I have an ongoing problems with nuisance RCD trips.

The supply is a TT system with a "whole house" 100mA RCD on the
incoming feeder.  Downstream of the RCD are two consumer units:

1. House circuits (Shower, cooker, socket ring main downstairs, socket
ring main upstairs, lighting ring main down stairs, lighting ring main
upstairs)

2. Electric Heating and Garage (electric heating ring downstairs,
electric heating ring upstairs, garage supply, immersion heater)

The garage supply is 2 x 2.5mm FTE and it also has a 30mA RCD at the
garage end (approximately 50 yards away)

Over the last month we have been continuously experince random nuisance
trips.  (it quietened down over the last couple of weeks, but picked up
again in the wet weather - significant?)  We have tried narrowing it
down, but with little success.  Differentiation using RCBOs has been
suggested, but that will require a new consumer unit.  (The current one
uses Wylex plug in mcbs.) It is interesting that sometimes just the
"whole house" 100mA RCD goes, at other times the garage RCD (some 50
yards away) will also trip out.

Last night I turned off Consumer Unit 2 at the isolator and we suffered
a trip at about 02:30.  Therefore there is nothing on those circuits
that can contribute to the trip.  (Apart from the tails from the "whole
house" RCD to the consumer unit).  Tonight I plan to turn off consumer
unit 1.  I am hoping for  a trip as that will rule out my house wiring
as a cause.

If that is the case, what's the next step?  I believe Western Power
Distribution are the local distribution company in the South West, I
assume that I aske them to check there supply side.

If it doesn't trip it means that it may be my problem.  In which case,
anybody in Wiltshire got a Megger I can borrow.....

Has anybody else had this sort of problem?  Have people experienced
trips due to "outside" influences?  How have they been resolved?

Cheers

Peter

puffernutter wrote:

> I have an ongoing problems with nuisance RCD trips.
> The supply is a TT system with a "whole house" 100mA RCD on the
> incoming feeder.  Downstream of the RCD are two consumer units:...
> The garage supply is 2 x 2.5mm FTE and it also has a 30mA RCD at the
> garage end (approximately 50 yards away)
> Over the last month we have been continuously experince random nuisance
> trips.  (it quietened down over the last couple of weeks, but picked up
> again in the wet weather - significant?)  


Quite possibly.


> It is interesting that sometimes just the
> "whole house" 100mA RCD goes, at other times the garage RCD (some 50
> yards away) will also trip out.


Is the 100mA RCD time-delayed? If it isn't, then a fault downstream of 
the garage RCD could trip either or both. Do not overlook the 
possibility you have two or more unrelated faults which are contributing 
to a total leakage enough to trip the RCD.


> Has anybody else had this sort of problem?  Have people experienced
> trips due to "outside" influences?  How have they been resolved?


The only real way is to Megger all the circuits individually, measuring 
their resistances, and if you don't get a clear indication first time 
round re-test periodically looking for a varying resistance (possibly 
weather related).

Assuming you have meter tails into the 100mA RCD, then output tails into 
Henley blocks splitting to your two CUs, you could get another 100mA 
(time dleayed) RCD, and split the meter tails *before* the RCDs, so 
givign each CU a 100mA RCD without having to buy new CUs. Socket 
circuits on TT do need 30mA protection though, so you could use a 30mA 
RCD for the house CU 1 (if your electric heaters on CU2 all use FCUs), 
although whole-house RCD protection is contrary to Regs too. This may 
reduce nuisance tripping.

This work is probably liable to Part P.

Owain