I have ajunction box which supplies an immersion heater.an rcd device on the main supply board protects this.I turned off the trip switch which protects the heater whilst preparing to take a supply from the junction for a heater of maximum 300watts.I connected a 3 core cable at the junction intending to connect this to a separate fused spur,but the main rcd on the supply board tripped out.this implies that even though the rcd which protects this circuit is off,the main rcd still detects an incomplete circuit.is this correct or am I missing something obvious(my electrician who will check my work is currently away so I cannot consult him)
The RCD should detect an imbalance and with active RCD's also lack of voltage. The imbalance will be caused if current from line or neutral finds another path such as earth. MCB's and RCBO's used in consumer units are often single pole so the neutral will still be connected so allowing the neutral to connect to earth would cause it to trip.
Before working on electrical items one should isolate all live supplies that is both line and neutral so the main isolator should be turned off not just a MCB or RCBO unless they are twin pole with is unusually in domestic systems.