This discussion is locked.

You cannot post a reply to this discussion. If you have a question start a new discussion

BS 3871 Miniature Circuit Breakers Let Through Energy

MG over 5 years ago

Currently involved in a project where I need to do the energy let through calculations for existing circuit breakers to prove that the increased fault current even though within the breaking capacity of the MCB, damage will not occur to the final circuit cables.

In a nutshell, the following formula must be true I²t<k²S². In BS 60898 MCBs the I²t is provided by the manufacturers as these are energy limiting devices.

As these happen to be BS 3871 and in the absence of such data can I get your recommendations? Use a definite trip time of 10ms and plug that in the equation?

Thanks

Mike

Parents

0 Former Community Member over 5 years ago

OK - I was assuming 90C/250C (ie XLPE insulation and K of circa 150) so you should be OK for 2.0mm2 and upwards - which should exclude a whole load of circuits from your assessment.

Which leaves you with the lighting circuits - how far down the cable do you need to go, if the 3kA value is present at the DB, using 1.5mm2 conductors before you are safe again - not far I suspect.

If you are looking for reason to keep the existing despite the mains upgrade, then you will need more data, but don't be surprised if it doesn't help much - do you know for example if it was compliant before the upgrade took place (ie at the previous fault level) - have you tried a random MCB is say Amtech (or data from a different supplier)

If you are looking for a reason to scrap the existing, then absence of that data is as good a reason as any

How accurate is the 3kA value, and is that subject to cut off characteristics from upstream devices

For information, if these are the old black cased loadmasters to BS 3871, they have an M3 (ie 3kA rating) and for all sizes to 70A, they have A2s values of 10,000, 40,000 and 90,000 for 1kA, 2kA and 3kA fault levels, respectively

Regards

OMS
Cancel
Vote Up 0 Vote Down

Cancel

Reply

0 Former Community Member over 5 years ago

OK - I was assuming 90C/250C (ie XLPE insulation and K of circa 150) so you should be OK for 2.0mm2 and upwards - which should exclude a whole load of circuits from your assessment.

Which leaves you with the lighting circuits - how far down the cable do you need to go, if the 3kA value is present at the DB, using 1.5mm2 conductors before you are safe again - not far I suspect.

If you are looking for reason to keep the existing despite the mains upgrade, then you will need more data, but don't be surprised if it doesn't help much - do you know for example if it was compliant before the upgrade took place (ie at the previous fault level) - have you tried a random MCB is say Amtech (or data from a different supplier)

If you are looking for a reason to scrap the existing, then absence of that data is as good a reason as any

How accurate is the 3kA value, and is that subject to cut off characteristics from upstream devices

For information, if these are the old black cased loadmasters to BS 3871, they have an M3 (ie 3kA rating) and for all sizes to 70A, they have A2s values of 10,000, 40,000 and 90,000 for 1kA, 2kA and 3kA fault levels, respectively

Regards

OMS
Cancel
Vote Up 0 Vote Down

Cancel

Children

No Data

BS 3871 Miniature Circuit Breakers Let Through Energy

Community Rules & Guidelines