6amp MCB type B or C

[sevetaz]

i have type B 6 amp mcbs protecting each of the four of my lighting circuits, kitchen, downstairs, upstairs and garage, most of the lights are gu10 type bulbs some LV. vitually everytime a bulb blows it kicks out the MCB someone has suggested that i fit type C as they wont kick in when simply a bulb goes. wanted to know peoples views before i change.

Dont know if its connected but tonight when i turned on a light on which the switch is a dimmer, one of the bulbs blew, the MCB tripped and the insides of the dimmer blew when i switched on... any ideas would be very much appreciated

 

[ColJack]

you must ensure that the Zs values of the circuit will support a C type breaker.. it's not just a simple case of a straight swap..

 

[ericmark]

In a modern house post 2008 there would be no real problem with C type magnetic trip but pre the use of RCD's on all circuits one has to be more careful.

With old style bulbs a fuse was built into the bulb because it is common when they blow to get ionisation and very high currents for very short time. The magnetic part of the 6A C MCB will trip at 60A rather than the 30A of a B type but the ionisation can draw 100's of amps so even with type D poor quality bulbs will still take out the MCB.

Some of the discharge lamps are just as bad and a CFU from Ikea melted the BA22d socket contacts when it went so seems no fuse included in the design.

So you may find changing make of lamp will help far more than changing the MCB.

 

[Taylortwocities]

Eric, I hope that you aren't suggesting that we can ignore EFLI if RCDs are installed?

 

[ban-all-sheds]

Another way to avoid MCBs tripping is to replace the B6 with a B20 (or 25 if you can get one) with 2.5mm² supplying an FCU adjacent to the CU, with a 5A fuse in it.

 

[ericmark]

Eric, I hope that you aren't suggesting that we can ignore EFLI if RCDs are installed?

Ignore no but the permitted parameters do change. With a RCD table 41.5 note 2 advises the permitted EFLI should not exceed 200 ohm however we also have volt drop to consider so the cable lengths are still limited.

It will be noted that many RCBO's are provided in the C rather than B magnetic release.

We did in the past use LOADMASTER MCB's which had no magnetic release and at the time it was felt these were an improvement on the fuse.

However the damage which could be caused under short circuit when considering the let through was of course a problem the main one was the blowing of up stream fuses and the problem of using D rated large MCB's in domestic does present the problem that before the MCB trips is can take out the incoming fuse.

However this is not a problem with 6A devices.

I would agree the BS7671 does not really explain what we may use a RCD for when it allows the use of an RCD when the earth loop impedance is high in TT systems 411.8.3 does state "or" not "and" but 415.1.2 The use of RCDs is not recognised as a sole means of protection and does not obviate the need to apply one of the protective measures specified in Sections 411 to 414.

However because there is a thermal trip the RCD is not the "sole" means of protection.

What does worry me is the use of passive RCD's remote from main incomer where under fault conditions volt drop may mean there is not enough volts for the RCD electronics to work and trip the device. I note the earth wire fitted to RCBO's to assist with this problem but where volt drop could be an issue then one should used active RCD's.

But returning to bulb blowing fault the extra current produced with C and D type MCB's can cause the contacts on BA22d bulb holders which in the main are only rated at 2A to fuse and so instead of just changing a bulb you need to change the bulb holder as well. So since I find it far easier to just change the bulb I will continue to use B6 MCB's in my house.

 

[JohnD]

on a tangent, since I moved to CFLs, I have not had any MCB trips.

I used to get them every time a spotlight blew in the kitchen. Now I have fitted Philips CFL spots which are not very good, but at least it saves me tramping down to reset. I expect I will move to LED spots as they get better and cheaper.

I would have thought that with filament bulbs going out of fashion this problem is going to disappear.