Hello everyone, 

I was wondering whether anyone knew / could point in the right direction of the information of small overloads and how it affects the temperature of cables.

When reading bs7671, according to 433.1.201 as long as the OCPD is one listed in the reg, eg. 60898 MCB’s, 61009 RCBO’s BS88 Fuses etc the cable automatically complies with I2 < 1.45 Iz if you follow In < Iz and the cable will not reach too high of a temperature during the overload period.

Things start to get a bit confusing when looking up the actual value of i2 and the conventional fusing time of all of the OCPD mentioned in 433.1.201 as not all of them have an I2 of 1.45, only the 60898 MCB’s do and even then, if they have an In > 63 the fusing time is then 2 hours rather than 1 hour for In < 63

then looking at BS 88-2 and BS 88-3 they have an I2 value of 1.6 and can also take up to 4 hours at the larger size of fuse (In > 400) to disconnect the circuit. I’m not sure how all of these different OCPD can protect the cable to the same degree with the different characteristics published in the electrical installation design guide.

you then have BS3036 fuse which has an I2 of 2, and a fusing time of 2 hours. When using this fuse looking at regulation 433.1.202 we have to apply the Cf of 0.725 to adjust from the 2 to the 1.45 to allow for larger currents being sustained during the overload period when using this type of fuse. However we don’t have to make adjustment for the BS88 fuse that takes 1.6 instead of 1.45 which would be a Cf of 0.9. Also the fact that they can take 4 hours rather than 2 to disconnect the circuit which could cause more heat to build up on the cable.

sorry it’s a bit long winded and I know I must be missing something here, any help would be greatly appreciated. 

Cheers.