MCB Nuisance Tripping

[Doctor D.]

We have just installed uplighting to a church hall.

The circuit consists of 4 x 300 watt halogens fed by their own 6 amp type B MCB via a 6amp switch and 1.5mm T&E.

When switching on initially we are suffering nuisance tripping of the MCB. After a couple of attempts the MCB stops tripping and everything works well.

I realise that 1200 watts equals just over 5 amps, and would appreciate some guidance on whether to split the load over two switches, and 2 x 6amp MCB's, or as we would prefer them all to be switched together, just swap the 6 amp type B to a type C MCB?

Thanks.

[Sparks]

The lamps will have a much higher inrush current when they are first turned on then the stated wattage... (especially if there are 4 of the buggers)

If the difference between a B and a C is a time delay, then I would think it would work

But as I don't know the difference[Yet!], then I cant be sure of that!

Is halogen the way to go, wouldn't sodium have been better (and cheaper to run!)

Sparks...

[Doctor D.]

Thanks.

We've already got 5 x 250 watt mercury there for direct lighting. The halogens are for bouncing off the ceiling (very high) for ambient lighting, and to use when passing through, as the mercury lamps take so long to reach any decent level of illumination. The halogens don't get that much use, and at £4 per fitting including the lamp I doubt we'll ever get the money back on sodiums.

[Niall]

C type is for high inrush loads so it should work, you could even go to a D if neccesary.

[Andrew Gabriel]

Providing the circuit length is not more than 33m, I would change the MCB to a 10A type C. Ensure the luminares have glass shields to trap all the material if one of the lamps explodes.

If I was designing the installation from scratch, I would have split into two circuits as a lamp failure at end of life has a high chance of tripping the breaker whatever its value (300W halogens have no integral fuse link). In a public building where there's a chance these might be the only lights switched on, you don't want any risk of losing them all when a lamp fails.

[Andrew Gabriel]

It's not time delay, but the short term overcurrent which is allowed through. From memory, Type B allows 3-5 times the MCB rating before instant tripping, and Type C allows 5-10 times the MCB rating before instant tripping. (Instant means within half a mains cycle.)

A large halogen lamp draws around 17 times its running current at initial switchon. (Actually, that's a figure for larger lamps than 300W -- 300W won't be as bad but I don't have its figure).

17 x 5A = 85A, although this is an over-estimate for several reasons. A 10A Type C will allow 50-100A, which given the 85A is an over-estimate, should be OK. As I said in my other posting, I would have designed it as two circuits anyway.

The 6A Type B will instantly trip at 18-30A, which is clearly less than the switchon surge from the lamps.

[Chris Oates]

I'd have used 2.5mm

[Doctor D.]

Andrew,

Many thanks for your very helpful notes. I agree with your thoughts about running the lamps over two circuits. We didn't do this because the hall also has another two halogens on another 6 amp MCB, so these 4 are not the only illumination, and we wanted to be able to switch the four together for asthetic reasons.

However asthetic or not, they are no use whilst they are nuisance tripping! We will redesign. The cabling in the main is clipped direct to the top of the open beams so it's not too much hassle.

We did consider using 2.5mm cable, but given the shortish runs and installation method decided it was a bit OTT for running 5 amps continuous load. May now re-consider that!

The cable run is less than 33 metres, and the fittings do have glass fronts (and very natty looking grills too.)

Thanks again.

DD

[Christian McArdle]

Use a 10A breaker. Maybe use Type C, or even a cartridge fuse holder (with spares available).

Christian.