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RCD tripping more since route of supply changed!


monkey5

Question

hi, please help

got a call last friday to an RCD tripping every 30 mins or so for no apparent reason. went to have a look turns out it use to have supply coming in over head but was changed by DNO on thurs to a TNCS, since then its tripped every 30 mins or so (longest time to stay on was 1hour 10 mins). it use to trip every now and then when windy but coz the cable came thru some trees the owner put it down to the tree rubbing sheathing off.

first off changed rcd, thought i had sorted got paid drove off within 30 mins of leaving gets a phone call to say it had tripped again.

next day goes back, big american fridge that seems to start up something (motor) every now and then, un plugs it puts it in to socket which is on non rcd protected side of board. explains it could be that waited bout an hour no trip..and await to hear from them. just as left it trips. goes back asked if theyve done anything, ie boil kettle, switch iron on. "no nothing".

basically removed all circuits on RCD side tested the circuits r1,r2,rn, R1+R2, IR - live to neutral, live to earth, neutral to earth, all reading were fine. checked Ze 0.33, PFC 0.71, PSCC 0.72.

put one circuit in at a time and waited 45mins no trip, removed it and did another circuit.. did this for them all no trip. removed all sockets found the odd minor problem like slight loose connection, fixed all sockets back and circuits back in board. no trip. again waited abit no trip so left. 2hours later gets the dreaded phone call its tripped.

DNO have been back and reted there side and said theres is all perfect.

circuits on the 80a rcd side 3x socket rings 32a and 1x socket radial 20a, dont think its overloading as been fine for last 6 years apart from the odd time when windy. going to check with clamp meter on tuesday.

going back on tuesday anyone have any ideas what it could be?

could it be anything to do with new supply?

seem to have written an assay again but like to give as many details as possible to paint the picture.

cheers wayne

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These things can be a pain but I'm guessing that the original earth was poor or non existing , the new TNCS now allows the RCD to operate. It has to be downstream of the device, its in the house somewhere, I know this doesn't help . As Specs has said, if your new RCD tests OK that rules it out so, its one of the circuits or a device plugged in . Is there an immersion heater,megger it , oversink heater, socket or light outside full of condensation , is there an earth fault on the central heating controls or pump?

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If IR readings dont help, you could always ramp test RCD in isolation (to get its tripping current), and ramp test it with load connected. may give an idea on how much is being leaked. if something is being leaked, go through again but with only certain circuits connected to try and narrow it down

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most ir results were 299 mohms (megger 1552) think 1 was 180 mohms had nothing lower.

didnt ramp test, but will on tues. what do you mean open up earth and test?

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could well be something thats only switching after a certain time?

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I forgot my favourite RCD hint, we had one that tripped when the door next to it was slammed. Tapped RCD (not hard) with screwdriver and it tripped, always worth a try.

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most ir results were 299 mohms (megger 1552) think 1 was 180 mohms

To earth ??

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To earth ??

live to earth, live to neutral and neutral to earth all results were very high.

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could well be something thats only switching after a certain time?

cant find anything thats switching on ie by timer

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would any of the circuits on the non rcd side cause any problems theres lights, immersion, fridge circuit. havent checked these as i dont think they'll cause any problems, does anyone else?

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if they have a borrowed neutral, then they could

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if they have a borrowed neutral, then they could

whats the best way and quickest to test for borrowed neutrals?

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usually when something trips the RCD on the other side!

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usually when something trips the RCD on the other side!

its a split board so only one rcd which has 3 socket rings and 1 radial ring. the lights, fridge etc are on non protected side.

its the tripping after 30mins that gets me coz everthing is on tv, sky etc and there just sat watching it when i trips. even trips in middle of night when all asleep.

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Just a thought, I solved a rcd tripping problem the other week where the rcd was going off intermittently. It turned out that the kettle which was the sort that you remove off the base which has power cord attached was faulty. The kettle was very slightly leaking a small bit of water into the base and then after a while it was getting inside the connector and causing the rcd to trip by a Line to earth fault.

However, the rcd was tripping only once or twice a day and it was happening a long time after anybody had used the kettle. Just to note though, when I meggered the kettle the reading quickly increased from virtually nothing and kept on going up in value whilst I held down the test button.

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most ir results were 299 mohms (megger 1552) think 1 was 180 mohms had nothing lower.

didnt ramp test, but will on tues. what do you mean open up earth and test?

If you wanted to read if there is any natural leakage occurring when appliances are in normal use, you would need to read the current flowing down the earth, CPC, But a normal clamp meter would not typically be able to read milliamp values..

so you would need to put a meter on a milliamp scale in series with the earth conductor.. (cuz as we all know... voltages read in parallel, Current read in series;))

Easiest place to do this is at the MET (Main earth connection.).

Turn installation Off

Disconnect main earth connection, put your meter probes on your two ends of the earth, (MET, and Earth conductor).

Re-energise installtion...

see what current if any is flowing down the Earth conductor....

If there is zero Milliamp flowing...

Its may imply there is one single fault event somewhere that is occurring & causing the trip..

If there is already a few miliamps... e.g. 10ma - 20ma,

could be a combination of various appliances each leaking a bit..

then one final one tips the balance!

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My money would be on something along the lines of what the others have said...

dodgy appliance... typically one permanently switched on OR one involving water, or damp external conditions..

Fridge..

Freezer..

Washing machine..

Dishwasher..

Central heating system..

Outside wiring..

Kettles..

Steam irons..

etc..

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If you wanted to read if there is any natural leakage occurring when appliances are in normal use, you would need to read the current flowing down the earth, CPC, But a normal clamp meter would not typically be able to read milliamp values..

so you would need to put a meter on a milliamp scale in series with the earth conductor.. (cuz as we all know... voltages read in parallel, Current read in series;))

Easiest place to do this is at the MET (Main earth connection.).

Turn installation Off

Disconnect main earth connection, put your meter probes on your two ends of the earth, (MET, and Earth conductor).

Re-energise installtion...

see what current if any is flowing down the Earth conductor....

If there is zero Milliamp flowing...

Its may imply there is one single fault event somewhere that is occurring & causing the trip..

If there is already a few miliamps... e.g. 10ma - 20ma,

could be a combination of various appliances each leaking a bit..

then one final one tips the balance!

Hi Specs,

is this method safe? didnt think it was a recommended method thats all and I am sure that I have read somewhere about not doing this.

I can appreciate in an ideal world we all would have one of those milliamp earth leakage clamp meters but we dont.

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Hi Specs,

is this method safe? didnt think it was a recommended method thats all and I am sure that I have read somewhere about not doing this.

I can appreciate in an ideal world we all would have one of those milliamp earth leakage clamp meters but we dont.

IMHO assuming you are a "Competent Person" its about as safe as doing a Ze test....Earth disconnected from installation!

Or

Doing a Zs on a live installation deliberately putting a voltage down the Earth/CPC!

Or

Opening up any live equipment to get you meter probes on to take some readings!

How else could you find out any leakage current value?

(other than Two clamp meters around each of the L & n tails! & minus any difference.)

If I were doing this, I would be at the CU, turn individual circuits on one at a time... whilst watching my meter to see if any readings appear...

Don't forget we are only talking milliamps.....

If you'd of had 1 amp or more flowing down the earth wire Your RCD would never set in the first place...

Most typical Clamp on Ampmeters dont get much good accuracy below 0.5amps!

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Gonna have to think where I read about this which was only over the last couple of days, I thought it was on the ECA website but just really quickly checked and couldnt find it.

However the article was recommending about not using a multimeter to measure earth leakage in the cpc as there was a good chance of it blowing up in your hand if not adequately internally fused and a large fault current occurred.

But now as per normal you've got me thinking cos my test leads are fused (not sure of fuse rating and too late now to check) and just so long as your method is followed ie isolate supply before putting mulitmeter in series with MET and earthing conductor and ensuring good connections with crocs then this should be ok.

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Other one to look at if it is every half hour or so is fridge/freezer, could be a fault on the compressor maybe ?

hi it was one of the first things i checked hoping it would be this but unfortunatly it still tripped when disconnected. thanks anyway

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My money would be on something along the lines of what the others have said...

dodgy appliance... typically one permanently switched on OR one involving water, or damp external conditions..

Fridge..

Freezer..

Washing machine..

Dishwasher..

Central heating system..

Outside wiring..

Kettles..

Steam irons..

etc..

have tried most of these except the washing machine and dishwasher, they'll be the first thing im checkin morning. thanks

just re read the post and i think your meaning it could be a few of these all with small amounts of earth leakage so all added together eventually gives me the 30mA needed to trip RCD.

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If you wanted to read if there is any natural leakage occurring when appliances are in normal use, you would need to read the current flowing down the earth, CPC, But a normal clamp meter would not typically be able to read milliamp values...

which meter would recommend to use special. do you mean a multi meter with fused leads on milliamp setting

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Thanks Specs, that was really good of you to explain that lot in detail and to make sure that everyone understands the safety aspects.

I'll pay for those drinks you got mate Guiness DrinkGuiness Drink:D

Just some extra info if anyone cares to read about multimeters

http://fluke.informationstore.net/efulfillment.asp?publication=10046-eng

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Hi Specs,

is this method safe? didnt think it was a recommended method thats all and I am sure that I have read somewhere about not doing this.

I can appreciate in an ideal world we all would have one of those milliamp earth leakage clamp meters but we dont.

its safer than it sounds. although your ammeter probably wont be able to take a full fault current to trip an MCB, it should be able to handle enough current to trip RCD. the ammeter wont really add much resistance to the main earth, so as long as power is turned off whilst you put meter into place/remove it, you should be safe

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