Within a trackside case, equipment known as the Functional
Supply Point (FSP) normally converts the 650V AC to 110V AC
using transformer(s). The FSP contains transformers and rectifiers
to convert the power distribution voltage to the AC and DC
voltages used by the track-side train control system equipment.
It may also accommodate lockable switchgear, to allow for safe
working practices on the power distribution and train control
equipment.
Location Cases (Locs) accommodate railway signalling
equipment to detect the location of trains, control the trackside
signals and switch the points. Locs and FSPs can be the same
physical asset, containing both power equipment and train
control equipment. The equipment may typically use both AC
and DC voltages less than 110V, so transformers and rectifiers are
required to obtain the correct voltage from the incoming supply.Class I and Class II protection
Protection arrangements in power distribution networks are
generally Class I or Class II. In Class I, exposed metal parts must
be connected together and connected to electrical earth by a
separate earth conductor (coloured green/yellow in the UK). The
basic requirement is that no single fault can result in dangerous
voltage becoming accessible so that it might cause an electric
shock and that, if a fault occurs, the supply will be automatically
disconnected. Traditionally, railway signalling power supply
distribution systems have been based on Class I individual earth
designs, which require an effective earth arrangement.
Class II, or double-insulated, electrical equipment is designed
in such a way that it does not require a safety connection to
electrical earth. The basic requirement is that no single fault can
result in dangerous voltage becoming accessible so that it might
cause an electric shock, and is achieved without relying on an
earthed metal casing. This is usually met by having at least two
layers of insulating material between live parts and the user, or
by using reinforced insulation. Not only is there a safety benefit
with Class II, but the availability is far greater as the supply is not
tripped due to a cable fault.In domestic situations, Class II power supplies (such as laptop
chargers) will have a two-wire power cord as opposed to a
three-wire power cord with a safety earth connection. Products
designed with Class II insulation are often labelled as “Class II” or
“double insulated” or will have a concentric square safety label
symbol.Earthing
BS 7671 (the IET Wiring Regulations - informally called “the
regs” by some) covers the primary types of power system
earthing arrangement called TN, TT or IT. These use the French
terms for Terre (earth), Neute (neutral) and Isolé (isolated).
The first letter indicates the connection between earth and the
power-supply equipment, so ‘T’ indicates a direct connection of a
point with earth (Terre) while ‘I’ means that no point is connected
with earth (Isolé).
The second letter indicates the connection between earth
or network and the electrical device being supplied, so ‘T’
corresponds to an earth connection by a local direct connection
to earth (Terre) or ‘N’ shows that the neutral connection is
supplied by the electricity supply network (Neutre).
The traditional way of distributing the power trackside along
the railway for signalling has been using the IT earthing system.
The output of the source transformer is isolated from earth, whilst
all exposed conductive parts of the installation are connected to
earth.
The source transformer output is isolated from earth, although
the system will be still earthed by the stray capacitances of
the cables. Should there be a fault with the cable insulation or
FSP equipment, creating a direct short circuit to earth, there is
no return path for the earth fault current with the equipment
remaining operational.58 FE ATURE