I visited a customer today who had an RCD tripping after a water spill on a hot plate.
It was a single RCD protecting the entire installation, Hager type AC, been trying to persuade her to change the board for a few years.
The bit that I find strange is that the RCD was seeing 15.9mA of earth leakage, measured with clamp meter across tails. RCD also tripped in 133mS with 30mA test from a socket.
I then did a ramp test on both 180 and 0 degrees, from a socket with the installation powered, expecting a very low trip current due to existing high leakage, but it tripped at 21mA on both directions. Either I missed something or there is something I don't understand about the RCD testing setup? The one thing I can see with hindsight is that I didn't repeat the tests with the MCB's switched off.
So why would the ramp have gone so high before tripping? This is on a TT earth with Zs @ DB of 17.5 ohms. Currently installations insulation resistance is only 0.47 M Ohms ar 250V, line / neutral joined to earth. I hope this improves as the hob dries out.
