Page 88 - Offshore Electrical Engineering Manual

Switchboard Construction 75

resulting from this technology is compact and light, with a much reduced maintenance
requirement. Foundation shock due to operation is very much reduced due to the light-
ness of contacts and the small distances they have to move apart for fault clearance.
Interrupters and contactors can be checked for adequate vacuum by applying a test
voltage across the contact when in the open position. 25 kV ac is required for testing a
12 kV interrupter. Due to the permeability of the various ceramic and metal construc-
tion materials, the operational life is limited due to a slow loss of vacuum, but by using
a ‘getter’ it is normally in excess of 20 years.
Short circuit ratings of up to about 40 kA are available for circuit breakers operat-
ing up to 12 kV. This may not be sufficient on some of the larger installations where
prospective fault currents and operating voltages may be higher. Because of the size
weight and space advantages of vacuum interrupters and contactors, and the reduced
fire hazard, this type of equipment is recommended for offshore use.

SULPHUR HEXAFLUORIDE (SF6) CIRCUIT BREAKERS
The SF6 medium voltage circuit breaker competes with the vacuum device for low
weight and bulk and can be considered as an alternative in most cases. Sulphur hexa-
fluoride has a dielectric strength several times that of air and good heat dissipation
properties. Low pressures are required to be maintained (usually less than 2 bar) and
it is unusual if topping up is required more than once every 2 years. Suitably high
fault ratings are available for most offshore situations.

SWITCHBOARD CONSTRUCTION
GENERAL
The following paragraphs list some important recommendations for switchgear pro-
curement specifications. The configuration of switchboards is discussed in PART 1
Chapter 2 and reliability aspects in PART 8 Chapter 1. In all cases, offshore switch-
boards must be highly resistant to the salt-laden corrosive atmosphere, and suffi-
ciently moisture protecting to prevent the ingress of water, particularly from above
(IP54 minimum). The enclosure must also physically protect operators from any arc-
ing, flames or flying fragments due to mal-operation.
Major changes to switchboards, once installed offshore, are extremely expensive,
and depending on the planned operational life expectancy of the offshore installa-
tion, as much spare equipped capacity should be incorporated into the switchboard
as possible. The spare auxiliary contacts of circuit breakers, contactors, switches and
relays should all be wired out to terminals as a matter of course by the manufacturer.
Whether racked or single mounting methods are used for protection relays, space for
additional relays or spare relay rack spaces should be provided.
Facilities for adding extra switchboard cubicles at either end should also be
provided.