Page 237 - Offshore Electrical Engineering Manual

224 CHAPTER 4 Busbar Protection

However, earth connections should be maintained and checked for resistance value
on a routine basis, particularly to prevent dangerous potentials appearing on the
switchboard frame during fault conditions. An earth or bonding bar interconnects the
framework of each cubicle, but is only earthed via the conductor monitored by the
frame earth relay. Where the circuit breakers are in trucks, the truck must also have
a heavy current earth connection to the bonding bar to allow earth fault current flow
from the circuit breaker. To prevent currents induced in cables from causing spurious
tripping, the gland plates at the switchboard end should be insulated from the switch-
board frame but earthed to a second separate earth bar which is directly earthed to
the main earth point.

DIFFERENTIAL PROTECTION
Kirchhoff’s first law may be directly applied by monitoring incoming and outgo-
ing currents at all the switchboard external circuit connections, summating them
and detecting the error, if an internal switchboard fault exists. The switchboard
may be divided into a series of zones, and each zone boundary should be treated
in the same way, as shown in Fig. 4.4.4. The relay settings must be such that the
transient flux produced by through-fault currents will not cause tripping because
of the imbalance due to unequal burden or saturation of CTs. By the use of high-
impedance relays with series stabilising resistors, the voltage across the relay dur-
ing through-faults, known as the stability voltage, may be kept below the relay
operating voltage. The minimum value of the stabilising resistor for stability may
be calculated as follows:

V S = Relay setting current × (R SR + R R )

where V , stability voltage; R , resistance of stabilising resistor and R , resistance
R
SR
S
of the relay coil.
An example calculation is given in PART 4 Chapter 7.
For fault calculations see PART 4 Chapter 6.
For relay discrimination topics see PART 4 Chapter 7.