The I2t (Joule integral, also more commonly known as the ‘let-through energy’) of a circuit-breaker is equal to the integral of the square of the fault current over a given time interval.

It may be used in calculations relating to protection of conductors against fault current, particularly where the magnitude of the prospective fault current is so high that a value of operating time (t) cannot be obtained from the time/current characteristic for the circuit-breaker. The I2t of a circuit-breaker may also be used to determine whether discrimination exists between the device and a fault current protective device on its supply side.
Manufacturers of circuit-breakers conforming to BS EN 60898 publish, or will provide on request, the I2t characteristic of such a device. Fig 1 shows a set of manufacturer’s curves for let-through energy. The characteristic is normally in the form of a line or curve giving maximum values of I2t as a function of prospective current under stated operating conditions.

On the basis of the ‘adiabatic equation’ given in Regulations 434.5.2 and 543.1.3 of BS 7671, cable protection under short-circuit or earth fault conditions exists up to the value of current where the I2t value let through by the circuit-breaker is less than value of I2t for the cable.

Discrimination of a circuit-breaker in relation to a fuse on the supply side exists up to the value of current where the I2t value let through by the circuit-breaker is less than the pre-arcing  of the fuse.

Energy limiting class number
An energy limiting class number (1,2 or 3) is marked on circuit-breakers of Types B and C having rated current up to and including 40 A, and rated short-circuit capacity of 3 000 A, 4 500 A, 6 000 A and 10 000 A. The number denotes the maximum I2t let-through by a circuit-breaker under short-circuit or earth fault conditions. This information may be used by the electrical installation designer for the same purposes as the information obtained from the I2t characteristic, as discussed above.

I2t  characteristic of a circuit-breaker

Fig 1