Electricity + Control June 2017

ELECTRICAL PROTECTION + SAFETY

Circuit Breaker Selection

Wynand Visser, CBI-electric: low-voltage

Miniature Circuit Breakers (MCBs) are ubiquitous safety devices in electrical distribution systems.

T he important role these devices play in protecting cabling from over currents and consequential electrical fire is often taken for granted or misunderstood. By revisiting the relationship between MCB ratings, conductor cross section and insulation type, the risk of failure can be mitigated. Electricity is dangerous, and so is driving a car. In the same way that traffic regulations make transportation relatively safe, so are the regulations that make electricity usage safe highly effective. In fact so effective, that it is easy to become over-confident and forget the real need for electrical protection and overlook key aspects of circuit breaker selection. The two main dangers of electricity is direct risk of electrocution of people and the fire-risk of overheating electrical circuits or arcing short-circuits as a result of over currents. Protection against the former is achieved by using proper insulation designed to electrically and mechanically keep a safe distance among conductors and people, in coordination with sound earthing techniques. Protection against over-currents is the domain of a circuit breaker. Current and heat Anywhere an electrical current is flowing in a conductor, it generates heat due to Ohmic losses. Heat generation can easily be reduced by increasing the conductor cross section, but with the high cost of cop-

per, designers are forced to use the smallest gauge possible that will tolerate the required current. The actual temperature attained by a conductor depends on the Ohmic resistance and the current flowing through it (commonly known as I 2 R losses) and the cable’s ability to dissipate heat to the environment, which is at ambient temperature. Large over-currents can cause temperatures of several hundred degrees. This will rapidly melt polymer insulations into a bubbling puddle and can spontaneously ignite nearby materials to start a fire. Moderate overload currents raises cable temperatures by tens of degrees and degrade the insulating materials over time. PVC (Poly- vinyl Chloride), a common insulating material, should not exceed about 70°C for extended periods [1]. The eventual loss of insulating function can lead to exposing people to live electricity, or a short circuit again leading to fire damage. Therefore a primary function of a circuit breaker is to protect cable insulation from heat caused by overcurrent conditions.

Circuit breaker parameters The key parameters of a circuit breaker are: Voltage rating; Current rating; Time delay curve; Short circuit rating; Temperature rating

Electricity+Control June ‘17

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