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Post by railtechnician on Jan 25, 2017 16:08:03 GMT
On Southern Region electrified lines the DC traction voltage induces a constantly changing ‘banshee wail’ background noise on telephone conversations. Apparently the problem existed as long ago as 1915 as contemporary correspondence between the GPO and LSWR indicates, the interference badly affecting GPO trunk lines.
These days the interference, commonly referred to as commutation noise still exists despite ever changing technology over the last century.
I have my own theories as to the cause but having never had anything to do with NR traction supplies, traction motors, or signalling track circuits and never having heard the interference myself I am wondering if anyone here can throw any light upon the elusive cause of this issue.
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Post by dave1 on Jan 26, 2017 15:19:33 GMT
I have a lot of noise on my phone line but I can't blame the SR as I am a nowhere near them. Apart from the obvious what are the difference's compared to LU. I have read that on the SSL lines they have increased the voltage to along the lines of the SR. The types of rolling stock differ though as there seems to be a mixture traction motors. No help to you I'm afraid.
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Post by railtechnician on Jan 27, 2017 4:07:45 GMT
I have a lot of noise on my phone line but I can't blame the SR as I am a nowhere near them. Apart from the obvious what are the difference's compared to LU. I have read that on the SSL lines they have increased the voltage to along the lines of the SR. The types of rolling stock differ though as there seems to be a mixture traction motors. No help to you I'm afraid. Apparently the explanation for the interference and how to mitigate it has flummoxed telephone engineers for the last hundred years, an issue I was unaware of until a contact on a comms list asked me if I had any ideas. My theories relate to the balanced return of traction voltage through the running rails and the impedance bonds which separate the traction return current from the track circuit current although I am unaware of the details of SR traction or signalling. It may be that the balance is lost due to change of track electrical characteristics, with changing weather conditions, with rerailing piecemeal and the loss of balance being amplified by the traction motors change of power draw through acceleration, coasting and braking. The interference frequencies are reported to be in the 500Hz range which is at the low end of telephone speech frequency band, 300Hz to 3400Hz. |
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Post by dave1 on Jan 27, 2017 8:26:25 GMT
I have a lot of noise on my phone line but I can't blame the SR as I am a nowhere near them. Apart from the obvious what are the difference's compared to LU. I have read that on the SSL lines they have increased the voltage to along the lines of the SR. The types of rolling stock differ though as there seems to be a mixture traction motors. No help to you I'm afraid. Apparently the explanation for the interference and how to mitigate it has flummoxed telephone engineers for the last hundred years, an issue I was unaware of until a contact on a comms list asked me if I had any ideas. My theories relate to the balanced return of traction voltage through the running rails and the impedance bonds which separate the traction return current from the track circuit current although I am unaware of the details of SR traction or signalling. It may be that the balance is lost due to change of track electrical characteristics, with changing weather conditions, with rerailing piecemeal and the loss of balance being amplified by the traction motors change of power draw through acceleration, coasting and braking. The interference frequencies are reported to be in the 500Hz range which is at the low end of telephone speech frequency band, 300Hz to 3400Hz. I think I have heard of unbalanced traction current protection on LU, I have seen something but can't remember where. I think the Victoria/Piccadilly (Heathrow) and Jubilee lines but perhaps I am going in the wrong direction. |
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Post by railtechnician on Jan 27, 2017 11:12:00 GMT
Apparently the explanation for the interference and how to mitigate it has flummoxed telephone engineers for the last hundred years, an issue I was unaware of until a contact on a comms list asked me if I had any ideas. My theories relate to the balanced return of traction voltage through the running rails and the impedance bonds which separate the traction return current from the track circuit current although I am unaware of the details of SR traction or signalling. It may be that the balance is lost due to change of track electrical characteristics, with changing weather conditions, with rerailing piecemeal and the loss of balance being amplified by the traction motors change of power draw through acceleration, coasting and braking. The interference frequencies are reported to be in the 500Hz range which is at the low end of telephone speech frequency band, 300Hz to 3400Hz. I think I have heard of unbalanced traction current protection on LU, I have seen something but can't remember where. I think the Victoria/Piccadilly (Heathrow) and Jubilee lines but perhaps I am going in the wrong direction. I am thinking of impedance bonds with regard to balanced traction return through the running rails, this is not found on the Picc Vic or Jube where the traction return is not via the running rails. I believe it exists on the Bakerloo north of Queen's Park and that is the only part of LU network that I can recall such being mentioned at the same time as blockjointless track circuits were installed. It is probably the case on those parts of the DR which are NR assets i.e. west beyond Putney Bridge and Turnham. |
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Post by dave1 on Jan 27, 2017 16:01:53 GMT
I think I have heard of unbalanced traction current protection on LU, I have seen something but can't remember where. I think the Victoria/Piccadilly (Heathrow) and Jubilee lines but perhaps I am going in the wrong direction. I am thinking of impedance bonds with regard to balanced traction return through the running rails, this is not found on the Picc Vic or Jube where the traction return is not via the running rails. I believe it exists on the Bakerloo north of Queen's Park and that is the only part of LU network that I can recall such being mentioned at the same time as blockjointless track circuits were installed. It is probably the case on those parts of the DR which are NR assets i.e. west beyond Putney Bridge and Turnham. I don't think that there was that on the lines I mentioned I must be talking about something else. The Bakerloo only has the third rail so like you say is to do with the bonding on that section and the others you mention. |
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Post by railtechnician on Jan 28, 2017 3:22:25 GMT
I am thinking of impedance bonds with regard to balanced traction return through the running rails, this is not found on the Picc Vic or Jube where the traction return is not via the running rails. I believe it exists on the Bakerloo north of Queen's Park and that is the only part of LU network that I can recall such being mentioned at the same time as blockjointless track circuits were installed. It is probably the case on those parts of the DR which are NR assets i.e. west beyond Putney Bridge and Turnham. I don't think that there was that on the lines I mentioned I must be talking about something else. The Bakerloo only has the third rail so like you say is to do with the bonding on that section and the others you mention. You are perhaps thinking of the continuous rail connection to both traction rails via resistors (usually around 10k and 20 k respectively ) to earth which is used to fix the reference voltages of +450v and -180v such that traction earth faults can be detected and indicated in the line control rooms for attention. A train can run with a traction earth fault if the fault is on the train but it will carry that fault moving it as it travels into each current section. On the other hand a traction earth fault can be a conducting path on the actual traction rails. anything from a discarded coke tin to lumps of metal, shopping trolleys etc thrown on the track by vandals. A moving traction earth fault will get a train taken out out service for repair, a fixed traction earth fault will get a Technical Officer hunting for the problem from the cab of a train running through the affected section at walking pace and in my time it was get out the cab and use a 2lb hammer to detach the usually burning tin can from the pozi (the most likely rail for such a fault) with the current still on. That often got a nice shower of sparks, especially in the tunnels, for larger items in open sections one would usually ask for traction to be discharged while the offending item was removed with bare hands. I suspect these days that the juice is always discharged to clear fixed traction earth contact faults. |
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