Allow me to explain that a 2[sup]nd[/sup] earth leakage unit can be a big help, particularly if its "mobile", like fitted to an extension lead. Earth leakage units measure the difference between the life and the neutral current. Ideally they should be exactly the same except in different direction. If they are NOT the same then - obviously - one of the circuits is showing a leak to earth. The trouble with earth leakage currents is that they are cumulative, ie: each earth fault adds to the total. So if you have a number of "suspect" appliances in operation they might together cause enough earth current to trip the unit. (Wet stove plates are notorious). Earth leakage units usually trip at 20mA, but you may have a number of devices with just 5 or 10mA fault currents. So, which appliance caused it to trip, and which appliance must be fixed or replaced? With just one earth leakage unit mounted fixed in the db board you will never - or with great difficulty - find out. This is where a 2[sup]nd[/sup] mobile unit will come in very handy in detecting the culprit, provided you have at least one outlet socket that is NOT connected through the ELU. It would work even better if you had a unit that actually measures the fault current, which will allow you to measure the extent of the fault. At more than 10mA you should have the appliance fixed or replaced.