Electrical
Circuit Breakers
By: L. W. Brittian, Mechanical-Electrical
Instructor
PART 6
IN THIS THE SIXTH PART OF THE ARTICLE COVERING
CIRCUIT BREAKERS, THE FOLLOWING TOPICS ARE COVERED:
MOLDED CASE CIRCUIT BREAKER
(MCCB)
The most common type of re-settable overcurrent protective
device is the molded case circuit breaker. The case functions
as both an outer wrapper and to retain in proper position
the breaker’s internal components. These cases are
made from various types of electrical insulating and fire
retardant plastic. Cases are typically not hermetically
sealed; this allows them to be subject to corrosion from
environmental factors. They are limited to 600 volts and
less. They are typically available in either single, two,
or three pole models. This type of circuit breaker is now
available as AFCI, GFCI, and magnetic, hydraulic-magnetic,
and thermal-magnetic types.
The book titled “Overcurrents and Undercurrents”
by Mr. E.W. Roberts, P.E. (ISBN: 0967432316) covers GFCI
type devices. The book is well written, and is the only
text that I am aware of that covers AFCI’s as well.
MOLDED CASE CIRCUIT BREAKER
MAINTENANCE (MCCB)
MCCB’s have many years of life built into them, requiring
little maintenance. This should not be understood to mean
that periodic maintenance is not required. NETA (InterNational
Electrical Testing Association Inc.) has developed and published
a book titled “Maintenance Testing Specifications”
(NETA-MTS-01) that provides some guidance as to how various
types of electrical equipment including MCCB’s, and
ICCB’s should be tested. Again you may have anticipated
my next statement. I recommend that you obtain a copy for
your reference (303.697.8441). I know that books cost money,
but not knowing how or what to do can cost a lot more than
just money, so try to talk the boss into purchasing it for
you. In the interim, the following is a short overview of
some MCCB maintenance tasks.
It is recommended that at least once a year a properly
trained and equipped qualified electrician perform the following
maintenance task:
-
Visually inspect the case to determine if any portion
indicates overheating; replace the breaker if overheating
indications are found.
-
Check connections for indications of overheating.
-
Cycle the breaker five times manually.
-
Check and record the voltage drop across the breaker
using a calibrated digital voltmeter (capable of reading
three places to the right of the decimal point).
-
The load should be operated at full load for three
hours, or until the breaker reaches normal load temperature;
scan the breaker with an IR type non-contact thermometer
and record the readings.
-
Record voltages and note any voltage imbalance from
phase to phase.
-
Current readings should be taken with a true RMS type
meter due to The increasing harmonic content in many
electrical systems in commercial/industrial facilities
today.
-
Current readings on equipment grounding conductors
(where required) for specific machines should be noted.
Clamp on type ground-rod circuit resistance reading
meters should be used for this task as they can detect
both the impedance and the level of current on the conductor
if any is present, as other clamp on type amp-meters
will not indicate Ma levels.
Breaker test sets are commercially available from several
sources (AVO Multi Amp is one source 800.723.2861). Testing
of circuit breakers is a very specialized area requiring
special training and test equipment and should be conducted
only by competent personnel. NEMA has published a valuable
guideline (AB-4-1991) that should be consulted when testing
MCCB’s.
In the seventh and final part of this article the following
topics will be covered:
• MICROCOMPUTER CIRCUIT BREAKERS
• FIELD SELECTABLE RATING CIRCUIT BREAKERS
• OPERATION OVERVIEW
• CURRENT SENSING
• CONTINUOUS AMPS
• LONG TIME DELAY
• SHORT TIME PICK-UP
• SHORT TIME DELAY
• INSTANTANEOUS CURRENT PICK-UP TRIP
• GROUND FAULT CURRENT
• GROUND FAULT PICK-UP
• GROUND FAULT DELAY
• VISUAL ANNUNCIATION-INDICATION LAMPS
• POWER CONSUMPTION MONITORING
• INTERNAL TEST FUNCTIONS
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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