Hello,

I'd be very grateful of some advice / guidance with respect to time delay RCDs. I've Googled, but not found
any definitive answers.


At present, the major wiring is as follows:

Meter Tails feed 100A Master Switch. This connects to a Henley Block, which feeds a 60A Switch Fuse, and the
house CU. The 60A SF is then connected via 16mm^2 SWA out to a garage CU, which is earthed separately. The
garage CU is a Split Load, with a 50mA time delay RCD for lights and freezer, and a 30mA RCD for the sockets.

Shortly, an additional SF will be added, feeding a summerhouse CU via 4mm^2 SWA, which will also be earthed
separately, and will have all its circuits protected by a 30mA RCD.

The new SWA will be very much more exposed thn the existing run, being laid along the edges of flower beds
etc, and it seemed a good idea to provide some form RCD protection to it. It also appealed to add some form
of RCD protection to the entire installation, so a good solution seemed to be to replace the 100A master
switch with a 100A 100mA time delay RCD. I am however concerned about discrimination between this, and the
50mA time delay RCD in the garage. Clearly, it will always discriminate based upon current, but will a 100mA
RCD also have a larger time delay built in? Are there standard delays associated with each value of leakage
current detection, obeyed by all manufacturers? Do I need to look for a 'extra time delayed' RCD?

I'm also concerned about nuisance tripping of this master RCD. Is a 100mA time delay RCD reasonably immune
from tripping under circumstances where there really isn't a problem?

Many thanks for any advice,

Chris Key

Christopher Key wrote:

I'm also concerned about nuisance tripping of this master RCD. Is a 100mA time delay RCD reasonably immune
from tripping under circumstances where there really isn't a problem?

We have an extensive electrical system (two large split load CUs)
protected overall by a single 100mA time delayed RCD. So far (in two
or three years) the time delayed RCD has never tripped.

The system comprises:-
Large house (6 bedrooms) with its own CU
'Outdoor' CU which supplies:-
Garage and games room, power and lighting
A second garage/workshop, power and lighting
Garden power for pond pump etc.
Small stable block, lights etc.
Outdoor lights

So I think my answer is - you shouldn't have a problem with nuisance
trips.

--
Chris Green

I'm also concerned about nuisance tripping of this master RCD. Is a 100mA time delay RCD reasonably immune
from tripping under circumstances where there really isn't a problem?

We have an extensive electrical system (two large split load CUs)
protected overall by a single 100mA time delayed RCD. So far (in two
or three years) the time delayed RCD has never tripped.

The system comprises:-
Large house (6 bedrooms) with its own CU
'Outdoor' CU which supplies:-
Garage and games room, power and lighting
A second garage/workshop, power and lighting
Garden power for pond pump etc.
Small stable block, lights etc.
Outdoor lights

So I think my answer is - you shouldn't have a problem with nuisance
trips.

Hi,

Thanks for the response. That certainly is an extensive installation, perhaps getting close to the 100A
limit. Does your installation contain three RCDs in 'series' anywhere, have you had any issues with
discrimination amongst them?

Chris Key

Christopher Key wrote:
I'm also concerned about nuisance tripping of this master RCD. Is a 100mA time delay RCD reasonably immune
from tripping under circumstances where there really isn't a problem?

We have an extensive electrical system (two large split load CUs)
protected overall by a single 100mA time delayed RCD. So far (in two
or three years) the time delayed RCD has never tripped.

The system comprises:-
Large house (6 bedrooms) with its own CU
'Outdoor' CU which supplies:-
Garage and games room, power and lighting
A second garage/workshop, power and lighting
Garden power for pond pump etc.
Small stable block, lights etc.
Outdoor lights

So I think my answer is - you shouldn't have a problem with nuisance
trips.

Hi,

Thanks for the response. That certainly is an extensive installation, perhaps getting close to the 100A
limit.

I doubt it, there's very little electric heating anywhere and there
aren't a lot of people in the house. I'm the only person to use any
power in the two garages. The only serious consumers of electricity
are the kettle, oven, dishwasher and washing machine - much the same
as typical houses everywhere.


Does your installation contain three RCDs in 'series' anywhere, have you had any issues with
discrimination amongst them?

There are several RCDs 'downstream' from the 100mA time delayed one
(but there are never three 'in series') :-

The 'split load' one in the outdoor CU protecting all sockets
outside and in garages.

The 'split load' one in the house CU protecting a couple of
circuits with sockets.

Two separate RCBOs on the 'non RCD' side of the house CU
protecting two ring circuits.

When these have tripped occasionally (for real faults in every case)
the 100mA time delayed one hasn't tripped.

--
Chris Green

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

"Dorothy Bradbury" wrote in message
...
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?

I've used the test button on a dedicated RCD to power down our barn every
night for years.
Never failed yet (famous last words :-)

I've used the test button on a dedicated RCD to power down our barn every
night for years. Never failed yet (famous last words :-)

It could be :-)

I wonder who designed the RCBO test-buttons, they seem designed to
frustrate their usage - lots of travel, near impossible to press (if like me
you do it at arms length, eyes shut, other finger in ear, awaiting blast).
--
Dorothy Bradbury

Does your installation contain three RCDs in 'series' anywhere, have you had any issues with
discrimination amongst them?

There are several RCDs 'downstream' from the 100mA time delayed one
(but there are never three 'in series') :-

The 'split load' one in the outdoor CU protecting all sockets
outside and in garages.

The 'split load' one in the house CU protecting a couple of
circuits with sockets.

Two separate RCBOs on the 'non RCD' side of the house CU
protecting two ring circuits.

When these have tripped occasionally (for real faults in every case)
the 100mA time delayed one hasn't tripped.

Hi,

Thanks very much for that. Certainly sounds like nuisance trips shouldn't be a problem then.

Chris Key

"Dorothy Bradbury" wrote in message
...
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.

Thanks very much, an awful lot of very useful information. Perhaps this should be part of the FAQ.

Chris Key