manufacturers' technical manuals, or NSTM,
The function of arcing contacts is not
necessarily impaired by surface roughness.
chapter 300.
Before working on a CB, de-energize all
Remove very rough spots with a fine file. Replace
arcing contacts when they have been burned
control circuits to which it is connected; the
procedures differ somewhat with the type of
severely and cannot be properly adjusted. Make
a contact impression and check the spring pressure
mounting that is used. On drawout CBs, ensure
they are switched to the open position and
following the manufacturer's instructions. If
information on the correct contact pressure is not
removed. When working on fixed-mounted CBs,
available, check the contact pressure with that of
open the disconnecting switches ahead of the
similar contacts that are functioning properly.
breakers. If disconnecting switches are not
When the force is less than the designed value,
provided for isolation, de-energize the supply bus
the contacts either require replacing because they
to the CB. Circuit breakers have different time
are worn down, or the contact springs should be
delay characteristics, such as short time, long
replaced. Always replace contacts in sets, not
time, or instantaneous trip. The adjustments for
singly, and replace contact screws at the same
selective tripping of most CBs are made and sealed
time.
at the factory. You should not normally make
Clean all surfaces of the CB mechanism,
changes to their trip settings, as these changes may
particularly the insulation surfaces, with a dry
completely disrupt their intended functions of
cloth or air hose. Before directing the air on the
protection. If improper tripping action occurs as
breaker, be certain (1) the water is blown out of
the result of a malfunction within the breaker, the
the hose, (2) the air is dry, and (3) the pressure
simplest means to correct the problem is to replace
is not over 30 psi. Check the pins, bearings,
the entire breaker.
latches, and all contact and mechanism springs
After a CB cover has been removed, you
for excessive wear or corrosion and evidence of
should check the interior components, such
overheating. Replace parts if necessary.
as contacts, overcurrent tripping devices, con-
Slowly open and close CBs manually a few
nections, and moving mechanical parts.
times. Be certain trip shafts, toggle linkages,
Contacts are the metal parts especially selected
latches, and all other mechanical parts operate
to resist deterioration and wear as a result of
freely and without binding. Be certain the arcing
arcing. The arcing occurs in a CB while its
contacts make-before and break-after the main
contacts are opening and carrying current at the
contacts. If you note poor alignment, slug-
same time. When firmly closed, the contacts must
gishness, or other abnormal conditions, adjust
not arc.
Contact materials have been subjected to
following the manufacturer's instructions for the
particular CB.
constant research. The result is various products,
Keep the sealing surfaces of CB contactor and
ranging from pure carbon or copper to pure silver.
relay magnets clean and free from rust. Rust on
Each is used alone or as alloys with other
the sealing surface decreases the contact force.
substances. Modern CBs have contacts made of
This may result in overheating of the contact tips.
a silver alloy coated with silver, or silver mixed
Loud humming or chattering will often warn of
with tungsten. The silver alloy is extremely hard
this condition. A light machine oil wiped sparingly
and resists being filed. Such contacts made of
on the sealing surfaces of the contactor magnet
silver alloy conduct current when discolored
will aid in preventing rust.
(blackened during arcing) with silver oxide. The
blackened condition, therefore, requires no
Always use oil sparingly on CBs, contactors,
motor controllers, relays, and other control equip-
filing, polishing, or removal. However, severe
ment. Do not use it at all unless it is called for
pitting or burning of a silver contact may require
in the manufacturer's instructions or unless oil
some filing with a fine file or with fine sandpaper,
holes are provided. If working surfaces or bear-
No. 00. Never use emery paper or emery cloth to
ings show signs of rust, disassemble the device and
remove raised places on surfaces that prevent full
carefully clean the rusted surfaces. Wipe light oil
surface closure of the contact surfaces. If
on sparingly to prevent further rusting. Oil has
necessary, wipe the contacts with a CLEAN cloth
a tendency to accumulate dust and grit. This will
moistened with INHIBITED methyl chloroform
cause improper operation of the device, mainly
solvent. To remove all DEADLY and TOXIC
if the device is delicately balanced.
fumes of the solvent, provide VERY LIBERAL
Clean arc chutes with a stiff brush and a cloth.
ventilation with exhaust fans or with portable
Replace broken or deeply burned arc chutes. Be
blowers.
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