Page 125 - Offshore Electrical Engineering Manual

112 CHAPTER 11 Power Electronics (Semiconductor Equipment)

this form of failure, as battery or distribution board failure will extinguish all the
lamps on the system.
Although luminaires with integral emergency batteries require slightly
more maintenance than luminaires without batteries, the benefit, in terms of
increased system reliability by avoiding the risk of common mode failure, is very
considerable.

DIRECT CURRENT SUPPLIES

The majority of secure supply systems are direct current (DC) and are required for
the following.

Engine-starting batteries: The normal criterion for generator starting is that at
least six consecutive start attempts each of 10 s cranking are possible from a
fully charged battery without further recharge. In the case of fire pump start bat-
teries, this is raised to 12 attempts from each of the two banks of batteries. This
would be further reinforced by a second means of starting, such as a hydraulic
pump device or compressed air motor.
Class A equipment: Fire and gas panels, emergency radios and public address
(PA) systems are usually called category A or class A equipment. They are pro-
vided with duplicate chargers and batteries, each battery/charger being capable
of supplying the rated load for the specified discharge time with mains failed. It
is important to ensure that the manufacturer’s design provides a fully duplicated
system. There should be no common components in the rectifier or voltage
regulation system, and the connection point for the two supplies, i.e., the distri-
bution board, should be protected by blocking diodes and fuses from failure of
one of the battery/charger units.
It is also normal practice to obtain the AC supplies for the two chargers from
different points in the supply system in order to increase overall reliability.
Class B equipment: All other DC supplies, with the possible exception of some
subsea equipment (see Part 2 Chapter 11), are supplied by single battery/charger
units. However, for improved reliability a compromise of two 100% rated char-
gers with one battery may be used. As most of the components of the system are
in the charger, an improvement in reliability can be obtained without the extra
weight or bulk of a second battery.
Switchgear tripping supplies: Offshore practice is similar to that for onshore
substations. For reasons of reliability, each supply should be dedicated
to one switchboard. Batteries should have sufficient capacity for the trip-
ping and closing of all circuit breakers on the switchboard in succession,
i.e., twice, followed by the simultaneous tripping of all the circuit break-
ers. There should also be sufficient capacity to supply the continuous drain
imposed by any control relays and indication lamps for a reasonable period
(e.g., 8 h).