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 . 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 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 . 2fC 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 , 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 . 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 4126.96.36.199 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  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 : (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 Existing IT Systems At present 10% of signalling power systems used on today’s railway infrastructure are the ‘existing’ type . 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 4188.8.131.52 (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% . 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 . 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 4184.108.40.206. 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 . 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 . [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’ . 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  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 . 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 . 6.1 Class II Power Transformers Network Rail is presently preparing a standard for Class II power transformers , . 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 . 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 . 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 , 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 . 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 . 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  Network Rail Industry Performance figures (2011);  BBC News – Office of Railway Regulation ‘frustrated’ at rail repairs – http://www.bbc.co.uk/news/uk13763264 (2011);  SSI 8503:2008 – SSI Application Manual – Earthing and Bonding of Solid State Interlocking (2008);  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);  Andrew Button CEng MIET MIRSE – A Concise Look at UK Main Line Railway Signalling Power Distribution Past, Present and Possible Future (2007);  Network Rail Standard – NR/SP/ELP/27243 Issue 1 (2006);  London Metal Exchange official copper price (£/tonne) for 2nd June 2011 (2011).  Network Rail Standard – NR/GN/ELP/27318 Issue 1 (2007);  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).  Network Rail Standard – NR/SP/SIG/30007 Issue 1 (2007);  Network Rail Standard – NR/SP/ELP/27244 Issue 1 (2006);  Network Rail DRAFT Standard – NR/L2/SIG/30007 Issue 2 (2011);  Tahir Ayub – Signalling Power Supply Distribution Strategy Proposal – STPE\EDS\EW12\GS4\CII Issue 3.1 (2009);  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.