LOW VOLTAGE BREAKERS
Low voltage circuit breakers are generally divided into two categories:
A) MOLDED CASE BREAKER - All molded case breaker are designed, tested and evaluated in accordance with
the rules of NEMA Publication AB-1 and Underwriters Laboratories, Incorporated Standards for Safety number UL
B) LOW VOLTAGE POWER CIRCUIT BREAKERS - These breaker are designed, tested and evaluated under
the specifications published by NEMA SG-3 and NASI C37-13.
MOLDED CASE CIRCUIT BREAKERS
All molded case breakers are divided into four categories:
3. Thermal Magnetic
4. Solid State
First let me state that you must choose very carefully the type of trip unit to be used in your particular
application due to the long range of trip time available. In thermal and some magnetic trips, these times can
range from a low of five seconds to a high of six hundred seconds. This is with a 300% overload. Although the
cable can stand this much of an overload, many pieced of equipment cannot. The magnetic trip on these
thermal/magnetic breakers set for instantaneous trips can be adjusted, the thermal unit cannot. This is why in
some applications the use of adjustable solid state trips are used.
I will now outline the proper method for the testing of molded case breakers. The one most important thing to be
considered in the acceptance testing of these breakers is heat. Since these breakers are 90% of the thermal
type, the control of any heat (except that of the thermal unit) is of utmost importance. The proper conductors
from the test set to the breaker under the test must be used. The connections to the breaker must also be
properly made. All breaker manufacturers, as well as the manufacturers of Multi-Amp test equipment, give
the proper cables. These cables must be of the right size. Any deviation from these cables could give you the
wrong results. I have never found any type of publication giving the proper or suggested procedure for testing of
molded case breakers; only the proper connections or methods. In the twelve years or more that I have been
testing breakers, I have found the following procedure to give the best and most accurate results.
A) First, be sure all test cables are of the proper length and size and that all connections are well made.
B) If the breaker is equipped with magnetic instantaneous units, these should be tested first, starting with "A"
phase and continuing through "C" phase. This test will allow the magnetic unit to work first before too much heat
has been put on the thermal unit. If the thermal unit is tested first, it takes quite a few minutes for the thermal
units to return to normal temperature. With the thermal units already heated, they could again trip the breaker
before the magnetic unit has time to work. The small amount of heat generated during the instantaneous test will
have time to dissipate on "A" phase during the testing of "B" and "C" phase and so on down the line. I have found
that this test routine gives you the best results and results that are closer to the manufacturers curves.
C) Many of the manufacturers of molded case breaker of larger amperage sizes have a removable front
cover. It is our recommendation that these covers be removed and the internal parts of these breakers be
checked before and after testing. The test personnel should check for:
1. Loose connections
2. Proper contact alignment
3. Arc chutes
4. Trip catching
5. To see after the test where arc was broken
It is a must that all testing work be done by extremely qualified personnel. These testing technicians should also
have their test truck a complete set of manufacturers test curves for the breaker under the test and also,
complete set manufacturers specification for the breaker under test.
D) Thus, the testing of molded case breakers can be broken down into these six steps:
1. Remove cover and visual input interior component. Replace cover.
2. Test instantaneous trip unit first. (If breaker contains one).
3. Overload each of the individual poles at 300% of rated value.
4. Verification that all test procedures were done in accordance with the manufacturers specifications.
5. Verification that breaker trip units actually perform according to the manufacturers recommendations.
6. Remove the cover and re-inspect interior parts.
It should be noted that the purpose for the use of 300% overload for testing purposes is :
1. The reaction of the circuit breaker to this overload is indicative of its reaction throughout its entire
Overcurrent tripping range.
2. It is relatively easy to generate the required current in the field and the wattage per pole from line to load is
large enough so the dissipation of heat in the non-active pole spaces is minor and does not affect the test results
E) Testing of molded case breaker with solid state trip. The method for the testing of the solid state units are
the same. However, as these units work from CT’s and logic systems and heat does not play the same role in
these, therefore the procedure can vary.
F) Testing of instantaneous - only molded case breakers. These breakers sometimes are referred to as MCP
(Motor Circuit Protectors) and contain a magnetic adjustable trip only. These breaker should never be loaded to
more than what they are set at. To do so could permanently damage these units.
Circuit Breaker Testing