90degrees C mcb

I presume you mean terminal screw temperature - not ambient air - as most MCBs are rated to a max ambient of 60C or even 55 in certain cases.

I fear you will draw a blank, as it it something I spent some time looking into a while ago, but if you ever do get a clear statement from some make I'd be most interested.

Meanwhile if you are asking so you can use the 90C rating of a multi core cable, I may be able to offer some anecdotal reassurance. Once the cable is unbundled into single insulated wires, so long as there is a reasonable opened out length, and cores are sensibly spread and looped, before they enter the terminal, the singles are much better cooled than the multicore, and the temperature at the terminal is considerably cooler than the cores of the cables.

The problem is that the extra cooling is quite dependent on uncertain things like the amount of free air that can convent in the enclosure, and what else is in there sweating away... We make use of maximum indicating temperature stickers in some cases as a reassurance, and also as a warranty diagnostic, and it is very rare that things are hotter than we expect, unless something has been horribly abused.

Mike.

Cheers Mike. I appreciate the anecdotal reassurance but this must be 100% compliant with manufacturers data. My search reveals no MCBs to 60898 that are available that allow conductors at 90C to be connected. 

You may well find there are not many that allow  terminals / ends of conductors at 70C either ..... 

Mike

I am bound to wonder why an MCB for "household and similar installations" would be used in this way. However, BS 60898 devices may be used in an ambient temperature of up to 40 ºC. That said, the reference temperature is 30 ºC and the test conditions are 20 ºC to 25 ºC. Under test, a 60 K temperature rise is permitted at the terminals. The MCBs must withstand a temperature test at 100 ºC.

Quite how this relates to normal use is a bit obscure, but to my mind, any BS 60898 device should withstand at least 85 ºC at the terminals.

Is the answer to terminate the cables outside the distribution board, then run singles into the DB, ensuring that the size of those singles means they will never go above the rating of the MCB?

Chris Pearson said:

"household and similar installations"

The term covers a wide range of installations, and is characterised by a range of factors including:
 - potential use by ordinary persons
 - typical 'pollution degree'
 - typical ambient temperature range in use
 - typical EMC environment
 - etc.

So, there are commercial applications (perhaps some parts of industrial installations too) for which it is correct to use 'household and similar' equipment.

Quite how this relates to normal use is a bit obscure

I agree. From what  little I've read from that standard I get the impression that it's more trying to control the temperature rise of the terminals from heat generated by the MCB itself (e.g. from the thermal element as well as it internal conductive parts), rather than deal with heat from the connected conductor - although clearly each is going to influence the other.

   - Andy.