BS 3871 Miniature Circuit Breakers Let Through Energy

Let through energy is a bit problematic for breakers - a fuse gets faster as the fault current rises,  and tends towards a constant let- through energy (I squared r times time, but the resistance r is a fixed parameter of the fuse, as is the weight of metal to be raised to melting point to start it breaking)

For an MCB, although at low currents increasing faut current increases breaking speed, there comes a mechanical limit of how fast the contacts can be separated,and it more or less stops getting faster.

For this reason, while fuses can be cascaded, and if the thin one blows, the fatter one behind it will not,  for all values of fault currrent, for MCBs no such statement can be made. Indeed it is common to find a random selection of breakers open under fault, and for really high current faults the fuse at the origin fails instead.

So I2t is actually a function of the prospective fault current, and is either depicted graphically, or listed as spot  values . For mcbs  made to the IEC standards since 1999 or so this is a spec parameter, and guaranteed by design. for earlier devices it isn't

IF you put in the maximum breaking time of 10ms you will come up with a very large minimum cable requirement,and may well (incorrectly) conclude the cable is not protected.

It may be cost effective to replace the breakers or back them with a fuse, but first post the make and rating of breaker here - some folk on here have breaker test data going back to mists of time and may be able to advise.

Let through energy is a bit problematic for breakers - a fuse gets faster as the fault current rises,  and tends towards a constant let- through energy (I squared r times time, but the resistance r is a fixed parameter of the fuse, as is the weight of metal to be raised to melting point to start it breaking)

For an MCB, although at low currents increasing faut current increases breaking speed, there comes a mechanical limit of how fast the contacts can be separated,and it more or less stops getting faster.

For this reason, while fuses can be cascaded, and if the thin one blows, the fatter one behind it will not,  for all values of fault currrent, for MCBs no such statement can be made. Indeed it is common to find a random selection of breakers open under fault, and for really high current faults the fuse at the origin fails instead.

So I2t is actually a function of the prospective fault current, and is either depicted graphically, or listed as spot  values . For mcbs  made to the IEC standards since 1999 or so this is a spec parameter, and guaranteed by design. for earlier devices it isn't

IF you put in the maximum breaking time of 10ms you will come up with a very large minimum cable requirement,and may well (incorrectly) conclude the cable is not protected.

It may be cost effective to replace the breakers or back them with a fuse, but first post the make and rating of breaker here - some folk on here have breaker test data going back to mists of time and may be able to advise.