Page 236 - Offshore Electrical Engineering Manual

Frame Earth Protection 223

arcing faults than medium-voltage switchboards because with increasing voltage, the
reduction in fault current due to arc resistance becomes less pronounced as the arc
voltage ceases to be a significant proportion of the total fault circuit driving voltage
(see Reference 1). Hence, the importance of earth fault protection for low-voltage
switchboards is stressed.

FRAME EARTH PROTECTION
This type of system has been used successfully offshore and consists of a frame earth
relay and neutral check relay arranged to monitor earth fault current flows into and out
of the switchboard. A simple frame earth system is illustrated in Fig. 4.4.3. Care must
be taken when installing the switchboard into the module such that it is and will remain
insulated from the steel of the module construction. The generator earthing resistor and
the switchboard frame should be earthed at the same point to avoid both series earth
connections and increasing the risk of the fault current being limited below the oper-
ating setting of the relay by a poor connection due to corrosion of the decking steel.
This is less likely to be a problem in an onshore substation using earth electrodes.

FIGURE 4.4.3
Busbar frame earth protection scheme.
Courtesy GEC Alsthom Measurements, now part of GE Grid Solutions.