most common cause of rcd tripping

[meady]

Hi fellas.........In your vast experience, what is usuall the most common cause of a rcd to trip? I have just installed a new cu on a tt system, The system was not earthed or bonded so I have installed a new spike with 16mm cable and bonded gas and water using a 10mm cable. done my dead tests R1+R2, ir tests etc and all results were passable. When I went live the rcd kept tripping. Any theories would be very helpful........thanks

[cj-sharpy]

Was the old system on an RCD? If so you may have made a mistake in the DB somewhere. All Neutrals and earths to the right place. If not did you do a Neutral to earth IR test. Do one as it sounds like a neutral to earth fault to me.

[darkwood]

Anything with an heating element especially if old as they break down with age, so water heater, shower, kettle etc and if applicable outside lights as rain and condensation get in.

[max power]

Have you done an earth electrode resistance test ;as its the general mass of earth you are testing as this can be sometimes be very up and down with the results.ARE you using the right rcd 30ma for a1667ohm earth resistance up to a 5ooma for a100ohm earth resistance.If your happy with your test its you installation thats the problem.A rcd will only trip when a fault occurs between the live and earth.Rcd cannot detect short circuits or overloads but they are very good at detecting very low EARTH leaking currents.Hope this helps if not come back

[darkwood]

Have you done an earth electrode resistance test ;as its the general mass of earth you are testing as this can be sometimes be very up and down with the results.ARE you using the right rcd 30ma for a1667ohm earth resistance up to a 5ooma for a100ohm earth resistance.If your happy with your test its you installation thats the problem.A rcd will only trip when a fault occurs between the live and earth.Rcd cannot detect short circuits or overloads but they are very good at detecting very low EARTH leaking currents.Hope this helps if not come back

Also dont forget rcd unit monitor neutral to earth as well even if the mcb is off for circuit.

[Bikeelec]

Check for crossed over neutrals at the C/U - easy misatake to make.