Protective devices

A switch has ----------------

• one state

• two state

• three state

• None of the above

The least expensive protection for over current in low-voltage system is

• Rewirable fuse

• isolator

• Circuit breaker

• Air break switch

Best protection is provided by HRC fuses in case of

• Open circuit

• short circuit

• both a and b

• none of the above

A relay used for protection of motors against overload is

• Impedance relay

• Electromagnetic attraction type

• Thermal relay

• Buchholz's relay.

Fuse protection is used for current ratings up to

• 10 A

• 20 A

• 30 A

• 100A

The acting contacts for a circuit breakers are made of

• Stainless steel

• Hard pressed carbon

• Porcelain

• Copper tungsten alloy.

Fuse wire, protection, system is usually not used beyond

• 10A

• 25A

• 50A

• 100A

A fuse wire should have

• Low specific resistance and high melting point

• Low specific resistance and low melting point

• High specific resistance and high melting point

• High specific resistance and low melting point.

Ionization in a circuit breaker is not facilitated by

• high temperature of surrounding medium

• material of contacts

• increase of field strength

• increase of mean free path.

Low voltage circuit breakers have rated voltage of less than

• 220 V

• 400V

• 1000 V

• 10,000 V.