Why Do The Simple Jobs Always Take The Longest?

[Crossy]

Why do all the easy jobs take on a life of their own, something that's particularly true in sunny Thailand?

OK, friend of the Missus who lives in the same complex asked me to look at her electrics (hubby is Japanese, nice guy but no clue about engineering), she'd heard of people being zapped in their showers and wanted to know it wouldn't happen to her.

All the condo. units have the same type of Square-D distribution board, should be neatly wired if ours was anything to go by.

A quick visual showed no RCD protection whatever, so off to HomePro to see what's available.

Brief exploration of the electrical bits department and a chat to a couple of cute (and knowledgeable) assistants showed they had the correct Square-D RCBOs to fit in that board, brilliant and at 1,900 Baht a pop we could get three for the same price as a single whole-house Safe-T-Cut.

They only had 10mA trip units so we would only put one on the water heater, bedroom circuit and kitchen circuit. The lounge has lots of technology with nice leaky switching PSUs, a 10mA would likely not stay closed. We'll check back later to see if they get 30mA breakers in.

Back home open up the board, as suspected nicely wired with numbers on the wires for each circuit.

But what's this? What are these white wires on the earth bar? Ah, that's the neutrals for the professionally installed aircon!

At this point I must note that the supply is TN-S with Live, Neutral and Earth being provided from a central closet where the meters live. There is about 0.5 ohms between N and E so there's a link somewhere but for our purposes it's TN-S.

Firing up an A/C netted about 4V between N and E with a suitably large spike when the A/C powered up. Nasty! This may explain why they had reportedly been using up PC power supplies and had odd problems with the TV too (now I've fixed it we shall see if things improve). It had been wired like that for at least five years!

Back to the job. Move the A/C neutrals to the proper neutral bar, luckily the installers had left plenty of length and they reached just fine with a little re-routing. The A/C remains un-grounded (there are limits to what I'll do for free) but the units are well out of reach and the lady of the house said she'll get the condo. sparks to run ground wires, we shall see what actually happens.

Installed three RCBOs, in the Square-D system these just clip on in place of the regular breakers. You have to remove the circuit neutral from the neutral bar and connect to the RCBO so a little re-routing and an extension of one wire that was too short was needed (screw connector and a bit of 2.5mm that I had in the box, it was even the right colour).

Quick visual that everything is in the right place and power on.

Now the fun starts, the RCBO serving the kitchen promptly dropped out! Everything unplugged, breaker still refuses to stay on. Turn off all the other breakers, now the kitchen breaker stays on. Turn on circuits one by one. Click!

Ah turning on the circuit for the washing machine drops the kitchen RCBO. Hmmm, washing machine breaker has wire No.7 on it, where's neutral number 7? Nowhere to be found. What we have is a borrowed neutral (two circuits sharing the same return path). Apart from the minor irritation of the RCBO not staying engaged this is a potential hazard for the sparks as the apparently isolated neutral could become live via another circuit. The proper fix would be to run another neutral, the quick (and still perfectly safe) fix is to move live number 7 to share the same breaker as the kitchen. A convenient side-effect is that the washer is now RCD protected.

Here we go, a nice Thai lady has an electrical system that's infinitely safer than it was, I have Brownie points with the Missus for helping her friend and Saito San will still have a wife when he returns from Japan.

NOTE. The couple in question own their unit and had previously asked the condo. people if we could add RCBOs. The maintenance man was 'busy' so had no issue with someone else looking in the box. I expect him to be 'inspecting' my work some time, and wondering what the screw connectors are for, I have no idea if he will recognise the RCBOs for what they are.

[electau]

Yes, fitting RCDs was a wise investment in electrical safety.

The main switchboard is MEN, the sub mains and all final subcircuits have a separate neutral and earth. (TN-S).

The 4 volts between the neutral and the earth (at the distribution board or at the airconditioner?) when an airconditioner is running. Did you find the actual cause of it? No earthing of the airconditioners may explain this. The 4 volts itself is not a hazardous value.

Crossed neutrals will cause RCDs to trip. (correct circuit connections)

Neutrals to earth will cause RCDs to trip. (insulation test)

Each final subcircuit must have its own individual neutral conductor. (correct circuit connections) for reasons of safety neutrals must not be shared between final subcircuits.

RCDs must trip at between 50% and 100% of their rated current in mA. Max leakage current should not exceed 30% of rated current of the RCD in mA, otherwise nuisance tripping may occur.

　

　

[Crossy]

Hi Electau.

Yeah, the 4 volts was when the A/C neutral was connected to the ground bar (and with just one of their three A/C units running, must have been a pretty hefty spike on startup), once correctly wired it's down to less than a volt under normal circumstances.

There was no way I was going to re-wire the washer outlet so putting it on the same breaker as the kitchen solved the borrowed neutral issue. It's all on 2.5mm and a 16A breaker so I may have to up to a 20A if they get tripping when the washer is on at the same time as the kettle and grill we shall have to wait and see.

[electau]

Yes one would agree Crossy that was the way to go with the common ( shared) neutral on the final subcircuit.

[Pib]

I'm pretty sure "Simple Jobs" are things of fairy tales. So many of the simple jobs I've started ended up taking at three times as long as planned and a trip or two to the hardware store for parts. It's like the time I had to replace an inside hinge on Pandora's Box....took me durn near forever to get most everything back into the box. Yeap, no such thing as a Simple Job, except maybe in fairy tales.

[bankruatsteve]

Yes, fitting RCDs was a wise investment in electrical safety.

The main switchboard is MEN, the sub mains and all final subcircuits have a separate neutral and earth. (TN-S).

The 4 volts between the neutral and the earth (at the distribution board or at the airconditioner?) when an airconditioner is running. Did you find the actual cause of it? No earthing of the airconditioners may explain this. The 4 volts itself is not a hazardous value.

Crossed neutrals will cause RCDs to trip. (correct circuit connections)

Neutrals to earth will cause RCDs to trip. (insulation test)

Each final subcircuit must have its own individual neutral conductor. (correct circuit connections) for reasons of safety neutrals must not be shared between final subcircuits.

RCDs must trip at between 50% and 100% of their rated current in mA. Max leakage current should not exceed 30% of rated current of the RCD in mA, otherwise nuisance tripping may occur.

　

　

Why would neutral to earth cause an RCD to trip? And, why is that an insulation test?

Cheers

[Sorridstroker]

Yes, fitting RCDs was a wise investment in electrical safety.

The main switchboard is MEN, the sub mains and all final subcircuits have a separate neutral and earth. (TN-S).

The 4 volts between the neutral and the earth (at the distribution board or at the airconditioner?) when an airconditioner is running. Did you find the actual cause of it? No earthing of the airconditioners may explain this. The 4 volts itself is not a hazardous value.

Crossed neutrals will cause RCDs to trip. (correct circuit connections)

Neutrals to earth will cause RCDs to trip. (insulation test)

Each final subcircuit must have its own individual neutral conductor. (correct circuit connections) for reasons of safety neutrals must not be shared between final subcircuits.

RCDs must trip at between 50% and 100% of their rated current in mA. Max leakage current should not exceed 30% of rated current of the RCD in mA, otherwise nuisance tripping may occur.

　

　

Why would neutral to earth cause an RCD to trip? And, why is that an insulation test?

Cheers

An RCD monitors difference in current between the phase and neutral conductors both values have to be balanced.

So if a neutral conductor is connected to earth the current value in the neutral is going to be lower than that of the phase conductor as you will get 'earth leakage' so if the values are inbalanced the rcd will trip.

If you carry out an insulation resistance test between conductors it will tell you if the insulation is sound or not hence no 'earth leakage' should occur

[David006]

So can you guys explain to me then how/why would the main board be MEN and not the sub by default?? All very confusing...

My house is what you call MEN I suppose and yes about every couple of months we get a nuisance trip at the domestic panel (usually during storms) which has an ELCB protecting the lot. The ELCB at the pump house for the wells and domestic pressure never trips and is also MEN.

That said the other month I reached inside the dog house size pump house to throw a pump switch on the inner wall ( i.e. blind to me) anyways I got a nice kick and on investigation found the neutral insulation had been eaten by something unknown and exposed at the entrance to the conduit switch box ..which I had touched I assume. The ELCB did not trip so methinks hmmm better investigate...so Google'd GFIs and found an article that seemed to indicate that if the fault is on the neutral side ie after the load then the GFI would not see it as an unbalance and therefore not trip...makes sense I suppose and the reason for just a "kick" I would assume is that the voltage has already been dropped across the load and I was a more attractive return....standing as I was in the mud although wearing wellies....

..yup nuffin is simple ...

[Crossy]

For safe and reliable operation there should only be one link between Neutral and Earth on an installation (more than one link won't actually make anything safer). This link MUST be upstream of any RCDs, if it's not then the RCD with either not stay energised, or (worse), not trip when you need it to.

Even if you have only a 2 wire connection to a pump house you must NOT insert a N-E link at the end of the run as this may result in odd tripping behaviour of other RCDs.

As to why David006 got a wallop of his neutral, it's likely the pump pressure switch is in the neutral line which would result in all wiring being hot, even with the pump not running. But without actually seeing the installation any diagnosis would of course be supposition. Perhaps the RCD didn't trip because it's stuck.

You should test all your RCDs regularly by pressing the 'test' button.

[electau]

In reply to post #8.

1. The MEN connection should be at the main switchboard only and an earth conductor run to any distribution sub boards within an electrical installation.

In the case of remote outbuildings supplied from the main switchboard an MEN may be installed at each distribution sub board with a main earthing conductor and electrode OR run an earth conductor to each distribution sub board.

A submain cannot be protected by an RCD if it has an MEN, the RCD has to protect the final sub circuit(s).

Ref AS3000.

2. The RCD at the pump may not trip because the first RCD upstream of it has tripped first.

You should check that the RCD protecting the pump is connected correctly and then test it using the test button. If it fails to operate replace the RCD.

The shock you received was due to insulation failure. The RCD should have tripped if the fault at the switch was downstream of the RCD.

　

[David006]

In reply to post #8.

1. The MEN connection should be at the main switchboard only and an earth conductor run to any distribution sub boards within an electrical installation.

In the case of remote outbuildings supplied from the main switchboard an MEN may be installed at each distribution sub board with a main earthing conductor and electrode OR run an earth conductor to each distribution sub board.

A submain cannot be protected by an RCD if it has an MEN, the RCD has to protect the final sub circuit(s).

Ref AS3000.

2. The RCD at the pump may not trip because the first RCD upstream of it has tripped first.

You should check that the RCD protecting the pump is connected correctly and then test it using the test button. If it fails to operate replace the RCD.

The shock you received was due to insulation failure. The RCD should have tripped if the fault at the switch was downstream of the RCD.

　

just looked at this tread again...so

actually I surmise that the belt I got may well have been the pump start capacitor discharge as a meter check failed to reveal any extraneous voltage...

.all RCD/GFIs are wired and operating correctly....

[thaibeachlovers]

If anyone has the time or inclination perhaps they would please explain to me why in NZ the neutral wires and the earth bar are connected in the new circuit boxes. I grew up in the old days of wired fuses and seperate neutral/ earth circuits. A few years ago I bought a new 3 breaker box to put in a shed and it had the neutral bar and the earth bar connected. I had to ring a sparky friend to see if that was correct ( it is ), but he didn't explain it to me in a language I could understand ( I'm not too up on technical jargon ).

From the previous discussion it would seem that an RCB would not work in NZ, which might explain why I'd never seen one before.

Thanks.

Edited July 30, 2011 by thaibeachlovers

[Crossy]

Linking Neutral and Ground either in the distribution board or the service head is called TN-C-S, if there is a local ground stake as well it becomes PME (Protective Multiple Earthing) or MEN (Multiply Earthed Neutral). MEN is theoretically required in new Thai installations. Try a Google to see the exact arrangements, start here http://en.wikipedia.org/wiki/Earthing_system

There is absolutely no reason for an RCD not to work in any of the above arrangements, they certainly work (and are required) in Oz who use the same (or very similar regulations) as NZ.

[thaibeachlovers]

Thanks Crossy. I stand corrected.

regarding NZ, there's certainly no requirement for existing buildings to fit an RCD, as no place I've lived in has one. I'll have to ask my sparky friend if it's a requirement for new builds.

What people do use are the plug in "circuit breakers" to use when outside. Can't remember their name now. Certainly easier than the heavy transformers we used to use ( I still do ).

[David006]

..hmmm I read someplace that NZ still has some power distribution networks that use only a single live wire and local neutral grounds ...bit like WW1/2 field telephones...another useless observation maybe..

[Crossy]

..hmmm I read someplace that NZ still has some power distribution networks that use only a single live wire and local neutral grounds ...bit like WW1/2 field telephones...another useless observation maybe..

Google Single Wire Earth Return.

Not used at outlet voltages as the ground resistance is too high, but a good way to halve the copper cost of a long run to an isolated home.

[electau]

If anyone has the time or inclination perhaps they would please explain to me why in NZ the neutral wires and the earth bar are connected in the new circuit boxes. I grew up in the old days of wired fuses and seperate neutral/ earth circuits. A few years ago I bought a new 3 breaker box to put in a shed and it had the neutral bar and the earth bar connected. I had to ring a sparky friend to see if that was correct ( it is ), but he didn't explain it to me in a language I could understand ( I'm not too up on technical jargon ).

From the previous discussion it would seem that an RCB would not work in NZ, which might explain why I'd never seen one before.

Thanks.

The neutral bar and the earth bar are connected by a link. This is the MEN link.

RCDs are used in NZ and NZ uses the MEN system. The wiring rules are AS/NZ3000.

The requirement to fit RCDs to older installations in covered by other legislation.

[electau]

Thanks Crossy. I stand corrected.

regarding NZ, there's certainly no requirement for existing buildings to fit an RCD, as no place I've lived in has one. I'll have to ask my sparky friend if it's a requirement for new builds.

What people do use are the plug in "circuit breakers" to use when outside. Can't remember their name now. Certainly easier than the heavy transformers we used to use ( I still do ).

Those "plug in corcuit breakers" are 30mA RCDs.

[electau]

..hmmm I read someplace that NZ still has some power distribution networks that use only a single live wire and local neutral grounds ...bit like WW1/2 field telephones...another useless observation maybe..

The SWER HV for distribution to remote rural properties. Uses a 12.7kV single phase 240/480 volt transformer. Limited to about 10kVA in size. One side of the of the HV winding is connected to the transformer tank and is earthed.

Edited August 1, 2011 by electau

[thaibeachlovers]

..hmmm I read someplace that NZ still has some power distribution networks that use only a single live wire and local neutral grounds ...bit like WW1/2 field telephones...another useless observation maybe..

Google Single Wire Earth Return.

Not used at outlet voltages as the ground resistance is too high, but a good way to halve the copper cost of a long run to an isolated home.

NZ has thousands of farms at the end of very long roads- also a useless observation maybe, LOL.

I stay a lot at a farm that is close enough to the road to warrent a 2 wire connection, but is far enough that when I turn on a bench saw the voltage drop dimms the lights and the computer shuts down!

If someone has the inclination, perhaps they could answer this question please- there is a shed about a hundred meters from the house and a three wire 6mm cable was buried to take power to it, which might be a bit thin to take power to everything that has been installed in the shed. Would it be possible to use both positive and neutral joined to take power to the shed and use an earth return as mentioned in a reply above. Thanks.

[Crossy]

There is a shed about a hundred meters from the house and a three wire 6mm cable was buried to take power to it, which might be a bit thin to take power to everything that has been installed in the shed. Would it be possible to use both positive and neutral joined to take power to the shed and use an earth return as mentioned in a reply above. Thanks.

Unfortunately no, SWER only works at high supply voltages (11kV) as the earth has quite a high resistance, at normal appliance voltage (220V) the voltage drop through the ground leg would make it impractical.

One could use a 1:2 transformer to increase the supply voltage to the shed (double the voltage and reduce the current to half) with a 2:1 at the other end of the 100m run to get back to 220V, not the cheap option but workable if it's not practical to replace the existing cable.

[thaibeachlovers]

There is a shed about a hundred meters from the house and a three wire 6mm cable was buried to take power to it, which might be a bit thin to take power to everything that has been installed in the shed. Would it be possible to use both positive and neutral joined to take power to the shed and use an earth return as mentioned in a reply above. Thanks.

Unfortunately no, SWER only works at high supply voltages (11kV) as the earth has quite a high resistance, at normal appliance voltage (220V) the voltage drop through the ground leg would make it impractical.

One could use a 1:2 transformer to increase the supply voltage to the shed (double the voltage and reduce the current to half) with a 2:1 at the other end of the 100m run to get back to 220V, not the cheap option but workable if it's not practical to replace the existing cable.

Thanks Crossy. Guess it's just going to be a case of only using one high power appliance at a time then. Power drops off a bit on start up, but seems to work OK after that.