Application Paper AP020006EN
Effective September 2020
Benefits of contactors vs. breakers for
medium-voltage motor control applications
Reduced footprint, clearances,
and front accessibility
Presently used MV MCCs, measure about
36 inches wide by 30 inches deep and only
require a front clearance of 36 inches. They are
front accessible and commonly available in a
single-high 800 A or a two-high 400 A starter
configuration, which can be installed against a
wall, back to back, or in a corner.
Introduction
With both power circuit breakers and contactors
able to start and stop a motor, how do you decide
which option makes the most sense?
The availability of fully rated 800 A contactors has
been a great advantage for medium-voltage (MV)
motor control. With contactors available at this
higher amperage, there is a reason to assess the
typical approach for motor control and understand
what are the advantages a contactor provides over
a circuit breaker.
Full-voltage starters are the most commonly used
method of MV motor control. Also known as
across-the-line starters, this equipment applies
line voltage to the motor in a single action via
contactors rated to connect and disconnect motor
in-rush currents. An assembly of motor controllers
under a common bus is often called a motor
control center (MCC).
On the other hand, an MV circuit breaker is a
power distribution protection equipment designed
to interrupt high levels of short-circuit current.
These devices are commonly found installed in
assemblies called switchgear.
This application paper will discuss common
application considerations when choosing a motor
starting method including: size of equipment,
endurance, impact on other equipment, and safety.
Here is the upshot—the contactor-based motor
starter solution comes out substantially ahead in
each of these parameters. The following pages
will describe the benefits of using contactors for
motor starting applications.
MV switchgear on the other hand, requires rear
access, is 96 inches deep, and has clearance
requirements of 70 inches at the front, 32 inches
on the side, and 36 inches at the rear. This
increased footprint and additional access
requirements can become costly, particularly
when the equipment is installed in locations
such as prefabricated electrical houses where
the cost per square foot is very significant.
For example, a typical MV MCC lineup with
ten 400 A starters will measure 180 inches in
width and would require about 83 square feet
of floor space, including front clearances. Using
circuit breakers in MV switchgear for the same
application would more than triple the overall
required footprint and clearances to 253 square
feet. This increase in footprint, clearances, and
overall cost is not necessary especially for
motors below 6000 horsepower (hp), which can
be controlled by a fully rated 800 A contactorbased starter.
Lower chopping current means there’s less
chance for adverse impact on your motor
When a vacuum interrupter opens, the level of
current produced by the arcing between the
interrupter’s contacts almost instantly falls to
zero instead of following a nominal current zero
decrease of the fundamental 60 Hz waveform.
This sudden interruption in current is defined as
current chop and depending on its magnitude
and frequency, it can cause serious insulation
degradation over the lifetime of a motor.
By design, vacuum interrupters in contactors have
a lower interrupting rating and are constructed to
operate more frequently than the ones used in
breakers. Vacuum interrupters in contactors are
manufactured using different metallurgy, which
allows them to exhibit a lower chop current (about
1 A) than the chop current produced by a breaker
interrupter (about 5 A). See Figure 1.
Application Paper AP020006EN
Effective September 2020
Depending on the characteristic impedance of the electrical system,
the transient voltage created by the chopping current could be
detrimental to equipment insulation. Figure 2 presents the transient
voltage created by a 1 A and 5 A chopping current in a simulated
circuit with a characteristic impedance of 3000 ohms. As shown, the
1 per unit overvoltage condition created by the 1 A current chop of
a vacuum contactor is well within the rating of most MV insulation
Figure 1. 1 A and 5 A chopping current waveforms
Figure 2. 1 A and 5 A voltage transient waveform (Z0 = 3000 ohms)
2
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Benefits of contactors vs. breakers for
medium-voltage motor control applications

systems and will not compel design engineers to use surge
arresters. Vacuum interrupters in circuit breakers, on the other hand,
have a 5 A current chop, increasing the voltage transient to levels
above four times the system voltage. These higher levels of transient
voltages often require the use of surge arresters as a mitigator to
protect insulation systems.
Application Paper AP020006EN
Benefits of contactors vs. breakers for 
medium-voltage motor control applications
Effective September 2020

Higher mechanical and electrical endurance
Motor starters using contactors have extended mechanical and
electrical life when compared to breakers used in the same
application. The impact here is substantial. For motor starting
applications, contactors can require little, if any, maintenance for
decades. In contrast, breaker-based solutions for motor starting
could require repeated maintenance beginning within a year of
operation.
As seen in Table 1, vacuum breakers have a mechanical endurance
of 10,000 operations with a manufacturer recommended
maintenance period of 500 operations. Vacuum contactors are
commonly rated for 25,000 electrical operations before maintenance
is recommended and a total electrical life up to 300,000 operations.
Table 1. Vacuum breaker and contactor rated operations
and maintenance
Device
Rated electrical
operations
Recommended
maintenance
operations
Vacuum breaker
400 A vacuum contactor
800 A vacuum contactor
10,000
300,000
200,000
500
25,000
25,000
Breakers are constructed to stop high levels of current in the event
of a fault. They are excellent interrupters, and as such, they are
better suited for power distribution applications instead of motor
starting applications where the number of operations significantly
increases. For example, an application where only four motor starting
operations are required per day would lead to maintenance periods of
four months if a breaker-based controller is applied. Using the same
scenario, we find that the contactor will not require any recommended
maintenance for more than ten years. Breakers can be used in this
context, but it’s important to understand manufacturers’ maintenance
recommendations in order to implement maintenance schedules that
accommodate for this usage.
Enhanced safety: faster clearing times
and lower let-through currents
Breaker-based motor controller short-circuit protection is usually
provided by the instantaneous overcurrent setting of the motor
protection relay. Although this type of protection operates with no
intentional time delay, it is important to note that inherit delays exist
due to relay and breaker operations or total clearing time.
To obtain the relay operating time, the protection engineer must
account for the relay’s output contact and instantaneous protection
operating time. Relays typically have an 8 msec contact operation
time with a 30 msec maximum pickup time. This means the relay’s
total operating time is about 38 msec or about 2¼ cycles. Once the
relay picks up and closes its output contact to trip the breaker, it
will take an additional 50 to 83 msec (3 to 5 cycles) in order to fully
open its contacts and clear the fault. Adding both the relay’s and the
breaker’s operating times, the total clearing time is 88–121 msec
(5–7 cycles).
Class E2 motor controllers use a main contactor to make and break
load and overload currents, in addition to MV current-limiting fuses
for interrupting fault currents that exceed the breaking capacity of
the main contactor.
Most 400 A MV contactors have interrupting ratings between
6000 A and 8500 A; and 800 A MV contactors have interrupting
ratings of 7200 A to 12,500 A. In order to obtain higher interrupting
ratings, current-limiting fuses are supplied as backup protection to
interrupt and limit short-circuit currents higher than the contactor’s
rating. The motor starter design must ensure the contactor does not
open above its interrupting rating, and instead allow the fuse to
clear this fault.
Table 2 tabulates the minimum and total clearing times as well as
the let-through current of common fuses used in 400 A and 800 A
MV motor starters. As seen in this table, the higher the levels of
prospective short-circuit current at the fuse location, the faster the
fuse takes to clear and limit the let-through fault current feeding
the fault. This current-limiting feature clears a fault within ½ cycle or
at least 10 times faster than a breaker’s instantaneous protection,
greatly reducing the amount of arc-flash energy produced during a
fault, making it safer for its users.
Table 2. 400 A and 800 A MV starter fuse characteristics [1]
400 A—24R
Prospective
short-circuit
current rms (A)
12,000
15,000
20,000
25,000
30,000
35,000
40,000
45,000
50,000
Minimum
melt (cycle)
2
1
Let-through
region
Let-through
region
Let-through
region
Let-through
region
Let-through
region
Let-through
region
Let-through
region
800 A—44R
Total
clearing
(cycle)
Peak let
through (A)
2.6
1.2
<0.5
800 A—57X
Minimum
melt (cycle)
Total
clearing
(cycle)
Peak let
through (A)
Minimum
melt (cycle)
Total
clearing
(cycle)
Peak let
through (A)
33,000
15
18
N/A
18
60
N/A
35,000
37,000
6
2
9
4
N/A
N/A
4.8
1.4
16
4.7
N/A
N/A
<0.5
40,000
1
2
N/A
0.6
2.4
N/A
<0.5
43,000
1
N/A
76,000
45,000
0.9
82,000
1.1
80,000
<0.5
47,000
<0.5
87,000
0.8
83,000
<0.5
50,000
<0.5
90,000
<0.5
86,500
<0.5
51,000
<0.5
93,000
Let-through
region
Let-through
region
Let-through
region
Let-through
region
Let-through
region
1.5
<0.5
Let-through
region
Let-through
region
Let-through
region
Let-through
region
Let-through
region
<0.5
90,000
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3
Application Paper AP020006EN
Effective September 2020
Benefits of contactors vs. breakers for
medium-voltage motor control applications

Conclusion
With the ability to pair a fully rated 800 A starter with an 800 A
induction motor, there’s an opportunity for an optimum solution to
starting and stopping MV motors. Applying a fully rated 800 A starter
instead of a 1200 A circuit breaker not only right-sizes the equipment
for the application but also translates to other benefits: reduced
footprint, lower chopping currents, higher endurance, and faster
clearing times. The contactor-based solution provides significant
advantages along each of these parameters:
1. One-third less footprint and clearance requirements.
2. Lower chopping current means reduced likelihood of adverse
impact on downstream equipment.
3. 20 times higher mechanical and electrical endurance and
50 times operations between maintenance intervals.
4. Faster clearing time, which further enhances safety.
All that said, the traditional approach can work, and equipment
can last until it becomes too expensive and too difficult to find
replacement parts. When that happens, there’s an opportunity
to assess the best approach available today.
References
[1] Eaton Corp., R-Rated Medium-Voltage fuses for
Motor Circuit Protection, 2018.
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Publication No. AP020006EN / Z24197
September 2020
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