Circuit Breakers : Selection Criteria

There are a few different criteria to consider when selecting a circuit breaker including voltage, frequency, interrupting capacity, continuous current rating, unusual operating conditions and product testing. This article will give a step by step overview on selecting an appropriate circuit breaker for your specific application.

a) Voltage Rating : The overall voltage rating is calculated by the highest voltage that can be applied across all end ports, the distribution type and how the circuit breaker is directly integrated into the system. It is important to select a circuit breaker with enough voltage capacity to meet the end application.

b) Frequency : Circuit breakers up to 600 amps can be applied to frequencies of 50-120 Hz. Higher than 120 Hz frequencies will end up with the breaker having to derate. During higher frequency projects, the eddy currents and iron losses causes greater heating within the thermal trip components thus requiring the breaker to be derated or specifically calibrated. The total quantity of deration depends on the ampere rating, frame size as well as the current frequency. A general rule of thumb is the higher the ampere rating in a specific frame size the greater the derating needed. All higher rated breakers over 600 amps contain a transformer-heated bimetal and are suitable for 60 Hz AC maximum. For 50 Hz AC minimum applications special calibration is generally available. Solid state trip breakers are pre-calibrated for 50 Hz or 60 Hz applications. If doing a diesel generator project the frequency will either be 50 Hz or 60 Hz. It is best to check ahead of time with an electrical contractor to make sure calibration measures are in place before moving forward with a 50 Hz project.

c) Maximum Interrupting Capacity : The interrupting rating is generally accepted as the highest amount of fault current the breaker can interrupt without causing system failure to itself. Determining the maximum amount of fault current supplied by a system can be calculated at any given time. The one infallible rule that must be followed when applying the correct circuit breaker is that the interrupting capacity of the breaker must be equal or greater than the amount of fault current that can be delivered at the point in the system where the breaker is applied. Failure to apply the correct amount of interrupting capacity will result in damage to the breaker.

d) Continuous Current Rating : In regards to continuous current rating, molded case circuit breakers are rated in amperes at a specific ambient temperature. This ampere rating is the continuous current the breaker will carry in the ambient temperature where it was calibrated. A general rule of thumb for circuit breaker manufactures is to calibrate their standard breakers at 104° F. Ampere rating for any standard application depends solely on the type of load and duty cycle. Ampere rating is governed by the National Electrical Code (NEC) and is the primary source for information about load cycles in the electrical contracting industry. For example lighting and feeder circuits usually require a circuit breaker rated in accordance with the conductor current carrying capacity. To find various standard breaker current ratings for different size conductors and the permissible loads consult NEC table 210.24.

e) Atypical Operating Conditions : When selecting a circuit breaker it is crucial to have in mind the end user location. Each breaker is different and some are better suited for more unforgiving environments. Below are a few scenarios to keep in mind when determining what circuit breaker to use:

• High Ambient Temperature: If standard thermal magnetic breakers are applied in temperatures exceeding 104° F, the breaker must be derated or recalibrated to the environment. For many years, all breakers were calibrated for 77° F which meant that all breakers above this temperature had to be derated. Realistically, most enclosures were around 104° F; a common special breaker was used for these types of situations. In the mid 1960s industry standards were changed to make all standard breakers be calibrated with 104° F temperature in mind.

• Corrosion and Moisture: In environments where moisture is constant a special moisture treatment is recommended for breakers. This treatment helps resist mold and/or fungus that can corrode the unit. In atmospheres where high humidity is prevalent the best solution is the usage of space heaters in the enclosure. If possible, breakers should be removed from corrosive areas. If this is not practical, specifically manufactured breakers that are resistant to corrosion are available.

• High Shock Probability: If a circuit breaker is going to be installed in an area where there is a high probability of mechanical shock a special anti-shock device should be installed. Anti-shock devices consist of an inertia counterweight over the center pole that holds the trip bar latched under normal shock conditions. This weight should be installed so that it does not prevent thermal or magnetic trip units from functioning on overload or short circuit scenarios. The United States Navy is the largest end user of high shock resistant breakers which are required on all combat vessels.

• Altitude: In areas where the altitude is over 6,000 feet, circuit breakers must be derated for current carrying ability, voltage and interrupting capacity. At altitude, the thinner air does not conduct heat away from the current carrying components as well as denser air found in lower altitudes. In addition to overheating, the thinner air also prevents the of building a dielectric charge fast enough to withstand the same voltage levels that occur at normal atmospheric pressure. Altitude issues can also derate most used generators and other power generation equipment. It is best to speak with a power generation professional before purchasing.

• Resting Position: For the most part, breakers can be mounted in any position, horizontally or vertically, without affecting the tripping mechanisms or interrupting capacity. In areas of high wind it is imperative to have the breaker in an enclosure (most units comes enclosed) on a surface that sways a bit with the wind. When a circuit breaker is attached to an inflexible surface there is a possibility of disrupting the circuit when exposed to high winds.