Page 103 - Offshore Electrical Engineering Manual

90 CHAPTER 7 DC Switchgear

dangerous situation of the arc remaining unquenched, outside the chute. Modern
circuit breakers are designed to avoid this condition.
Typical DC loads and fault currents are highly inductive, and circuit breakers
must be capable of dissipating all the energy stored in the circuit until arc extinction.
Such highly inductive loads will maintain the arc for longer whilst the stored energy
in the load circuit decays. This will place more stress on the contacts, so two-stage
contacts are now used, where the magnetic coils ensure that the arcing contacts take
the onerous arcing duty whilst the main contacts will carry the load current when the
circuit breaker is in the closed position.
All circuit breaker mechanisms (including alternating current) should be ‘trip
free’, i.e., if a fault occurs during closure (making fault), the tripping action must
still be available and the circuit breaker does not need to close first. Obviously, once
the circuit breaker is closed and latched, all necessary protection device trips must
still be operationally live. Trip circuit supervision should be provided, where the trip
circuit continuity is monitored and provided with an alarm facility in a manned area.
Fig. 2.7.1 shows a typical modern DC circuit breaker.
Other arc quenching methods may be used.

• ‘Arc runners’ will assist with avoidance of critical current by leading the
arc away from contacts, also driven by electromagnetic forces from blowout
coils.
• A ‘puffer’ stream of air may be used to assist moving the arc into the arc
chute.

SPECIFICATION

Modern DC circuit breakers should be specified for

• rated voltage and impulse voltage,
• maintaining dielectric strength,
• rated switching current for load and overload,
• trip time performance, i.e., high speed, and current limiting,
• current sensing – directional or bidirectional,
• thermal performance for continuous rated current,
• fault performance, i.e., arc containment of pressure/safe venting of gasses and
heat dissipation.

Variable-frequency inverter drives are covered in PART 2 Chapter 11.