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Topic Title: Wisdom sought
Topic Summary: Intermittant RCD fault
Created On: 11 August 2012 03:06 PM
Status: Post and Reply
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 11 August 2012 03:06 PM
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esspeegee

Posts: 127
Joined: 06 January 2008

I attended a call out to look into the cause of intermittent tripping of an RCD which covered about 10 circuits. New property so everything under 12 years old. I took the cover off the board and tested the Wylex 30mA RCD trip times with my Fluke. All satisfactory. After this I pointed, using my WDE driver, to the RCD, while talking through the problem with the customer. Although my driver didnt touch the RCD, it tripped. I thought it was a co-incidence, but I repeated this several times and every time the blade of my driver got within 4mm of the RCD it tripped. If I used my finger or the driver handle, it didnt trip.
My driver isn't magnetic (as far as I can tell).

Im going to change the RCD before stripping out the many circuits but wondered if anyone had any feedback on this? The CU cover is metal and I wonder if it, like my driver is interfering with the RCD internals?

Thanks for any feeback
Simon
 11 August 2012 05:18 PM
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Jaymack

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Originally posted by: esspeegee
Im going to change the RCD before stripping out the many circuits

Well now, thats one way! Not recommended though! Much better to attempt to find the problem with some procedural steps.

If a ramp tester is available, this may help to isolate the problem. It may be the case that there is a standing earth leakage on the RCD, just below the tripping threshold.

A. Ramp test at output side of RCD with all ccts switched OFF - say it trips at 28mA.

B). Repeat with circuit 1 only, switched ON, (no appliances plugged in or loads switched ON), say it trips at 25mA. The leakage for the fixed wiring of circuit 1, is assumed to be then 28 - 25 = 3mA.

C). Repeat for circuits 2/3/4 etc. and record individual circuit trip values. Sum the circuit mA tripping values e.g. 3mA + 2mA + 3mA and subtract the total (8mA) from the value in A, i.e. 28mA - 8mA = 20mA, this is the assumed earth leakage value in mA for the fixed wiring.

D). Repeat for all individual circuits but now with circuits ON and take individual mA readings with each appliance plugged in and switched ON in turn.

E) Taking circuit 1 as the final ring circuit for appliances, say the RCD trip value with a class 1 fridge switched ON is 23mA, leakage current for this appliance is then 25mA (from the previous reading for the fixed wiring) - 23mA = 2mA.

Repeating the above procedure for other equipment and appliances, may determine the problem if it is on the premises.

For earth leakage detection of appliances, there are alternatives if the correct equipment is available, e.g. an earth leakage tester reading down to 1mA, in conjunction with a flexible cable breakout plug/socket arrangement, this is useful for measuring earth leakage on running units; or by using a proprietary appliance tester with mA readings.

Regards
 11 August 2012 07:04 PM
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esspeegee

Posts: 127
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Thanks Jaymack. I have a ramp tester and will give the procedures you have specified a go although I've never tried this before. However, my question is asking for your thoughts on the behavior of the RCD I have described. Does it sound like a faulty RCD or is the tripping caused soley by the proximity of the screw-diver blade normal?
What do you think?
 11 August 2012 08:44 PM
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Jaymack

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Originally posted by: esspeegee
is the tripping caused soley by the proximity of the screw-diver blade normal? What do you think?

Spooky, does the hair on the back of your neck stand up as well? I doubt if they're related.

Screw-diver? The mind boggles!, growling at the badger?

Regards
 11 August 2012 09:49 PM
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PG

Posts: 185
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Originally posted by: esspeegee

My driver isn't magnetic (as far as I can tell).


Easy to check if you put it near a magnet? Mine are.

How about testing the RCD (full test) with the screw driver next to it to see if there are any differences?

If the introduction of general metallic implements in the vicinity of the RCD causes a trip/change in performance then it isn't really fit for purpose?
 11 August 2012 10:17 PM
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primo

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I take it the customer doesn't usually stand near the RCD pointing a screwdriver at it when it is intermittently tripping?!!

(Apologies for un-helpful post!)
 11 August 2012 10:25 PM
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PG

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Originally posted by: primo

I take it the customer doesn't usually stand near the RCD pointing a screwdriver at it when it is intermittently tripping?!!


But he/she might place a metal object near the consumer unit - not right for the electrics to extinguish?
 11 August 2012 10:47 PM
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primo

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Originally posted by: PG

Originally posted by: primo



I take it the customer doesn't usually stand near the RCD pointing a screwdriver at it when it is intermittently tripping?!!




But he/she might place a metal object near the consumer unit - not right for the electrics to extinguish?


Sat night...
 12 August 2012 06:36 AM
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ebee

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Although ramp testing might help sometimes when testing an RCD and its problems t has got to be remembered that a ramp test is just that = a ramp - not much resolution so to speak.
Mine on a 30mA setting tests at 15 then 18,21,24,27 & 30 and records the time.
therefore any adjacent readings could have a 0 to 6mA error.
Adding a few things up with a 0 to 6mA error might be misleading I would think.

All I can think of is would moving an "iron core" near the RCD effect the amplified difference on this particular RCD?

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 12 August 2012 09:23 AM
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esspeegee

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There might be an intermittent fault in one of the circuits. I haven't started to look yet because I was taken aback by the behaviour of the RCD. Judging by the feedback so far this seems an unusual incident.
I moved the blade of the driver around the RCD but it only ever tripped when 4mm away from the front face and below the test button. And let me stress I didnt need to make contact with it to cause the trip. And I repeated the problem a dozen times so its not like I've imagined it
It must be the metal of the driver interfering with the magnetic field inside the thing and that is odd enough to warrant swapping the RCD. I'll re-evaluate my options then.
In the meantime thanks a bunch for the feedback
Simon
 12 August 2012 10:52 AM
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potential

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Originally posted by: ebee
All I can think of is would moving an "iron core" near the RCD effect the amplified difference on this particular RCD?

That is my feeling too.
The screwdriver metal is interferring with the magnetic field in and around the RCD.
It may or may not be a fault of the RCD.
If there is an imbalance of current through the RCD very near to the tripping point (where it disconnects the circuit) the metal may distort/enhance the magnetic field and cause a trip.
 12 August 2012 01:30 PM
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alancapon

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Originally posted by: esspeegee
. . . I moved the blade of the driver around the RCD but it only ever tripped when 4mm away from the front face and below the test button. . .

. . . that is odd enough to warrant swapping the RCD. . .

Why? An RCD works by creating a magnetic field within a toroidal core. Under fault-free scenarios, the magnetic field from the phase and neutral conductors (which pass through the toroidal core) cancel out. Introducing another magnetic field adjacent to the core, is likely to cause an output current into the control circuitry, which may operate the device.

Regards,

Alan.
 12 August 2012 03:35 PM
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esspeegee

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I have just tested the screw driver and there is just enough magnetism to pick up a tiny nail. I tried to repeat the effect with several other RCDs and non of them were affected, but non of them was Wylex so it might be the difference in design rather than a fault. I think I just learned something and owe an apology for wasting your time folks.
I'm going to try and learn something else now - How the devil to locate the intermittent problem using the ramp method described by Jaymack. Or I might just phone in sick...
Simon
 14 August 2012 12:29 AM
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PG

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Originally posted by: esspeegee

I just learned something and owe an apology for wasting your time folks.
Simon



To the contrary - no time wasted - especially if you keep the forum posted on progress.
Regards
 17 August 2012 11:04 PM
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esspeegee

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Update for anyone interested. I replaced the RCD like for like. (Wylex WRS 80/2) The new one didn't trip when my driver was close or even touching it. And the intermittent trip that led to the call out in the first place has not occured since.
Simon
 19 August 2012 11:11 PM
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PG

Posts: 185
Joined: 17 October 2011

Originally posted by: esspeegee

Update for anyone interested. I replaced the RCD like for like. (Wylex WRS 80/2) The new one didn't trip when my driver was close or even touching it. And the intermittent trip that led to the call out in the first place has not occured since.

Simon



Thanks Simon - any response from Wylex?
Regards
 20 August 2012 04:26 PM
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esspeegee

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no reply from Wylex _ I didnt and dont have time to follow that up. Just gl;ad the customers happy and the jobs a good un
simon
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