MCB tripping on lighting circuit

14 Jun 2006
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United Kingdom
We have a small outbuilding which has a 6A MCB (B6) protecting the lighting circuit. This consisted internally of 8 x 50W halogen downlighters each with its own transformer and 2 x 100W external lights switched through a PIR.

For security reasons I have replaced the 2 x 100W external lights with 2 x 500W floodlights. The MCB now has a tendency to trip. I realise that a 6A rated MCB is right on the limit for the new load. 1mm cable has been used throughout the installation.

Am I in order to upgrade the MCB to 10A to prevent the tripping on this basis?
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or you could fit 300W tubes in the floods.
Am I in order to upgrade the MCB to 10A to prevent the tripping on this basis?

You could if you can achieve the required disconnection times - Zs values.

Other solution would be to change to a C curve 6A MCB but Zs still would need to be checked.#
Do the two 500 watt lamps come on at the same time with one PIR controlling them ? If so the in-rush current before the lamp reaches full temperature may be the reason the MCB is tripping.

Using two PIRs ( one per lamp ) will stagger the lights and reduce the in-rush current. And if a PIR fails the other will still operate.
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Thanks for the replies.

Yes, one PIR controls both lights. I think the suggestion to fit 300W tubes is probably the easiest solution.

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