What is the purpose of protective devices?
The purpose of a protective devices is to open the circuit before any damage can be done to the conductor and equipment
What is an overload?
a moderate increase in current beyond the normal rated current value.
Typical values of overload currents may range as high as________ the normal currents.
6 times
Typical causes for an overload are:
- too much equipment connected to a circuit, referred to as the
octopus effect - use of improper size equipment—for example, small
equipment for a big job - a worn-out or seized motor bearing, or excessive mechanical load.
What is an overcurrent?
an extreme increase in current well beyond the normal rated current value.
The ways overcurrent can result when:
- A motor starts- When a motor starts, it initially draws a current much larger
than its normal full-load current. - A short circuit occurs. Within a few thousandths of a second, a short-circuit current can
become hundreds of times larger than the normal operating current.
Two causes of a short circuit are:
- Failure of the conductor’s insulation, which causes the conductor to
touch a metal casing and create a line-to-ground short - a foreign object makes an accidental contact, causing a line-to-line
short
Before selecting or replacing a protective device for an installation, the four critical factors are______, _______,________ and ________
Voltage rating, current rating, interrupting rating , and total clearing time
Voltage rating relates to the ability of the protective device to:
- quickly extinguish the arc after the fuse element melts or the circuit breaker opens
- prevent the open-circuit voltage in the system from re-striking across the open
fuse element or across the circuit-breaker contacts.
Standard fuse ratings are:
15A, 20A, 30A, 40A, 60A, 100A, 200A, 300A, 400A, 600A, 800A,
1,200A, and 1,600A.
what rating is the maximum short-circuit current an overcurrent protective device can safely interrupt without damaging itself.
Interrupting rating
Many circuit breakers and older-style fuses have an interrupting rating of 10,000 A
HRC (High Rupturing Capacity fuses) commonly have an interrupting rating of _________
200,000A
The time-delay fuses must be marked with the letter “_” for delay.
Marked “D”
The non-time-delay fuses must be marked with the letter “_” for low melting point.
Marked “P”
What are the two low-voltage fuses?
- plug fuses
- cartridge fuse
The plug fuse is sometimes called the ________________.
screw-base or Edison-base fuse.
the maximum rating of any plug fuse 30A
The ratings generally available are 15 A, 20 A, 25 A and 30 A
true or false
True
The maximum rating of any plug fuse is
30A
The Code states that plug fuses with different ratings must be interchangeable
True or False
False, it need to be non interchangeable
