On 07/11/2010 02:01, Dave Osborne wrote:

Ah, I wasn't suggesting that the RCD becomes over sensitive, rather that
is is and has always been over sensitive.

An RCD that trips at 17mA is likely to be noticeably more of a nuisance
than an RCD that trips at 27mA.

Fairynuff, but you shouldn't consider one that trips at 17 mA as
over-sensitive - the standard allows them to trip anywhere between 15
and 30 mA. In my experience though they all tend to trip at around the
20 mA mark, ±2 or 3 mA.

The test button is of limited facility.

Agreed, but it will (usually) alert you to a device that isn't going to
trip at all. If the test resistor circuit has gone o/c you'll get a
false negative result, which errs on the safe side.

There's no substitute for testing with an RCD Tester.

Agreed, but, OOI, how many times have you found an RCD that failed on
elongated trip times but did actually trip on the test?

--
Andy