Why and when to choose CBEs

13 February, 2008

A circuit breaker for equipment (CBE) is a special type of circuit breaker designed to provide carefully matched protection for an individual piece of equipment including those in electronic, electrical, communications and industrial systems. They can be found in a range of applications including air, sea and land transport, process control, computers and telecommunications, medical equipment, domestic appliances, machine tools and robotics.

Fuses and miniature circuit breakers (MCBs) mainly guard against catastrophic failure – usually for wiring in domestic or industrial buildings. They are used to avert serious damage or fire in the event of high value short circuit currents, but are not necessarily designed to offer protection against low level overcurrents, the effects of which can be equally serious.

Circuit breakers have a variety of types and trip profiles, and therefore can be more precisely matched to loads and the working environment. The high sensitivity of CBEs safeguards equipment against low but potentially harmful overloads, such as slowly increasing currents caused by gradual circuit or component failure, or equipment malfunction or abuse, while allowing normal current surges such as switch-on transients. One important property of circuit breakers is that they can be quickly reset, enabling the circuit to be restored with minimum downtime.

Circuit breaker performance is also stable over time. However, as fuses age, their blow characteristics can change, which may lead to nuisance blowing and increased downtime. In addition, there is no assurance that a replacement fuse will be of the correct rating. If a fuse is replaced by one of a higher rating, overheating and catastrophic equipment failure may occur.

Reduced ratings
Using CBEs for circuit protection allows designers to reduce the ratings on wiring, connectors and other components to achieve cost, space and weight savings. Unlike a fuse rating, a circuit breaker rating gives the maximum current that the circuit breaker can carry continuously at normal ambient temperature. A 10A circuit breaker, for example, can carry 10A without nuisance tripping. Further, a typical 4A circuit breaker with a slow trip profile can tolerate a temporary 10A current surge without nuisance tripping, but will still trip if a genuine overload is experienced. Designers should also consider points such as protected equipment value, cost of down-time and warranty costs.

Different types of CBE
Thermal CBEs provide a positive snap action, suited to motor and transformer windings, automotive and marine installations, and battery powered equipment.

Hot wire thermal CBEs are suited to printed circuit board applications, and for other requirements where fast switching times are necessary.

Magnetic CBEs offer fast and precise tripping and are suited to the needs of sensitive electronic systems.

Thermal magnetic CBEs combine delayed thermal operation for low level overcurrents, with fast magnetic operation on higher level overloads. Temporary current surges can be absorbed without over specifying the CBE or degrading its sensitivity to even small sustained overloads. Communication and control systems are typical of the high value capital equipment that can benefit.

Hydraulic magnetic CBEs utilise the damping effect of a solenoid to accurately achieve defined delays and to eliminate the effect of short-term surges.

Most engineers are concerned about nuisance tripping, which is often caused by in-rush currents associated with certain components – primarily motors, transformers, solenoids, and large capacitors. In such cases, the designer needs to specify a circuit breaker that has a delay. Thermal CBEs have a natural delay, and magnetic circuit breakers can have added hydraulic delays. The designer can use the CBE specifications to match the delay to the duration of the expected in-rush currents. To avoid nuisance tripping, circuit protection should be specified using the normal current (nominal current) with suitable trip delays to match the application.

Important characteristics and specifications for designers include:
• The time-current characteristic plots tripping time against overcurrent, expressed as a percentage of normal rating.
• Temperature compensated CBEs are available for applications where stable characteristics are required over a varying temperature range.
• Very low voltage circuits may require the use of special models to reduce power loss difficulties.
• CBEs for aerospace, defence, marine, industrial and automotive designs need to operate successfully under conditions of shock, vibration, humidity, water splash and temperature extremes.
• Using CBEs with LVD certification enables compliance with European CE requirements.

Undervoltage and overcurrent protection
Restarting without warning can be unsafe in many applications, including industrial machinery, hobby and garden equipment. Many applications now require undervoltage as well as overcurrent capability to ensure that equipment is not automatically re-powered after a power failure or other disconnection.