EC (Fuse,Disconnet, Etc) Chp 2

Question 1

What two factors must be provided for in any electrical power or control system?

Answer 1

•Protection (from overcurrent, short circuits, faults)

• Control (start/stop functions, regulation, sequencing)

Question 2

Under what conditions would you use a time-delay fuse? Give an example.

Answer 2

Starting motors or transformers, where high initial current is normal but short-lived.

Example: Protecting a motor circuit that has a high starting current.

Question 3

What are the four types of circuit breakers?

Answer 3

• Thermal
• Magnetic
• Thermal-magnetic
• Electronic (solid-state or digital)

Question 4

When would you use the magnetic trip feature in a circuit breaker?

Answer 4

When instantaneous protection from short circuits or very high overcurrents is needed.

Question 5

List at least five important factors when selecting a protective device:

Answer 5

1. Voltage rating
   
   2. Current rating
   3. Interrupting capacity
   4. Time-current characteristics
   5. Environmental conditions (temperature, enclosure, etc.)

Question 6

6. Are more than two poles available on disconnecting devices? If so, how many?

Answer 6

Yes, disconnecting devices are available with up to 6 poles or more, depending on the application (commonly 1 to 4 poles).

Question 7

What is meant by the interrupting capacity of a fuse?

Answer 7

It is the maximum current the fuse can safely interrupt without exploding or failing destructively.

Question 8

What does quartz sand filler in a fuse provide?

Answer 8

Effective heat transfer
It also helps extinguish arcs quickly during high-current interruption.

Question 9

Why is the inverse time characteristic of a fuse important?

Answer 9

Because it parallels the characteristics of conductors, motors, and transformers, providing selective and time-coordinated protection.

Question 10

What are two measures of the degree of current limitation provided by a fuse?

Answer 10

1. Peak let-through current (Ip)
   
   2. I²t (energy let-through)

Question 11

How are I²t values of a fuse derived?

Answer 11

They are calculated from testing under standard conditions by integrating the square of the current over the operating time during a fault.

Question 12

When will the highest overvoltage occur from a switching surge?

Answer 12

When switching no-load transformers or long, lightly loaded transmission lines, particularly with capacitive or inductive loads.

Question 13

How does a lightning arrester protect equipment?

Answer 13

It diverts surge current to ground by becoming conductive during overvoltage (like lightning), clamping the voltage and protecting equipment

Question 14

Determine the interrupting capacity for a 15-kVA, 1-phase transformer with 4% impedance on 480V, 60Hz.

Answer 14

FLC = 15,000
——— = 31.25 A
480