Full thread missing, just ref re time-delay RCD suffering nuisance trip.

Nuisance tripping can be a faulty RCD
o I've been there, had the RCD, had RCD-Tester/MEM confirm it

More usually it does suggest you have a problem
o CU Wiring
---- packed cables, tight bend radius
o Wiring Installation
---- damaged cable, nail, screw, installation damage
---- this can include outside if external run, corrosion etc
o Wiring in a Socket
---- cold insulation flow due to tight bend radius against a sharp screw
---- too many cables, old butyl rubber cable somewhere re tail oxidation
---- earth wiring in sockets without green sleeving, pressure contact etc
---- socket terminals flex on plug insert/removal re insulation against metal
o Wiring in a Switch
---- same as above, but extends into appliances
o Appliance Level
---- Leakage problem -- some old SMPS leaked 3-3.5ma each (eg, filter in PSU)
-------- less common with recent SMPS, easier to overload a ring than reach it
---- Insulation problem -- marginal problem only apparent occasionally
------- eg, when connected to the circuit, or "irritating" socket wiring
---- Portable wiring -- vacuum cleaner cables eventually break internally
------- eg, near the plug from twisting, at the cord-grip, localised overheat
o Bonding Level
---- Heater element -- failing mineral insulation in heater
-------- storage heaters, immersion heater, electric heater, toaster, kettle, oven
-------- if bonding is incomplete this can upset things in a fault condition
---- Supply problem -- RCDs should only trip re downstream, but N-E probs exist
-------- RCD which have an Earth tail can also detect N-E faults (eg, MEM)
-------- thus they can trip where other 2-wire RCDs do not trip (eg, MK, Merlin Gerin)
o Water in fitting
---- lights like to fill with water, algae, goldfish at this time of year
o All of the above
---- RCDs only care about *total* N-L current imbalance
---- several circuits & a faulty one can leak enough to sensitive, not trip an RCD
---- then it comes down to time or another small leakage appliance being added
---- 3-wire RCDs do add other factors into the equation

Some installations, eg, TT, require 100ma time-delay master RCD:
o 100ma rating = protect the supply cable & installation against damage and fire
---- they do not provide electrocution protection
o time delay = discriminate by allowing downstream non-time delayed RCD to trip
---- typical downstream RCD are 30ma non-time delayed for electrocution protection

Tracking down nuisance tripping = time = cost:
o Persistent RCD trips are easily identified
---- a) turn off each MCB in turn until the RCD will reset
-------- then you powering each MCB singly to identify which circuit
-------- if the RCD will not reset at all, disconnect all N, all E & prove RCD ok
---- b) use a megger to measure each line to E resistance on each circuit
-------- then prove by a) just to verify
o Nuisance tripping RCD are not easily identified
---- an actual L/N to E path may be a tiny contact area, which carbonises on shorting
---- so once it occurs the localised heating/carbonising re-insulates it until the next
time
---- all circuits may pass a megger, although that should not be assumed (may point
finger)

If you have a split-load CU with some circuits not on a 30ma non-time-delayed RCD,
then a fault on those circuits will trip out that master time-delayed 100ma RCD.

If you have a whole house RCD, ie, all circuits on a 30ma non-time-delayed RCD then
any fault on the house side should involve that RCD (since the master has time-delay).
Where that is not the case a) verify whether the master is a 3-wire type, b) verify that
the 30ma non-time-delayed RCD is actually working (test-button, RCD tester etc), c)
verify that the master RCD doesn't supply more than 1 CU which itself has a problem,
for example an N-to-E or N-Soil problem. I've seen that one at a rural friends house.

An RCD is supposedly a downstream protection devices (not upstream re supply).
o 3-wire RCDs (MEM) do detect more faults than 2-wire RCDs (MK, Merlin Gerin)
o 3-wire RCDs have a flying Earth connection, and so detect N/E problems too
---- typically electronic devices as well as electromechanical
o 2-wire RCDs just use the usual imbalance between L & N
---- typically just electromechanical devices

IIRC conceivably there could be a fault which will may only trip a 3-wire RCD, and
if that happens to be the master, that it is 100ma & time-delayed doesn't matter.

With downstream RCD tripping:
o Minimise the culprits -- unplug any unnecessary appliances (turning off not enough)
o Physical inspection -- of all sockets, switches & CU wiring re damage, screws & butyl
o Wiring inspection -- verify rings are rings, they can break = overload 2.5FTE = damage
= trips
o Use a split-load CU -- rotate circuits to an RCBO over time (individual
discrimination)

RCBOs can be handy for discrimination:
o You can get away with just using one, slowly pinning a circuit down
BUT
o RCBO Rating must be above the average current draw on the circuit (ie, usable)
o RCBO rating must be below the cable rating on the circuit (ie, protection)
o Typically that means a 16A will do for most circuits, except for lighting

RCDs technically are not supposed to be used as on/off devices re cycle life:
o I have often wondered how long they survive used as a main switch
o Particularly where the test-button is used to power off the house, 250 cycles?

Nuisance tripping isn't normal - but it can be an RCD fault, albeit rare.
--
Dorothy Bradbury