I have a standard 4 bed house with an outbuilding which has power fed to it.
The electrics are in "generally good shape" (i.e. no rubber wire etc) and I
have had a new split load consumer unit fitted with each side on an RCCB main
switch, with the lights not on a RCCB and the fridge / freezer on its own RCCB,
so that another circuit tripping does not take them out with it.

Occasionally, like two or three times a year, one of the circuit trips out - it
might do this two or three times (re-setting in between) over a period of a
couple of hours then be fine again for another few months.

The problem is that it this occurs so infrequently that I can't build up a
sample of tests to narrow down where the problem is occurring. If I had a
selection of specialist test equipment I guess I could measure each circuit and
item to test for earth leakage, but only have "basics" like a multimeter, scope
(and a working but basic old uncalibrated PAT tester).

Can anyone suggest a course of actions and tests to help identify the problem
with what I have got, or can I hire the necessary piece of kit ?

Thanks

Nick

nick smith wrote:

I have a standard 4 bed house with an outbuilding which has power fed to it.
The electrics are in "generally good shape" (i.e. no rubber wire etc) and I
have had a new split load consumer unit fitted with each side on an RCCB main
switch, with the lights not on a RCCB and the fridge / freezer on its own RCCB,
so that another circuit tripping does not take them out with it.

Occasionally, like two or three times a year, one of the circuit trips out - it
might do this two or three times (re-setting in between) over a period of a
couple of hours then be fine again for another few months.

The problem is that it this occurs so infrequently that I can't build up a
sample of tests to narrow down where the problem is occurring. If I had a
selection of specialist test equipment I guess I could measure each circuit and
item to test for earth leakage, but only have "basics" like a multimeter, scope
(and a working but basic old uncalibrated PAT tester).

Can anyone suggest a course of actions and tests to help identify the problem
with what I have got, or can I hire the necessary piece of kit ?


You may actually have no problem at all.
RCD's seem to trip on quite small imbalances, and I spent ages trying to
ascertain whgat was causing mine to go - In teh end I put i a 100mA one
and its been fine since, except

- tyhe washing machine debveloped an earth short in the motor
- I left and extension plugged into an outside socket, and it rained...

Ther is alwys some earh leakage: Its damned hard to identify what it is.

There are things you can do, like check for earth neutral shorts. One of
these somewhere in the house will cause random tripping on a seemingly
arbitrary basis.

The way to check this is to unhook the erath wire (on a dead CU of
course) from every circuit and check it shows no neutral connection -
use a sensitive analog ohmmeter - e.g. cheap dial based meter.

If those check out OK, I duno what else to suggest. What one really
wants is a very sensitive clamp on ammeter that you can pass both live
and neutral through and measure current impbalance, and then switch
everything on and off to try and work out what is causing it. If only
RCD's hade meters on them...

A little damp in e.g. a cooker can be the culprit, or voltage spikes
coupled with RFI filters on e.g. TV's etc.


Thanks

Nick

On Fri, 25 Feb 2005 09:51:36 GMT, "nick smith"
wrote:

I have a standard 4 bed house with an outbuilding which has power fed to it.
The electrics are in "generally good shape" (i.e. no rubber wire etc) and I
have had a new split load consumer unit fitted with each side on an RCCB main
switch, with the lights not on a RCCB and the fridge / freezer on its own RCCB,
so that another circuit tripping does not take them out with it.

Occasionally, like two or three times a year, one of the circuit trips out - it
might do this two or three times (re-setting in between) over a period of a
couple of hours then be fine again for another few months.

The problem is that it this occurs so infrequently that I can't build up a
sample of tests to narrow down where the problem is occurring. If I had a
selection of specialist test equipment I guess I could measure each circuit and
item to test for earth leakage, but only have "basics" like a multimeter, scope
(and a working but basic old uncalibrated PAT tester).

Can anyone suggest a course of actions and tests to help identify the problem
with what I have got, or can I hire the necessary piece of kit ?

Thanks

Nick

hi nick,
sounds like you have a minor fault, first off you may as well PAT
test all the appliances cus you got the kit, mind you this is not
foolproof especially on things like washing machine, because a fault
could be present only on a certain cycle of the programe.

the next thing to do would be to carry out an insulation test of the
fixed wiring in the house, an insulation test cannot be carried out
with a multimeter, it requires an insulation tester, these units test
at 500 Volts thereby putting extra strain on the circuit being tested
showing up even the slightest insulation break down and or earth
leakage. i wont bother giving any more details on this as it requires
a sparkie to do it anyway.

how worried about it are you ?
regards
bob

burbeck wrote:
On Fri, 25 Feb 2005 09:51:36 GMT, "nick smith"
wrote:

I have a standard 4 bed house with an outbuilding which has power
fed to it.
The electrics are in "generally good shape" (i.e. no rubber wire
etc) and I
have had a new split load consumer unit fitted with each side on an
RCCB main
switch, with the lights not on a RCCB and the fridge / freezer on
its own RCCB,
so that another circuit tripping does not take them out with it.

Occasionally, like two or three times a year, one of the circuit
trips out - it
might do this two or three times (re-setting in between) over a
period of a
couple of hours then be fine again for another few months.

The problem is that it this occurs so infrequently that I can't
build up a
sample of tests to narrow down where the problem is occurring. If I
had a
selection of specialist test equipment I guess I could measure each
circuit and
item to test for earth leakage, but only have "basics" like a
multimeter, scope
(and a working but basic old uncalibrated PAT tester).

Can anyone suggest a course of actions and tests to help identify
the problem
with what I have got, or can I hire the necessary piece of kit ?

Thanks

Nick


youve got all the kit you need there!
I pretty much agree with Bob, except I'd say one can make good use of
low voltage testing as well.

1. PAT test appliances (500v), assuming you know how to handle the HV
aspect of the PAT tester safely. If you dont, test them with multimeter
instead. Low voltage testing, while less accurate, still picks up the
majority of faults.

Test resistance L-E and N-E. Write a list of the results for all
appliances - this is important, as its comparison youre after. When LV
testing the actual figures you get are often not a reflection of what
the figs would be at higher voltage.

LV testing is not perfect, but it does normally pick out the culprit.
When testing wash mc, if it has a mechanical program timer, run it thru
the clicks while testing it. With cooker, switch each ring etc on in
turn, and so on.

Again, only if you know how, its easy to test L-E resistance of the
ring circuit without undoing anything: the CU MCB just needs to be off
and all appliances unplugged, one can connect to L and E at a wall
socket.

N-E can only be tested if you open the CU and disconncet the right
wires, and I gather youre not happy about doing that safely.

I wasnt clear about your terminology. Best option is to put each ring
on its own RCBO, and replace the RCD with a plain switch, if and only
if youre not on a TT system.

This has 2 advantages:
1. Power out only affects one ring circuit
2. Less chance of trips due to the leakage being divided over several
RCBOs.

Make sense?


NT


hi nick,
sounds like you have a minor fault, first off you may as well PAT
test all the appliances cus you got the kit, mind you this is not
foolproof especially on things like washing machine, because a fault
could be present only on a certain cycle of the programe.

the next thing to do would be to carry out an insulation test of the
fixed wiring in the house, an insulation test cannot be carried out
with a multimeter, it requires an insulation tester, these units test
at 500 Volts thereby putting extra strain on the circuit being tested
showing up even the slightest insulation break down and or earth
leakage. i wont bother giving any more details on this as it requires
a sparkie to do it anyway.

how worried about it are you ?
regards
bob

On Fri, 25 Feb 2005 10:14:51 +0000, The Natural Philosopher
wrote:

You may actually have no problem at all.
RCD's seem to trip on quite small imbalances, and I spent ages trying to
ascertain whgat was causing mine to go - In teh end I put i a 100mA one
and its been fine since, except

It doesn't meet the regulations.

30mA is the maximum acceptable leakage current in a domestic
installation.

- tyhe washing machine debveloped an earth short in the motor
- I left and extension plugged into an outside socket, and it rained...

An earth short?

You mean the windings fused? Well that'd do it - should have tripped
the MCB too.

And rain will do it too.....

Ther is alwys some earh leakage: Its damned hard to identify what it is.

White goods like washing machines are leaky anyway.

It isn't unusual to see leakage in the order of 10mA from a typical
washing machine. You can expect similar from tumble driers.

There are things you can do, like check for earth neutral shorts. One of
these somewhere in the house will cause random tripping on a seemingly
arbitrary basis.

Well, you could, save for the fact that if you've a TN-C-S system the
earth and neutral are the same conductor anyway at the incomer.

That won't cause an RCD to trip by itself.

The way to check this is to unhook the erath wire (on a dead CU of
course) from every circuit and check it shows no neutral connection -
use a sensitive analog ohmmeter - e.g. cheap dial based meter.

?

Why would you want to do this, precisely?

If those check out OK, I duno what else to suggest. What one really
wants is a very sensitive clamp on ammeter that you can pass both live
and neutral through and measure current impbalance, and then switch
everything on and off to try and work out what is causing it. If only
RCD's hade meters on them...

Nope. A clamp meter is unlikely to tell you much either - above and
beyond the fact that there is or is not a degree of current flowing -
it isn't the correct tool for the job, however.

What you really need to do is to start with some facts, some
knowledge, and then use the correct tool to identify the problem -

- which is probably, in all likelihood the RCD itself.

I was suggesting some facts - an RCD is rated at a given trip current,
e.g. 30mA.

Manufacturing tolerances (this directly from MK) are such that so long
as that RCD operates within + or - 50% it is acceptable.

In simple terms, your 30mA RCD probably trips at nearer 15mA than
30mA.

The proper solution is to fit an independent 30mA RCD, or to use an
RCBO rated at 30mA, for the circuit containing the washing machine.

Under pretty well NO circumstances should you fit a 100mA RCCD as the
main switch in a domestic installation.

A little damp in e.g. a cooker can be the culprit, or voltage spikes
coupled with RFI filters on e.g. TV's etc.

How?

Sugar Free wrote:


Manufacturing tolerances (this directly from MK) are such that so long
as that RCD operates within + or - 50% it is acceptable.

Nah. The spec is: (a) shall not trip at 50% of nominal - so should allow
15mA imbalance indefinitely; (b) shall trip within 200ms at 100% of
nominal; (c) (if a 30mA device, being used for supp. protn agin direct
contact) shall trip within 40ms - two full mains cycles - at 500% of
nominal. (This directly from the OSG, referencing regs 713-13-01 and
412-06-02; I'll see your MK tech-sales desk and raise you an IEE pub ;-)
That said,

In simple terms, your 30mA RCD probably trips at nearer 15mA than
30mA.

this is very much the case - to meet the above specs it's only sensible
for MK and others to build for an expected trip around 20mA, so few will
be oversensitive (tripping at 15mA) but practically all will meet the
30mA rating.

The proper solution is to fit an independent 30mA RCD, or to use an
RCBO rated at 30mA, for the circuit containing the washing machine.

Agreed - the more circuits fed by a single RCD, the more difficult it is
to track down which one(s) have the small leak(s) causing a nuisance
trip, *and* the more little leaks accumulate from multiple appliances
(particularly anything with an interference suppressor with capacitors
L-to-E and N-to-E, as most have).

Under pretty well NO circumstances should you fit a 100mA RCCD as the
main switch in a domestic installation.

Well... TT installations aren't *that* rare, expecially out in t'sticks,
and 100mA-time-delay RCDs are just about always needed there, since it's
really really really REALLY rare that you can guarantee a sub-one-ohm
earth impedance with any economically feasible earth rod(s)...

Stefek

"burbeck" wrote in message
...
On Fri, 25 Feb 2005 09:51:36 GMT, "nick smith"
wrote:

I have a standard 4 bed house with an outbuilding which has power fed to
it.
The electrics are in "generally good shape" (i.e. no rubber wire etc) and
I
have had a new split load consumer unit fitted with each side on an RCCB
main
switch, with the lights not on a RCCB and the fridge / freezer on its own
RCCB,
so that another circuit tripping does not take them out with it.

Occasionally, like two or three times a year, one of the circuit trips
out - it
might do this two or three times (re-setting in between) over a period of
a
couple of hours then be fine again for another few months.

The problem is that it this occurs so infrequently that I can't build
up a
sample of tests to narrow down where the problem is occurring. If I had a
selection of specialist test equipment I guess I could measure each
circuit and
item to test for earth leakage, but only have "basics" like a multimeter,
scope
(and a working but basic old uncalibrated PAT tester).

Can anyone suggest a course of actions and tests to help identify the
problem
with what I have got, or can I hire the necessary piece of kit ?

Thanks

Nick

hi nick,
sounds like you have a minor fault, first off you may as well PAT
test all the appliances cus you got the kit, mind you this is not
foolproof especially on things like washing machine, because a fault
could be present only on a certain cycle of the programe.

the next thing to do would be to carry out an insulation test of the
fixed wiring in the house, an insulation test cannot be carried out
with a multimeter, it requires an insulation tester, these units test
at 500 Volts thereby putting extra strain on the circuit being tested
showing up even the slightest insulation break down and or earth
leakage. i wont bother giving any more details on this as it requires
a sparkie to do it anyway.

how worried about it are you ?
regards
bob



May not be relevant to your case but I've just pinned down an infrequent RCD
trip to the washing machine. The motor brushes were very worn and the motor
had a lot of graphite dust around the communtator, brush housings, and field
coil
connectors. New brushes and a thorough clean up seems to have cured it.

Chris K

Sugar Free wrote:

On Fri, 25 Feb 2005 10:14:51 +0000, The Natural Philosopher
wrote:


You may actually have no problem at all.
RCD's seem to trip on quite small imbalances, and I spent ages trying to
ascertain whgat was causing mine to go - In teh end I put i a 100mA one
and its been fine since, except


It doesn't meet the regulations.

So? fit RCBO'sd where necessary. Will get round tuit eventually.


30mA is the maximum acceptable leakage current in a domestic
installation.

Bollox.
There is no requrement for RCD'sd at all except on specific sockets. My
domestric siitation is about 4 times the average domestic situatrion in
terms opf cable runs and electrnic eqyupment.


- tyhe washing machine debveloped an earth short in the motor
- I left and extension plugged into an outside socket, and it rained...


An earth short?

Probably.


You mean the windings fused? Well that'd do it - should have tripped
the MCB too.


No. about 2k to earth from windings. BNo I can't figure it either, but
thats what my meter showed, and the repalcement motor showed perfect
insulation...

And rain will do it too.....


Ther is alwys some earh leakage: Its damned hard to identify what it is.


White goods like washing machines are leaky anyway.

It isn't unusual to see leakage in the order of 10mA from a typical
washing machine. You can expect similar from tumble driers.


So three tunble drires and you are ****ed. So much for regulations.

There are things you can do, like check for earth neutral shorts. One of
these somewhere in the house will cause random tripping on a seemingly
arbitrary basis.


Well, you could, save for the fact that if you've a TN-C-S system the
earth and neutral are the same conductor anyway at the incomer.


You don't seem to understand the issues here at all do you?

Any earth neutral short upstream of the CU, even if the neutral and
earth are bondede there, will result in some neutral current flowing
down the earth instead of teh neutral, and tripping the RCD.


That won't cause an RCD to trip by itself.

It will. Been there done that.

Take a screwdriver on any part of an installation under power and short
neutral to earth. 30mA RCD is about 70$ guaranteed to trip.




The way to check this is to unhook the erath wire (on a dead CU of
course) from every circuit and check it shows no neutral connection -
use a sensitive analog ohmmeter - e.g. cheap dial based meter.


?

Why would you want to do this, precisely?


To make sure there are no earth neutral shorts upstream of the CU.
Because as you pointed out if you don;t unhook they will show a dead
short if ou neutral and earyh are bonded at teh CU or downstream.
..


If those check out OK, I duno what else to suggest. What one really
wants is a very sensitive clamp on ammeter that you can pass both live
and neutral through and measure current impbalance, and then switch
everything on and off to try and work out what is causing it. If only
RCD's hade meters on them...


Nope. A clamp meter is unlikely to tell you much either - above and
beyond the fact that there is or is not a degree of current flowing -
it isn't the correct tool for the job, however.

I know. Thats why I saif what you want is...aa sensituve clamp on
meter...that will measure ciurrent differentials of the order of 10-15mA.

After all an RCD is basically a clamp on ammeter of a sort anyway.


What you really need to do is to start with some facts, some
knowledge, and then use the correct tool to identify the problem -


Sadly lacking in yoir case it seems.

- which is probably, in all likelihood the RCD itself.

Doubtful.


I was suggesting some facts - an RCD is rated at a given trip current,
e.g. 30mA.

Yes?

Manufacturing tolerances (this directly from MK) are such that so long
as that RCD operates within + or - 50% it is acceptable.

so?

In simple terms, your 30mA RCD probably trips at nearer 15mA than
30mA.

And?

The proper solution is to fit an independent 30mA RCD, or to use an
RCBO rated at 30mA, for the circuit containing the washing machine.

Under pretty well NO circumstances should you fit a 100mA RCCD as the
main switch in a domestic installation.



Bollox. Its a backup. You should strictly also RCBO those parts of the
system that have to gave 30mA RCDS on them - exterior sockets IIRC ONLY.

However that leaves the majority of the rest of teh house unprotected:
Legfgal, but I prefer a bit of protection. Anything that blows a 100nA
RCD is seriously out of whack. In my case there wer definite faults. The
30-mnA whole house RCD tripped for no detecable reason other than the
overall leakage was marguinal. Any surge would set it off.


A little damp in e.g. a cooker can be the culprit, or voltage spikes
coupled with RFI filters on e.g. TV's etc.


How?

Damp puts resistance across terminals, RFI filters because they have
capacitors beween live, nuetral and earth, Enough of these and a voltage
spike will trip an RCD. Even more and you don't even need the spike.

For example a 10nf capacitor by my hasty calculation, between live and
Earth will conduct slightly less than 0.8mA. Thats a not unreasonable
sort of value to have on an RFI filter in a a bit of electronics.

Multiply that by two computers, two monitors, a router, a pabx, five
televisions, and half a dozen assorted hifis and radios, not to mention
the ten light dimmers, all RFI equipped, throw in two cookers, a
klargester down the garden, some outside lights that get a little damp,
and you cam easily see where 30mA comes from...


Ive ascretained that 30mA is untenable, 100mA is rock solid. I still
have to - and will - add 30mA RCBO's to teh three circuits that have
oustide cabling.

Stefek Zaba wrote:

Well... TT installations aren't *that* rare, expecially out in t'sticks,
and 100mA-time-delay RCDs are just about always needed there,

Not necessarily time-delayed; the
main-switch-feeding-two-RCDs-in-parallel method has much to commend it.

since it's really really really REALLY rare that you can guarantee a
sub-one-ohm earth impedance with any economically feasible earth
rod(s)...

On a public mains supply you might just as well write "NEVER" there.
The impedance to earth on the supplier's side is allowed to be as high
as 20 ohms (usually is much less of course) - giving a max. Ze value of
21 ohms when you add on a notional one ohm for the transformer and
metallic conductor impedances [see sec. 1.1(iv) in your OSG].

--
Andy