A
railway must be supported with a safe and reliable
power system for signals, points, interlockings and
communications. This power system must be tolerant
against failure and provide a constant, stable supply in
order for the control and communications equipment to operate
correctly.Often either taken for granted or overlooked, power systems for
railways can be difficult to provide, given that the infrastructure
is long and narrow and not always close to an external power
source. In addition, the earthing arrangements differ from
domestic, industrial and commercial electrical systems, a situation
that has caused compliance issues with safety requirements.
The signalling power supply must always be available, with a
continuity of supply similar to other safety-critical applications, for
example airport ground lighting or hospital operating theatres.
So, it is not acceptable to turn everything off when a fault arises,
which has been a traditional electrical engineering solution for
such hazards.
The power system must be safe, both for maintenance staff and
for members of the public, when the equipment is located on
platforms or at level crossings, even should faults occur that result
in accessible metal becoming live.
Railways have invested a lot of time, effort and money in
providing reliable power supplies with multiple sources of energy,
but, in some cases, they have neglected the power distribution
network that moves the power to the current-using equipment.
However, over the last few years, new maintenance processes and
the introduction of Class II equipment have improved matters,
after a lot of effort by both signalling and power engineers in
Network Rail and the supply industry.Railway signalling equipment
PAUL
DARLINGTONPrincipal Supply Point
A signalling power supply comes onto the rail infrastructure
from a Distribution Network Operator (DNO) network via a
Principal Supply Point (PSP). DNOs are companies licensed to
distribute electricity in the United Kingdom. At the PSP, the
incoming supply is converted to the distribution voltage of,
typically, 650V. The incoming supply is usually 400V, but can be
other voltages.
To maintain continuity of supply, PSPs use additional sources
in addition to the 400V DNO supply, including the railway’s own
three-phase high voltage traction-power system and standby
generators.
A PSP for an important route may also have an uninterruptible
power supply (UPS), so that the supply is maintained during the
changeover between the two sources. In simple terms, a UPS is
a big battery fitted with an inverter that converts the battery’s
DC output to AC. It may also provide insulation monitoring
equipment.Functional Supply Point
The major elements of a railway signalling power system are
the power sources, the distribution network and electrical loads
(sometimes referred to as current-using equipment). The power
supply voltage is often transformed down to a lower voltage to
minimise voltage drop when distributed over longer distances.
Historically, a two-wire 650V AC distribution system has been
used for railway signalling power in the UK.POWER SYSTEM EARTHING
56 FE ATURE