Traction current is metered at the feeder station per section, because a section drawing power with nothing in it is a fault to earth, and a fault to earth on twenty five thousand volts is a fire. Section 12 draws forty amps from about one in the morning until the first train, every night, and there is no fault.
I am an overhead line engineer and the feeder station that supplies my sections is where this starts.
Why a section drawing current with no train is serious.
The wire carries twenty five thousand volts. A train draws current from it through a pantograph and returns it through the rails, and the current transformer at the feeder sees that as a load that comes and goes with the trains.
With no train in a section the current should be near enough zero. A few amps of leakage across dirty insulators in the wet is normal. Forty amps at twenty five thousand volts is a megawatt, and a megawatt going somewhere that is not a train is going into the ground through something, and things that a megawatt goes through catch fire.
So a section drawing forty amps with nothing in it gets a call to the control room and an inspection.
Section 12 draws forty amps, plus or minus a couple, from about one in the morning until the first train enters it at about five, every night.
The current comes on over a few seconds and goes off the same way. It has done it for at least six years, which is as far back as the feeder station's logs go.
The first night anybody noticed, in 2019, they tripped the section and sent a team out expecting to find a fire.
There was no fire. There was nothing. The section was walked in the dark with lamps and thermal imagers and the wire and the insulators and the return path were as they should be.
They reclosed the breaker. Forty amps.
What has been done since, and this is a subject the railway takes very seriously.
Insulation tests on every insulator in the section, twice. Clean.
The section isolated at both ends and the feeder measured with no section connected. Zero. Connected again. Forty.
A megawatt has to go somewhere and it makes heat where it goes. Every structure, every earth bond, every return conductor and every rail joint in that section has been thermally imaged at three in the morning, in three different winters. Nothing is warm.
The current transformer itself. Replaced. A second one added in series as a check. Both read forty.
And the thing that makes it what it is: the return.
Traction current goes out on the wire and comes back on the rails and the return conductor, and those are metered too. On section 12 at night, forty amps goes out on the wire and forty amps comes back on the return.
A fault to earth does not come back on the return. It goes into the ground. A megawatt that goes out and comes back is not a fault. It is a load. It is something between the wire and the rail, in that section, drawing a megawatt for four hours, that a thermal camera cannot see.
The load is a train's load.
Forty amps at that voltage is about what one of the units on that route draws when it is standing with its auxiliaries on and its heating running, on a winter night, with the pantograph up.
I have put that sentence in a report once and taken it out again.
What is done about it.
The electrical control room has a standing note that section 12 shows a known overnight load and that it is not to be tripped for it, which was the right decision after the third team walked the section for nothing.
The energy is billed. The railway pays for what the feeder meters and the feeder meters a megawatt for four hours a night, which is a few hundred pounds, which nobody has ever asked about.
What I do.
I do the section inspections and the insulator maintenance on the schedule.
The one thing I have changed is that I do not do section 12 on a possession that runs past one in the morning.
A possession is when the line is closed and the wire is isolated and earthed so we can work on it, and the earthing is the thing your life depends on. You put the earths on, you test dead, and the wire is safe.
In November 2022 we had a possession on section 12 from midnight to four, earths on at both ends, tested dead, and I was on a platform under the wire at about ten past one with a colleague, on a contact wire that was earthed and dead and that I had tested myself.
The feeder station logged forty amps on section 12 at eight minutes past one, into a section that was isolated at the breaker and earthed at both ends, and my colleague, who was holding the wire in a gloved hand at the time, said it had gone warm, and we came down and we did not go back up.