Problems Replacing Main Breaker Switch With An Rcd

[paul1970]

Due to having a small child in the house I decided to have a quick check of my rented house's electrics. When I checked the consumer unit I found that it only had a main switch and no RCD (which I didn't like the thought off considering we have 2 electric showers and 2 water heaters).

Asked the landlord about putting in a RCD and of course the answer was "you hab safety cutout already" while pointing at the 8 MCB's, despite all manner of explanations he wasn't having it.

So I decided to bite the bullet and found the correct RCD for the Square D consumer unit for 2900 baht at Homepro and went to fit it myself.

As soon as i replaced the main switch with the RCD the RCD started tripping, first I thought it may be the AC's but with the MCB's for them all turned off the RCD would still trip. Eventually had everything turned off except the lights and still it was tripping but not instantaneously but maybe after a few minutes.

Then a strange thing happened where the RCD tripped with ALL the MCB's switched off, by now it was getting dark so the main switch has gone back in until tomorrow.

So I'm thinking what the problem could be, now I know an RCD trips by detecting an imbalance in the current between the live and neutral but with all the MCB's off there should be zero current supplied so zero current coming back along the neutral, so I'm scratching my head a bit know as to where the problem could be ?

Any advice appreciated.

[Crossy]

Sounds like a N-E fault, or possibly that a MEN link is incorrectly installed between E and N.

Check here for the correct installation of the MEN if you have an RCD main switch http://www.crossy.co.uk/wiring/Consumer.html the link MUST be installed on the incoming neutral before the RCD.

Many Thai homes are wired with the neutral and ground connected by feeding the incoming neutral via the ground bar, something I really don't like as it makes removing the link for testing impossible.

It's also quite possible that you have L & N reversed on your incoming supply which can cause interesting effects with RCDs.

Carefully trace what you have and post a sketch here if you're confused.

Do you have any test gear?

It is of course possible that your RCD is faulty.

[paul1970]

^ Thanks for the reply, great link (no pun intended) too, Thanks.

The only test gear I have is a DVM though I could possibly get my hands on other pieces of kit here in Thailand.

I don't see a MEN link installed in the consumer unit but to make matters worse there are 3 white wires attached to the earth bar and all the neutrals on the neutral bar are all white (except for one green, even have 2 greens connected to the live from one of the MCB's, so whoever wired this house did so with no regard to any colour codes). Possible the link has been done in the attic, I'll disconnect the wires from the earthing bar and make a continuity check between each one to the neutral bar to see if any are acting as the MEN link.

I've had a look around the house for an earth rod, but there doesn't seem to be one unless they buried it somwhere (normally I would assume as they've utilized the earthing bar that they would wire it to a ground spike but with this being Thailand, god knows.) I remember once when I lived in a BKK condo that I got a tingle every time I touched the oven only to find that the oven was "earthed" by someone screwing the earth connection into the plaster on the wall.

i tested the polarity with a DVM by measuring the voltage from L to E and N to E and got 230 and about 1V respectively.

I think I'll try to wire a single test load to the RCD to check it in isolation from the rest of the house wiring.

Edited September 19, 2010 by paul1970

[electau]

It sounds like that you have low insulation between the a neutral and earth on one (or more) of your subcircuits. You will need to check the insulation resistance with a 250/500V insulation tester on each circuit. You canot check insulation resistance with a VOM.

You should retain the main switch and install the RCD betwen the load side of the main switch and the line side of the circuit MCBs.

There must be no earth connection to the neutral on the load side of an RCD.

Edited September 20, 2010 by electau

[bdenner]

Sounds like a N-E fault, or possibly that a MEN link is incorrectly installed between E and N.

It is of course possible that your RCD is faulty.

Is the MEN system used through out Thailand? Certainly not here in the rice paddies. The only Earth Neutral bonding is at the star point of the transformer some 1/2 km down the road. I installed dual pole circuit breakers for that reason having measured the neutral to 50 Volts above Earth at times. Much better now, 2 to 5 volts, as they installed a new transformer 2 years ago. I have full earthing and RCD protection house is wired using 2c+e TPS Cable.

From what I'm reading the OP needs to get an electrician to look at the installation.

Edit: As stated above you should not remove the main isolator. The RCD is a safety device not an isolator.

Edited September 20, 2010 by bdenner

[paul1970]

I found the fault today, measured continuity between the Neutral and Earth bars in the consumer unit and it was 0.5 ohms.

First I disconnected all the neutral wires from the neutral bar one at a time while measuring the continuity between the neutral earth bars to see when the continuity between the two went away. It only went away with all the neutrals removed.

So next I reconnected all the neutrals and did the same step with the connections to the earth bar, disconnecting one of them made the reading jump to a few megs. What had happened is that whoever had wired this house before he had decided to wire the neutral from the kitchen to the earthing bar in the consumer unit.

Moving the kitchen neutral to the neutral bar stopped the tripping and everything now works but it now only leaves 2 connections to the earth bar, one to a water heater and another that goes god knows where but I really don't think it's to an earth rod despite there being some 3 pin sockets in the house with the earth connected. Will have to try and see if I can follow that one up in the attic to see where it leads.

There is definitely no MEN link and I'm unsure if I should put one in, called PEA to try and find out what kind of earthing system they had in the part of Pattaya where the house is and I may have well been talking martian to them.

[Spoonman]

Ask Local Yokul, he will sort it for ya.

[Crossy]

Good that the fault has been located.

Assuming our OP has an RCBO (which includes overload protection) rather than a plain RCD (which does not) he can simply replace his main switch, otherwise, as Electau notes, he needs to leave the main switch in circuit to provide overload protection.

Paul1970, please post details of your RCD/RCBO device.

MEN is not implemented all over Thailand. This being the case my advice is always to NOT install a MEN link unless you KNOW MEN is implemented. Fitting a MEN link in a non-MEN area can lead to a very hazardous situation.

[paul1970]

Yes it's an RCBO, Schneider (Square D) QO-MGBX Main Circuit Breaker with Residual Current Protection, model no QO263MBGX30 as shown in the Schneider catologue linked to below

Schneider catologue PDF

Crossy, when I used to do electrical work (many years ago now) I was taught that an earth rod shouldn't be used with the PME system as that there should only be one connection to earth and that if you did install an earth rod that you would be providing an earthing point for the whole grid that could possibly become dangerous during a grid fault or nearby lightning strike. While in Thailand I have seen advice on installing Earth rods on PME systems, what is your take on this ?

[Crossy]

Confusing innit ^^^ ?

The hazard of an open neutral on a PME system is one of the reasons PME has only relatively recently become acceptable in the UK (and then only on underground supply systems). All N-E links are provided by the supply company and are always sealed in the service head. The multiple earths are also provided by the supply company. Customers are not permitted to link N and E.

In Thailand PME is implemented in a different way, the N-E link is in the CU and fully accessible, the multiple earths are provided by the individual ground stakes (although many systems also have neutral grounds on every 3rd pole or so).

Provided there are plenty of other grounds available a neutral failure won't have serious effects, however, if yours is the only N-E link on the network, everyone else's neutral current will try to return via your ground rod, not a good situation (every earthed piece of metal in your home will become live!!).

My advice is always, do NOT add a MEN / PME link to your installation unless you KNOW MEN / PME is implemented. I also always advise an RCD even though one is not strictly necessary to prevent indirect contact in a MEN system.

[AllanB]

I have just installed an RCD consumer unit (garage unit) for my pool, for the pool pump and lights.It will power the pump timeswitch, but when I turn the lights on the RCD trips and likewise when I start the pump manually using the timeswitch. I have tried bypassing the RCD and everything works, so my wiring appears to be okay and I have sunk a 1 metre earth bar and wrired it into the CU.

Early this year I installed one in my boat in the uk for for 240v shore supply and it was fine, I bought this comsumer unit from the UK too.

[Crossy]

Allan, since you've already got a thread running in Swimming Pools I suggest everybody answers there.

Please post a clear photo of the wiring in your consumer unit, RCDs tripping as soon as a load is applied are often the result of a wiring error.

The circuit may still operate if the RCD is bypassed but will not be safe.

[electau]

There is minimal risk of a hazard of an open circuited neutral if there is a compliant main earth and electrode. The MEN system has been used in Australia under AS3000 since 1931 and is mandatory.

However.using it in Thailand is not advised because of the following.eg.

1. The integrity of the main supply neutral can not be guaranteed.

2. The integrity of the MEN link can not be guaranteed.

3. There is no regime of testing of electrical installations.

4. Polarity tests may not be carried out prior to connection of supply.

5. The earthing of the distribution network neutral may be non compliant.

Therefore the TT or Direct system of earthing should be used and all circuits protected by 30mA RCDs.

So Crossy the TT system of earthing is still used in the UK for an overhead supply with the PME (MEN) system used for an underground supply to an electrical installation.

Malaysia uses the TT system with RCDs.

[Crossy]

So Crossy the TT system of earthing is still used in the UK for an overhead supply with the PME (MEN) system used for an underground supply to an electrical installation.

Indeed, UK supplies on LV overheads are invariably TT.

Underground may be TNC-S (with or without PME) or TNS. The supply authorities often convert TNS systems to TNC-S when the armouring of the underground cable (which is used as the ground conductor) becomes degraded and no longer provides a functional earth.

Just to throw confusion into the mix your local supply company may not be able to tell you which earthing arrangement you have. TT is fairly obvious, but TNS and TNC-S look the same at the service head (any N-E link is sealed within and not accessible).

[electau]

So Crossy the TT system of earthing is still used in the UK for an overhead supply with the PME (MEN) system used for an underground supply to an electrical installation.

Indeed, UK supplies on LV overheads are invariably TT.

Underground may be TNC-S (with or without PME) or TNS. The supply authorities often convert TNS systems to TNC-S when the armouring of the underground cable (which is used as the ground conductor) becomes degraded and no longer provides a functional earth.

Just to throw confusion into the mix your local supply company may not be able to tell you which earthing arrangement you have. TT is fairly obvious, but TNS and TNC-S look the same at the service head (any N-E link is sealed within and not accessible).

Would agree there Crossy.

With the UK PME system the MEN link is the responsibilty of the supply authority and does not form part of the consumers electrical installation. For test purposes the incoming earthing conductor can be disconnected at the main switch board. TN-S.

With the Australian system under AS3000 the consumers main neutral, the MEN link and the main earth and electrode form part of the consumers electrical installation. TN-C-S.

The TT system was never used in Australia although a hybid version combining a V-ELCB and was used in the 1930s as an option it was discontinued by the 1940s and was last mentioned in th 1961 Ed of the now AS3000. There were considerable practical problems with its use.

All were converted back to a conventional MEN connection at the main switchboard. The TT system itself was never seriously cosidered. Leaving out the MEN link is regarded as a serious offence under current electrical legislation.

　

　

　

　

　

Edited December 15, 2011 by electau