Why Earthing Of An Electrical Installation Is Important For Safety.

[electau]

With an earth fault the potential (voltage) of the earthing system may rise to line voltage depending on the resistance of the main earth. Where this touch voltage exceeds 50VAC automatic disconnection must occur in less than 0.4secs. If a person is in contact with the earthing system AND conductive metal, eg, a metallic pipe that enters the ground some distance away they may receive a shock.

Equipotential bonding will minimise this risk as the earthing system as the metallic pipe will be at the same potential.

( The difference between bonding and earthing is important. Usually a system relies on bonding to keep exposed conductive parts at the same potential. The earth connection connects the bonded system to the general mass of earth, but the resistance of the earth electrode ( to the general mass of earth) is not controlled, and can be from a few ohms to a thousand ohms).

With the MEN system and with a broken incoming neutral automatic disconnection will not occur, and the installation must be isolated at the main switch.

With the MEN system and reversed polarity, which should NEVER occur if correct polarity testing is carried out prior to initial connection or connection after repairs or alterations, eg. meter replacement, the installation cannot be disconnected from the distribution system at the main switch as the line is connected directly to the earthing system.

The earthing system and electrode maintains the touch voltage to less than 50VAC.

With the TT system a broken neutral does not cause a potentially hazardous situation, but reversed polarity is as lampholders and permanently connected equipment will still have supply connected to them.

[electau]

Most electrical fatalities and injury by the effects of electric shock are caused by indirect contact, not direct contact.

Direct contact is when a person comes into contact with a live terminal, busbar or bare conductor. Earthing can not protect a person in this circumstance. An RCD under certain conditions will if primary protection methods fail.

Indirect contact is when a person comes into contact with conductive material which is not normally live under normal conditions. eg, an electrical appliance, metallic switchboard, lighting fitting, airconditioner etc. The conductive material becomes live under earth fault conditions, which is caused by insulation failure.

Thus it is essential that continuity of the PE conductor is maintained to all electrical equipment that is required to be earthed and 3 pin socket outlets are used where practicable although this may not always be possible in Thailand.

Because of the risk of possible damage to the PE conductor in portable and semi portable appliances and (inThailand) the use of 2 pin plug tops and 2 core flex RCD/RCBOs should be installed.

[T_Dog]

Good information electau. One point to make is to inspect appliances that get a lot of human contact to ensure they have an effective ground. It surprises me that many refrigerators, electrical cooking tops, rice makers, and other appliances that are used daily often come with only a two-wire power cord and no ground. Our new Samsung refrigerator was delivered with a green wire in an envelope for grounding. Doubtful many people ever connect them, but you do hear of deaths in Thailand occasionally from refrigerators having a line-to-chassis short. Appliances that have a high frequency of human use should be the first point of concern with a good earthing system.

[electau]

Good information electau. One point to make is to inspect appliances that get a lot of human contact to ensure they have an effective ground. It surprises me that many refrigerators, electrical cooking tops, rice makers, and other appliances that are used daily often come with only a two-wire power cord and no ground. Our new Samsung refrigerator was delivered with a green wire in an envelope for grounding. Doubtful many people ever connect them, but you do hear of deaths in Thailand occasionally from refrigerators having a line-to-chassis short. Appliances that have a high frequency of human use should be the first point of concern with a good earthing system.

This is perfectly true, those appliances with a 2 core lead and 2 pin plug top comply with TIS (Thai Industrial Standards) not IEC.

The earth wire supplied is to provide a form of equipotential bonding to earth to minimise the effects of potential leakage currents and mild shocks ( tingling sensation).

This is reason that RCDs need to b e installed as they will provde protection from earth faults where the earth impedance is high.

[Forkinhades]

Good information.

Could you also highlight how important it is to install an RCD on a TT system (95% of Domestic installations in Thailand), and how one should not rely on just an earth being installed.

[electau]

Good information.

Could you also highlight how important it is to install an RCD on a TT system (95% of Domestic installations in Thailand), and how one should not rely on just an earth being installed.

RCD/RCBOs should be installed on TT electrical installations as the fault loop impedance will not be low enough in the event of an earth fault to operate a conventional protective device within the required tripping time, 0.4 secs for an MCB.

Where the touch voltage exceeds 50VAC auto disconnection must occur. Touch voltage only exists under earth fault conditions.

Edited November 30, 2012 by electau

[Forkinhades]

Good information.

Could you also highlight how important it is to install an RCD on a TT system (95% of Domestic installations in Thailand), and how one should not rely on just an earth being installed.

RCD/RCBOs should be installed on TT electrical installations as the fault loop impedance will not be low enough in the event of an earth fault to operate a conventional protective device within the required tripping time, 0.4 secs for an MCB.

Where the touch voltage exceeds 50VAC auto disconnection must occur. Touch voltage only exists under earth fault conditions.

Very good. IMO should must be a must!

[David006]

So what are you saying here?

The inference seems to be that every conductive household item should be bonded ..sound idea perhaps but a bit impractical tying together stainless steel sinks , pipes,taps, hobs, etc.?

With the price of electronics so low one wonders why all appliances do not have mandatory inbuilt LCBs like shower heaters....guess it will come.

Often thought there should be a market for a piggy back LCB plug adapter .maybe they are out there already someplace?

Never seen the replacement outlet versions here...would be a start?

[Forkinhades]

So what are you saying here?

The inference seems to be that every conductive household item should be bonded ..sound idea perhaps but a bit impractical tying together stainless steel sinks , pipes,taps, hobs, etc.?

With the price of electronics so low one wonders why all appliances do not have mandatory inbuilt LCBs like shower heaters....guess it will come.

Often thought there should be a market for a piggy back LCB plug adapter .maybe they are out there already someplace?

Never seen the replacement outlet versions here...would be a start?

what are you saying? LCB?

I am saying minimum requirements would be a main RCD/ELCB

[electau]

So what are you saying here?

The inference seems to be that every conductive household item should be bonded ..sound idea perhaps but a bit impractical tying together stainless steel sinks , pipes,taps, hobs, etc.?

With the price of electronics so low one wonders why all appliances do not have mandatory inbuilt LCBs like shower heaters....guess it will come.

Often thought there should be a market for a piggy back LCB plug adapter .maybe they are out there already someplace?

Never seen the replacement outlet versions here...would be a start?

All PE conductors to the earth bar, earth socket outlets 3 pin, HWS, Airconditioners, Hot plate units etc. Light fittings as required.

Bond the metallic water pipe if installed to the main earth bar ( only at one point, the pipe should enter the ground). Install a main earth and electrode or equivalent.

Install RCDs /RCBOs on all final subcircuits at the switchboard.

Isolated items of metal work are not required to be bonded. eg metallic taps, sinks, containers etc.

Ref. AS3000.

Edited November 30, 2012 by electau

[David006]

So what are you saying here?

The inference seems to be that every conductive household item should be bonded ..sound idea perhaps but a bit impractical tying together stainless steel sinks , pipes,taps, hobs, etc.?

With the price of electronics so low one wonders why all appliances do not have mandatory inbuilt LCBs like shower heaters....guess it will come.

Often thought there should be a market for a piggy back LCB plug adapter .maybe they are out there already someplace?

Never seen the replacement outlet versions here...would be a start?

All PE conductors to the earth bar, earth socket outlets 3 pin, HWS, Airconditioners, Hot plate units etc. Light fittings as required.

Bond the metallic water pipe if installed to the main earth bar ( only at one point, the pipe should enter the ground). Install a main earth and electrode or equivalent.

Install RCDs /RCBOs on all final subcircuits at the switchboard.

Isolated items of metal work are not required to be bonded. eg metallic taps, sinks, containers etc.

Ref. AS3000.

Not challenging anybody ( .....brought this issue up last year sometime if ya care to search?)...anyways as you mentioned all grounded items would be at the leakage potential.......be they taps or pipes.......your words not mine....yes?

Sorry DID mean RCD for our pedantic friends......let's not get into a pissing contest here!!.....I call 'em. GFIs....

[Forkinhades]

Sorry DID mean RCD for our pedantic friends......let's not get into a pissing contest here!!.....I call 'em. GFIs....

Pedantic? Electricity is serious stuff, installations need to be installed correctly, otherwise it will kill, either by shock or possibly fire.

If you can them GFIs, which I am sure we would have all understood, why did reference to LCB on 2 occasions?

[doglover]

Unfortunately those little bagged ground wires only seem to protect manufacturers. I know better and not one of the bagged ground wires ended up in use in my house. I did set them aside with the intention of doing it tomorrow.... All to easy to justify to oneself that the RCD/RCBO will do it's job if called upon. Any of you 'electrical guys' ever get around to bonding every appliance with a two prong cord end your homes?

[doglover]

^^^^

Grounding your appliances, I meant.

[Forkinhades]

Take a minute too read these 2 examples I post here.

Example 1 has the kitchen sink bonded (earthed) on a TT system, with NO RCD

Example 2 does NOT have the kitchen sink bonded (earthed) on a TT system with an RCD installed.

You are using an appliance near to the sink that now has a fault to earth, so in effect has become live. So whilst you are holding the LIVE appliance, and are not touching anything that has been earhed (kitchen sink), You will NOT get a shcok, as there is no potential difference. On both systems

However once you touch the kitchen sink on system 1 you WILL get a shock, but on example 2 the RCD will trip, and you will not recive a shock. Given that the kitchen sink will have a value of less than 1667 ohm (highly probable). If the value was higher then you would not receive a shock, as the sink is indeed 'floating' and has no path to earth.

So for all you DIYers you must realise how important it is that an RCD is the first line of defence, and that earthing comes afterwards, and indeed needs to done correctly.

[bbradsby]

for the layperson, please describe what is a 'TT" system, and how to 'bond' items? Also, 99%+ of residentlal water supply piping in LoS is blue PVC, so is of no use for electrical safety.

[Crossy]

for the layperson, please describe what is a 'TT" system, and how to 'bond' items? Also, 99%+ of residentlal water supply piping in LoS is blue PVC, so is of no use for electrical safety.

Everything you need to know about TT and the other grounding systems http://en.wikipedia.org/wiki/Earthing_system

If you don't know what bonding is about it's actually safer not to do it, bonding metalwork that does not require it can actually reduce safety. Ensure you have a decent ground rod linked to the ground terminal of outlets, water heaters and aircons as well as RCD protection on all outlets and you will be fine.

[bbradsby]

thx Crossy, one area of interest in the article:

"The TT earthing system ... can impose added requirements on variable frequency drives..."

As the newer Inverter A/C units have variable drive motors, is anything different required for wiring these loads?

[T_Dog]

Take a minute too read these 2 examples I post here.

Example 1 has the kitchen sink bonded (earthed) on a TT system, with NO RCD

Example 2 does NOT have the kitchen sink bonded (earthed) on a TT system with an RCD installed.

You are using an appliance near to the sink that now has a fault to earth, so in effect has become live. So whilst you are holding the LIVE appliance, and are not touching anything that has been earhed (kitchen sink), You will NOT get a shcok, as there is no potential difference. On both systems

However once you touch the kitchen sink on system 1 you WILL get a shock, but on example 2 the RCD will trip, and you will not recive a shock. Given that the kitchen sink will have a value of less than 1667 ohm (highly probable). If the value was higher then you would not receive a shock, as the sink is indeed 'floating' and has no path to earth.

So for all you DIYers you must realise how important it is that an RCD is the first line of defence, and that earthing comes afterwards, and indeed needs to done correctly.

Although I agree that RCD protection combined with effective grounding techniques is a good approach, this post has some misleading information. The amount of current required for a fatal outcome varies from person to person, but I can guarantee from personal experience that a path resistance of 200,000 ohms will give one quite a shocking experience. 240 volt systems in Thailand need to be respected as electau is trying to point out, and the energy that could injure is four times that of a 120 volt system.

[Forkinhades]

Although I agree that RCD protection combined with effective grounding techniques is a good approach, this post has some misleading information. The amount of current required for a fatal outcome varies from person to person, but I can guarantee from personal experience that a path resistance of 200,000 ohms will give one quite a shocking experience. 240 volt systems in Thailand need to be respected as electau is trying to point out, and the energy that could injure is four times that of a 120 volt system

The shock in amps would have been 240/200000 =0.0012 (1.2 milliamps)

RCDs are normally set at 30 milliamps, although you can get some as low 10 milliamps

The threshold for sensation is 0.001 Amps (1milliamp).

While any amount of current over 10 milliamps (0.01 amp) is capable of producing painful to severe shock,currents between 100 and 200 mA (0.1 to 0.2 amp) are lethal. Currents above 200 milliamps (0.2 amp),

while producing severe burns and unconsciousness, do not usually cause death if the victim is given immediate attention. Resuscitation, consisting of artificial respiration, will usually revive the victim.

The actual resistance of the body varies depending upon the points of contact and the skin condition (moist or dry). Between the ears, for example, the internal resistance (less the skin resistance) is only 100 ohms, while from hand to foot is closer to 500 ohms. The skin resistance may vary from 1000 ohms for wet skin to over 500,000 ohms for dry skin.

Would be interesting to note how you received this shock, and how you was informed that the resistence was 200,000 ohms? maybe a nail in the wall attached to an earth wire that was connected to a shower heater?

All this really does highlight the fact that the earth must be tested, and on a regular basis, together with the RCD.

An earthing system that has a value of over 200 ohms is considered unstable, and I would certainly not want a value as high as that. You should be looking for a value of bewteen 3 and 20 ohms.

A Fluke 1654B tester is one that I personally use, and is available for around 50,000 baht http://www.fluke.com/Fluke/m3en/Installation-Testers/Fluke-1654B-Multifunction-Installation-Tester.htm?PID=72324

[Forkinhades]

I would also just like to mention a job that I done quite recently back in the UK.

The job was do a periodic test on an Factory installation, which was indeed quite old, say 1950's. The system was a TNCS system, 3 phase. On doing an insulation resistence test the reading was 0.2M ohms, bewteen L1 (phase) and earth. The system was working fine, and nobody was getting shocks of any discription. With the installation being quite old, there was a lot metal containment etc etc. As already stated the system was working fine, BUT the UK regs state that all insulation readings, should be of at least 0.5 M ohms. The fault was on the main cable, and because it did not comply, the cost was £16,000 to replace it!

Edited December 3, 2012 by Forkinhades

[electau]

With an earth resistance should not exceed 1 ohms from any part of the electrical installation earthing system to the main earth connection. This is a guide only. In practice it will be much less less than 0.5ohms depending on the impedance of the conductors.

There is no protection if you contact a live terminal, bare wire etc.( direct contact) (In certain cases if an RCD is installed on a final sub circuit it will provide protection by disconnecting the supply).

Earthing provides the primary method of protection, an RCD provides additional protection to meet the auto disconection requirements.

If there is NO earthing and the fault current will flow through your body hand to hand or hand to foot or both and the result may be severe injury or cardiac arrest. You may receive up to full line voltage. An MCB will not trip. If an RCD/RCBO is installed to protect the circuit it will trip in less than 0.4 secs, generally 20msecs and you MAY experience a slight shock only. The RCD has protected you.( indirect contact).

With earthing system intact and a earth fault occurs you will most probably not receive shock and MAY receive a mild shock not exceeding 50VAC. Voltages less than 50VAC are not regarded as hazardous. The protective device must auto disconnect and if the impedance is too high RCDs must be employed, eg TT systems.

With residential/domestic type installations earth bonding may be carried out on the steel reinforcing ( re bar) at one point ( during construction if possible) and the metallic water pipe where in enters the building at one point. Bonding conductor to be the same size as the main earth. Thailand tends to use PVC pipe for water so you can dispense with this connection.

Edited December 4, 2012 by electau

[electau]

Your chances of being revived with a cardiac arrest is 1% with CPR commenced immediately and 15% with a defibriilator.

Any currrent through over 10mA is hazardous and disconnection must occur in less than 0.4 secs. That time period is important.

The effects of leakage current will be minimised by the earthing system.

Edited December 4, 2012 by electau

[electau]

Take a minute too read these 2 examples I post here.

Example 1 has the kitchen sink bonded (earthed) on a TT system, with NO RCD

Example 2 does NOT have the kitchen sink bonded (earthed) on a TT system with an RCD installed.

You are using an appliance near to the sink that now has a fault to earth, so in effect has become live. So whilst you are holding the LIVE appliance, and are not touching anything that has been earhed (kitchen sink), You will NOT get a shcok, as there is no potential difference. On both systems

However once you touch the kitchen sink on system 1 you WILL get a shock, but on example 2 the RCD will trip, and you will not recive a shock. Given that the kitchen sink will have a value of less than 1667 ohm (highly probable). If the value was higher then you would not receive a shock, as the sink is indeed 'floating' and has no path to earth.

So for all you DIYers you must realise how important it is that an RCD is the first line of defence, and that earthing comes afterwards, and indeed needs to done correctly.

That shock ( touch) voltage must not exceed 50VAC in practice it will be far less than that with a compliant earthing system, disconnection time must be less than 0.4 secs.

Earthing first, then fit RCDs. Earthing is the primary protection.

Edited December 4, 2012 by electau

[Forkinhades]

With an earth resistance should not exceed 1 ohms from any part of the electrical installation earthing system to the main earth connection. This is a guide only. In practice it will be much less less than 0.5ohms depending on the impedance of the conductors.

Of course, but we are talking about combined values on a TT system, that information will only confuse the laymen here.

In the UK it is mandatory that a main RCD is fitted on a TT system, the earthing arrangements come after.

I am only talking about TT systems as this is Thaivisa, and we at most in Thailand, where 95% of domestic properties have a TT installation.

[Forkinhades]

Here is another point to consider.

In 2008 The UK introduced the 17th Edition, which REMOVED the regulation for bonding a sink! and highlighted the need for RCDs on final circuits in wet areas, and power circuits etc etc (PME systems). TT system would still require a main RCD.

With that fact in mind, what would you need to bond in a dwelling in Thailand? I am struggling with this, as the water is done in plastic, I have not seen any gas, and whislt the building earth could be earthed, in theory the connection would be within the fabric of the building and thus beyound the reach of touch. Personally I would do this on a TT system just to get the reading as low as possible, and in turn connected to the main earth rod.

If you was to bond parts of your dwelling that you felt appropiate, then I would suggest using 16mm cable which has a resistence of 0.0023 ohms per metre, which in turn will allow just over 20 meters in order to stay below 0.05 which is great for PME systems. However most systems in Thailand are TT, so this does fall down as it is near on impossible to get the low readings that a PME system would offer.

All final circuits should have an CPC (Circuit Productive Conductor),

PME groups together these systems TNS, TNCS, TNC http://en.wikipedia.org/wiki/Earthing_system

[linno2]

Boy, I hope the Thai sparky has a handle on all this! He has not started working yet but is a polytech college graduate from Korat with a good reputation. I will see if he has any ideas about the grounding and TTs RCD etc. Before the house commenced ist lifting off the ground as flood protection we didn't have earth as the laptop tingled on its metal parts. Always worried me with washing machine and hot water heater etc. Thanks Electau and Forkinhades for the info and advice which I will work hard at getting installed.

Cheers!

[Forkinhades]

Boy, I hope the Thai sparky has a handle on all this! He has not started working yet but is a polytech college graduate from Korat with a good reputation. I will see if he has any ideas about the grounding and TTs RCD etc. Before the house commenced ist lifting off the ground as flood protection we didn't have earth as the laptop tingled on its metal parts. Always worried me with washing machine and hot water heater etc. Thanks Electau and Forkinhades for the info and advice which I will work hard at getting installed.

Cheers!

Quite simple really, just make sure he runs a 3 core to all final circuits, the extra cost is minimal, 16mm to the earth rod, and installs a main RCD. Also ask to WITNESS the earth test!, and advise us here what the reading is. Would love to see the look on his face, when you ask that, but if he can do it, then he is surely worth his money on that alone.

I would recommend that you install a 100mA Main RCD,

30mA RCBO's on the final circuits that are critical, this will stop nuisance tripping.

Advise here what you are going to install, and I will gladly advise back.

Will also point out that the above is one of many different configurations you can have, it all comes down to cost, and indeed what you require.

best regards

ps you can always pm me

Edited December 5, 2012 by Forkinhades

[linno2]

Very much appreciated! I will be back from work in Pakistan this coming Sunday - sparky to start work next week so I can make sure all happens properly. Will keep you posted and take up your generous offer to comment on my discussion, findings etc

cheers!

[Forkinhades]

Very much appreciated! I will be back from work in Pakistan this coming Sunday - sparky to start work next week so I can make sure all happens properly. Will keep you posted and take up your generous offer to comment on my discussion, findings etc

cheers!

more than welcome