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Maintenance Planning
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Electric Switch Heating
Switch heating exists for the sole purpose of making sure that points continue to work in all weather conditions, specifically to prevent freezing in winter. Heating is used only in the winter period since this is the time when the weather is at its most adverse. There are set temperatures at which the heating will come on, these being 2°C for wet weather (snow and rain) and -1°C for dry weather. This is decided because, if there is no moisture present then there cannot be any possibility of ice forming, and freezing the points. All switch heating is controlled from a control cubical, usually found on the line side. Pictures of a control cubical can be seen in Figs 21 and 22. Each cubicle in turn controls transformers, which drop the voltage to 110V. Each set of points has its own controlling transformer.
On top of the control cubical, a moisture sensor can be found. This determines if there is any precipitation in the air. The moisture sensor can be seen below.
These photographs show both the individual switch voltage transformer
and the inside.
Switch heating voltage transformer
The control unit within the control cubicle has an LED display. This gives several pieces
of information. It records:
Temperature at which the heating comes on
Cold rail temperature
When the terms hot and cold rail is used, this refers to the temperature sensors present on
the track. A sensor next to a strip heater measures what is called the hot rail. A sensor
3 m away from the heat strips measures what is called the cold rail. This is 3 m away so that
there is no heat transfer from the strips to the cold sensor.
Electrical switch heating
The picture on the left shows a typical electrical switch heating installation. The thin strip travelling the length of the rail is the
heating strip.
Also visible is the distribution, junction box. This splits the incoming 110V into two for each heating strip.
Although one control cubical may control several sets of points, there is usually only one
set of temperature sensors. These measure the temperature for all sets of points.
Maintenance work is general monthly inspections of all plant and electrical equipment. A set
procedure is undertaken when testing lineside equipment. This is as follows:
The cold rail temperature sensor is sprayed by the engineer with one of two things; chewing
gum remover or a freezing spray. This brings the temperature down to approximately -25°C.
Then, to prevent the heating immediately switching itself off again, the hot rail sensor is
sprayed.
Spraying the temperature sensor
The switch heating strips are further tested by spraying water on them. If the water
evaporates straight away, the heating is working.
An alternative method of testing them is to manually override the heating from the control
cubical, and then testing the current running through the wires in the transformers. This
method does not require going on the track, and is just as effective.
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