44 / 108
44 E L EC TR I C AL CONNEC T I ON
AU T UMN 20 1 6
GIVE ‘EM A BREAK
C
ircuit breakers play a vital role in
all electrical systems – residential,
commercial and industrial.
Breakers are rated in amps, the
amount of which is plainly printed on the
end of the handle.
The purpose is to automatically open
a circuit in the event of over-current,
by which is meant electrical current
in excess of the rated ampacity of the
conductors.
If the ampacity is exceeded, the
temperature of the wire will rise. This
is usually due to an increase in supply
voltage, short circuit caused by a line-
to-line or line-to-ground fault, or an
increase in the connected load.
Prolonged or repeated temperature
rise will damage the wire insulation.
In a severe case, the copper can get
red hot inside the wall, igniting nearby
combustible material and causing a
widespread fire.
Breakers perform the additional
function of acting as a manual switch.
After a fault has been corrected, the
tripped breaker can easily be reset.
Fuses are also effective over-current
devices. A moulded-case switch looks
just like a breaker and snaps into a load
centre in the same way, but it has no
over-current interrupting capability.
Codes require that if you brace the
handle of a breaker in the on position
the device will still trip without moving
the handle in the event of overload.
Circuit breakers have been around
for a long time and they are extremely
reliable. They were used in early
telegraph networks long before the first
power distribution system was built.
If a breaker becomes faulty, it is
preferable for it to trip when there is no
overload rather than fail to trip when
needed. In fact, that is how they are
designed. Breakers do not stick or lock
in the on position. The price we pay for
that kind of performance is that they
occasionally nuisance trip.
To prevent damage to a computer’s
hard drive, an inexpensive
uninterruptible power supply (UPS) will
provide continuity of power when a
circuit cuts out.
Life support equipment, either in
a health-care facility or in the home,
generally has elaborate automatic back-
up power. Increasingly, this set-up is
seen in family homes.
When an over-current device powers
down a branch circuit, the first move is
to determine whether it is performing
its protective function or just nuisance
tripping. For the most part, circuit
breakers are reliable and rarely fail. It is
a simple matter to swing the wires over
to a good breaker.
An individual circuit outage generally
falls into one of three categories,
although there are grey areas.
The breaker may fail to reset, thereby
signifying a dead short, or it may hold
for a short interval before tripping
again. Otherwise, it may hold for quite
a long time, sometimes hours, before
cutting out.
The latter is because breakers in
common use are inverse time devices,
with the ability to tolerate a small
amount of over-current for a long time
and a greater amount of current for a
short time.
An accurate picture can be gained
from current measurement using
a clamp-on ammeter (trade name
Amprobe). If the breaker holds long
enough, you can check the current at
each outlet until the fault is located.
If there are loads plugged in, one of
THE CIRCUIT BREAKER HAS
SAVED A LOT OF LIVES.
DAVID
HERRES
OUTLINES THE ROLE AND
CHARACTERISTICS OF A SIMPLE
BUT EFFECTIVE DEVICE.
This residual current device interrupts both conductors.
RCDS