Grid Vs Cu

[bankruatsteve]

OK - so Crossy doesn't want us to get off topic in other threads and why I start this one.

There have been MANY posts mentioning the grid system that is in use/should be in use/etc. TT/IT/MEN etc. wrt where grounding occurs.

To my thinking the more grounding there is in the grid (like MEN) the "healthier" the system - but I suppose there are pro/con for any of the systems and I've never really gotten into how the "grid" really works.

So, for me, the CU (the local consumer home system) is what we can control. And, for that, I don't see what diff it makes the grid system. For ground fault protection, what diff does it make if G is connected to N? RCD don't care about ground but some devices like GFI do. What diff does it make the grid or the G/N connection?

C'mon Elk/Electau/Crossy whoever - this could be something to get into when you get bored.

Cheers

[InterestedObserver]

For new construction or rework use three-prong receptacles and run the ground, green/green-yellow wire, to a nice big ground rod (stake) in the yard. Use a RCD (RCCB) wherever possible, especially for all circuits located in wet or damp locations.That should get us started.

Edited August 8, 2010 by InterestedObserver

[bankruatsteve]

OK - thanks. Actually, in this thread I was looking more for - you know sitting around the table with a beer or whatever and discussing more like the technology than the how to of - well, what I was talking about in my OP. Cheers.

[InterestedObserver]

In the US, domestic electrical installations are configured as Multiple Earth Neutral (MEN) connections. The source transformer neutral is grounded along with a ground rod installed at the residential service entrance. The neutral and ground conductors are bonded (linked) together at the service disconnect.

Edited August 8, 2010 by InterestedObserver

[powderpuff]

I don't claim to be an expert though I did wire up my house.

If Neutral MUST be earthed as you claim why then would we need 2 wires coming into our house?

Live & ground would be all we would need.

I know Crossy talks about running a wire between ground & neutral in your CU or load center as I prefer to call it. He tells neophytes not to do it.

I'll stay with my discreet 3 wire system.

I am with electau.

[electau]

International LV Power Distribution.

Unfortunately in the absence of a common international power distribution system, it is important to understand that there are significant differences in the way power is distributed within many countries.

Here are the five most common variations.

The TN-C System has the neutral and protective functions combined in a single conductor throughout the system.

The TN-S System has separate neutral and protective conductors throughout the system. This system is used in 230/400V systems throughout Asia.

The TN-C-S System has the neutral and protective functions in part of the system. This is known as the MEN (Multiple Earth Neutral) System in Australia and New Zealand. The neutral is connected to earth at the main switchboard.

The TT system has the exposed conductive parts of an installation connected to earth electrodes electrically independant of the earth electrodes of the source.This system is also used in 230/400V systems throughout Asia.

The IT system has no direct connection between live parts and earth, the exposed conductive parts of the installation are earthed.

With the TN-C-S (MEN) system the earth fault path is from the point of the fault through the PE conductor to the earth bar at the switchboard through the MEN link to the main neutral conductor and to the source of supply. This is a low impedance earthing system

With the TT system there is NO link at the neutral and earth bar, in this case the fault path is not through the main neutral but returns to the source of supply by way of the main earth conductor, earth electrode and the general mass of earth to the source of supply. In most cases this resistance will never be low enough to allow a protective device such as an MCB to operate. An earth fault can cause the earthing system to rise to a dangerous potential ( over 50VAC) For this reason RCDs must be employed. This is a high impedance earthing system.

The IT system is also a high impedance earthing system and RCDs are required for protection from earth faults.

With the MEN system the current in the main earth conductor and electrode is minimal and can for practicable purposes be ignored. In the event of the main neutral failing or becoming open circuited the main earth will minimise the touch voltage to earth.

　

　

I

[Crossy]

Thanks Electau saved me typing it all out

The important thing to realise is that many local distribution systems here seem to be IT. IT is not permitted for LV distribution systems in most countries so many of us have no experience of it leading to misunderstanding and confusion.

The big issue is that because the neutral is not grounded at any point, a L to E fault will cause no ground current to flow so neither an MCB nor an RCD will trip, it requires two ground faults (either N-E or another phase-E) before the breaker will open. This is why IT is used in high-reliability supplies in locations such as data-centres (and these places have special ground fault detection systems).

Unfortunately, this additional reliability has the side-effect in the event of a L-E fault of making the neutral 220V above ground and the other two phases 380V above ground, not a happy situation if you touch either.

IMHO, it is safest to assume an installation is TT and use the lowest RCD trip current you can get away with (30mA is a maximum value). Most definitely do NOT ground your local neutral unless you are certain that your distribution system really is MEN configured.

[electau]

The overall impedance to earth on an IT system used as a distribution system will be low enough for an RCD 30ma or less to operate. But in Thailand it should be assumed that supply neutral is earthed at at least one point on the distribution system.

It might explain why RCDs in Thailand have adjustable sensitivity.

Schneider Electric (France) has some interesting information on the subject.

http://www.electrical-installation.org/wiki/Selection_criteria_for_the_TT,_TN_and_IT_systems

Thailand may have justified the use of the IT system before 1940 on the basis of economics,and the use of 2 pin socket outlets. But today they now have problems with the advent of 3 pin socket outlets. Thus they compromise by installing RCDs as the main protective device in the meantime on existing electrical installations.

(Remember Japan is one country that has influenced Thailand in many ways including electrical systems and distribution).

　

　

[stgrhe]

Here are some pictures to electau's explanations above.

TN-C System

TN-S System

TN-C-S System

TT System

IT System

[InterestedObserver]

I don't claim to be an expert though I did wire up my house.

If Neutral MUST be earthed as you claim why then would we need 2 wires coming into our house?

Live & ground would be all we would need.

I know Crossy talks about running a wire between ground & neutral in your CU or load center as I prefer to call it. He tells neophytes not to do it.

I'll stay with my discreet 3 wire system.

I am with electau.

IAW electrical standards, ground cannot be a load current carrying conductor. Ground is a safety conductor to equalize potentials and carry fault current to trip the MCB only. Therefore it's line (live), neutral, and ground conductors.

Edited August 9, 2010 by InterestedObserver

[electau]

I don't claim to be an expert though I did wire up my house.

If Neutral MUST be earthed as you claim why then would we need 2 wires coming into our house?

Live & ground would be all we would need.

I know Crossy talks about running a wire between ground & neutral in your CU or load center as I prefer to call it. He tells neophytes not to do it.

I'll stay with my discreet 3 wire system.

I am with electau.

IAW electrical standards, ground cannot be a load current carrying conductor. Ground is a safety conductor to equalize potentials and carry fault current to trip the MCB only. Therefore it's line (live), neutral, and ground conductors.

The general mass of what is termed ground/earth is inself too high a resistance to trip an MCB on an earth fault, this is the major disadvantage of the TT system. The MEN system does not have any fault current to earth as it returns to the source by way of the neutral conductor.

There is one instance where the ground /earth IS used as a return path and that is the 12.7kV SWER ( single wire earth return ) supply to a single phase 3 wire 240/480V typically 10kVA for rural supplies in some parts of Australia for farms.

　

[bankruatsteve]

Well, this is what I was looking for. Thanks! Esp Electau and Crossy for the good explanations.

As an aside: I don't know what the "system" in the states is called - I suppose it is closest to MEN. But, what the CU gets is 2 of the phase lines with neutral grounded at the 1/2 winding of the transformer giving the 220/110v service of which all socket/lighting are one one side or other and the 220 (no neutral) is only used for dryers, ovens, etc. designed for that. Anyway, that's why N-E is a given there.

And, yes Crossy, I now accept there are some things I don't understand about the grid systems used here. Anyone can help me understand the following?

1. So, with IT there is not even ground to neutral at the source?? What then is relative zero potential the the system?

2. Perhaps answer for #1 will explain this, but since AC "current' just does a little dance (vs DC taking a trip) why do you say it needs to "return to source"?

As usual (for those who know me), I probably would have lost some bets had we all been at the table with a beer in hand but this is "fun" for me.

Thanks.

[powderpuff]

(Remember Japan is one country that has influenced Thailand in many ways including electrical systems and distribution).

Good old Japan where they are running a 90 or 100 volt system.

[InterestedObserver]

1. It's called a 3-wire, single-phase, mid-point neutral or split phase connection. Provides 120/240 for residential and light commercial usage.

2. In electrical power systems, earth is always reference zero potential.

3. If the neutral, or any single conductor is not grounded/earthed then the system will float relative to earth (except for capacitive coupling voltage).

4. You always need a return path for the current to get back to it's source. Electrons flow around the closed path, if you have an open circuit then no current can flow.

Edited August 9, 2010 by InterestedObserver

[electau]

(Remember Japan is one country that has influenced Thailand in many ways including electrical systems and distribution).

Good old Japan where they are running a 90 or 100 volt system.

Japan uses a 100V/170V 50Hz system. They also use 60Hz.

Japan would have influenced electrical design in Thailand between 1920 and 1945. Other countries such as Germany with the old VDE standards.

Thailand adopted the European electrical voltages and frequency 220/380/ 50Hz. The standard socket outlets were the 2 pin European (round pin) and the 2 pin flat blade type as used in Japan and the USA. Fused knife switches were the standard protective device. There was no concept of protective earthing.

Edited August 9, 2010 by electau

[electau]

Earth fault clearance, an example.

A 20 amp MCB standard C curve must trip in less than 0.4secs with a fault current of 7.5* 20A which is 150A. ( AS3000)

The max total earth fault loop impedance must not exceed 1.53ohms and can be easily obtained using the MEN system as the fault current returns to the source by way of the consumers neutral and distribution neutral.

If however the TT system is used or the MEN link is removed ( which converts the system to TT) the earth impedance will no longer be low and may be over 100ohms. In this case the fault current would be 2.3A. which is within the normal operating parameters of the MCB.

To overcome this problem an RCD/RCBO is installed. A RCD will trip in less than 0.3secs on a current of 30mA.( AS3000).

With an IT system as may be used in many areas of Thailand RCDs must be used to clear earth faults as again the impedance to the source neutral is high. The RCDs operate because of the capacitive and general relative impedance of the distribution system itself and all installations connected to it.

　

　

[Crossy]

The RCDs operate because of the capacitive and general relative impedance of the distribution system itself and all installations connected to it.

It's all rather worrying when an unknown and variable factor becomes an essential part of a fault clearance system.

Steve.

The potential relative to ground in an IT system is another unknown, it will actually be 'about' ground due to leakage but you only need a L-E fault to overcome any clamping effect that the leakage provides.

Whilst technically incorrect it's convenient to picture AC as 'flowing' from L to N (electrons really flow from - to + so even our normal picture of DC flowing from + to - is incorrect), there must always a complete path for the current to flow around otherwise nothing useful happens.

As usual (for those who know me), I probably would have lost some bets had we all been at the table with a beer in hand but this is "fun" for me.

Bugger!! missed out on a beer

Edited August 10, 2010 by Crossy

[InterestedObserver]

George Westinghouse and Thomas Edison were perfectly satisfied with conventional current flow from positive (+) to negative (-), the basics of the electrical power industry were defined using conventional current flow direction and it's still used today. Curses on the those electronics types that need actual electron flow direction to make their semiconductors work.

[Crossy]

Curses on the those electronics types that need actual electron flow direction to make their semiconductors work.

But we did invent the "hole" to keep the Conventional Current chaps happy

Besides, without those pesky semiconductors we wouldn't be typing this today

Edited August 10, 2010 by Crossy

[Khonwan]

<snip>The important thing to realise is that many local distribution systems here seem to be IT<snip>

Is that the case? From my observations, at least within Nakhon Sawan province, the neutral appears always to be earthed at frequent points along the LV transmission run - not IT systems. My own system is now like this, with the neutral earthed approximately every 200m. I installed a consumer earth (2.4m copper clad steel rod) many years ago when I built my house. I am still confused, however, as to whether I should create a MEN link in my CU. I did so for a year then recently replaced my MCB with a RCBO, at which time I removed the link as the RCBO instructions seemed to indicate to me. MEN link or not - what do you (et al) think?

Edited August 10, 2010 by Khonwan

[Crossy]

Is that the case? From my observations, at least within Nakhon Sawan province, the neutral appears always to be earthed at frequent points along the LV transmission run - not IT systems. My own system is now like this, with the neutral earthed approximately every 200m. I installed a consumer earth (2.4m copper clad steel rod) many years ago when I built my house. I am still confused, however, as to whether I should create a MEN link in my CU. I did so for a year then recently replaced my MCB with a RCBO, at which time I removed the link as the RCBO instructions seemed to indicate to me. MEN link or not - what do you (et al) think?

If your neutral is grounded every 200m or so then you've got MEN and installing the link will add safety Note that the link must be on the supply side of any earth leakage devices if they are to function correctly.

Unfortunately you can't assume anything here. I've seen IT, TT and TNC-S (MEN) within a few hundred metres within the same village, just depends upon when each Tx was installed

In the event you can't determine your supply type it's safe to assume TT (and install a ground rod and RCD).

Edited August 10, 2010 by Crossy

[bankruatsteve]

Earth fault clearance, an example.

A 20 amp MCB standard C curve must trip in less than 0.4secs with a fault current of 7.5* 20A which is 150A. ( AS3000)

The max total earth fault loop impedance must not exceed 1.53ohms and can be easily obtained using the MEN system as the fault current returns to the source by way of the consumers neutral and distribution neutral.

If however the TT system is used or the MEN link is removed ( which converts the system to TT) the earth impedance will no longer be low and may be over 100ohms. In this case the fault current would be 2.3A. which is within the normal operating parameters of the MCB.

To overcome this problem an RCD/RCBO is installed. A RCD will trip in less than 0.3secs on a current of 30mA.( AS3000).

With an IT system as may be used in many areas of Thailand RCDs must be used to clear earth faults as again the impedance to the source neutral is high. The RCDs operate because of the capacitive and general relative impedance of the distribution system itself and all installations connected to it.

　

　

OK. BUT - if the CU is grounded then impedance to ground is low and can clear earth faults (touch voltage etc.).. I understand that circuits need, well circuits (and if that's a "flow" in AC circuits - I'd like to see that explained.). But faults are different. Maybe I'm too pragmatic but I just can't see the expense and nuisance of RCD circuit protection just b/c some 1/100000000 circumstance might happen.

[elkangorito]

OK. BUT - if the CU is grounded then impedance to ground is low and can clear earth faults (touch voltage etc.).. I understand that circuits need, well circuits (and if that's a "flow" in AC circuits - I'd like to see that explained.). But faults are different. Maybe I'm too pragmatic but I just can't see the expense and nuisance of RCD circuit protection just b/c some 1/100000000 circumstance might happen.

If you want to know about these things, I would suggest you undergo an appropriate course, which will take about 4 years if you also do the required practical content.

If you want quick answers then I suggest you do some quick reading. Start with:

1] Ohms Law.

2] Kirchhoff's Law.

3] Lenz's Law.

Once you're familiar with these, move on to AC theory, which details the vast differences between AC & DC.

The next thing you'll need to accomplish is Impedance (an AC property). Once you have a brief understanding of this, Transformers (AC Machines) would be the next possible step in your 'fast track' education.

From understanding Impedance & Transformers, you will be able to possibly understand a thing called Fault Current.

If you're really game, you could further investigate Temperature Rise & how this affects electrical equipment.

Good luck in your new 'hobby'...& I mean this sincerely.

It's only taken me about 30 years to learn what I know...& I have lots more learning to do.

Your questions are not dumb or stupid but you need to realise that the few of us here on TV that make technical comments, do so from lots of experience. We didn't learn this stuff in 5 minutes. I, for one, do not enjoy explaining electrical theory to people. It's laborious & unnecessary.

[Khonwan]

Crossy, thanks for this – clear to me now; I’ve got TNC-S (which I thought was most likely) and my CU earth should link to my neutral before the RCBO (not to the N-bus-bar as before).

Bankruatsteve, I’m not an electrician…just a natural philosopher like yourself; I believe the problem is that the CU earth (your earth rod) can have very high impedance that may not clear faults fast enough to save your life (I’m sure that you like me have never checked the impedance), particularly in stony ground, or very dry sandy-loam.

[bankruatsteve]

LOL

[eraymos]

I don't know if I should have started a new post with this, but I guess this is as good of a place as any.

When I was trying to figure out what type of Squre D CU I should use I took contact with Schneider Electric Thailand (S.E.T).

I was thinking about Crossy's split service unit. The good people of S.E.T had no idea what I was talking about and wanted me to send them a diagram.

I hope you don't mind Crossy, but I sent them a copy of your split service unit.

We all remember what that looks like. Any how S.E.T sent me back a slightly different configuration with 2 CUs side by side. Note that they have the incoming neutral connected to the ground bar first.

What do you guys think about this?

Edited August 11, 2010 by eraymos

[electau]

I don't know if I should have started a new post with this, but I guess this is as good of a place as any.

When I was trying to figure out what type of Squre D CU I should use I took contact with Schneider Electric Thailand (S.E.T).

I was thinking about Crossy's split service unit. The good people of S.E.T had no idea what I was talking about and wanted me to send them a diagram.

I hope you don't mind Crossy, but I sent them a copy of your split service unit.

We all remember what that looks like. Any how S.E.T sent me back a slightly different configuration with 2 CUs side by side. Note that they have the incoming neutral connected to the ground bar first.

What do you guys think about this?

The first diagram is correct, and would comply with AS3000 and BS7671.

The second diagram does not comply for the following reasons.

1. The Main Neutral must be connected to the main neutral bar not the earth bar

2. The Main Earthing conductor to the earth electrode must be connected to the main earth bar.

3. The MEN link must be connected at only one point, that is between the main neutral bar and the main earth bar.

The third diagram does not comply for the folowing reasons.

1. No earth bar

2. PE conductors connect to the neutral bar.

3. No MEN link.

4. Main earth conductor termination at earth electrode should be readily accessible for inspection and testing purposes.

That third diagram is not very clear with the actual circuit connections.

　

[Crossy]

No problem eraymos, glad the drawings were of use, I've got the original Visio drawings somewhere if you want them

I agree electau, I REALLY don't like the Thai way of linking N and E although that's how it's shown in all the local drawings I have and in the PEA guidance booklet. It makes it very difficult to remove the N - E link for testing purposes

Hmmm, 'testing' and 'Thailand', sadly two words not normally associated

EDIT I'd also add to electau's comment on diagram 2 that there is no single point of isolation. i.e. no one breaker to turn off the entire installation. Whilst not an absolute requirement of BS7671 it is considered good practice (at least in domestic installations) to have one breaker that kills everything.

EDIT 2 Actually, diagram 1 would not comply with BS7671:2008 (17th Edition) as that requires all circuits to be RCD protected, it does however comply with the previous 16th Edition.

Edited August 11, 2010 by Crossy

[stgrhe]

The third picture shows an earth rod of 2.4m length dug 0.3m in the ground. I have previously on TV been advised that the rod, according to AS3000 that is, should stick 0.3m above the ground so clamped earth wire is visible, where the above ground part be painted with acrylic paint.

Can someone clarify this please.

Edited August 12, 2010 by stgrhe

[Crossy]

UK Regs say that the connection to the ground rod must be accessible for maintenance. To protect the rod (and stop people tripping over it) we tend to use an access box so the end of the rod can go below ground level and still be protected.

Like this:-

Edited August 12, 2010 by Crossy