BS 3871 Type 2 30 A MCB (allowed installation method)

ianphilip007 · Aug 15, 2011

hi guys

after a little advice re a 30 A Type 2 BS 3871 MCB. Now i know they are old but still compliant , just checked the OSG p 49 Table 7.1 for maximum length and installation methods but there not listed.

i have the maximum ZS of 0.88 from page 103 table 2d OSG where they are listed.

can anyone advise where the allowed installation methods would be listed? or are they the same as for a type b BSEN 60898 ?

many thanks

ianphilip007 · Aug 22, 2011

be interested to know what people think. i come accross BS3871 fitted boards quite alot. are they non compliant and need to be replaced ? or are they compliant but been superceeded ?

platty1005 · Aug 22, 2011

I assume you have a spare way on the DB and you are installing a New 30A radial circuit. The requirement is that all new circuits must comply with current edition of BS7671. I cannot find any reference to BS3871 CB's being non-compliant it is in fact an overcurrent device.
Providing the design criteria are met for disconnection time IE Zs is low enough to get 0.4s, the rated short circuit capacity of the CB is not exceeded IE PFC. The Volt Drop is compliant and at 14 metres this all looks Ok.

Your big problem is the circuit needs RCD protection as the cable is concealed so providing you can fit an RCD Adjacent to DB, using the old CB should be Ok. The info for BS3871 has probably been left out of new regs because they are no longer easily obtained having been replaced by BSEN 60898 CB's. So providing you can get the old info on max Zs and it complies then I agree it should be OK. Measure the PFC at the DB compare to the M number on the CB if max PFC is lower than M number - then I don't think it is a problem.

ianphilip007 · Aug 22, 2011

thanks platty thats what i thought. the max zs is actually listed in the osg so no problem there when calculated. pfc is well below the 3ka rating of the breaker. RCD is present on the exisiting board, but it is not ideal as it is a rcd mainswitch. so no segregation of circuits, but the property is a bungalowe with only 1 lighting and 1 ring main and a cooker circuit.