Hi All just a bit of advice in you please. My Mate has a garage ( business ) car repairs etc, the setup inside is very basic, 7 or 8 twin socket outlets, two 3ph feeds to two car lifts and about 8 single florecents. the incoming supply is 3PH 4 wire. the REC have installed a 30ma RCD. Now the strange bit. The garage opens at 8am, there is a industrial unit about 10Mts away, they open at 9am. So the garage is all up and working fine, the people then arrive at the unit next door and switch their lights on ( Bigger unit more lights ), it trips the RCD in the garage. I have looked and I can no problems with the cabling in the garage, The garage owner has contacted the REC about the problem, they came out checked their bit in his garage but then said they can not do anything or check the premiss next door unless they request it. Could it be for some reason the line voltages are being pulled down enough to cause an imbalance in the search coil , or the RCD is over sensitive, but would not explain why the RCD is tripping when next door turn their circuits on. It only happens the once a day first thing, then the rest of the day it is fine