Sparks Electrical News

by Rhodam Evans, Major Tech

Circuit breakers are automatically operated electrical switches that are designed to protect electrical circuits from damage due to overload or short circuit. The basic function of the circuit breaker is to identify a fault and then interrupt the flow of current. All circuit breakers have common features in how they work, but specifications vary substantially depending on the voltage class, current rating and type of circuit breaker.

Certain ranges of low voltage circuit breakers are din rail mounted and classified as low voltage thermal
magnetic. Thermal magnetic circuit breakers incorporate two switching mechanisms, a bimetallic strip (thermal) and a solenoid (electromagnet). The bimetallic strip serves as a means of handling over-currents and responds to less extreme currents than the solenoid but to longer term current conditions. Electrical current exceeding the circuit breaker overload rating heats the bimetal strip enough to bend it towards the actuator mechanism which causes it to push on the actuator mechanism and open the circuit. The time the bimetal strip needs to bend and trip the circuit varies inversely with the current and so the higher the current the faster the bimetal strip will cause the circuit breaker to trip.

The magnetic part of the circuit breaker handles short circuits and consists of an iron core with a wire coil around it, forming an electromagnet. The electromagnet responds instantaneously to large surges in current (short circuit) with the pulling force increasing as the current increases. The circuit breaker contacts are held closed by a latch controlled by the actuator lever and as the current in the electromagnet increases beyond the rating of the circuit breaker, the pulling force created by the electromagnet releases the latch, which lets the contacts open through a spring action.

The international standard, IEC60947-2, defines the rated current In of a circuit breaker as the maximum current that the breaker is designed to carry continuously (at an ambient air temperature of 30 °C). The most common values for the rated current of miniature circuit breakers are 6 A, 10 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A and 63 A. As well as maximum continuous current, circuit breakers are also subject to instantaneous tripping current which is defined according to the curve of the circuit breaker and identified as B, C, D, K or Z curves.

The different curves indicate the instantaneous tripping current, which is the minimum value of current that causes the circuit breaker to trip without intentional time delay in less than 100 ms and expressed in terms of In. C curve indicates that the circuit breaker trips instantaneously above 5 In and up to and including 10 In whereas a D curve indicates that the circuit breaker trips instantaneously above 10 In and up to and including 20 In.

Thermal magnetic circuit breakers are therefore often installed where it is important to quickly limit short circuit current. This is because the electromagnet in these devices can extinguish the arc between breaker contacts in as little as four milliseconds. This compares favourably to the speed of interruption available from other types of breakers, such as hydraulic-magnetic, which generally energise an electromagnet to interrupt short circuit currents. It may take hydraulic-magnetic breakers 10 milliseconds or more to stop current flow completely. One point to note is that thermal-magnetic breakers are sensitive to temperature and in sufficiently warm environments, the normal current handling capacity must be de-rated to compensate for the temperature difference.

The final and possibly most important safety aspect is the fault current rating (kA rating) of the circuit breaker. The value of the kA rating determines the maximum amount of current a circuit breaker can handle under fault conditions. The circuit breaker only has to withstand this amount of current for the amount of time it takes to trip. For example, a value of 6 kA means the circuit breaker can withstand 6 000 A of current during the brief time it takes to trip. Under the fault conditions described above, much more current can flow through the circuit than what it was designed for. A circuit designed for a maximum of 20 A may suddenly be drawing hundreds, if not thousands of amps.

If, during a short circuit, there is more current flowing through the circuit than the kA rating of the circuit breaker then the circuit breaker will fail in one of two ways. Either the contacts in the circuit breaker will weld together and prevent the circuit breaker from tripping, thus leaving the circuit live. This would hopefully only result in damage to the cable, but could easily start a potentially fatal fire. Alternatively, the circuit breaker will explode as a result of the copper in the circuit breaker overheating, which could be very dangerous to anyone nearby who turned on the circuit breaker after a fault. Many companies offer 3 kA and 6 kA to cover most applications downstream from the main circuit breaker, but the appropriate kA rating must be calculated through cascade tables. Cascading is what happens when you place a smaller kA rated circuit breaker on the load side of a larger kA rated circuit breaker. In such cases, the larger circuit breaker limits a certain amount of the fault current, thus enabling you to safely use smaller rated circuit breakers downstream. Unfortunately many electricians are unaware of this concept, and it remains one of the most common design faults found in electrical circuits.

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Title: Editor
Name: Karen Grant
Email: sparks@crown.co.za
Phone: +27 11 622-4770
Fax: +27 11 615-6108

Title: Advertising Manager
Name: Carin Hannay
Email: carinh@crown.co.za
Phone: +27 11 622-4770
Fax: +27 11 615-6108

 
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