Nuissance tripping on a 30mA Main RCD on a TT system

[toploader2010]

Hi all, hoping some 1 out there can help me on the following problem. just recently qualified and have this nightmare situation with no experience on the following problem.

I have nuissance tripping on a 30mA RCD. it trips at random times and can go as long as 5 days before it goes again. the customer has kept a log of items thats on at the time of the trip but nothing seems to be the same everytime it goes.

It has one 30mA rcd protection covering all the circuits. The board was installed 7 years ago and its a TT system.

I have carried out a full test and here are some of the reading.

Ra of electrod= 235ohms

lowest value for IR was 79M ohms all the rest was above 200M ohms

RCD test, this would trip on the half times test and 1 * was 29ms and 5* was 19ms.

my Zs reading was between 54ohms and 55 ohms on all circuits.

I have replaced the RCD for a a new 1 but still the same. The house is built on top of rock so ive had problems trying to get a new rod into the ground so the only way ive been able to get the readings down is by adding another rod a meter away from my 1st and runing a 16mm from the board to my 1st, looping onto the rod and goin to the next rod(one continiuos run, removed insulation and looped to the next rod) is this allowed???

Also when im taking PSC reading, im getting a a message on meter hi value. is this the norm for TT systems.

Any answers would be greatly appreciated.

 

[alectrician 1]

have you tried a continuity test for loose conections on ring main i.e. different values, seems more like the fixed wiring if youve changed the RCD

 

[the pict]

I may be wrong but a TT system may require a 100mA S, RCD, and bang in as many rods as you can get into the ground, and do a ramp test to find the exact tripping current
Pict

 

[i=p/u]

as above .. back to basics.. good luck im sure il need it someday

 

[i=p/u]

he has already changed the rcd and still happening , but as you say do a ramp test and the then get the clamp meter out to find where your earthleakage is comming from.....

 

[Richard Burns]

If the RCD is tripping on half load is this with other circuits connected?
If you test the Rcd without any outgoing connections and it still trips on half load then there is either a problem in the CU or another faulty RCD.
More likely is there is a high earth leakage from one particular circuit and when other circuits come in it overloads the RCD.
Check which circuit is always on when it trips, if any. Other wise test the RCD connecting in circuits each time, one of them (or possibly a combination of them) will cause the RCD to trip early.
Could just change it to 100mA but does not resolve the cause, and woudl need 30mA split further down.

 

[Deleted member 19807]

Earth rods 1m meter apart are too close in my book do a google on it.

when it trips has it been raining? are there any outdoor electrics, have they just started using the immersion heater, have they got a lot of computers & electronics, just some of the things to think about.

 

[Engineer54]

As hifly stated 1 metre is too close together, they must be at a minimum of the length of the ground rod apart. ...Or the area of influence will over lap and cancel out a certain portion of each of the rods protection value. And....a Ra of 235ohm ain't going to be doing much for this installation at all!! You can connect the rod(s) with 240mm it won't make a jot of a difference (unless it's bare copper that is lol!!) 6mm is fine and all you really need...

On rocky ground, you will be much better of using a ground lattice plate 1m X 600mm, or in extreme cases trenched flat copper tape. Both systems will require digging i'm afraid.

I suspect however, you have yet another duff RCD device myself, change it for a known good one and see what happens. still bad your going to have to start tracing the leakage currents on the installation, and your then going to need an leakage amp clamp....

 

[i=p/u]

what about a clamp meter of line and neutral if you have the resoloution...................

 

[Engineer54]

what about a clamp meter of line and neutral if you have the resoloution...................

Few clamp meters will read mA with any decent resolution, i don't know of any in fact!!

 

[Deleted member 26818]

Sounds like cumulative earth leakage.
You will most likely have to provide each circuit with individual 30ma RCD protection.

 

[toploader2010]

I have checked the continuity of the ring main and all is well. i have isolated all the circuits and switched back on and it doesnt trip. It is tripping with randomly circuits and not a particular circuit unfortunatly.

The RCD is tripping on half load with other circuits connected.
i will test the Rcd without any outgoing connections and see if it still trips on half load.

I will have to see if i can borrow a meter with ramp testing, mine is a old robin that doesnt have the ramp test function.

Can i use a current clamp and modify a extension lead so the earth is accessible and connect the ext lead to socket then connect my device to the ext, put my current clamp on the earth and measure the current leakage when the device is on. will this be ok?

 

[Engineer54]

I have checked the continuity of the ring main and all is well. i have isolated all the circuits and switched back on and it doesnt trip. It is tripping with randomly circuits and not a particular circuit unfortunatly.

The RCD is tripping on half load with other circuits connected.
i will test the Rcd without any outgoing connections and see if it still trips on half load.

I will have to see if i can borrow a meter with ramp testing, mine is a old robin that doesnt have the ramp test function.

Can i use a current clamp and modify a extension lead so the earth is accessible and connect the ext lead to socket then connect my device to the ext, put my current clamp on the earth and measure the current leakage when the device is on. will this be ok?

Not with a conventional current clamp meter, you need a leakage amp clamp that will have a microhm resolution. Also a lot of MFT kit do not ramp in 1mA steps, but in maybe 2 or 3 mA so not a very accurate method of testing leakage trip levels.

 

[Octopus]

If you can't locate one issue, which is very likely, I'd be talking to the client about a new dual RCD board or if the house is old, a board with a full set of RCBO's

Having one 30Ma RCD up front is a very undesirable situation for any household!

 

[toploader2010]

thanks for you response