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單項(xiàng)選擇題

It （） for an emergency generator to require paralleling.

A.is usual
B.is not usual
C.is necessary
D.is not impossible

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單項(xiàng)選擇題

Usually the panels are interlocked with （） so that the devices can be made dead before the doors are opened.

A.protective earth
B.isolators
C.ground switch
D.circuit-breakers

單項(xiàng)選擇題

In （） the control action is independent on(of) the output.

A.a closed loop control system
B.an open loop control system
C.a boiler control system
D.a steering gear control system