Title says it really.
Yesterday we had some lightning which obviously led to some supply
issues resulting in flickering supply.
I thnk I reset the breaker 10x yesterday
I've tested the RCD and it trips at exactly 30mA.
[Tested by connecting 1 through 6 47K resistors from live to earth -
each adds 2mA of leakage.]
I had a visit from EDF a few months ago. The first guy found something
that I believe is key. [Not certain of the exact numbers here as I'm
working from memory.] He said my earth imedance was 0.37 ohm to
substation, and the spec limit is 0.38 ohms..
Then [just to get out I feel] he said he would arrange for a voltage
recorder for a week or so. Of course everything was perfect during
that period, so no action.
I called up EDF again today and got through to someone who insisted it
is my problem and that their supply could not possibly cause my
breaker to trip. Further the earth impedance is in spec - even if
1/100 ohm from spec. I really didn't like his atitude.
Any suggestions ?
I'm at the end of my tether and tired of re-setting the clocks!
My ideas (pssibly off the wall) include:
1. 100mA breaker instead of 30mA - but I don't like the loss of safety
protection so it's a last resort.
2. I believe it may be related to capacitance of the wiring as it's a
large extended 70s house. I'm thinking about adding a 240VAC rated
capacitor(s) at the entry point to try and swamp my wiring
3. Add my own earth spike to reduce earth impedance.
A good start would be to stop trying to blame someone else for what's
happening on your system. It wont achieve anything useful.
2&3 wont help any. You need to begin by determining where the problem
is. Since the RCD works ok, the next thing I'd do is leakage test all
your appliances. A megger is the right tool, but if really necessary
you can often spot the faulty one with a multimeter.
He went to work on the breakup of the electricity network in the
If your supply is TN-C-S (PME) the limit is 0.35ohms however the
measurement tools have quite a substantial error when you get down
that far, never mind the test lead contact resistance. It would only
become an issue if the figure rose sufficiently high at the furthest
point on your install to mean a circuit breaker will not disconnect on
a L-E fault within a reasonable time (which is about 0.4sec). Thus
that is not a concern.
There is a Wiki / uk.d-i-y FAQ on RCD tripping. Basically unplug
everything (not just turn off), then see if the RCD trips.
If you have a split-load RCD system you may want to create a dual RCD
setup (RCD1 RCD2) or convert to split load, or convert to a couple of
RCBO thereby (say) keeping lights (up & hall) and one power circuit on
(cooker) if the main RCD trips out.
RCDs will trip on N-E faults - damp socket, screw touching a cable,
trapped cable in a floorboard, water penetration into an external
circuit, or on L-E faults such as failing immersion element, kettle
element, heater element, washing machine or dryer or cooker element,
clothes iron, even microwaves are not unknown.
If you do buy an IR tester from say Ebay to test each cable
individually, you need to ensure any burglar alarm & smoke alarm is
disconnected, along with all other appliances. Neon's will give fake
low IR (eg, 0.67megohm) so they need to be out of circuit (fuse out or
JOOI with a PME system where the leccy co supplies the "Earth" thats
just a connection to the neutral of the incoming cable is it not?.
Well it is here..
So if thats providing the "Earth" as such is it that critical if the
resistance to actual "Earth" (being the ground in this instance) is high
After all if a cable hits anything thats "earthed" then the fault
current is flowing from Live to wherever that Earth is. Course if thats
connected to the mains incomer earth then thats a full short across the
incoming cable and that should take the fuses to Kingdom come very
However if that live should short to some other metal in the gaff then
the resistance of that to "Earth" can either be via a PME system or
wherever that bit of metal is or is not connected to?..
And the fault current can then be lots or rather little?..
So seem to me best protection is via RCD in most all cases..
It is, although they can only do that if they have designed the neutral
to the property to be used as a combined Protective and Neutral (PEN)
Generally with PME, the PEN conductor will be earthed at multiple
points to "real" earth along the way - not just at the substation. Hence
its likely to be closer to real earth at the consumers point of use than
is the case with TN-S
The requirement for main equipotential bonding should generally ensure
they are much the same, apart from when said "other metal" is not
capable of introducing a potential to the point of contact.
No, it needs to be lots (i.e. said extraneous metal is capable of
introducing a potential - hence needs bonding) or none (i.e. said bit of
metal is isolated and "floating" electrically)
A well designed and *maintained* TT system should be safe, yes. PME is
probably better for fit'n'forget (although the 17th edition brings new
installs closer to being like a TT in design anyway).
Are you using precision resistors then? 20% is the tolerance of common
resistors. This also assumes a stable voltage which you already state
is varying (flickering).
You need to understand just how a 30mA RCD works. It compares the L &
N currents and trips if there is a difference in the order of 30mA -
(more comonly around 25 to 30) A minor variation in external earth
loop impedance is immaterial to its operation.
You were getting factual advice - attitudes regardless
Stop poking around without the requisite knowledge, you might do well
to get hold of a copy of the "On Site Guide to BS7671", or engage
someone competent. Obtain proper test equipment and carry out a ramp
test on the RCD with its load terminals disconnected, carry out
insulation resistance tests L & N to earth on your installation but
take precautions not to apply 500v dc to electronic equipment as it
will in all probability die if you do.
A lot of electronic equipment is supplied with noise spike filtering
circuitry and many appliances have suppressor circuits. This can and
does create an earth leakage path - enough of these and you are into
nuisance tripping country.
I have yet to find a domestic installation with sufficient fixed
wiring inter-conductor capacitance to cause enough leakage to cause
tripping. I think you can forget the theory that capacitive leakage is
your problem. If capacitive leakage is the problem its in an appliance
or several of them combined.
Domestic electrical systems are relatively simple but do need someone
with competence not the "a little knowledge is a dangerous thing" as
per the old adage.
I had a very sensitive main trip: Too many compueters and filters.
Replace the main trip with 100mA..if you are running on the edge thats
only 70mA..and that's enough for any common faults. Then fit RCBO's to
the rings that REALLY need 30mA protection. Yes it costs, but it
works.. On the way check any rings or spurs for neutral earth shorts.
Frankly I dont think a main single 30mA fast trip is suitable for modern
installations with a lot of electronics connected.
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