When selecting the proper circuit breaker for an application, the ratings and environment need to be considered.
Ratings
The voltage rating of a circuit breaker is determined by the maximum voltage that can be applied across the terminals, the type of distribution system and how the breaker is being applied in the system.
The voltage system of 480Y/277V is the most common found in commercial and institutional buildings. It has a solidly grounded neutral. This system is also very prevalent in industrial plants and some high-rise residential buildings.
When a breaker is applied in a panelboard, it is important that it have the lowest possible voltage rating that will do the job and meet the specifications. It can save the customer a lot of money if the breaker is chosen.
A 2-pole, 480/277V breaker can be used on this system because it is a three-phase, 4-wire, grounded system. The maximum line to ground voltage is 277 volts across one pole of the breaker.
That is not the case in a three-phase, 3-wire Delta system.
Here, a fault condition could occur that would allow the breaker to see the full 480V across one pole. UL requires that each pole of the breaker be rated to interrupt this full 480V.
The continuous current rating of a molded case circuit breaker is the amount of current it is designed to carry in open air. The breaker has a specific ampere rating and is Ambient Compensated. Most manufacturers calibrate their breakers for a 40°C (107°F) ambient. The National Electric Code (NEC) allows a breaker to be applied to a maximum of 80% of the breaker's continuous current rating. Some manufacturers offer breakers that can be used at 100% if they are specifically designed and tested for such use. They must also specify the minimum size enclosure, ventilation needs and conductor size for the application.
The Interrupt Rating of a molded case circuit breaker is the amount of fault current it can safely interrupt without damaging itself. The interrupt rating must be equal to or greater than the amount of fault current available at the point in the system where the breaker is applied. The interrupt rating always decreases as the voltage increases. The interrupt rating is one of the most critical factors in the breaker selection process.
Most molded case circuit breakers retain the same tripping characteristics whether they are applied to a 50 Hz or 60 Hz system. On higher frequency systems, the breaker may need to be specially calibrated or derated. A molded case circuit breaker that has a thermal magnetic trip unit might not have the same thermal or magnetic performance at a higher frequency than 60 Hz. MCCBs with electronic trip units require special derating factors and cables or Bus at higher frequencies.
The number of poles of a molded case circuit breaker is determined by the type of distribution system in which it is applied. Except in certain special applications, each hot conductor is considered a pole. For single-phase applications with a grounded neutral, a single-pole breaker can be used. Two-pole and three-pole breakers are used in three-phase systems.
Environment
Thermal magnetic breakers can be affected by large differences in the ambient temperature. At ambient temperatures below 40°C, the breaker carries more current than its continuous current rating. The mechanical operation of the breaker could be affected if the temperature is significantly below the 40°C standard. The breakers will carry less current than their continuous rating if the temperature is above 40°C, and could cause Nuisance Tripping. It could also cause unacceptable temperature conditions at the terminals of the breaker.
Electronic trip circuit breakers often have a wider temperature range (-20° - 55°C) and so are less susceptible to ambient temperature fluctuations. At very low temperatures, the mechanical parts of the trip unit could require special lubrication. At very high temperatures, the electronic circuitry components could be damaged. Some MCCBs with electronic trip units have special self-protection circuitry to trip, should the internal temperature rise to an unsafe level.
An atmosphere that has a high moisture content or the presence of corrosive elements should be avoided. Electrical equipment should be mounted in clean, dry environments. When moist conditions cannot be avoided, special fungus treatments may be needed. Although the glass-polyester molded cases may not support the growth of fungus, terminals and other parts may. If changes in temperature create condensation, space heaters in the enclosures may be required.
Because the air is thinner at high altitude, it reduces the cooling and dielectric characteristics from those of denser air found at lower altitudes. Circuit breakers must be derated for voltage, current and interrupting ratings at altitudes above 6000 feet.
Special shock resistant breakers must be used for installations subject to high mechanical shock. Special installed anti-shock devices hold the trip bar latched under shock conditions, but don't inhibit the proper functioning of the breaker for short circuits or overload conditions.
A large steel manufacturer was experiencing a lot of nuisance tripping of circuit breakers located near the smelting side of their factory. Upon investigation, it was determined the circuit breakers had thermal magnetic trip units. The ambient temperature at that end of the factory was often around 110°F (43°C), particularly in the summer. The thermal magnetic breakers were only rated for 104°F (40°C).
The solution? They changed to breakers with electronic trip units that had a wider temperature range (up to 131°F/55°C) which easily handled the ambient heat, even on the hottest summer day.
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Circuit Breaker
Ratings
The voltage rating of a circuit breaker is determined by the maximum voltage that can be applied across the terminals, the type of distribution system and how the breaker is being applied in the system.
The voltage system of 480Y/277V is the most common found in commercial and institutional buildings. It has a solidly grounded neutral. This system is also very prevalent in industrial plants and some high-rise residential buildings.
When a breaker is applied in a panelboard, it is important that it have the lowest possible voltage rating that will do the job and meet the specifications. It can save the customer a lot of money if the breaker is chosen.
A 2-pole, 480/277V breaker can be used on this system because it is a three-phase, 4-wire, grounded system. The maximum line to ground voltage is 277 volts across one pole of the breaker.
That is not the case in a three-phase, 3-wire Delta system.
Here, a fault condition could occur that would allow the breaker to see the full 480V across one pole. UL requires that each pole of the breaker be rated to interrupt this full 480V.
The continuous current rating of a molded case circuit breaker is the amount of current it is designed to carry in open air. The breaker has a specific ampere rating and is Ambient Compensated. Most manufacturers calibrate their breakers for a 40°C (107°F) ambient. The National Electric Code (NEC) allows a breaker to be applied to a maximum of 80% of the breaker's continuous current rating. Some manufacturers offer breakers that can be used at 100% if they are specifically designed and tested for such use. They must also specify the minimum size enclosure, ventilation needs and conductor size for the application.
The Interrupt Rating of a molded case circuit breaker is the amount of fault current it can safely interrupt without damaging itself. The interrupt rating must be equal to or greater than the amount of fault current available at the point in the system where the breaker is applied. The interrupt rating always decreases as the voltage increases. The interrupt rating is one of the most critical factors in the breaker selection process.
Most molded case circuit breakers retain the same tripping characteristics whether they are applied to a 50 Hz or 60 Hz system. On higher frequency systems, the breaker may need to be specially calibrated or derated. A molded case circuit breaker that has a thermal magnetic trip unit might not have the same thermal or magnetic performance at a higher frequency than 60 Hz. MCCBs with electronic trip units require special derating factors and cables or Bus at higher frequencies.
The number of poles of a molded case circuit breaker is determined by the type of distribution system in which it is applied. Except in certain special applications, each hot conductor is considered a pole. For single-phase applications with a grounded neutral, a single-pole breaker can be used. Two-pole and three-pole breakers are used in three-phase systems.
Environment
Thermal magnetic breakers can be affected by large differences in the ambient temperature. At ambient temperatures below 40°C, the breaker carries more current than its continuous current rating. The mechanical operation of the breaker could be affected if the temperature is significantly below the 40°C standard. The breakers will carry less current than their continuous rating if the temperature is above 40°C, and could cause Nuisance Tripping. It could also cause unacceptable temperature conditions at the terminals of the breaker.
Electronic trip circuit breakers often have a wider temperature range (-20° - 55°C) and so are less susceptible to ambient temperature fluctuations. At very low temperatures, the mechanical parts of the trip unit could require special lubrication. At very high temperatures, the electronic circuitry components could be damaged. Some MCCBs with electronic trip units have special self-protection circuitry to trip, should the internal temperature rise to an unsafe level.
An atmosphere that has a high moisture content or the presence of corrosive elements should be avoided. Electrical equipment should be mounted in clean, dry environments. When moist conditions cannot be avoided, special fungus treatments may be needed. Although the glass-polyester molded cases may not support the growth of fungus, terminals and other parts may. If changes in temperature create condensation, space heaters in the enclosures may be required.
Because the air is thinner at high altitude, it reduces the cooling and dielectric characteristics from those of denser air found at lower altitudes. Circuit breakers must be derated for voltage, current and interrupting ratings at altitudes above 6000 feet.
Special shock resistant breakers must be used for installations subject to high mechanical shock. Special installed anti-shock devices hold the trip bar latched under shock conditions, but don't inhibit the proper functioning of the breaker for short circuits or overload conditions.
A large steel manufacturer was experiencing a lot of nuisance tripping of circuit breakers located near the smelting side of their factory. Upon investigation, it was determined the circuit breakers had thermal magnetic trip units. The ambient temperature at that end of the factory was often around 110°F (43°C), particularly in the summer. The thermal magnetic breakers were only rated for 104°F (40°C).
The solution? They changed to breakers with electronic trip units that had a wider temperature range (up to 131°F/55°C) which easily handled the ambient heat, even on the hottest summer day.
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