單項(xiàng)選擇題Ammeters and voltmeters used in sinusoidal AC power systems indicate which of the following values of the waveforms measured?（）

A.Peak value
B.Root-mean-square value
C.Average value
D.Maximum value


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1.單項(xiàng)選擇題Fuses placed in series with a thermal trip-type circuit breaker are used for （）.

A.time-delay protection
B.short-circuit protection
C.short duration surge protection
D.sustained overload protection

2.單項(xiàng)選擇題The main difference between a motor control circuit containing low voltage protection and low voltage release is that the latter contains （）.

A.a magnetic operating coil
B.normally open line contacts
C.thermal-overload protection
D.a momentary-contact start button

4.單項(xiàng)選擇題The part of a fuse that melts and opens the circuit is made of （）.

A.copper and antimony
B.steel and babbitt
C.aluminum or beryllium alloy
D.zinc or an alloy of tin and lead

5.單項(xiàng)選擇題Time delayed or delayed action-type fuses are designed to （）.

A.prevent grounds in branch circuits
B.prevent opens in motor circuits
C.permit momentary overloads without melting
D.guard lighting and electronic circuits