Protection Devices

Question 1

What is meant by overcurrent?

Answer 1

An excess of current.

Caused by either short circuits or overloads

Question 2

What do overcurrent devices protect against?

Answer 2

Short circuits
Overloads
Ground faults
Lightning surges.

Question 3

What is the active function of a fuse?

Answer 3

To open a circuit under abnormal current conditions.

Question 4

What is the most common type of fuse used?

Answer 4

Cartridge fuses.

Question 5

Do most non-time delay fuses contain a thermal cut-out?

Answer 5

No.

The thermal cut out provides overload protection through a soldered connection to the fuse link.

Question 6

What percentage of rated continuous current would a thermal cut out provide a time- delayed response?

Answer 6

Up to 500%.

At fault levels above 500%, the fuse link would open quickly.

Question 7

What are the two basic types of circuit breakers?

Answer 7

Instantaneous (Magnetic)
Thermal magnetic (time-limit).

Question 8

What is the most common type of breaker?

Do these provide short circuit, or overload protection?

Answer 8

Thermal magnetic.

Both.

Question 9

What does “trip free” mean?

Answer 9

It means that a breaker is “free to trip” even if mechanically held open.

Question 10

Do most motors require overload protection?

Answer 10

Yes.
Due to a high inrush current on start the overcurrent device must be oversized and therefore only works to interrupt short circuit current.

Question 11

What are the two main types of overload devices?

Answer 11

Thermal.

Magnetic flux.

Question 12

What are the end blades of a fuse called?

Answer 12

Ferrules.

Question 13

What kind of material are fuses generally filled with?

Answer 13

Arc quenching material,

Question 14

Which table in the CEC is used to size overcurrent devices for a motor?

Answer 14

Table 29.

Question 15

What are the two amperage ratings on an overcurrent device?

Answer 15

Continuous rating.

Interrupting rating.

Question 16

Under short circuit conditions, how fast would a thermal magnetic breaker trip?

Answer 16

3-5 milliseconds.

Question 17

What are the primary factors which determine fault current?

Answer 17

Circuit voltage.
Impedance of short circuit path.
kVA rating of supply transformer.
percent impedance of the supply transformer.

Question 18

Can a fault current be higher than expected in a circuit with motors?

Answer 18

Yes.

Motors act as generators when decelerating and can increase the fault current.

Question 19

What criteria must be observed to select a proper motor disconnect?

Answer 19

Manual action.
Lockable.
Break all ungrounded conductors @ locked rotor current.
Acceptable horsepower rating.

Question 20

What are acceptable motor disconnects?

Answer 20

Fused/unfused switch.
MMS (manual motor starter/switch)
Circuit breaker
Plug (portable motors).

Question 21

When are manual motor starters frequently used?

Answer 21

For small motor loads under 1/3 hP.

Question 22

What is a motor magnetic?

Answer 22

An assembly consisting of a contactor and an overload relay.

Question 23

When sizing an overload heater, what values must be considered?

Answer 23

Motor FLA (full load current),
Motor service factor.