The Hazard Of An Open-Circuit Neutral

[Crossy]

Please note, this thread is intended as information for those with some electrical knowledge who may not be aware of the simple technique described. As always please take care when dealing with anything related to your power supply.

The problem

Losing the neutral in a conventional TT supply system (just a ground stake, no MEN link between N and E) simply results in all your lights going out with little or no serious issues.

However with an increasing number of expat homes requiring 3-phase supply and the requirement for new installations in Thailand to be MEN connected the loss of the neutral can lead to significant and potentially lethal consequences.

Losing the neutral of a 3-phase system results in the most lightly loaded phase voltage increasing, potentially to over 400V with the associated effects on your delicate technology none of which will be particularly impressed.

Loss of the neutral in a MEN system can result in all the ‘earthed’ metalwork in your home becoming live, potentially at 220V, an even more unpleasant situation, particularly with small children about.

The solution

So, it would be nice if we could detect a lost neutral and open the supply in order to protect ourselves, our family and our technology. There are commercial supply monitors that will provide this facility (along with over/under voltage and other detections) but these are expensive and can be difficult to find in sunny Thailand, they also require installing by someone who knows what he’s doing.

All is not lost, whilst researching something else I came across a method whereby a regular RCD (ELCB) can be used as a lost-neutral detector, as an aside it also detects a phase-neutral reversal (another potentially damaging and worryingly common occurrence) . This technique was developed in South Africa where copper theft is endemic and the neutral is usually the bottom of the wires on the pole.

All you need is an additional RCD (4-pole if you have a 3-phase supply) and a second ground stake (you can do away with this if your system is MEN wired).

Examine the diagram below, all you need to do is install the new RCD before your distribution board and add a separate ground stake to your neutral as shown, the new RCD can also act as an incoming supply isolator.

How it works.

Consider normal operation (top diagram), the neutral is at or near ground potential so no current flows down the ground stake and the RCD remains in balance. Open the neutral (second diagram), now there is no current in the neutral leg of the RCD and it opens protecting your supply. Reverse phase and neutral (third diagram) and we have a similar effect, again the RCD is imbalanced and trips.

IMPORTANT NOTES. This RCD is only there to detect an open neutral, it will NOT that’s NOT protect you from shock, you need another conventionally connected RCD in your distribution board to provide shock protection. Also if your supply is IT (neutral is not earthed at all) you gain nothing from adding this system.

[doglover]

Thanks for sharing that Crossy. Very appropriate and affordable protection. Could you elaborate on something though....

All you need is an additional RCD (4-pole if you have a 3-phase supply) and a second ground stake (you can do away with this if your system is MEN wired).

Why wouldn't one just wire in the MEN link into their consumer unit even if the distribution system is not MEN and not install the second ground stake? Afterall, as far as I can see, the second ground stake is really just an MEN link with a higher impedance.

If the second ground stake is required (which it most likely is and I've just missed something) then is there a certain distance this should be from the the existing ground stake already in place? Would there be any hazard with leaky appliances draining through the ground rod and returning through the second stake?

[InterestedObserver]

The second ground stake is required to complete the circuit when the neutral is open, thus causing the RCD to trip. The two ground stakes must span the neutral break.

Edited April 2, 2011 by InterestedObserver

[doglover]

The second ground stake is required to complete the circuit when the neutral is open, thus causing the RCD to trip.

If you re-read my initial post you will see I realize that. I am asking why one wouldn't just use an MEN link in leau of a ground stake tapped onto the neutral even if the distribution system was not an MEN system?

The two ground stakes must span the neutral break.

Yes, two ground stakes must span the neutral break. But I am refering to the two consumer ground stakes, which do not need to span the break.

[Crossy]

Indeed ^^^.

The only requirement for this circuit to work is that the neutral has a ground stake on the consumer side of the new RCD. Whether that comes from a second stake or via a MEN link is immaterial.

That said, we tend to say that we should NOT install a MEN link if MEN is not specifically implemented due to the issues of an open neutral (which we are guarding against anyway).

Personally, since RCDs are not absolutely reliable, unless MEN is definitely implemented I'd use two stakes.

[doglover]

Thanks Crossy, that makes sense. Can someone answer my other two questions?

If the second consumer ground stake for the neutral is required (as it is when there is no MEN), is there a certain distance this should be from the the existing consumer main ground stake already in place? Generally, a domestic installation would only have one ground stake for safety reasons. Are there any hazards to consider, or be aware of, with this second ground stake tapped on to the supply neutral?

[InterestedObserver]

The second ground stake is required to complete the circuit when the neutral is open, thus causing the RCD to trip.

If you re-read my initial post you will see I realize that. I am asking why one wouldn't just use an MEN link in leau of a ground stake tapped onto the neutral even if the distribution system was not an MEN system?

The two ground stakes must span the neutral break.

Yes, two ground stakes must span the neutral break. But I am refering to the two consumer ground stakes, which do not need to span the break.

Crossy's diagram clearly indicates that two consumer ground stakes are "not required if MEN".

[Artisi]

Crossy,

Would you care to expand on your statement "since RCDs are not absolutely reliable", there has been an ongoing discussion in another place regarding the need / no need of appliances (water heater) being earth connection even though the unit is fitted with a RCD. This is for my own understanding as I believe that an earth connection is necessary whereas others are saying no need.

Thanks

[doglover]

Crossy's diagram clearly indicates that two consumer ground stakes are "not required if MEN".

Yes, it certainly does InterestedObserver. And my question in my initial post clearly asks something different from what your mind heard when you read it. If you could please re-read my initial post you will hopefully see where the miscommunication is.

I am aware that you are knowledgeable in the electrical field and I am not challenging that. I am only stating that you read something other than what I typed.

[Crossy]

Thanks Crossy, that makes sense. Can someone answer my other two questions?

If the second consumer ground stake for the neutral is required (as it is when there is no MEN), is there a certain distance this should be from the the existing consumer main ground stake already in place? Generally, a domestic installation would only have one ground stake for safety reasons. Are there any hazards to consider, or be aware of, with this second ground stake tapped on to the supply neutral?

There are instances when two ground rods could constitute a hazard. This would occur when there's a fault current flowing in one and nothing in the other, this would mean that one ground could be at a significantly higher voltage than the other (ground isn't always 'ground'). In order to reduce this possibility some countries regulations permit only one ground stake (US, Oz) whilst others insist on bonding the rods together (UK), Thailand appears to have no rules either way.

In our case the safest method would be to ensure that the two rods are far enough apart to prevent metalwork connected to the house rod being touched at the same time as the auxiliary neutral ground. In reality, any fault current should be so short lived (because the RCD opens) that there is no significant hazard, certainly less hazard than that introduced by not having the RCD.

EDIT In the case of farms where livestock are kept things become more complex as it's not unknown for even a small potential between front and back legs to startle or even kill large animals. Most countries have special provision in the wiring regulations for earthing around farm buildings.

[Crossy]

Would you care to expand on your statement "since RCDs are not absolutely reliable", there has been an ongoing discussion in another place regarding the need / no need of appliances (water heater) being earth connection even though the unit is fitted with a RCD. This is for my own understanding as I believe that an earth connection is necessary whereas others are saying no need.

RCDs (and MCBs) are electromechanical devices, they can and do fail. RCDs particularly can fail to open or open at a much higher current than intended if they are not regularly operated with the 'Test' button and how many of us do that? (Guilty as charged M'Lud).

Consider a regular 3.5kW Class 1 (ground required) water heater connected correctly to a solid (MEN) ground and protected by a 30mA RCD and 20A MCB. If you get a fault in the heater (say 5 Ohms) between the element and heater metalwork (grounded) a large current will attempt flow (220/5 = 44A), however, the RCD will operate long before the current has had chance to get anywhere near this value. If the RCD does not open for any reason the 20A MCB will open (more slowly) and kill the circuit. SAFE OUTCOME.

Now let's consider the same water heater with the same protective devices but this time ungrounded. Now add our 5 ohm fault to the heater metalwork, this time no current will flow as there is no path to ground. If someone now touches the metalwork a ground current will flow and if it's greater than 30mA the RCD will open and protect the person. This time if the RCD fails to operate the current will continue to flow through the person but will be limited by body resistance to way less than that required to open the 20A MCB (but still easily lethal) so current flows forever or until you let go of the metalwork because you're dead. QED

The observant will notice that in the case of the grounded heater a person does not at any time form part of the electrical circuit whereas when ungrounded current must flow through a persons body before the safety device operates.

Once upon a time a ground only was considered sufficient for safety, now a ground plus RCD is considered necessary, an RCD alone is not considered adequate to protect from shock in the case of Class 1 appliances.

Class 2 (double insulated) appliances are considered safe without a ground, in this case an RCD provides additional protection from shock.

I'm not aware of any Class 2 water heaters being available in Thailand (or for that matter anywhere else), but I would love to be proved wrong in this case as they could save many lives.

You may repeat this information with or without attribution wherever you like

[Artisi]

Would you care to expand on your statement "since RCDs are not absolutely reliable", there has been an ongoing discussion in another place regarding the need / no need of appliances (water heater) being earth connection even though the unit is fitted with a RCD. This is for my own understanding as I believe that an earth connection is necessary whereas others are saying no need.

RCDs (and MCBs) are electromechanical devices, they can and do fail. RCDs particularly can fail to open or open at a much higher current than intended if they are not regularly operated with the 'Test' button and how many of us do that? (Guilty as charged M'Lud).

Consider a regular 3.5kW Class 1 (ground required) water heater connected correctly to a solid (MEN) ground and protected by a 30mA RCD and 20A MCB. If you get a fault in the heater (say 5 Ohms) between the element and heater metalwork (grounded) a large current will attempt flow (220/5 = 44A), however, the RCD will operate long before the current has had chance to get anywhere near this value. If the RCD does not open for any reason the 20A MCB will open (more slowly) and kill the circuit. SAFE OUTCOME.

Now let's consider the same water heater with the same protective devices but this time ungrounded. Now add our 5 ohm fault to the heater metalwork, this time no current will flow as there is no path to ground. If someone now touches the metalwork a ground current will flow and if it's greater than 30mA the RCD will open and protect the person. This time if the RCD fails to operate the current will continue to flow through the person but will be limited by body resistance to way less than that required to open the 20A MCB (but still easily lethal) so current flows forever or until you let go of the metalwork because you're dead. QED

The observant will notice that in the case of the grounded heater a person does not at any time form part of the electrical circuit whereas when ungrounded current must flow through a persons body before the safety device operates.

Once upon a time a ground only was considered sufficient for safety, now a ground plus RCD is considered necessary, an RCD alone is not considered adequate to protect from shock in the case of Class 1 appliances.

Class 2 (double insulated) appliances are considered safe without a ground, in this case an RCD provides additional protection from shock.

I'm not aware of any Class 2 water heaters being available in Thailand (or for that matter anywhere else), but I would love to be proved wrong in this case as they could save many lives.

You may repeat this information with or without attribution wherever you like

Thanks for the response which makes it perfectly clear why an earth is necessary.

I will pass this on but I am aware that some people do not want to hear - but that's at their peril.

How does the saying go - "there are non so blind that don't want to see"

As a suggestion - do you think is worthwhile adding to your comprehensive data on electrics.

[doglover]

If I were you Artisi I would repeat it without attribution and you'll look brilliant ! ( )

Excellent answers in an easy to understand lingo as usual Crossy. Sorry for the 20 questions but I have another (although likely my last on the subject)...

Could there be value in utilizing this protection even with a single phase TT installation? Let's say a consumer with a single phase TT installation has a second ground stake pounded and taps it onto the supply neutral just as one would do for a 3 phase system where a MEN has not been implemented. First of all, it would better to have this RCD disconnect the mains if there has been a break in the neutral than to have the electricity searching for a return path inside the residence. Also if Somchai had a little welding job on the premises and somehow crossed the mains wiring when he taps in at the meter (as an example) one would be aware of the situation immediately. Secondly, in the event of a fault originating from within the installation there would be a much greater possibility of having a low impedance path back to the transformer with this second ground stake tapped onto the neutral. Particularly if the fault was on a non RCD protected circuit (some appliances that will be typically leaky and the installer may have opted not to use a RCD for example) or if a RCD was malfunctioning.

On the flip side.... If there was a lightning strike on the mains neutral there could be some high step voltages around the ground area that this second ground stake has been pounded. But at least this would ground some of the strike though. A more desireable situation than full force of the strike entering further into the residence and then blasting a path to ground.

No doubt I have missed something and my thinking is flawed. However, by posting my questions we can all learn more about this protective measure. Thanks for your time.

[Crossy]

I see no reason whatever why you should not use this arrangement on a simple TT system, it would indeed protect against L-N reversal and an open neutral (although this is nowhere near as detrimental as it would be on a MEN connected system).

A word or two of warning:-

This connection will not be of any help if you have an IT system (ungrounded neutral), a number of posters have reported that they appear to have this kind of system.

Also, there are some areas in Thailand where the supply is 3-phase 3-wire with the 220V being taken phase-to-phase, again if you have this type of arrangement you'll gain nothing from using this technique, indeed if you ground one of the phases you could be introducing nasty failure modes into other parts of the system.

I'll add both these tomes to my website, it's not been updated for a while.

[InterestedObserver]

All this talk about intentional multiple ground paths (loops). Not good engineering practice. What electrical code encourages the practice, I know the Canadian and American electrical codes don't.

[doglover]

I see no reason whatever why you should not use this arrangement on a simple TT system, it would indeed protect against L-N reversal and an open neutral (although this is nowhere near as detrimental as it would be on a MEN connected system).

A word or two of warning:-

This connection will not be of any help if you have an IT system (ungrounded neutral), a number of posters have reported that they appear to have this kind of system.

Also, there are some areas in Thailand where the supply is 3-phase 3-wire with the 220V being taken phase-to-phase, again if you have this type of arrangement you'll gain nothing from using this technique, indeed if you ground one of the phases you could be introducing nasty failure modes into other parts of the system.

I'll add both these tomes to my website, it's not been updated for a while.

I realize Crossy (and some others) is/are aware of what I am about to say but others may find it helpful (as I suspect every DIYer and his dog is going to want one of these fancy new gadgets). With an IT sytem, not only would it not be any help but it would change the characteristics of the system and have repercussions. Anyway, for those who are interested in differentiating between the distribution systems I have come across a great link on the subject. The OP provides very detailed information and pictures with his enquiries and the responses are top notch. Here is the link... http://www.thaivisa....stem-do-i-have/

Personally... I originally thought the distribution network in my area was TT. I now believe it to be IT but have not confirmed this with my own two eyes. Which is the only way I can be sure what it is (as opposed to just asking the PEA and taking the answer for gospel). Whatever you do, DO NOT take a ladder and climb up closer to the transformer for better photos as the OP did in the linked thread. High voltage cables have minimum safe approach distances and it is very dangerous (yes, even if you have one of those high tech bamboo ladders, it is still dangerous ).

Here is my plan to see it with my own two eyes. I will make an offer to the local kids that live around the transformer of 100 baht if they can race over to my place and let me know the next time the PEA bucket truck is at the transformer. If the truck is still there when I arrive the kids get their reward. I'll grab my point and shoot camera on the way out the door. I'll then offer the PEA gents a big bottle of lao cao (or maybe something with a little more class) if they can go up in the bucket and take some good shots of the transformer connections..... Great chance to talk with them (or attempt to) about what I see on the display afterwards as well....

Edited April 3, 2011 by doglover

[bankruatsteve]

Except for neutral swap (which I just can't believe really happens) why would RCD care about the system or grounding method?

[InterestedObserver]

Except for neutral swap (which I just can't believe really happens) why would RCD care about the system or grounding method?

The RCD trips when the current differential exceeds its rating. The open or swapped neutral RCD uses the system grounds to complete the circuit through the RCD and back to the source transformer. RCDs always use ground current to trip, sometimes a persons body is part of that ground circuit.

Edited April 3, 2011 by InterestedObserver

[CDNinKS]

My supply here in Hua Hin is definitely a MEN system as the neutral at the transformer and at the first pole after that is earthed (the next earthed neutral is not for 8 more poles)

So I thank Crossy for the advise on the "open" neutral. My plan is to bring 3 phase into the property and use one phase on the guest house, one phase to run all the pumps (pool, house supply) and the last phase for the main house.

Does that sound reasonable?

[doglover]

^^^^^^

No it does not make sense. The phases will surely be imbalanced. May I suggest you could do a search on '3 phase' or 'three phase' as it is quite involved and will take a fair amount of reading to understand three phase installations.

[CDNinKS]

No it does not make sense. The phases will surely be imbalanced. May I suggest you could do a search on '3 phase' or 'three phase' as it is quite involved and will take a fair amount of reading to understand three phase installations.

OK, took your advise and this is what I found:

Three phase power is a very efficient form of electrical power distribution. All three wires carry the same current and have a constantly balanced power load. Three phase power does not generally power domestic houses, and when it does, a main distribution board splits the load. Most domestic loads use single phase power.

So that was my intention, a main distribution board where I would split off the phases to supply single phase power to the 3 different areas.

As to an imbalance, on my soi there are at least 6 houses drawing from the same phase, while nobody draws from the other 2 that I can see.

[billd766]

A couple of evenings ago I switched on my gate lights as it was raining and getting dark

The cct breaker on my power board popped open so I reset it and tried again with the same result.

Yesterday morning after it had dried out I tried again and got the same result.

So I got a couple of new lights and climbed a small ladder on the fist lamp of two. I took the globe off and removed the bulb, cleaned out all of the dead fly wings, went back up to the house and cleaned the globe and switched the light on with the same result.

Bugger it, so I went down and did the same for the other side with the same predictable results.

the right side has only 1 cable and the left has 2 so I tried to take the base off but it was rusted and I had the wrong size screwdriver.

Back up to the house for a bigger screwdriver and some rust remover and I finally got the base off and untwisted the cables (but they were covered in insulation tape) and looked them over.

There ate the bottom of 1 pair was the fault.

When the sparky wired the lamps up he cut the outer insulation back around the wires instead of along them and nipped through the inner insulation which caused the problem.

It took 8 years to become a fault.

The repair is easy as I went back to the house once again and got a terminal block and trimmed the faulty cable insulation back properly but as I was cutting the inner insulation I got a short sharp belt off the wire even though the cct breaker was tripped.

It wasn't enough to throw me off the ladder but enough to wake me up and of course the sweat is pouring off me and I thought bugger this for a lark.

I then did the wiring more carefully and put it all back together and it worked just fine.

Lessons to be learned are just because the cct breaker isolates the cct, just because the Safe-T-Cut is in the system and just because you think you know that it is safe and you know what you are doing, electricity sometimes has a way of sneaking up on you and giving you a sharp bite when you least expect it.

On the good side as the gate is about 40 metres away at least I got some exercise.

Edited April 7, 2011 by billd766

[InterestedObserver]

My supply here in Hua Hin is definitely a MEN system as the neutral at the transformer and at the first pole after that is earthed (the next earthed neutral is not for 8 more poles)

So I thank Crossy for the advise on the "open" neutral. My plan is to bring 3 phase into the property and use one phase on the guest house, one phase to run all the pumps (pool, house supply) and the last phase for the main house.

Does that sound reasonable?

If you are going to use the three-phase electrical system as you describe then I'd invest in a proper under-over voltage/phase loss/phase reversal relay. There are a couple of excellent posts on ThaiVisa Forum that cover the subject in detail, Namm uses such a system.

Edited April 9, 2011 by InterestedObserver

[CDNinKS]

If you are going to use the three-phase electrical system as you describe then I'd invest in a proper under-over voltage/phase loss/phase reversal relay. There are a couple of excellent posts on ThaiVisa Forum that cover the subject in detail, Namm uses such a system.

Yes, but I don't have Naam's money

[electau]

If one is using the MEN system and a compliant earthing system that conforms with (for example) AS3000.

If the neutral conductor is broken the potential of the earthing system rises to full line voltage.

The touch voltage to earth by a person in contact with the earthing system will not exceed 50VAC. A mild tingling effect may be experienced.

If this should occur isolate the power at the main switch and the have the problem rectified.

This is the same as having reversed polarity but in this case the power cannot be isolated at the main switch. Reversed polarity should never occur if correct testing procedures are carried out.

With the TT and IT systems if the neutral is broken there will not be a rise of potential within the earthing system.

The main earth or MEN bond must never be disconnected (except for testing purposes).

The first signs of a faulty neutral developing are flickering lights when additional load is switched on. A faulty neutral develops over time.

This is why the main earth is an important feature of the MEN system and affords protection.

Edited April 19, 2011 by electau

[InterestedObserver]

If one is using the MEN system and a compliant earthing system that conforms with (for example) AS3000.

If the neutral conductor is broken the potential of the earthing system rises to full line voltage.

The touch voltage to earth by a person in contact with the earthing system will not exceed 50VAC. A mild tingling effect may be experienced.

If the touch voltage to earth does not exceed 50VAC, then how can the potential of the earthing system rise to full line voltage?

[CDNinKS]

So, after a trip to the PEA office they informed us that since we have 2 houses on the property, we can simply get two Tambien Baan's then we can have 2 meters. I was wrongly told in the past that the only way to have 2 meters was to split the chanote. That eliminates my problem of running one phase to the small house, it can be on it's own meter. I will still get 3 phase run to the main house just to distribute the load.

[electau]

If one is using the MEN system and a compliant earthing system that conforms with (for example) AS3000.

If the neutral conductor is broken the potential of the earthing system rises to full line voltage.

The touch voltage to earth by a person in contact with the earthing system will not exceed 50VAC. A mild tingling effect may be experienced.

If the touch voltage to earth does not exceed 50VAC, then how can the potential of the earthing system rise to full line voltage?

Equipotential bonding. The actual touch voltage is difficult to calculate but the main earth and equipotential bonding minimise the voltage rise to less than 50VAC.

The earthing system is at line voltage relative to the neutral at the source of supply, the transformer.

The touch voltage is the voltage between the earthing system and the general mass of earth.

Edited April 20, 2011 by electau

[electau]

With reference to Crossys post #1.

If you have any any earth on the load side of an RCD the RCD will trip. In this case the earth is on the neutral.

The only known method is to monitor the neutral to earth voltage and the protective device set to operate at a set value below 50VAC, perhaps 10 to 12 volts. The device would require a functional testing requirement. It would have to protect against high resistance connections in the neutral also.

A compliant earthing system minimises the touch voltage to safe levels.

However, where the neutral integrity ( and for that matter the earthing system) can not be guaranteed the MEN system should not be used. This is the most logical reason to use the TT system and RCDs.

Faulty neutrals are a supply authority problem (PEA/MEA) which they should address.

[InterestedObserver]

If one is using the MEN system and a compliant earthing system that conforms with (for example) AS3000.

If the neutral conductor is broken the potential of the earthing system rises to full line voltage.

The touch voltage to earth by a person in contact with the earthing system will not exceed 50VAC. A mild tingling effect may be experienced.

If the touch voltage to earth does not exceed 50VAC, then how can the potential of the earthing system rise to full line voltage?

Equipotential bonding. The actual touch voltage is difficult to calculate but the main earth and equipotential bonding minimise the voltage rise to less than 50VAC.

The earthing system is at line voltage relative to the neutral at the source of supply, the transformer.

The touch voltage is the voltage between the earthing system and the general mass of earth.

OK, equipotential bonding. So then the main earth and equipment bonding limit the voltage rise (touch voltage) to less than 50VAC by design and presumably AS3000 for the MEN system. Since the neutral is also earthed at the source in the MEN system, seems to me that you can only have a voltage rise of 50VAC maximum relative to the neutral.