Sparks Electrical News February 2018

DISTRIBUTION BOARDS, SWITCHES, SOCKETS AND PROTECTION

13

CIRCUIT BREAKERS: DESIGNED TO PROTECT ELECTRICAL CIRCUITS FROM DAMAGE

by Rhodam Evans, Major Tech

C ircuit breakers are automatically oper- ated electrical switches that are de- signed 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 break- ers 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-cur- rents 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 bi- metal strip enough to bend it towards the ac- tuator 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 cur- rent 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 han- dles short circuits and consists of an iron core with a wire coil around it, forming an electro- magnet. The electromagnet responds instanta- neously 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 le- ver and as the current in the electromagnet in- creases 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 I n 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 instanta- neous 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 I n . C curve indicates that the circuit breaker trips instan- taneously above 5 I n and up to and including 10 I n whereas a D curve indicates that the circuit breaker trips instantaneously above 10 I n and up to and including 20 I n . 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 con- tacts in as little as four milliseconds. This compares favourably to the speed of inter- ruption available from other types of break- ers, such as hydraulic-magnetic, which gen- erally energise an electromagnet to interrupt short circuit currents. It may take hydraulic- magnetic breakers 10 milliseconds or more to stop current flow completely. A point to note is that thermal-magnetic breakers are sensitive to temperature and in sufficiently warm environments, the normal current han- dling capacity must be de-rated to compen- sate 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 cur- rent 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 fa- tal fire. Alternatively, the circuit breaker will explode as a result of the copper in the cir- cuit breaker overheating, which could be very dangerous to anyone nearby who turned on the circuit breaker after a fault. Many com- panies offer 3 kA and 6 kA to cover most ap- plications downstream from the main circuit breaker, but the appropriate kA rating must be calculated through cascade tables. Cas- cading 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 ena- bling 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.

WRAPTOR: SAVES COSTS AND ENHANCES IDENTIFICATION

A great tool to control manufacturing costs and enhance identification qual- ity by automatically printing and apply- ing labels, Brady Corporation’s Wraptor A6500 identifies cables in five seconds, eliminating the need to print labels in advance and the time needed to apply them by hand. The Wraptor’s technology enables 300 dpi prints for bar- codes, logos and diagrams, and a tight label wrap around the wire, with minimal bubbling or wrinkling. The Wraptor A6500 can identify cables from

rable B-427 self-laminating vinyl and B-499 ny- lon cloth wrap-around cable labels, designed to last in challenging industrial environments. The self-laminating label offers excellent abrasion, fuel and oil resistance, great outdoor durability, has self-extinguishing properties and is available in multiple colours. The nylon cloth wrap-around label offers abrasion, fuel, oil, dirt, heat, cold and chemical resistance. Complete technical data sheets for both labels are available.

1.52 mm to 15.24 mm in diameter and apply labels with varying widths up to 50.8 mm, and lengths between 19.05 mm and 76.20 mm. Its full colour touch screen and modern user interface increase the Wraptor’s ease-of-use while USB-ports and Ethernet connectivity make it practical to implement in a variety of production environments. Coupled with Brady Workstation apps for easy label design, bar- coding and serialisation, the Wraptor increas- es process efficiency. The Wraptor is designed to apply Brady’s du-

Enquiries: +27 (0)11 704 3295

SPARKS ELECTRICAL NEWS

FEBRUARY 2018