Page 80 - Offshore Electrical Engineering Manual

CHAPTER

Generation and 5

Distribution Switchgear

and Transformers

SWITCHGEAR – GENERAL REQUIREMENTS
Switchboards are the nodal points in any electrical system. They must be designed to
provide a safe, reliable means of carrying and directing current flows where they are
required and must be able to withstand the thermal and magnetic stresses involved in
clearing faults. This fault capability must include:

1. With the circuit breaker initially connecting two healthy sections of the system,
the ability to interrupt a fault immediately after it occurs in a fast and reliable
manner. This is known as the fault breaking capability.
2. The ability to connect a live section of the system to a faulty section and then to
immediately interrupt the resulting fault, in a fast and reliable manner. This is
known as the fault making capability.

For an in-depth discussion on the operation of circuit breakers, the reader is rec-
ommended to refer to the J&P Switchgear Book (See Bibliography). The following
is a brief introduction to the principles and should assist the engineer in selecting
switchgear of adequate performance for the application.
Switchboards on vessels and platforms are normally classified in one of the fol-
lowing categories:

• Main switchboards, to which the generators are connected.
• Primary distribution switchboards, supplied either directly or via transformers
from the main switchboard.
• Emergency switchboards, fed normally from the main switchboard, but with
a direct supply from the emergency generator for emergencies. On ships
emergency generators have traditionally not been provided with parallel-
ing facilities, as it was considered that the emergency generator should be
reserved for emergencies. (Note: On platforms, emergency generators are
increasingly being fitted with paralleling facilities. However, the benefits of
easy load testing of the emergency generator and flexibility of supply have to
be weighed against cost of upgrading the emergency switchboard fault rating
to allow this.)
• Main and emergency distribution boards fed from the appropriate supply and
located close to the supply users.

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