Cable Testing Basic guide to cable testing for newly qualified SMTH staff or trainees. This is for information only. The SMTH MUST ALWAYS BE FOLLOWED AT ALL TIMES. F. M. Spowart June 2018 v1
Cable Testing Page One of Ten Why do we test cables? Copper is an extremely good conductor of electricity, but if it becomes unprotected or damaged, it can either fail to operate a circuit, allow voltages to escape to earth where circuits in the same cable or circuits in another faulty cable could be incorrectly energised. Therefore we carry out three tests to see how healthy the cable is and to prevent potential wrong-side failures. Continuity Test: We carry out a continuity test to ensure the core is terminated in its correct place according to the diagrams, is jointed correctly, and to ensure it is continuous and intact. A damaged core that has only a few strands left will only carry some of the flow of current and eventually fail to energise whatever is fed by that core. The lower the ohms reading the more the core is conductive. A core that is cut will show a zero (or O/L) reading. Insulation Test Core to Core We also carry out one of two insulation tests to ensure that the protective rubber that surrounds the inner cores and the main outer sheath is intact. A damaged outer sheath allows water to penetrate which over time will cause the cores to corrode and may allow circuits in the same cable to be false fed. Insulation Test Core to Earth If both inner and outer protective sheaths degrade, this may cause earth faults to occur where circuits close by could be falsely energised by escaping voltages. So a core to earth test is carried out to see just how healthy or poor the outer sheathing is. Insulation tests are carried out using an insulation tester (more commonly known as a megger ) that can test up to 1000 volts. The higher the reading the healthier the cable. What happens if the cable fails the Insulation Test and all or most of the cores are flat? With both C- E and C-C testing, we require a pass of 1 meg ohm* or above. Anything lower than 1 meg (more commonly referred to as being flat ), and the cable MUST be signed out of use and be investigated. This could be caused by a number of things; a damaged section, wet joint, earthy connections or termination blocks, a short or simply that the cable is life expired and requires renewal. *This is for existing in-service cables. New cables require a pass of 10meg ohms. Only a signal engineer can give permission to leave a cable in service under 1 meg, and in some cases below 250 ohms. If you are involved in testing a cable and it fails, you MUST inform control and the on-call supervisor which will want to know the area, the actual signalling equipment affected, the readings, the train service and how it will be affected, and how many spares are in the cable for diversion purposes. The engineer on call may give permission to keep the cable in service and arrange an urgent renewal. It will all depend on what equipment is affected, just how low the readings are and how it will impact the train service. Some equipment may also be restricted whilst others remain in service such as indications only. It is therefore imperative that the exact reading is written down on the test sheet no matter how small it is. A decision is then made on the actual reading.
Cable Testing Page Two of Ten What happens if just a few cores fail? If you attend a fault and find that the core causing the fault is disconnected ( dis ), in order to fix that fault you will have to divert that core to a healthy one and that means a full cable test MUST be carried out to ALL of the working cable, unless a deferral is granted by the on-call supervisor, this is where they will give permission to test just 10% of the spares, as long as there are sufficient. If the rest of the cable or a good portion of it tests well, you will be given permission to divert those faulty cores. See the SMTH Part 2 Appendix D paragraph D2 for information on diverting faulty cores. What happens after the diversion? See SMTH Part 2, Appendix E for the process of monitoring and periodical testing of the faulty cable. How is cable testing carried out? Only training and frequent practise suffice in the real world to get used to testing a cable. The continuity and insulation tests are straight forward enough providing the correct cable is tested and it is disconnected fully at both ends and you understand what the readings mean. The function test which is the final test to ensure the correct equipment is tested and working as designed. This may be more complex due to what equipment is supplied by the cable, what is required to energise those circuits or how the cores are wired in the loc(s) as some cables are simply not terminated in any order by core number. Also some areas have their own unique functions and identification letters. Where should I test? Unfortunately over the years and through regions, cable terminations aren t to a specific (single) standard and whilst some diagram link analysis can be straight forward, others are more complicated. Choosing the wrong cable can have consequences when testing, but this should be apparent by carrying a identifying continuity test on a few cores to ensure you are on the correct cable well before you megger it. However, serious consequences can arise if you terminate a cable on the wrong side of links and in the wrong order. Circuits can be polarity conscious and also pick up contacts from a relay, so if these are incorrect, you could quite easily cause a wrong side failure and potential serious accident. These are samples below from various diagrams over the years: 1 2
Cable Testing Page Three of Ten 3 4 5 6 7 As you can see they are all different in their own way. On each one except example numbers 3 & 6, they have a column labelled LEFT and RIGHT or L1/L2 and R1/R2 so you can see which cable is terminated on the left or the right of the links. On numbers 3 & 6, some of the cores are split by a forward slash or diagonal line. The letter or number on the left of the slash or diagonal line is terminated on the left side of the links. It is imperative you get the correct side of the links when testing a cable, but more importantly when it comes to removing a cable for renewal, you do not want to cut off the wrong cable!
Cable Testing Page Four of Ten Diverting circuits if cable tests poor Correctly diverted core: 145/62A Incorrectly diverted core: 145/70A 146/25B Faulty Core 523 TPR B112 HR B112 HRN Row A Loc 145/62A Row B Loc 145/70A Loc 146/25B The two diagrams above show a correct and incorrect diversion. The top diagram shows the cable in question coming from loc 145/62A to loc 145/70A and in and straight out again. The diversion wiring is done correctly with no issues. The second diagram now shows the same diversion. However on this occasion it shows that there are intermediate contacts of a TPR relay. (Shown here as a simple example and not as it would be seen on the actual link analysis).this diversion is dangerously incorrect and will have serious consequences. This diversion has bypassed the contacts for 523 TPR. Therefore a train stood on track circuit number 523 wouldn t be detected, which would allow signal B112 to clear on to the train which could result in a serious collision by a following train. It is imperative that the circuit to be diverted MUST NOT miss out any contacts in between. You must ensure that diagrams are read correctly and that you and your assistant are at the correct locations. The same error could occur if loc 145/70A was missed out entirely and the diversion was carried out in loc 146/25B instead. A walk through checking all locations in between may be the only option to ensure that no part of the circuit is missed. If there are any doubts, you must STOP and inform your supervisor. Note: when carrying out a diversion you MUST ensure that the diagrams are marked Maintenance Copy. If the diagrams are marked with anything else such as Commissioning, New Work etc, you MUST STOP and advise the on-call supervisor. This is because there may be ongoing work and circuits or wiring may be temporary.
Where Binding posts are used Cable Testing Page Five of Ten Dotted lines signify example showing internal wiring Binding Post B LHS 5143 T1PR Binding Post B RHS Cable A Cable B Loc 51/33 Normally cable cores are terminated onto links, pick up internal relay functions via internal wiring and then leave on a separate pair of links (as shown in diagram on page 4 for loc 145/70A). However in some locations binding posts are used instead, but these are becoming rarer and are now not preferred. This is where the cable cores are terminated onto the left hand side of a pair of links, pick up internal functions using internal wire and then leave on the right hand side of the same pair of links. Binding posts MUST always have red dome nuts fitted to prevent a link being inserted. If a link was inadvertently fitted, the contacts of that function would be bypassed and could result in a serious wrong-side failure. When removing a cable for renewal, as SMTH tester, you MUST ALWAYS make a note of any links inserted AND removed, as well as any red dome nuts fitted on your SMTH log slip, as you or the next tester could inadvertently insert a link where a binding post is used. Where a faulty core on a binding post is to be diverted, you must ensure that all contacts in that circuit are included and are function tested before the cable is returned to service. Always consult the SMTH on the correct way to divert faulty cores with red wire as this may change in future.
Cable Testing Page Six of Ten Procedure for testing a cable Continuity Test: Two Core Cable Only: If you are testing a two core cable, you MUST use a DC source (such as a small 9v battery or the 50v scale on a megger) and MUST not rely on an ohms reading on a meter. This is because the cable must have the correct polarity at both ends especially when feeding a DC circuit. A cross in the cable, such as incorrect jointed cores will not be picked up by a meter on ohms. However, you still need an ohms reading with a meter as this will show how healthy the connection is. Multi-Core Cables: Anything above a two core cable is tested with a meter as a reading must be obtained for the cable test sheet. 1. You must ensure you are working on the correct cable by physically tracing or by bell test, this is by choosing a good pair of spares (usually the already dissed test lines which are usually cores one and two) and asking your assistant at the other end of the cable to short them out whilst you obtain a reading. Ask your assistant to remove the strap and reapply it to be sure you are on the correct one. It is also best practise to do this on some other spare cores (if available) and also to go through the cable diagram and check together that the same circuits apply on that cable. Do not rely on link row numbers as these are likely to be different at each end. 2. Obtain permission from the signaller to disconnect the cable (only if you haven t received a deferral to test 10% of the spares). 3. Put your meter on cores one and two. 4. Ask your assistant to do the same with the strap. 5. Take a reading on ohms and record result. 6. Ask your assistant to take it off, make sure you lose the reading and repeat. 7. With the strap still on core one, ask your assistant to move the other end to core three. 8. Take a reading on ohms. If reading is good, use core one as the reference core. 9. Repeat this on all cores recording the results. If at some point, you don t get a reading. Make a note and move on. If you later come to the same scenario, for example no reading on cores 6 and 9, it s likely they have been crossed. To find a cross, leave the reference core as number one and with the meter go onto the suspect core and ask your assistant to go down each core with the strap (again with one end of the strap on core one) until you obtain a reading. Either have the problem corrected in the loc or joint and retest ALL of the cable again.
Cable Testing Page Seven of Ten Insulation Test: 1. Taking great care on where each core is terminated, put on the 24-way test straps on all the cores of the affected cable. 2. If there are any straps that are spare, these must be bagged up and kept clear of any other wire, loc case or earth and any body parts. 3. If both sets of straps are used (for example for a 27-48c cable), plug both plugs of the test straps together. 4. Turn on and test the megger for battery power. 5. Test the earth point in the loc. To do this plug in both pos and neg leads into the megger and put one end onto the test earth and the other into the ground outside of the loc. On 1000v range obtain a reading. This MUST be ZERO, if not disregard the test earth point and use the ground instead. 6. Turn the megger off. 7. Plug in the test strap plug(s) into the neg side of the megger. 8. Plug the red test lead into the megger. 9. Turn on megger and set to 1000v. 10. Tell anyone near the cable to stand clear to avoid electric shock risk. 11. Test the whole cable to earth and record result on cable test sheet. 12. If the reading is above 1 meg, it is a pass*, however if the reading is a fail you must test the cores to earth individually at the end of the core to core test But DO NOT remove the test straps yet. Note: Even if the cable is just above 1 meg, it is advisable to test each core individually. *10meg for new lineside cables. 13. Turn off the megger. 14. Remove one of the leads from the 24-way test strap from core one. Place it WELL CLEAR of all other cores and any metallic parts. 15. Turn on the megger, set to 1000v and put the red pos lead onto core one. 16. Obtain a reading and record result. 17. Return the first lead on to core one and remove the lead from core two. 18. Obtain a reading and record result. 19. Repeat by removing each lead, record result, replace same lead, then remove the next and so on. 20. Once all cores are tested individually, remove all test straps very carefully. 21. Unless an individual C E test is required (at step 12 above), pack away all the test straps and megger. Function Test: This is very important that this is carried out correctly, with people placed in specific locations watching what is happening as voltage readings are taken as disconnections and reconnections are made. You must, depending on what equipment is tested, that signaller s indications are observed, points are checked on site on the state of their lie, signal aspects are observed on the ground and relay positions are observed. Make sure that any other equipment in the cable such as level crossings or any other equipment is observed for the correct position or indication. Anything on the ground that can physically change state MUST be checked in person. You must also ensure that no stray voltage is present at the first function or at the final function when circuits should be de-energised. The first function is the first item of equipment after the affected length, such as a motor, relay, indicator, signal etc. The final function is the indication at the signaller s panel or the equipment on site fed by the circuits under test. Correct polarity is extremely important in function testing. Most circuits have their own positive and negative supply; however some are either shared, or are reversed. For instance, a signal circuit may share a negative for the indications going back to the signalbox, or the circuit polarity is reversed and so the pos and neg wires are swopped over indicated by a number 1 & 2.
Cable Testing Page Eight of Ten Referring to the this diagram, you will see that some circuits have a 1 & 2 after their function, this means that neither core is a dedicated pos or neg and the polarity will be swopped over depending on what the points (in this case) are doing. This could be both indication and controlling circuits so it is extremely vital it is correct. If these cores were terminated incorrectly or jointed incorrectly, this could have serious consequences as they could move the wrong way or indicate normal when they are laid reverse and thus potentially derail a train at high speed, so that s why the polarity must be observed and the actual lie of the points on the ground and the signalbox is corresponded together. Signals, and other circuits such as AHB indications and block also use polarity conscious circuits. Always check the diagrams. Before function testing, you will also need to know which way the voltage is fed in order to see who needs to observe the meter and who needs to slip the links. A basic rule of thumb is that control circuits (circuits that power over points, change signal aspects or operate barriers etc ) usually feed out from the signalbox or RR, so WR s, HR s etc, will feed out through the loc out to the equipment, whereas, indications from points, signals, tracks etc will be feeding back. So if you are at the loc the furthest from the signalbox or RR, you will need the meter on the circuit feeding out. But be aware that not all circuits are energised and some may require some routes or special functions setting by the signaller. Some people test the circuits in their own way. For example, some may put all of the links in and then slip them individually. This makes it easier to test as all the circuits will be in, so tracks will be clear to move points or clear signals. However, if done in this way, you must make sure you make a physical note of all circuits that are tested and those that still need doing, especially if you will test to the signaller separately later on. The other method is to leave all the links out and test them individually; however, you can only then test in a specific way. For example, you need to put in tracks first, then points, then signals, and then barriers, plus any other equipment* or each signalling item will not move/clear with the others disconnected. *Some equipment may also have to be done in a specific order. With everyone in position with a meter each (with 150k ohm shunt fitted) and appropriate nut spinners/link extraction tool, follow the steps on the next page.
Cable Testing Page Nine of Ten 1. Choosing the first circuit and depending on feed direction, ask your assistant to put on their meter (with 150k ohm shunt fitted) on the same circuit. Use core numbers as a reference and confirm you are on the same circuit as each other. Do not use link termination numbers as these will probably be different in each loc. 2. Ask them if they have any voltage. Confirm they have none. If voltage is present, STOP work immediately and investigate why. DO NOT proceed until the erroneous source is found. 3. Ask the signaller what the equipment is showing. DO NOT ask leading questions such as are those points showing normal. Always ask can you tell me what.. points are showing, that way they will have to look rather just lazily answering with yes. Signallers are nearly always distracted by other operations happening at the same time and therefore what they see as the easiest and quickest answer may have serious consequences later on for you. 4. Confirm that the equipment is indicating what you expect. 5. Ask your assistant(s) what position the corresponding relays are in. 6. If any of the equipment on the ground will change state, you must also observe this yourself or ask another assistant to check at all stages of the testing. 7. Reconnect first link. Ask your assistant what their reading is and confirm that there is still no voltage present. 8. Confirm signaller s indication hasn t changed and ask them to observe any changes as soon as they happen. Note: older style interlockings such as early SSI systems take a little longer to register any changes. 9. Reconnect the second link. 10. At this point you would expect a voltage AND the polarity to be confirmed, the signaller to state any changes and your other assistant(s) to state any relay/equipment physical change. However it s up to YOU to confirm clearly each change of state by asking each person individually. 11. Disconnect the first link. 12. Observe from all parties that the equipment has changed state. 13. Disconnect the second link. 14. Observe from all parties that the equipment has not changed state. 15. Reconnect first link. 16. Observe from all parties that the equipment has not changed state. 17. Reconnect second link. 18. Observe from all parties that the equipment has changed state and it is what you expect to receive. 19. Repeat all steps for each piece of equipment. 20. Repeat all steps for other equipment that is fed the opposite direction (this time by you observing the meter and your assistant slipping the links). To make this easier, you could test all equipment with your assistants, then after, disconnect all links and you can then test them to the signaller. You will probably find that they are either too busy to stay on the phone for the entirety of the full test with all parties or they will get bored and start saying in, out, back again, rather than stating the signal aspect colour, or points normal, reverse, etc. Make sure they repeat the functions correctly, NOT in or out etc as these may mean something totally different to you or to what you are expecting. This is also the same for your assistant. Having an assistant say up or down for a relay is not good enough. You need to know which relay is doing what, so 123 NKR de-energised and 123 RKR energised should be spoken. Also having your assistant on the meter saying polarity is right way around/correct is not good enough. He must be stating positive polarity on core., negative polarity on core... He may be unsure himself on which way around the polarity actually should be, so you need to hear it spoken. This applies to ALL circuits not just polarity conscious ones. YOU are responsible for signing that cable back into service, so the signaller and your assistants MUST understand what you require from them. Remember that ALL phone calls are recorded, which also means the actual duration of the test will be logged too, so it should never be rushed.
Cable Testing Page Ten of Ten If you are using back-to-back radios, you must also be aware of loss of communication during a critical talk over and may miss vital information, so make sure comms is repeated if using radios. Also due to the lag in time when using radios and especially mobile phones, this may seem that some equipment has energised before others, for instance, a relay could seem to energise after indication is obtained if your assistant is at the relay with a mobile and you are next to the signalbox.!!!remember THE SMTH MUST BE FOLLOWED AT ALL TIMES!!! F. M. Spowart June 2018 v1.