Working on existing circuit safely

[amateur]

I read an existing post on rcd tripping whilest working on an mcb isolated circuit. It mensions to use a resistor in parallel with the meter, I was wondering what resistor would be suitable because I too have noticed this issue whilest adding an extra light fittin in the loft. HOwever I thought nothing of it because it's way below the touch voltage anyway. I'm also wondering is possible for 230v to inudced onto that cable then?

It certainly was a challenge to try wire the fitting without the bloody rcd tripping. When it did trip i was plunged into darkness. Not fun!! They could of told us this at college.

 

[lgpmachine]

If the circuit you are working on is RCD protected, the RCD will trip if you cut through a cable even when the MCB is isolated. As long as the MCB for that circuit is isolated there shouldn't be any danger of voltage being present on that circuit. The RCD trips because when you cut through the cable, you are creating a neutral to earth fault, which causes the RCD to operate.

 

[endoftheworld]

If your test meter trips the rcd it must be drawing more than 30ma, so to limit mains current to less than that (assume 250v to cover variation) and with V=IR etc. you need at least a 250/0.03=8333 ohm resistor (8K3) rated at (W=VA this time) 250*.03=7.5w, although a lower rating is fine for short duration tests.

Of course this is based on not knowing the internal resistance of the meter, and assuming 0 ohms.

Sorry about the maths lesson

Of course if your using the meter to do loop impedance that resistor will send all your results to pot. Just bridge the RCD out for the duration of the tests.