I've had a complete rewire to my commercial premises and a new consumer unit fitted Dec 2012. I started trading Dec 2013. After 12 weeks from moving in I started having problems with my lighting in my shop sign.

I kept the original fluorescent light fittings but changed all the tubes and starters before opening the shop. There are three 5 foot tubes in the main sign and three 2 foot tubes in the side light box. The electrician dropped a cable into the main light box and wired up the original fittings.

The sign lights were on a 16 amp timer. All was working well until one day, when the sign lights were meant to come on it tripped the main RCCB (63A 30mA) disabling all the devices on that circuit - timer (controls sign lights), indoor lights, smoke alarm, sockets, water heater and burglar alarm. None of the MCBs tripped. I tried putting the RCCB on but it kept tripping, the only way it came on was if switched off the MCB controlling the timer- sign lights.

Called the electrician, he said all his work is ok. Thinks the fault is with the original sign fluorescent fittings which he hadn't touched, claiming that water may have got into the light box and/or the fittings are shot.....all this without even inspecting the light box outside!!! He quoted me a price to replace all the fittings but I dubious...at least get a ladder out an inspect the damn thing!!

I swapped the timer for a normal light switch to eliminate the timer...still the same problem! Once you switch the switch on to put the lights on the RCCB trips. Also I opened the box and was found to be dry.

However, I have noted that if I put on the sign lights first thing in the morning and then put on all the lights inside, switch on all the plugs, the RCCB doesn't trip...all works well! The other way round switch all electrics inside, then switch on my sign lights the RCCB trips. At the end of the working day if all electrics switched off inside and I attempt to put my sign lights on..same problem RCCB trips..(something to do with heat/conductance??). Also same thing in evening if I switch off all the MCBs apart from the one controlling the external lights.

If the outside lights were faulty and need changing surely they would trip if used in isolation? I rang this so called sparky, and he says he's baffled..he will get back to me!

The MCB for the external fluorescent lights is a B type 6A. I have read that for commercial fluorescents a C type MCB should be used? Do I need a higher rated B? 6A Type C? or a larger type C?

Indoor lights consist of 38 LED GU10 7W each. Sockets powering a desktop. lcd, till, phone, router, pdq. I did notice once that when I switched one of the sockets off the standby light remained on, on the monitor. I pressed the socket switch a few times and it appeared to sort it out.

Can anyone please help solve this mystery regarding my sign lights. Thanks in advance.

Baggyboy, April 2014

all you have to do is get your electrician to isolate and megger the outside lights,its not rocket science mate its only a simple light circuit,i am a spark of 40 years and have never heard such rubbish,if you don't trust your electrician ,then get another one,but don't insult the man by calling him a so called sparky,shame on you

mm, April 2014

If there are 2 rcd's or there are some circuits not protected by the rcd and the live is taken from the protected side but the neutral is taken from the unprotected side you can get some strange things happening.but you said it worked for several weeks so i think I'm wrong on that one.I still think the problem is a slight earth fault on the sign, a quick test on that circuit is easy to determine if there is a problem,I don't think replacing all the fittings is necessary

toptrish, April 2014

Thanks Toptrish for your reply. Can you explain what you mean by the live and neutral not going to the same as the live and neutral the RCD is controlling? Is it the live and neutral from the light fitting? Sorry if it's a daft question but I would like to be able to explain to the sparky.

Also, I don't want to tempt fate but I switched off and on the RCD several times yesterday also the same to each MCB on that circuit, and all seems ok at the moment! I'm puzzled!!

Baggyboy, April 2014

At a rough guess [and its only a theory]the live and neutral are not going to the same live and neutral that the RCD is controlling.Or there could be a slight earth fault that is tripping it,but I would have to test your theory about the way you switch lights on, it could be a coincidence.I can see the sparky's point of view,the sign being the only thing he did not renew,the light fittings don't have to be dripping in water to be faulty,damp will do it just as effectively and outside lights are very prone to it

toptrish, April 2014