IEC 60947-2 obviates risk and ensures safety and reliability. Its requirements have grown in number to keep pace with evolving technologies and reliability needs of electrical distribution.
Circuit breaker categories A and B
The standard sets out two main circuit breaker selectivity categories: A and B.
Category A: These circuit breakers are typically miniature circuit breakers (MCB) and molded case circuit breakers (MCCBs). They operate at the socket outlet end of final distribution. In the event of a short circuit they trip immediately.
Category B: These circuit breakers have withstand breaking capability. They don’t necessarily trip in the event of a short circuit, which allows downstream circuit breakers to switch off. Their place is in incoming switchboards.
Icu and Ics: essential to operational reliability
Across both categories, circuit breakers must deliver reliability. The two IEC 60947-2 requirements which ensure that reliability are:
• Ultimate breaking capacity (Icu)
• Service breaking capacity (Ics)
Icu is the maximum short-circuit current that a circuit breaker can break without damage. That maximum might be 6,000 or 10,000 amps. Or when it comes to MCCBs, as high as 200kamps. However, in a sense, Icu is essentially as statement of quality for a specific voltage rating.
And that’s where Ics comes in. It is expressed as a percentage ratio of Icu and tells you the maximum short-circuit current if a circuit breaker can break three times and still resume normal service. The higher the lcs, the more reliable the circuit breaker.
Suitability for isolation: seeing is believing
Among the requirements that must be visible even on an installed circuit breaker is suitability for visible isolation. It is particularly important. If a circuit breaker is turned off, it should indicate so visibly. It should not be able to indicate otherwise if the contacts are not open. In other words, it offers proof of isolation.
Pollution degrees: should be in all specifications
A requirement that I think is particularly important point and is not often found in general specs is degree of pollution. It determines in what kind of environment circuit breakers can be installed. In a building where there is no dust no humidity, the circuit breaker is comfortable. But in an outdoor public installation, for example, or manufacturing facility there’ll be dust. It might cause leakage currents, affect electrical properties, and lead to dangerous arcs.
I say a useful rule of thumb for household applications it that devices should be pollution degree 2. In my opinion, it’s the right degree for low-pollution non-conductive applications. For more heavy-duty applications, like incoming switchboards for utilities and factories, pollution degree 3 is mandatory. It means circuit breakers withstand conductive pollution dust, humidity, you name it.
Conformity assessment and certification, independent and indispensable
IEC 60947-2 sets out many more requirements. I’ll be running through them in Part 3. But there’s a point I’d like to emphasise first.
It is that all circuit breakers should be conformity assessed and certified compliant in independent, highly reputed third-party laboratories. The standard actually sets the conformity tests – very close to operating conditions – that circuit breakers should undergo.