I pressed the "Press to Test" button on our Wylex RCD in the CU yesterday and it failed to trip! Tried several times, but it would not trip the device.
Switch the RCD off by using the DP switch bult into it - then switched it back on again. After doing this the PTT button works fine. I shall replace the thing anyway - but is this "usual" for such devices? It is only about 3 years old. I must admit I hadn't tested it for some 6 months so is that the possible cause?