Circuit Breakers
Perhaps the most common of all protective devices circuit-breakers are found in modern single-phase domestic consumer units and three-phase industrial / commercial distribution boards.
The British Standard BS EN 60898 applies to these devices and they usually come in sizes of 2 A up to 125 A.
Apart from its nominal rating (In) 10 A, 16 A, 23 A etc we usually find two other values shown on the circuit breaker, they are Ics and Icn.
Ics is the amount of current that the circuit breaker can safely withstand without being damaged for example 6 kA or 6000 A and remain usable.
Icn is the amount of current the circuit-breaker can safely handle but would not remain usable.
So, if we have a 32 A circuit breaker with an Ics of 6 kA and an Icn of 10 kA this means that any fault current up to 6 kA (Ics) would not damage the device and it would still be usable. However, anything above 6 kA up to 10 kA would damage the device and render it unusable. Anything above 10 kA (Icn) can be dangerous and could cause the circuit-breaker to burst into flames or even explode.
This is why prospective fault current values (Ipf) are so important and should always be measured at the origin of the installation where ‘thanks to Ohm’s Law’ they will always be the highest.
See BS7671 Table 41.3 for common value ratings (In) of BS EN 60898 circuit-breakers as well as their maximum Zs values.
There is a balancing act that we perform when it comes to fault current. We need it to be high enough to trip the device within its maximum disconnection time but we don’t want it to be so high that it damages the installation/devices.