Type D 40A MCB upstream of BS88 63A Fuse

[Ziggy Sparks]

Can anyone help with the time current characteristics of which will operate first? Essentially a 40A type D crabtree pole star fed from a BS 88 g 63A fuse. Looking at the time current characteristics I reckon that the BS88 would operate at a fault current of 500A for a 0.4s disconnection whereas a Type D operates at 800A for 0.4. Am I right and if so how can you justify using one?!:dunce2:

 

[Guest55]

dont have the time graphs to hand but i do know that any circuit with a type D mcb needs a super low Zs , so i'd be checking that as well.

 

[Darkwood]

Why is there a 40A type D downstream of a 63amp bs88 what is the set-up for needing a type D in series with a bs88 general use fuse

 

[Ziggy Sparks]

It's upstream. Sub main from meter cabinet via 63a bs88's feeds db. 40a type d mcb in db feeds psu. It is a battery charging equipment.

 

[Darkwood]

Is this the requested mcb as per manufacturers instructions for the PSU if this was the case i would also be weary that although the BS88 although higher in current rating may operate on inrush as from you OP you said BS88 'g'.... gg are for general use and gm are usually for motor rated or other circuit with inrush currents, but both knowledge of the inrush peak of the psu and the inrush tolerance of the bs88 are needed usually sorted from manufacturers tech' help.
At a glance though you have inverse partial discrimination in favour of the upstream device and due to the nature of it supplying a DB then it shouldn't be set up this way.
If a fault current of 2ka flows it will exceed tripping points of both devices by a good margin then the decider to which operates is down to many variables but favouring the bs88 to rupture first.

 

[MarkieSparkie]

From the B7671:2008+A1:2011 protective device characteristics it would appear that discrimination is not assured, however, you will need to consult the manufacturers data for the specific devices to be certain.

 

[Adam]

It's upstream. Sub main from meter cabinet via 63a bs88's feeds db. 40a type d mcb in db feeds psu. It is a battery charging equipment.

It's actually downstream.Upstream is towards supply. Downstream is away from supply.D40 MCB is downstream of BS88: 63A.