Year 2 H6

Question 1

Define: Overload

Answer 1

moderate increase in current beyond normal rated current value

Question 2

Some problems caused by overload

Answer 2

Annealing
Rapid oxidation
burning insulation
cracking insulation

Question 3

Some causes of overload

Answer 3

octopus effect
improper size equipment
worn-out or seized motor bearing
excessive mechanical load

Question 4

Define: Overcurrent

Answer 4

may range from 6 to many 100 times the normal rated current. fast

Question 5

An overcurrent can result when:

Answer 5

A motor starts

Short cct occurs

Question 6

Short cct

Answer 6

larger-than-normal current that flows outside the normal current path.
parallel path of low resistance

Question 7

Ground fault

Answer 7

another form of short cct. It is a short cct to ground. GFCIs, and GFI

Question 8

The effects of short ccts

Answer 8

Mechanical forces: bent bus bars and equipment

Thermal energy: I^2R x t

Question 9

Voltage rating of a protective device is to

Answer 9

Quickly extinguish the arc after the fuse element melts or the cct breaker opens
Prevent the open-cct voltage from restriking

Question 10

Interrupting rating

Answer 10

max short-cct current an overcurrent device can safely interrupt without damaging itself

Question 11

time-delay fuses must be marked

Answer 11

with the letter D for delay

Question 12

non-time delay fuses must be marked

Answer 12

with the letter P for low melting point

Question 13

inverse time-current characteristic curve

Answer 13

shows that the higher the current, the quicker the fuse opens

Question 14

a fuse my occasionally open when

Answer 14

ambient temp is high
excessive vibration
loose or dirty connection

Question 15

When replacing a fuse make sure and observe the correct

Answer 15

voltage rating
Current rating
Interrupting-capacity rating
Time-current characteristics

Question 16

Plug fuses is sometimes called

Answer 16

screw-base or Edison-base fuse
W for standard non-time delay
D for time delay

Question 17

A plug fuse is not to be used

Answer 17

in a cct with more that 125 V between conductors unless

the cct is supplied from a system with a grounded neutral and there is no conductor at more than 150 V to ground

Question 18

three types of cartridge fuses

all three types come in standard and HRC

Answer 18

Ferruale-contact
Knife
bolt-on

Question 19

dual-element fuse

Answer 19

Time-delay fuse
marked with D
Code requires it on cycling loads such as motor cct

Question 20

Type P fuses

Answer 20

Low melting point

interrupting capacity of 50 000 A

Question 21

Two types of HRC

Answer 21

HRCI (form 1) overload and short cct protection or wires and equip
HRCII short cct protection only

Question 22

List the four main types of cct breakers

Answer 22

Thermal
Thermal-magnetic
Magnetic
Solid State

Question 23

Function of circuit breaker

Answer 23

protects connected apparatus against overloads and short cct in low-voltage under 750V
Advantage is that it is resettable

Question 24

Operation of cct breaker

Answer 24

quick-make, quick-break type
trip-free
some have thermal and mag trip not all

Question 25

GFI breaker

Answer 25

detects ground currents of 5 mA or greater