單項(xiàng)選擇題Since fuse elements are made of zinc or any alloy of tin and lead, the melting point of the fuse element must be ().

A.higher than that of copper
B.lower than that of copper
C.equal to that of copper
D.reached when the conductor it is protecting becomes "white hot"


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1.單項(xiàng)選擇題The most common source of excitation for synchronous motors is a/an ().

A.low voltage battery
B.motor attenuator set
C.DC exciter generator
D.AC supply

2.單項(xiàng)選擇題Before reassembling any machinery, you should ().

A.replace all bearings regardless of length of service
B.apply a heavy coat of oil to all mating surfaces
C.clean any corroded surfaces and file all burrs smooth
D.coat all parts with alemite grease

3.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

4.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

5.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