by ericmark »
Wed Feb 15, 2017 8:15 pm
All I can do is explain how a RCD works in the hope it triggers something which helps.
It does not measure current to earth, it actually measures current in the line and neutral and compares them, if equal then all is OK. If current escapes from either line or neutral to any other place, be it an earth, or another circuits line or neutral, over 15 mA then it may trip, there are some RCD's rated to trip 90 ~ 100% but most are rated 50 ~ 100% so a 30 mA RCD can trip at just over 15 mA.
With a TN-C-S supply earth is the same polarity as neutral and there has to be both a strong leak neutral to earth and a large current drawn before it with trip, but with TN-S you need to draw less current, and with TT it's quite easy, reason for this for any current to transfer from neutral to earth they need to be at a different polarity or voltage. If voltage or polarity is the same no current will flow even with a direct short.
With a earth neutral fault with a TN-C-S supply you need a load, this can cause some odd effects, for example a bit of damp toast stuck in the toaster does not trip the RCD until the kettle is turned on.
The problem with a MCB is it only turns off the line, the neutral is left connected. So start putting the MCB's in left to right and the last MCB on right may trip the RCD, but start putting the MCB's in right to left and the last MCB on the left can trip it.
It is just the last MCB just puts it over the limit.
So you could be looking for a neutral to earth fault or line to earth fault, the electrician will use an insulation tester using 500 volt to find the fault, all you can do is some detective work.
In the main a FCU has a double pole switch, so switching off also important even when fuse is removed. Wet walls could cause problems, immersion heaters, etc.
Likely it will need an electrician but with a little more understanding maybe some thing will twig with you.
