Thx for the 2 replies. It was a qualifed electrician who installed it but didn't know the answer. As my friend is an elderly widow who wanted a 2nd opinion I said I'd try on line for her.
I will pass on the info given by 'Sparx' thank you.
my friend had a 2nd ceiling light installed in the hall feeding from the original, I think. A 2nd light switch has also been added, so that you can switch off either end. During a recent short powercut, the new light remained on, but dim. (she had used a dimmer bulb by mistake). What caused this and...