Electrical
Circuit Breakers
By: L. W. Brittian, Mechanical-Electrical
Instructor
PART 4
IN THIS THE FOURTH PART OF THE ARTICLE COVERING CIRCUIT
BREAKERS, THE FOLLOWING TOPICS ARE COVERED:
• TIME CURRENT CURVES
• AVAILABLE FAULT CURRENT
• SERIES RATED DEVICES
• SELECTIVE COORDINATION
• LINE AND LOAD TERMINAL CONNECTIONS
• AMBIENT COMPENSATED CIRCUIT BREAKERS
TIME CURRENT CURVES
The proper design of an electrical system involves many
detailed tasks, such as selection of the circuit breakers
that will protect the conductors, equipment, and people
who operate the equipment. Proper selection and coordination
of breakers for a specific system is facilitated by the
use of time current curves. Reading these curves is quite
technical and will not be covered in adequate detail to
allow someone to properly select OCPD’s; my intention
in this short paper is to provide a brief overview only.
Time current curves are plots of the amount of current
(vertical scale) flowing in the circuit to the time (horizontal
scale) required for the breaker to clear the fault current.
Curves are listed by some manufactures as being instantaneous,
ultra-short, short, medium, and long.
Melting (eutectic alloy) element type electrical fuses
have no moving parts, so no inertial forces need to be overcome
for the fuse to open the circuit. Breakers, on the other
hand, have parts that must be moved from one position to
the other to open the circuit. Generally speaking, fuses
(particularly current limiting fuses) can open a circuit
faster than circuit breakers. Some solid-state components
can be damaged beyond repair in less time than a circuit
breaker may be able to open the circuit. For this and other
reasons, fuses and not electrical-mechanical circuit breakers
best protect some types of electrical components. Restated,
you cannot always replace a fuse with a circuit breaker
of the same voltage and amperage values.
The time an overcurrent protective device operates can
be divided into the following four segments. sensing time:
magnetic elements are quicker than thermal elements (which
intentionally add delay). Opening time: fuses are quicker
than breakers as they have no parts to move. Arcing time:
the time during which an arc is present both fuses and breakers
have to extinguish the resultant arc. And finally arc extinguishing
time: the time the protective device takes to extinguish
the fault’s arc varies with the type of device, amperage
rating, AIC rating, voltage, and the amount of short circuit
or overload current developed.
AVAILABLE FAULT CURRENT
When selecting circuit breakers it is important to know
both the maximum continuous amperage and the available fault
(short circuit) current. The NEC in article 110.9 provides
the following guidance, “ Equipment intended to interupt
current at fault levels, shall have an interrupting rating
sufficient for the nominal circuit voltage and the current
that is available at the line terminals of the equipment.
“ There are two methods commonly used to comply with
this NE code requirement.
The most conservative method is to select all OCPD’s
based upon the fault current available at the electrical
service (or source of supply). For example if 50,000 amps
of fault current could be supplied to a building at the
service, even the most distant (from the service) branch
circuit breaker should be selected to have the ability to
safely open the circuit with 50,000 amps of fault current,
even though that amount of current would not be available
to the line terminals of the most distant circuit protective
device. Depending upon the specific nature (such as arcing,
or bolted) of the fault, the total amount of fault current
available may or may not be developed during operation of
the nearest upstream protective device.
SERIES RATED DEVICES
The second method of breaker selection, which is more realistic
and more first cost economical, is to select the device(s)
based upon the level of fault current that engineering level
calculations determine can be potentially available at the
device's line terminals.
One may question why spend the extra money purchasing breakers
that have a higher AIC than the system can deliver? When
it is reasonable to anticipate that the power supply’s
capacity will be increased, the initially more costly selection
may be justified based upon anticipated system capacity
growth.
SELECTIVE COORDINATION
Selective coordination is the selection and application
of circuit protective devices in series such that under
overload or fault current conditions, only the device just
up stream from the overload or fault will open to clear
the fault. The remainder of the circuit’s protective
devices will remain closed passing power to their individual
loads. Selectivity can be based upon time or current levels.
This method of selection allows two devices to be connected
in series with each other, and seeing the same current level
to respond in differing times, the one closest to the fault
with the shortest operating time would open the circuit.
The device up stream from it, while having the same current
level trip point, would have a longer trip delay time, allowing
the closer device to react first to open the protected circuit.
If not properly coordinated, the device closest to the
fault could have the longer time of response (both having
the same current level trip values), and the next protective
device up stream could open the circuit, resulting in a
potentially more wide spread circuit outage to be experienced
by the facility. When the breaker nearest the circuit’s
faulted point does not trip yet the one above it does, a
review of the degree of coordination should be undertaken.
LINE AND LOAD TERMINAL CONNECTIONS
The terminals at the top of a breaker (when installed in
a vertical position) are for connection to the source of
supply and are called the line connections (NEMA markings
L-1, L-2, L-3, or IEC markings 11, 21,31). The terminals
at the bottom of the breaker are for connection to the load
(NEMA markings T-1, T-2, T-3, or IEC markings 12, 22, 32).
Most breakers must be installed with the source of supply
connecting to the top terminals.
Some breakers are listed such that they may be connected
to the source of supply either at the top (line) or the
bottom (load). These breakers can then be used in a back-fed
type of application; that is power can be connected to the
bottom (load) of the breaker and the breaker can be used
to supply power via its line (top) connections to a bus
bar. When a breaker is marked line or load, it must be installed
in that manner only. That is line to the source of power
and load terminations connected to the utilization equipment.
The NEC requires that back-fed type breakers be so installed
that it takes more than a pull on the breaker to remove
it. See article 408.16 (F) of the 2002 edition.
AMBIENT COMPENSATED CIRCUIT
BREAKERS
There are some common installations where the electrical
load to be protected will be located in an area that is
subject to a different range of environmental conditions,
particularly ambient temperature. An extreme example of
this type would be where a fan motor is located in a minus
40 degree ice cream freezer and its protective circuit breaker
is located in a poorly ventilated motor control center room
where the air temperature routinely exceeds 100 degrees
F. during hot summer months. This could result in the breaker’s
thermal element trip point being reduced due to its hotter
ambient. The breaker could experience nuisance tripping.
To avoid temperature related offsets breakers are available
with an ambient compensation feature. This design enhancement
feature allows the breaker to open the circuit, with out
deviation caused by changes in the ambient air temperature
within a listed ambient temperature range.
In the next part of this article the following topics will
be covered:
• INSULATED CASE CIRCUIT BREAKERS
• ACCESSORIES
• SHUNT TRIP
• AUXILIARY-REMOTE ALARM SWITCH
• GROUND FAULT SENSOR
• UNDER VOLTAGE TRIP
• LOCK-OUT-TAG-OUT PROVISIONS
• REMOTE OPERATOR HANDLE
• STORED ENERGY BREAKER OPERATOR
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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