Electrical
Circuit Breakers
By: L. W. Brittian, Mechanical-Electrical
Instructor
PART 3
IN THIS THE THIRD PART OF THE ARTICLE COVERING CIRCUIT
BREAKERS, THE FOLLOWING TOPICS ARE COVERED:
• Methods Of Mounting Circuit
Breakers
• Fixed Mounted Circuit Breakers
• Removable Mounted Circuit Breakers
• Drawout Mounted Circuit Breakers
• Methods Of Securing Circuit
Breakers
• Stab Lock Type Circuit Breakers
• Bolted Type Circuit Breakers
• Din Rail Mounted Circuit Breakers
METHODS OF MOUNTING CIRCUIT
BREAKERS
For our study purposes we will divide the methods used
to mount circuit breakers into three general groups: Fixed,
Removable, and Drawout. A review of these mounting methods
follows.
FIXED MOUNTED CIRCUIT BREAKERS
A circuit breaker that is bolted in its enclosure and wired
to the load frame, is called call a fixed mounted circuit
breaker. These units are typically rated 600 volts or less
and are front mountable. Power is provided to the breaker
typically by wires or sectional type bus bars. Power feeding
the circuit breaker must be turned off in order to physically
remove the fixed mounted breaker.
REMOVABLE MOUNTED CIRCUIT
BREAKERS
A removable circuit breaker has two parts, a base, which
is bolted to and wired to the frame, and the actual breaker,
which has insulated parts that electrically mate with the
base. This means of mounting allows the unit to be replaced
with out re-wiring the unit on the line side of the breaker.
This type of mounting is typically used for breakers rated
600 volts or less.
DRAWOUT MOUNTED CIRCUIT BREAKERS
A drawout circuit breaker also has two parts, the base,
which is bolted and wired to the frame and the actual breaker,
which slides into and electrically mates with the base.
This allows the unit to be replaced without having to turn
off the power feeding the breaker. The load must be turned
off in order to test or remove the unit.
As a safety feature these units are interlocked to automatically
turn the power off just before removal of the breaker begins.
By design, only the circuit breaker’s load must be
turned off to remove the breaker. This method of mounting
allows for a single breaker to be disconnected from the
power supply. That is to say that it does not require that
all of the power be disconnected from all of the breakers
installed in the larger enclosure such as a motor control
center.
There are various designs used to facilitate the “racking-in”
(installation)
and “racking out” (withdrawal) of the drawout
type circuit breakers. Some utilize some form of jacking
screw to initially move and thus electrically disengage
the breaker, then a traveling trolley type of hoist (somewhat
like a small boat winch) supports the breaker during removal
and re-installation. A transient supporting device is necessary
as these large (physical size) breakers are too heavy and
too bulky to be safely moved into and out of position by
one person.
METHODS OF SECURING CIRCUIT
BREAKERS
Circuit breakers are typically secured in place by one
of the following methods:
• Through bolts
• Stab locked to the busbar or some type of receptacle
connection
• Bolted to the busbar
• Din rail mounted
“STAB-LOCK” TYPE
BREAKERS
This method of circuit breaker mounting utilizes a male-female
type of plug and receptacle connection to a metal busbar
on one end. The opposite end of the breaker is mated to
the enclosure housing and does not make electrical contact
with the busbar and is not part of the electrical circuit.
These types of breakers are found in homes and light commercial
applications installed in loadcenters. With this method
of mounting, some movement of the circuit breaker case is
normal. This small amount of breaker case movement is typically
1/8 of an inch or less on the busbar end. Single pole breakers
tend to exhibit the greatest amount of movement due to their
greater aspect (width to height) ratio. The circuit conductor
termination lug may (but not necessarily) also exhibit some
minor movement of the termination lug; again this movement
is less than about 1/8 of an inch.
On occasion the line connection(s) may lose some gripping
force overtime; when this occurs one may be tempted to re-pinch
the connection closed just a bit. This pinching should be
avoided, as repeated bending results in the metal losing
its strength. When a loose connection has become evident,
the breaker should be replaced. The busbar and its insulation
should also be inspected for indications of overheating
related damage. With this mounting method, voltage drop
test should be done from the loadcenter or panelboard’s
main lugs to the breaker’s load terminal(s).
BOLTED TYPE BREAKERS
When a longer service life breaker is wanted, a bolted
type is typically used. These types have a metal tab (one
for each phase) sticking out from one end that is bolted
to the busbar with a machine screw (bolt type fine threads
and not sheet metal screw type steep pitch threads). When
replacing these types of breakers, the retaining bolts or
machine screws will have power on them unless power to the
entire panelboard has been removed.
It is not uncommon for some individuals to determine that
it is necessary to replace these types of breakers with
power still applied to the busbars. I am not a big fan of
working any thing above 12 volts hot, for I have witnessed
too many good folks get hurt doing what was initially anticipated
as being a quick and simply task. When this type of breaker
must be replaced with power still applied to the busbars,
it should be done only under strict safety procedures; using
proper personnel protective equipment and double insulated
tools (everyday plastic handle screwdrivers must not be
used). A detailed job safety analysis should be conducted
before any hot work is undertaken. On more than one occasion
I have witnessed some highly competent professionals conducting
rehearsals of this type of activity several times until
each safety step was done correctly. Take the time you need
to be safe.
DIN RAIL MOUNTED BREAKERS
With this method, a mounting rail is secured to the enclosure
and the breaker is snapped onto the mounting rail. This
allows replacement to be done quickly as the device can
be unclipped and a new one clipped on to the DIN rail. Conductors
for the supply and load are typically secured to the breaker
using pressure connectors that are tightened by some type
of threaded fastener. While not as easy to replace as a
stab-lock type breaker, this method does allow for some
saving of time, both during panel building and individual
breaker replacement later on.
The letters DIN stand for German Industry Standards. DIN
rails are available in more than one physical size. The
DIN rail mounting method is increasingly replacing thru-bolt,
foot, and plate mounting methods once more commonly used.
In the next part of this article the following topics will
be covered:
• TIME CURRENT CURVES
• AVAILABLE FAULT CURRENT
• SERIES RATED DEVICES
• SELECTIVE COORDINATION
• LINE AND LOAD TERMINAL CONNECTIONS
• AMBIENT COMPENSATED CIRCUIT BREAKERS
If you have any questions or comments, please send me an
E-mail.
Remember Work Smarter, Not Harder
L. W. Brittian
Mechanical-Electrical Instructor
lwbrittian@hot1.net
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