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Electrician near the low-voltage cabinet. Uninterrupted power supply. Electricity.

Low-Voltage Circuit Breakers James G Stallcup and James W Stallcup – Part 1 of 2

An insulated-case and molded-case circuit breaker consists of two general parts. First part consists of the current-carrying conductors, contacts, and proper operating mechanism necessary to perform the circuit-switching functions. The second part consists of the protective element, including the tripping mechanism associated with the circuit breaker.

Note: Insulated-case and molded-case circuit breakers including enclosures carrying the UL label are factory-sealed,

Low Voltage Circut Breaker Requirements NEC 240.80 thru 240.85

Low-voltage circuit breakers must be installed so that they are trip free and capable of being closed and opened by manual operation of the hands. As noted, this means that normal operation by other than manual means, such as electrical or pneumatic is permitted if means for manual operation are also provided. Circuit breakers must clearly indicate that they are either in the open “off “or closed “on” position.

Circuit breaker handles that are operated vertically and not rotationally or horizontally, the “up” position of the handle indicates “on” position and down position “off.”

Circuit breakers must be marked from manufacturer with their ampere rating in a manner that will be durable and visible after their installation. However, markings are permitted to be visible by removing a panel trim or cover. Circuit breakers rated at 100 amperes or less rated at 1000 volts or less are required to have the ampere rating molded, stamped, etched, etc. into their handles. Circuit breakers having an interrupting rating other than 5000 amperes must have its interrupting rating on the circuit breaker. This interrupting rating is not required to be marked on circuit breakers used for supplementary protection of equipment.

Circuit breakers can be used as switches in 120-volt and 277-volt fluorescent lighting circuits where they are listed and marked SWD or HID. Circuit breakers must be marked with a voltage rating not less than the nominal system voltage. This rating indicates the circuit breakers capability to interrupt fault currents between phases or phase-to-ground. It is important to remember that circuit breakers with a straight voltage rating, such as 240V or 480V can be installed in a circuit in which the nominal voltage between any two conductors does not exceed the circuit breakers voltage rating. A two-pole circuit breaker must not be used for protecting a three-phase, corner-grounded delta circuit unless such circuit breaker is marked 1Ø –3Ø to indicate it is for such use. A circuit breaker with a slash rating, such as 120/240V or 480Y/277V can be installed in a solidly grounded circuit where the nominal voltage of any conductor to ground does not exceed the lower of the two values of the circuit breakers voltage rating and the nominal voltage between any two conductors does not exceed the higher value of the circuit breakers voltage rating.

17.8 Inspection and Cleaning NFPA 70B – 17.8

Insulated-case and molded-case circuit breakers should be kept clean of external contamination so that internal heat can be dissipated in a normal manner.

Note: A clean case reduces a possible arcing condition between exposed live conductors and between energized conductors and ground.

The structural strength of the case is important in withstanding the stresses during fault-current conditions. Naturally, the case should be inspected for cracks and replaced where necessary.

Loose Connections NFPA 70B- 17.9

Excessive heat in a circuit breaker can cause a nuisance tripping and possibly lead to a failure. Loose connections are the most common cause of excessive heat. Maintenance inspections for loose connections or evidence of overheating should be checked as deemed as necessary. Tightness of loose connections should be performed and comply with NFPA 70B-8.11. Insulated-case and molded case circuit breakers having non-interchangeable trip units are appropriately adjusted, tightened, and sealed at the factory. Interchangeable trip units installed and maintain improperly may overheat if not tightened properly during installation. Manufacturer’s recommended maintenance procedures of connections should be followed.

Mechanical Mechanism Exercise NFPA 70B – 17.10

Circuit breakers with moving parts require periodic inspections for loose connections and overheating problems. Manual operation of the circuit breaker will help keep the contacts clean and aid in the lubrication performance. Although manual operation exercises the breaker mechanism, none of the mechanical linkages in the tripping mechanisms are moved with this exercise. Some circuit breakers have push-to-trip buttons that should be manually operated to exercise the tripping mechanism linkages in a proper manner.

Note: Annex K, covers long-term maintenance procedures and Annex L deals with maintenance intervals. Where testing is necessary, see NETA guidelines and specifications for such procedures.


Maintenance personnel must remember that insulated-case and molded-case circuit breakers will trip from exposure to continuous currents that are greater than their amperage ratings. Circuit breakers can also trip from unduly high ambient temperatures and improper connections can be a problem as well as other conditions that transfer undue heat to the internal parts of the breaker. Many of these conditions violate the installation specifications. Maintenance personnel must realize that an insulated-case and molded-case circuit breakers installed in a panelboard should not be loaded in excess of 80 percent of its continuous current rating (125 percent times the load). Continuous load is defined for in NEC Article 100 is operating for 3-hours or more.


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