Line I

overload

is a moderate increase in current beyond the normal rated current value, overloaded equipment ect

overcurrent

an overcurrent may range from six times to many hundred times of normal rated currents, short circuit , ground fault ect

short circuits

is a larger than normal current which flows outside the normal current path. it is a parallel path of low resistance

short circuit damage to circuits

thermal and magnetic stress damage

fault current is

not normally present in a cct, but is the maximum current that can flow should a fault occur

fault current can be calculated by

Isec / z (percent impedance) in the secondary of a transformer

if overloading is permitted it can cause

annealing, rapid oxidation, burning insulation, cracking insulation

typical causes of an overload are

too much equipment connected, improper size equipment, worn out or seized motor bearing / excessive mechanical load

the role of protective devices is to

open the cct before any damage can be done to the conductor and equipment

Table ___ in the cec to find maximum ampacity of fuses and breakers

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voltage rating in protective devices relates to

quickly extinguish the arc after the fuse elements melt or the cct breaker opens

prevent the open circuit voltage in the system from restriking across the open fuse element or cct breaker contacts

voltage rating of a fuse or circuit breaker must be

equal to or greater than than the voltage of the circuit or system

the ampere rating in a protective device relates to

the rating the current device can carry continuously without opening

when selecting a fuse or breaker your ampere rating should

not exceed the current carrying capacity of the current

interrupting rating

is the maximum short circuit current an over current protective device can safely interrupt without damaging itself

time current characteristics

for any fuse the more severe the fault(the higher fault current), the faster the fuse responds. time current characteristics compare fault magnitude and speed of response

time delay fuses must be marked the letter

“D”

non-time delay fuses must be marked with the letter

“P” for low melting point

besides where current exceeds the fuses current rating, a fuse may occasionally open by

high ambient temperature

when the connection is loose or dirty

excessive vibration

when replacing a fuse consider these

voltage rating

current rating

interrupting-capacity rating

time-current characteristic

two general types of fuses

plug fuses

cartridge fuses

max rating voltage rating and amp rating of a plug fuse

150v to ground, 30a

the fuse is designed to be

the weakest link in the circuit. its job is to break the circuit before damage is done to the conductors

arc quenching powder in a fuse is

a fire-extinguishing powder that prevents an arc from continuing or bursting through the tubing of the fuse

all renewable fuses are

non time delay

overload situation in a fuse

when a overload exceeds a fuses current rating for a sufficient length of time, the fuse melts

short circuit situation in a fuse

the links melt instantly, the large current causes portions of the links to vaporize

a fuse with time delay

delays opening in a overload situation, the time required for the fuse to open depends on its time-current characteristic curve

time delay fuses are sometimes referred to as

dual element fuse

standard fuse interrupting capacity is

10,000 a

hrc fuses are

high rupturing capacity fuses that interrupt large short circuit currents without disintegrating, at the same time, they protect cables and equipment

hrc fuses are

non interchangeable, that is they are designed in such a way that other fuses will not fit into their holders

hrcI fuses

provides protection from both overloads and short circuits, has a high interrupting capacity of 200,000a

hrcII

fuses only give short circuit protection only

a fuse with current limiting ability

opens the circuit before the fault current reaches its max value

most standard fuses have a rating of

600a, 600v

circuit breakers

an electro-mechanical device designed to open a current carrying capacity under both overload and short circuit conditions without injury to the device

trip free in a circuit breaker

this means that the internal tripping mechanism must trip an overload or short circuit even if the operating lever is held in the on position

greatest advantage of circuit breakers

that is they are resettable

circuit breakers have 5 major components

frame or case

operating mechanism

trip elements

contacts

terminal connection

circuit breaker operation

the handle turns the circuit breaker on and off. it is quick make, quick break type

thermal element in circuit breakers

is a heat sensitive element bimetallic strip , excessive current or overload causes one of the metals to bend and force the other over it and opens the circuit

magnetic element in a circuit breaker

short circuit or ground fault protection element uses an electromagnetic device, when a short circuit occurs, the fault current passes through the electromagnet to attract an armature mounted on the trip bar, and opens the contacts

magnetic only breaker

is an instantaneous trip circuit interrupter , this only provides short circuit protection but no overload protection. used for motors ect

hic breaker

high interrupting capacity breaker, used when the available short circuit current often exceeds the interrupting capacity of standard breakers.

75.000 a and higher

gfi breaker

the ground fault circuit breaker is a thermal-magnetic breaker that incorporates a solid state, ground fault sensing circuit to detect ground currents of 6ma and higher