Electricity + Control June 2017

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ELECTRICAL PROTECTION + SAFETY
Circuit Breaker Selection
Wynand Visser, CBI-electric: low-voltage
Miniature Circuit Breakers (MCBs) are ubiquitous safety devices in electrical distribution systems.
The 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 I2R 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
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