)
[P] Relative to the other systems, there is less earthing assets
to maintain. This is due to the legacy IT system operating
without a CPC and SWA. The issues caused by this are is
discussed later in section 5.5.
[C] Maintainability of this system is particularly poor as
insulation resistance testing is a ‘dead test’ and requires a
power supply outage, which is hard to obtain. The only
method available is visual inspections, which is a difficult
and time consuming process. For this reason, signalling
power cable conditions were rarely assessed on legacy IT
systems. Over time, this would allow a gradual increase
in high impedance single PEFCs to occur due to a
decrease in cable insulation resistance until it is at such as
low value that permits a double PEFC. Without a form of
fault protection, this has a serious health and safety
implication. IMDs have been retrospectively installed in
legacy IT systems as discussed in section 5.4.
SAFETY = 2/3 (AVERAGE)
[P] The high impedance fault path presents a low fire risk
solution.
[P] The legacy IT system does not utilise a steel-wire-armour
(SWA) or CPC. This removes the possibility of exporting
faults across the entire signalling supply (existing) rather
than being localised at the point of fault (legacy). Both of
these points are detailed in section 5.5 under the negative
aspects of safety.
[P] The safety is reliant on ensuring there is a maximum of
10Ω for RE [3] in order to meet touch voltages during
single PEFCs. However, the existing IT system is even
more reliant on RE to ensure ADS is achieved in the event
of a double PEFC. The reason for this is explained in
section 5.5.
[C] The principal drawback of this system, as discussed
earlier, is personal safety due to electric shock caused by
excessive touch voltages. It is at its most dangerous state
in a double PEFC situation as there is no fault protection.
This is in accordance with the signalling power design
philosophy at the time towards reducing the risk to
availability and reliability of signalling power supplies to
ALARP. However, this negative point is counterbalanced by the fault being localised at the double earth
faults, rather than being exported throughout the
signalling feeder as is the case with the existing IT
system.
[C] The inability to detect the deterioration of cable
insulation also affects the safety of the system. This has
been resolved by the retrospective installation of IMDs,
which is discussed in section 5.4.
3.5
Other Legacy IT Systems Issues
Legacy IT systems were designed with BR924 type
650V/110V transformers. On early examples, the values of
inrushes on these transformers were often untested, which
meant that feeder fuses were undersized for the peak transient
inrush currents generated. During energisation of a legacy
signalling power feeder, feeder fuses would often rupture for
this reason. Later BR924 models did specify inrush.
However, at a minimum of 15-20 times inrush, the feeder
fuses were sized extremely high to allow for fuse
discrimination. This has an impact of increasing cable sizes
far beyond the nominal current being supplied on the
signalling feeder.
4
Application of Electricity
Regulation and BS7671
at
Work
As mentioned earlier, the legacy IT systems may have been
compliant to the electrical installation standards of the time.
However, both standards and legislatures have evolved;
resulting in legacy IT system only achieving partial
compliance.
The EaWR 1989 came into force on the 1st April 1990 and
is supported by the Health and Safety at Work Act 1974. Both
are statutory laws which are set down by legislature. EaWR
1989 advocates that the application of BS7671 for low
voltage installation is likely to achieve compliance. Two key
principles are referred to throughout both EaWR and BS7671
[5]:
(1) An electrical installation is to be fit for its intended
operational function;
(2) An electrical installation is to be safe during normal
operation when faults occur.
Responsibility of signalling power distribution and
systems was transferred to the Head of E&P in 1999. The aim
of this move by Railtrack was to improve delivery of
infrastructure performance, efficiencies and safety. The
application of the EaWR 1989 and BS7671 to signalling
power design, installation and testing on the railways were
enforced by E&P in a bid to improve these key principles in
railway signalling power supplies. This is still valid in current
practices today [5].
5
Existing IT Systems
At present 10% of signalling power systems used on today’s
railway infrastructure are the ‘existing’ type [4]. A key
difference between this and the legacy IT system is that it
uses a ‘distributed’ SEB/ CPC which is either an integral 3rd
core or external single core cable. It also utilises SWA for
mechanical protection. This will have an effect on double
PEFC scenarios as discussed in section 5.2. With the
provision of CPCs, ADS is certainly achievable in a double
PEFC situation.
The existing IT system improves on the feeder protective
device by utilising electronic protection relays (EPR). These
provide a cost saving on the size of cable as is discussed in
section 5.3.
5.1
Basic Protection
Basic insulation and Class I still provide basic protection in
the event of direct contact causing electric shocks.
However, power cables are now constructed and tested to
BS5467. These are either stranded copper or aluminium
cores, insulated with cross-linked-polyethylene (XLPE) to
provide increased conductor heat capacity, steel-wirearmoured (SWA) for increased mechanical protection and
finally a poly-vinyl chloride (PVC) sheath. Network Rail
standards only permit signalling power cable routing in lidded
troughs.
5.2
Double PEFC
Prior to a single PEFC, the fault protection is similar that of a
legacy IT system; it still utilises insulation of live parts from
earth. However, additional protection now operates a
distributed SEB, where the whole signalling distribution
network is bonded throughout. It is also earthed locally at the
trackside enclosures and single PEFC touch voltage is still
reliant on RE.
Following the event of a single PEFC, the IT earthing
system is transformed into a TN (Terra-Neutral) earth
referenced system. The distributed SEB conductor has
become a protective multiple earth (PME) conductor. This
change of conductor function is believed to exist only in
railway signalling power.
Figure 3 shows the simple circuit breakdown of a worst
case double PEFC occurring at the PSP and in a LOC at the
end of a signalling feeder.
Figure 3: Existing IT system – Double PEFC
The touch voltage can be calculated by:
Z  R  RL1  RCPC , 2 nd PEFC 
U
and
Z
U C  2 nd PEFC  RE (see BS7671 regulation 411.6.2)
RCPC = Resistance of the CPC.
The replacement of REA and RE with RCPC allows the
double PEFC to be a low impedance/ high current fault. An
EPR will now be able to disconnect a double PEFC in a
compliant time of 5 seconds for a distribution circuit and 0.1
seconds for a final circuit based on the Z calculation. This
meets BS7671 regulations 411.6.4 (i) and 411.3.2.3 (for a TN
circuit). Again, this is emphasised by the Network Rail
project advice notice, PAN/E/EP/PRO/0035.
5.3
Electronic Protection Relays
In existing IT systems, the use of EPRs is universally
practised as the primary source of overcurrent protection. It is
backed up by HRC BS88 fuses in case a problem develops on
the EPR causing it not to operate.
EPRs allow the user to manually set a disconnection time
when a certain fault current (or greater) is detected. This
allows the EPR to be ‘fitted’ around the ZS of a signalling
feeder circuit. In other words, the cable CSA sizing is no
longer dictated by the protection device. Now the greatest
influence is ensuring there is a maximum voltage drop (Vd)
of 10% [6]. This is far less stringent on the cable CSA, with a
reduction of typically up to 70% (based on a cable CSA
reduction from 120mm2 to 35mm2) being achieved. With
copper prices at £5786.46 per tonne, EPRs provide major cost
savings to a resignalling scheme [7].
Another safety benefit of an EPR is that they will
automatically disconnect both line conductors of an existing
IT system in the event of a double PEFC, whereas in certain
fault situations, only one fuse on one line will disconnect.
This is a safety requirement of BS7671 regulation 531.1.3.
A negative trait of the EPR is that they require a higher
degree of maintenance and testing compared with fuses.
5.4
Insulation Monitoring Devices
IMDs provide two levels of warning to prevent an indirect
contact electric shock from occurring. These are required on
all IT systems where supply availability is essential under
BS7671 regulation 411.6.3.1. The purpose of an IMD is to
monitor the efficacy of the insulation of an IT signalling
distribution cable, which over the course of time will
inevitably deteriorate. The IMD works alongside a remote
condition monitoring (RCM) logger to provide two levels of
fault warning for maintenance.
In the event of insulation resistance being reduced to
below 65kΩ, an ‘alert’ is recorded by the IMD information is
passed to the local maintenance via email or a fault message
on the control centre technician’s terminal. This notifies
maintenance of a reduced operational performance of the
signalling distribution cable and that closer investigation and
monitoring is required.
Below 20kΩ, an ‘alarm’ is sent to maintenance via the
method previously mentioned. However, this is a notification
of an imminent failure of the signalling distribution cable and
a response is required within 24 hours.
Both alerts and alarms are available during the normal
operation of the signalling power system. This provides an
advantage of not requiring an insulation resistance test to be
performed. This is a ‘dead’ test and therefore requires a
power outage, which would cause a disruption to the service
of trains. Furthermore, periods where trains are not running
are often difficult to obtain. Both the figures suggested are
recommended values. However, testing is required to fine
tune these values in practise [8]. Overall, IMDs increase the
reliability, safety and maintainability of all forms of IT
systems at a nominal cost.
5.5
the signalling feeder rather than remain at the point of
fault, as is the case of a legacy and proposed IT system. If
either the CPC or the local earth electrode becomes open
circuit, hazardous touch potentials may be reached [9].
[C] The liability of high fire risk is also to the detriment of
safety.
PRAMS
RELIABILITY = 2/3 (AVERAGE)
[P] Reliability of the system has been improved with the
introduction of remotely monitored IMDs.
[P] The PVC cable sheathing has a longer life span than
rubber/ EPR. In addition to this, the reliability of the
cable is also improved with the provision of SWA
mechanical protection.
[C] However, reliability is reduced, as the risk of high
current/ low impedance fault resulting in fires has
increased. Despite this, it is worth remembering that the
IT earthing system is a high impedance circuit. Therefore
the probability of a double PEFC occurring on two
separate line conductors is ‘extremely low’ [9].
AVAILABILITY = 1/3 (LOW)
[C] The provision of ADS is in keeping with the two key
principles of the EaWR 1989. However, the knock-on
effect of this is a reduction in the availability of the
railways due to an increase in the risk of disconnection of
supply.
MAINTAINABILITY = 1/3 (LOW)
[P] Maintainability has been improved with the introduction
of remotely monitored IMDs. This has reduced the
reliance on a low impedance earth electrode of at least
10Ω providing a compliant touch voltage. It has also
removed the need for an insulation resistance test.
Section 5.4 provides further information.
[C] To ensure the safety of the electrical installation, an
increased level of maintenance is required to ensure that
ADS (fault protection) is achieved. This is due to the
addition of earthing assets such as the EPR, CPC and
SWA, along with the earth electrode.
SAFETY = 1/3 (LOW)
[P] IMDs provide an improvement in safety (see section 5.4).
[C] In general, the IT system is expected to suffer from high
impedance faults rather than low impedance faults. The
maintenance required for the existing IT system is higher
than the others as it relies on the EPR and CPC for
compliance with ADS. Therefore, the likelihood of a high
impedance faults is greater for this type of system if its
condition is allowed to deteriorate. This may result in
non-compliant touch voltage being attained at the point of
fault and transferred across the entire feeder by the CPC
and SWA. Therefore, this system is highly reliant on
ensuring there is a maximum of 10Ω for RE [3] to
maintain a compliant touch voltage. As mentioned earlier,
a compliant earth electrode resistance is very difficult to
achieve and even more challenging to maintain.
[P] The benefits of the distributed SEB conductor/ CPC and
introduction of EPRs regarding safety and allowing ADS,
albeit at the cost of high maintainability, has been
discussed in sections 5.2 and 5.3.
[C] Unfortunately, the inclusion of this conductor has come at
a cost of decreasing safety by exporting faults. In the
event of single PEFC, excessive fault currents flowing in
both the CPC and SWA will be transferred throughout
5.6
Other Existing IT Systems Issues
and Developments
The requirement to calculate ADS times based on touch
voltages leads to an increase in design time and costs. On
large resignalling schemes, this may introduce significant
costs, which otherwise would be unnecessary on the legacy IT
system and as shall be examined, the proposed IT system.
Existing signalling power supplies designed today use
650V/110V transformers with a maximum inrush of 10 times
the route mean squared (RMS) current rating of the
transformer rather than the 15-20 times inrush of BR924 type
transformers [10]. The benefit of this is to reduce the impact
of transformer inrush on signalling feeder cable sizing, thus
providing economical savings in cable.
6
Proposed IT systems
How can the positive attributes of the legacy and existing IT
systems with regards to PRAMS be combined? The solution
is the proposed IT system.
This uses double or reinforced insulated equipment and
cables, also known as class II, as the provision for basic and
fault protection. The use of class II has previously been
applied to final circuits such as electric toothbrushes and
irons. However, this will be the first application of its kind on
a distributed circuit. The principle of class II is that as long as
all E&P equipment enclosures and cabling meet class II
ingress, impact, earthing and other protection requirements,
basic and fault protection is fully compliant under BS7671. In
addition, significant PRAMS advantages and cost savings can
be achieved. These are detailed in 6.1 to 6.7 [13].
6.1
Class II Power Transformers
Network Rail is presently preparing a standard for Class II
power transformers [12], [13]. The key requirements which
have been identified are:
 The primary side of the isolation transformer shall meet
or exceed the requirements of BS EN 61558-1. This
standard provides guidance on the performance
requirements of the power transformer regarding issues
such as insulation resistance, protective earth conductor
current and provisions for protective earthing.
 Supplementary insulation for fault protection shall
enclose the primary terminals and windings. It should be
possible to determine the transformer tapping connections
without the necessity for isolation, i.e. a transparent
supplementary insulation.
 The transformer core and the primary and secondary
insulated concentric copper earthing shield (interwinding
screen) shall be connected to an Earth Terminal integral
to the transformer. This is for the purpose of functional
earthing for local supplementary equipotential bonding.
 Both the core and windings shall be vacuum pressure
impregnated with an ingress protection rating of IP8X.
This shall prevent ingress of water in the event of total
immersion such as in the case of flooding.
 Labelling shall adhere to BS EN 61558-1 including the
external and internal warning labels.
 The power transformer inrush requirement is still 10
times the RMS current rating of the transformer.
This standard is currently in draft format and these key
requirements maybe changed, altered or removed.
6.2
Class II E&P Switchgears
Class II switchgear have been developed, based on
requirements of BS EN 61140 and BS2754:1976. These have
been design to exceed BS7671 ingress protection
requirements (IPXXB/ IP2X) by allowing for total immersion
due to flooding [9].
6.3
Class II (Unarmoured) Cables
The use of double or reinforced insulation as a form of fault
protection has to meet cable requirements to BS7671
regulation 412.2.4. This specifies that:
 the rating of the cable has to be equal to the nominal
voltage of the system as a minimum,
 mechanical protection shall be provided by the nonmetallic sheath of the cable.
This is in line with a recent study conducted by Cobham
Technical Services (CTS) which recommended the use of
unarmoured two-core cables for the distribution of signalling
power cables in the proposed IT system [9]. The proposed use
of unarmoured cable is still a highly controversial at this
moment in time. However, the greatest benefit of not utilising
an armoured cable is to mitigate the transfer of faults
throughout a signalling distribution system, which would
occur in an SWA cable.
So far, only one British standard unarmoured cable has
been identified for the proposed IT system signalling
distributed power cable, namely BS7889:1997. This is ideal
as it is aluminium or copper stranded core, XLPE insulated
and PVC sheathed cable which is rated to work up to
600V/1000V. These cable properties satisfy the class II
requirements. However, currently it is only available in a
single core with a minimum CSA of 50mm2. A two core cable
would be required for the proposed IT systems.
There is a full revision to the standard which is due for
public comment in 2011 and full release in 2012, specifically
BS7889:2012. The revision is currently undergoing
manufacturers’ comments. Its scope will cover cables of up to
5 cores and a CSA of between 1.5mm2 to 120mm2. Along
with this change, the cable marking arrangements and testing
methods are expected to harmonise with the European
standards (CENLEC). The insulation and sheathing type are
expected to remain the same as before. A low smoke fume
(LSF) sheathed version, BS8573:2012, is also due for release
with the same CSA, core number and voltage rating
properties. Both these cables are ideal for the proposed IT
system.
An enhancement to the unarmoured signalling power
cable can be achieved by routing the cable in troughing and
high impact PVC conduit for interconnections between
trackside enclosures and the troughing.
6.4
Basic Protection
By definition, class II equipment and cables have basic and
supplementary insulation (double layer) or reinforced
insulation. Therefore the risk of direct contact electric shock
is reduced to an ALARP level by the design of the cable.
6.5
Fault Protection
In the event of a single PEFC, a TT earthing system is
created. This follows the fault path of a legacy IT system. The
risk of touch voltages rising above 60V has to be mitigated, in
order to comply with the ‘permanent condition’ under BS EN
50122-1. This situation is no different to both incumbent
signalling power IT earthing systems.
However, the double PEFC protection has changed from
the existing IT system. The removal of the CPC has a
consequence of significantly increasing the disconnection
times to the point where ADS as a form of fault protection
can no longer be relied upon. The risk of an electric shock is
at the same level as a legacy IT system in this circumstance.
The risk of a single insulation failure and to an even
greater degree a double insulation on a different line
conductor has been reduced, by utilising a class II design, to
ALARP, as described in section 6.3.2. Despite some
scepticism to the use of class II as a form fault protection on
distributed circuits, compliance with BS7671 is ultimately
achieved. In addition to this new protective measure, existing
IT system protective measures encompassing the use of EPRs
and remotely monitored IMDs are used.
6.6
PRAMS
The positive attributes of the legacy IT system have been
combined with the similar attributes of the existing IT system.
RELIABILITY = 2.5/3 (AVERAGE/HIGH)
[P] The implementation of an IMD allows maintenance to
react to an alert/ alarm indicating poor insulation. This is
described in section 5.4.
[P] With a high impedance fault path in use, the risk of fires
caused by overheating cables and current arcing due to
high impedance connections is low. This is an
improvement for reliability as well as safety.
[C] In comparison with the BS5467 armoured cable used in
the existing IT system, the unarmoured cable has a lower
resistance to impact.
AVAILABILITY = 3/3 (HIGH)
[P] Class II is considered compliant to BS7671 for providing
fault protection. Therefore ADS as a form of fault
protection, is not required, equating to a high quantity of
supply availability and train performance. This is similar
to the legacy IT system and is explained in detail
throughout section 3.
MAINTAINABILITY = 3/3 (HIGH)
[P] Compared with the legacy IT system, maintainability has
been improved with the introduction of remotely
monitored IMDs, as cable condition assessments can now
be measured whilst in operation rather than based on
visual inspections and insulation resistance ‘dead’ testing.
[P] The removal of the CPC and SWA means that there are
fewer assets to maintain.
SAFETY = 3/3 (HIGH)
[P] The reduction in fire risk with the implementation of a
high impedance path provides a high degree of safety.
[P] As discuss earlier in section 5.3, EPRs, unlike fuses,
allow a safe disconnection of both line conductors.
[P] IMDs allow a high degree of safety with the warning of
poor cable insulation. This should alert/ alarm
maintenance of insulation degradation/ failure before an
indirect contact electric shock transpires. For more
information, see section 5.4.
[P] As discussed in the negative safety attributes of the
existing IT system, in section 5.5, faults are exported
away from the point of fault (LOC area) to other LOCs
and REBs along the signalling feeder by the CPC and
SWA. The proposed IT system does not utilise these, and
as a consequence does not suffer this safety issue.
[P] Class II mitigates the risk of electric shock to ALARP
levels.
[P] The proposed IT system is less reliant on the RE being a
maximum of 10Ω [3], as there are fewer earthing assets
to be maintained and no possibility of transferring a
dangerous touch voltage throughout the feeder.
6.7
Economical Benefits
Based on quotations from a leading cable manufacturer, the
savings on a:
 3rd copper core is approximately 35%.
 SWA is approximately 26%.
There are currently 29 type A signalling projects taking
place across the country in CP4 and a further 38 type A
signalling projects in the pipeline for CP5. With a typical type
A project requiring around 35km of signalling power cable, a
saving of £26 million is estimated across CP4 and CP5 by
utilising a 2c unarmoured cable compared with a 3c armoured
cable. This is based on a typical signalling power cable of
70mm2. This headline savings figure only takes into account
major enhancement projects (type A) and not smaller
signalling schemes. To compel this argument further, the cost
of raw copper has increased by more than 5 times over the
last 13 years and shows no signs of abating [7]. Therefore, the
savings from reducing the usage of copper is incremental
year-on-year.
However, a greater saving is expected as a consequence of
the performance improvements of the proposed IT system
with the advantage of supply availability combined with a
BS7671 compliant method of safety against electric shocks.
However, it is very difficult to quantify the savings in
reduction of train delay minutes.
Cable theft is a major problem on the railway networks
along with many other industries. This has cost the rail
industry £43 million as a result of 16,000 hours of train delays
over the past three years [14]. With the price of metals
continuing on an upward trend, this problem is unlikely to go
away. The proposed IT system removes the CPC from the
signalling feeder cable. This is expected to decrease the
incentive of copper theft as only line conductors are now
distributed. Disconnection of either or both of these will cause
an immediate signalling power outage which would alert the
maintenance and limit the time for committing this costly
crime.
A key improvement on the existing IT system is the
advantage of reduced maintenance because of the need for
fewer earthing assets. There is also a safety and
maintainability benefit in that the proposed IT system is less
reliant on the earth electrode. This will provide efficiencies by
driving down maintenance costs.
Finally, a reduction in costs can be achieved by driving
down the time and effort in the design of signalling power
supplies. This is due to the move away from ADS fault
protection and not having to consider the design intricacies of
the CPC and SWA.
7
Summary
Britain is more aware than ever of the importance of its
railways but at the same time the industry’s costs are under
scrutiny as never before.
This paper proposes a means by which the availability of
signalling power supplies, failures of which are a significant
source of delays, can be improved in a cost-effective manner.
The unarmoured class II system takes the strongest train
performance attributes of the legacy IT system in terms of
safety and maintainability. It then combines this with the
positive safety features of the existing IT system including the
introduction of IMDs and EPRs, improves protection against
electric shock and – finally – delivers significant PRAMS
benefits as is highlighted in section 6.6.
The use of class II systems can therefore be expected to
deliver significant savings and performance improvements at
a time when cost reductions are being demanded throughout
the railway industry.
Acknowledgements
[A] British Approvals Service For Cables (BASEC) and
British Cable Association (BCA) for information
provided on BS5467 and BS7889 cables.
[B] Messieurs Ron Checkman, Richard Allen, Tahir Ayub,
Andrew Button, Richard Dunsford, John Alexander and
Graeme Christmas of Network Rail, Pete Duggan of
Invensys and Mark O’Neill of Atkins for all their
contributions.
[C] City Electrical Factors Ltd. for information provided on
costs of cables.
References
[1] Network Rail Industry Performance figures (2011);
[2] BBC News – Office of Railway Regulation ‘frustrated’
at rail repairs – http://www.bbc.co.uk/news/uk13763264 (2011);
[3] SSI 8503:2008 – SSI Application Manual – Earthing
and Bonding of Solid State Interlocking (2008);
[4] Cobham Technical Services – Generic (Design Basis)
Safety Case Report for the use of Class II Equipment as
Protective Measure Against Electric Shock – 2011-048
Issue 1.0 (2011);
[5] Andrew Button CEng MIET MIRSE – A Concise Look
at UK Main Line Railway Signalling Power Distribution
Past, Present and Possible Future (2007);
[6] Network Rail Standard – NR/SP/ELP/27243 Issue 1
(2006);
[7] London Metal Exchange official copper price (£/tonne)
for 2nd June 2011 (2011).
[8] Network Rail Standard – NR/GN/ELP/27318 Issue 1
(2007);
[9] Cobham Technical Services – Final Technical Report
and System Design Specification for the use of Class II
Equipment as a Protective Measure against Electric
Shock – 2010-0762 Final Report (2010).
[10] Network Rail Standard – NR/SP/SIG/30007 Issue 1
(2007);
[11] Network Rail Standard – NR/SP/ELP/27244 Issue 1
(2006);
[12] Network Rail DRAFT Standard – NR/L2/SIG/30007
Issue 2 (2011);
[13] Tahir Ayub – Signalling Power Supply Distribution
Strategy Proposal – STPE\EDS\EW12\GS4\CII Issue
3.1 (2009);
[14] British Transport Police – Cable Theft Issue http://www.btp.police.uk/passengers/issues/cable_theft.a
spx (2011);
[*] BS7671:2008 Requirements for Electrical Installations
(2008);
[*] BS EN 50122-1:1998 Railway applications – Fixed
Installations – Part 1: Protective provisions relating to
electrical safety and earthing (1998);
* Continually referenced throughout paper;
[P] Positive (pro) PRAMS attributes;
[C] Negative (contra) PRAMS attributes.
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