Consumer unit switch tripping despite RCD

[Dundonald]

Radial spur off of main consumer unit in garage. The spur feeds exterior lighting via a dusk till dawn unit - 3 x 40 watt halogen lamps and 1 x 150 watt halogen security lamp. It was cheaper for me to buy an RCD plug than a separate RCD and fuse box so I simply plug the exterior lighting in to the radial spur using the RCD plug.

When I first plugged in the circuit the consumer unit fuse tripped, but the RCD didn't cut out. I'm not sure I understand why, can anyone help? I thought the RCD would cut out before anything else and therefore protecting the main consumer unit.

In attempt to find out the cause of the fuse trip on the consumer unit I checked every single connection on the dusk till dawn, junction boxes, and lamps, and they all looked fine. So I flicked the switch on the fuse and this time I heard a pop outside and again the fuse tripped. One of the lamps had popped - identifiable from a black ring on the brick around the lamp. Again I checked the connections in the junction box leading to that lamp and again also inside the lamp. They all are fine, but I noticed more carbon around the manufactured attachments where the cable runs in to the lamp unit. So I'm figuring the lamp is faulty. I'll be replacing this lamp unit and of course I do not leave the circuit on. But that said, strangely, I flicked the trip switch on the fuse in the consumer unit back to on and the circuit works fine now - not sure why either!

So my questions are -

1. why didn't/doesn't the RCD trip before the fuse on the consumer unit
2. any ideas what the likely cause of the problem is with the lamp - just out of interest? Like I say I'll just get a replacement from B&Q anyway but just curious.

[Ellis Greensitt]

The RCD only protects against a fault to earth What you had was a short circuit in a faulty lamp, this would not produce a fault to earth and therefore not operate the RCD. This fault would product a current sufficient to operate the fuse in the CU. Hope this helps

[Dave Liquorice]

An RCD does not provide any overload protection. It only trips if there is an inbalance in the current flowing in the live an neutral wires connected through it. If you have live/neutral short an RCD won't see that as a fault condition as 6kA if going and 6kA is coming back.

[Stefek Zaba]

As Ellis said, an RCD only detects faults to earth, most obviously L to E. If your wiring or lamp had an L-to-N short, that'll make the MCB pop, but the RCD will be happy as Larry, since just as much current - in this case, lots and oodles - is flowing out down the L as is coming back up the N.

As mentioned above, a fault making the L shorted to N. Either a fault in the lamp, or - more likely, I'm afraid - you wired it up wrong (or ended up with bare L and N conductors touching when you sqaushed them back in place). The blackening of the brick, and the popping of the MCB, says you had quite a lot of electricity flowing for a short time. When big fault currents like this flow, they can 'clear' the fault by burning away the metal bits wot are touching where they shouldn't be - so explaining your 'well now it doesn't trip the MCB any more!' observation.

HTH - Stefek

[RichardS]

it does however raise the worrying question as to what would have happened to any insulation in the immediate vicinity of this self-righting extraneous-metal-bit-vapourising fault, though.