Discuss BS 3871 Type 2 30 A MCB (allowed installation method) in the Electrical Forum area at ElectriciansForums.net

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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
 
Ian are you installing a new circuit or is this a PIR.

If it's a new circuit then design the circuit taking into length of it and use a cable that will give you a sufficiently low enough R1+R2 to achieve that 0.88 value, and be with-in your VD requirements. Remember the OSG is a guide, and those maximum lengths are just that guides, you can extend those lengths with good design.

If it's a PIR, then measure your Zs through the circuit and whatever the highest value is, is the one you use, and again if this is below 0.88 ohms, then your fine.
 
its a new circuit Malcolm. ZS will be fine and is not a problem. 14 meters ( 4/1.5mm cable) ZE (0.12) it was the fact that the BS3871 was not listed in OSG am i right in assuming its allowed installation methods are the same as a BSEN60898 Type B MCB ?
 
R1=R2 = 16.71 x 14/ 1000 = 0.23 ZS = ZE+(R1+R2) =0.12+0.23 = 0.35

Volt drop = 11x 30 x 14/ 1000 = 4.62 v ( below 6.9v) fine.

ZS will be way below 0.88 Just no listing for allowed installation method which will be clipped direct/buried.
 
Ian, BS 3871 is a withdrawn standard replaced by BS EN 60898 and therefore there is no design information in the 17th BRB and BGB or their OSGs. Strictly they should not be used for new designs as they don't comply with the current regulations.
The BS 3871 MCB design data you require is in the 16th Edition Wiring Regulations and OSGs.
 
Last edited by a moderator:
Max length for Zs = (0.88 - 0.12) ÷ ((16.71 ÷ 1000) x 1.2) = 37.9m

Max length for VD = (11.5 x 1000) ÷ 30 ÷ 11 = 34.85m

So max length = 34.85m
 
Can you add a new circuit to a cu which takes BS3871 MCB's ? surely you dont have to replace the board so that BSEN 60898 MCB's can be used ??

i thought the BS3871 were discontinued or replaced by 60898 not that 3871 were now obsolete or non compliant with reguations
 
Should be a din rail fitting so just find a BS EN 60898 that will fit the front ( as you put it on)..........as long as the circuit that you pull in complies with the current regs (17th) then i dont see a problem: remember, that to comply with the current version of BS7671 final circuits designed to be used by unskilled and/or uninstructed persons not under supervision MUST have ADDITIONAL protection by means of an rcd with a residual opperating current of not more than 30mA
 
the board is an old wylex nn board which takes the nb mcb's i dont think any of the new BSEN60898 will fit.

do you think that using a BS3871 mcb with a 17th ed compliant circuit would not comply ?
 
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.
 
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.
 

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