Which Earthing System Do I Have?

[stgrhe]

In my house that is currently under construction in Pranburi, the development has just got its 3-phase transformer connected. I am a bit confused, and although the developer has asked PEA for information, I am unsure of what earthing system they are providing.  

When looking at the power cables it appears to be only three wires, which would indicate to me that it is a delta network without both neutral and earth.

This picture shows how our branch is connected to the main branch down the Petchkasem Road with a three-wire system leading to our transformer at the development.

From the new transformer they have run four cables across to a temporary pole with the main breaker and the meters. The forth cable must be neutral but how do I know whether it is PEN/MEN, i.e. earthed, or not?

It does not seems to be a proper TN-C system to me as I have no idea of whether there is any earthing somewhere. Could it be that PEA is installing some home made TN-C derivate?

My concern is whether I shall put in a MEN-link in the distribution board, thus connecting the neutral to the earth bar or not?

The meter (temporary outdoor version) looks like this

[Crossy]

The 3-wire system on the big insulators is your 25kV supply, normally does not use a neutral

Your 3-phase 4-wire will certainly have the neutral grounded at the Tx.

The Thai version of MEN relies on multiple properties having earth spikes and MEN links, difficult to tell if you cant access the other properties on your feed, only real way to know is to ask PEA (good luck).

Whilst new installations are supposed to be MEN I would NOT install a MEN link at this stage, provided you have RCD protection you'll be fine running as a TT system until you can confirm TN-C / TN-C-S

You are very close to a big Tx, I would be worrying about potential fault current exceeding the rating of your incoming breaker, are there any PEA fuses between you and that Tx?

[stgrhe]

 

The 3-wire system on the big insulators is your 25kV supply, normally does not use a neutral .

Your 3-phase 4-wire will certainly have the neutral grounded at the Tx.

Noted!

The Thai version of MEN relies on multiple properties having earth spikes and MEN links, difficult to tell if you can't access the other properties on your feed, only real way to know is to ask PEA (good luck).

OK, I will try to find out. Are you saying that if PEA have put in earth rods on the consumer side of the transformer it is a MEN installation and then I shall connect the neutral to the earth bar in the DB?

Whilst new installations are supposed to be MEN I would NOT install a MEN link at this stage, provided you have RCD protection you'll be fine running as a TT system until you can confirm TN-C / TN-C-S.

All circuits will have their separate RCBO, ABB type DS271 (30mA with 10kA breaking capacity), which is a single wire only that doesn't break the neutral in the event of an earth fault.

You are very close to a big Tx, I would be worrying about potential fault current exceeding the rating of your incoming breaker, are there any PEA fuses between you and that Tx?

The cable distance between the meter and the DB is 100 metres. As I said, there is a fairly large breaker at the PEA side before the meters but I have actually not looked closely enough to see whether they have installed any HRCs.

My MCCB is the ABB type T1max B-version that has a 160A uniterrupted current rating (lu) with a 40A thermomagnetic limiter (TMD). The limiter can be set to 0.7 - 1.0 of 40A, which means the thermomagnetic cut-off can be adjusted between 84A and 120A. The breaking capacity (luc) is 25kA for this MCCB version.

Does this provide you with a clue of whether I shall be needing further protection?

[BlackGrape]

Crossy

3 wire, 4 wire!!! Get a loop impedence tester ur sorted!

[david96]

One should verify that the neutral terminal of the distribution transformer is earthed by observation.

The PEA will earth the transformer tank but that is not earthing the neutral terminal.

The distribution neutral should be earthed at regular intervals on the supply network.

You should also check with the PEA and the developer.

The distribution neutral may well be earthed - at some point remote from the transformer and at only one point.

(It is the PEA who should be advising you on what earthing system to use)

[david96]

A 300KVA 240/415V transformer has a PSC of about 13900Amps. This is at the terminals of the transformer and is equivalent to a bolted short cicuit fault.

Your transformer is 220/380V and the PSC would be less by about 10%.

The PSC at the meter would probably be about 9000A to 10000A.

The PSC at the main switchboard would be less than 6000A based on size of conductors and their route length.

A PSC of a one phase to neutral fault is of course much lower.

(PSC- Prospective Short Circuit Curent ).

The PEA circuit breaker will have the fault current it is rated at on its nameplate, look for the 380V rating in kA.

[david96]

A 300KVA 240/415V transformer has a PSC of about 13900Amps. This is at the terminals of the transformer and is equivalent to a bolted short cicuit fault.

Your transformer is 220/380V and the PSC would be less by about 10%.

The PSC at the meter would probably be about 9000A to 10000A.

The PSC at the main switchboard would be less than 6000A based on size of conductors and their route length.

A PSC of a one phase to neutral fault is of course much lower.

(PSC- Prospective Short Circuit Curent ).

The PEA circuit breaker will have the fault current it is rated at on its nameplate, look for the 380V rating in kA.

Should be 500KVA not 300KVA

300KVA has a PSC of about 8300Amps and at the Main switch board the PSC would be about 4000Amps.

My error.

[stgrhe]

 

The PEA circuit breaker will have the fault current it is rated at on its nameplate, look for the 380V rating in kA.

The transformer's rating is 25OkVA and there are no HRCs fitted by PEA, I checked that yesterday. I shall today check the rating of the PEA circuit breaker.

The reason for my initial question is because neither the developer nor the PEA representative have been able to come up with any satisfactorily answers to my questions on which earthing system they are providing. It is really a shame how unprofessional these PEA people are.

[stgrhe]

The fault current for 200-230V A/C is 85kA and 50kA for 380V A/C according to the plate on PEA's main breaker. This breaker is however an provisional one since only three users, and their meters, are connected at this time. When the development is fully built there will be between 13 and 15 houses.  

PEA Main Circuit Breaker

The transformer has its body earthed and so are some other parts up top, but I cannot see whether the neutral point of the star actually is earthed or not, it doesn't appear to be so. In that case I suppose it is a TT system and if so I might have a problem at hand since all my RCBOs and the MCCB only switch the active cores. Any suggestions please?

Earth wires at the transformer

[elkangorito]

I have found a document that leads me to believe the impedance of a 250kVA transformer is 4% (see attachment at bottom of page).

PSC calculation.

Find full load current of transformer.

P = 1.732 x E x I

I = P / 1.732 x E

= 250000 / 658.16

= 380 amps.

Find the transformer impedance.

Z = E (short cct volts at secondary) / I

= (4% x 380v) / 380A

= 15.2v / 380A

= 0.04 Ohms.

Find the PSC at the transformer terminals.

I (short circuit) = E / Z

= 380 / 0.04

= 9500 amps.

It must also be pointed out that these calculations are based on an "infinite supply capacity", which will not be the case in reality. As a result, the PSC will be smaller than 9500 amps.

If the distance from the transformer to your Consumer Unit is considered, the cable impedance will further reduce the PSC.

The conclusion is that a 10kA Main Circuit Breaker is more than adequate for your needs with the existing transformer. As a

matter of fact & now knowing the size of the tranny, I would say a 6kA Main Circuit Breaker would have been adequate.

There is only one possible problem...

You say that the transformer will ultimately supply 13 to 15 homes. Let's take the worst case scenario of 15 future homes.

The FLA (full load current) of the existing transformer is 380 amps. 380 divided by 15 equals about 25. That's 25 amps per

phase for each home, assuming that the homes will be supplied with 3 phase & of a similar size to each other. If this is the

case & each home only uses single phase equipment, each home will have about 75 amps to play with (25 x 3).

If the maximum capabilities of the transformer are exceeded or approach this limit, the transformer may need to upgraded. Of

course, this will affect the PSC if this should happen.

From your pics, I can't see that the neutral has been earthed. As a matter of fact, it looks very much like most of the transformers I've seen in Thailand...no earthed neutral. I wonder if they really meant it to be an IT system?

BTW, the PEA breaker is rated for 50kA at 380v.

Other info below.

PEA_M_005A_2009.pdf

Customer Service Standards - PEA & MEA.

http://www.eppo.go.th/power/FF-E/pw-reform-2-appx1-E.html

[stgrhe]

 

There is only one possible problem...

You say that the transformer will ultimately supply 13 to 15 homes. Let's take the worst case scenario of 15 future homes.

The FLA (full load current) of the existing transformer is 380 amps. 380 divided by 15 equals about 25. That's 25 amps per

phase for each home, assuming that the homes will be supplied with 3 phase & of a similar size to each other. If this is the

case & each home only uses single phase equipment, each home will have about 75 amps to play with (25 x 3).

Elk, thanks for the PCS clarification, much appreciated.

Yes, all houses in the development will be provided with 3-phase supply. The two that are already built were originally using single phase supply but prepared for 3-phase as I have been told.

Apparently the max number of house will be 13 since four plots will be merged into two larger plots of about 1 rai each.

Now, if the neutral remains unearthed at the transformer, my MCCB and RCBOs will only switch the active, is there anything additional I need to observe apart from the obvious of ensuring that the polarities are correct throughout the installation?

[david96]

Transformer neutral not earthed.

I have never seen a neutral of a transformer earthed in Thailand. I suspect that they use the IT system annd this will explain the poor earthing conditions of Thai electrical installations. May be this why the PEA cannot tell you what earthing system they use, they just do not know.

Your earthing system should remain TT.

Reversal of polarity should never occur if testing is carried out in the correct manner.

[stgrhe]

Transformer neutral not earthed.

I have never seen a neutral of a transformer earthed in Thailand. I suspect that they use the IT system annd this will explain the poor earthing conditions of Thai electrical installations. May be this why the PEA cannot tell you what earthing system they use, they just do not know.

Your earthing system should remain TT.

Reversal of polarity should never occur if testing is carried out in the correct manner.

David, please explain what you mean by "Your earthing system should remain TT." How can the system be a TT if the neutral of the transformer is not earthed? Am I missing something?

Your earthing system should remain TT.

This is a TT system, right?

and this is an IT system.

[Crossy]

That ^^^ is my interpretation too, TT has the neutral grounded at the Tx (and nowhere else), there is no N-E link.

IT has no grounding of the neutral anywhere.

Our Machines and Control lab at uni had a 3-phase IT supply, the lab-tech demonstrated its 'safety' by regularly grasping one of the phase conductors and not dying.

[bkkbill]

^^^^ Now that safety tip would be burned into the memory ^^^

[Crossy]

^^^^ Now that safety tip would be burned into the memory ^^^

It was when on one occasion (the last) there was a phase-earth leak (luckily a high resistance one) caused by a dodgy mercury-arc rectifier resulting in the phase he got hold of being at 380V above ground !!!!!!! I learned some new and interesting words at the tender age of 19.

Thus is demonstrated that an IT system can introduce a false sense of security, a leaky device somewhere can lead to lethal voltages elsewhere.

Grounded neutral in one of its many forms is by far the safest system under most fault scenarios.

[stgrhe]

So apparently my installation is an IT system. One final question, should a MEN link still be used or in such a system? I would appreciate an explanation to the answer so I can learn something.

[Crossy]

So apparently my installation is an IT system. One final question, should a MEN link still be used or in such a system? I would appreciate an explanation to the answer so I can learn something.

No, you should not install a MEN link, to do so would ground the neutral and convert the system to TT via your (inadequate for a TT main ground) earth stake.

[stgrhe]

So apparently my installation is an IT system. One final question, should a MEN link still be used or in such a system? I would appreciate an explanation to the answer so I can learn something.

No, you should not install a MEN link, to do so would ground the neutral and convert the system to TT via your (inadequate for a TT main ground) earth stake.

Thanks a lot Crossy!

[elkangorito]

Transformer neutral not earthed.

I have never seen a neutral of a transformer earthed in Thailand. I suspect that they use the IT system annd this will explain the poor earthing conditions of Thai electrical installations. May be this why the PEA cannot tell you what earthing system they use, they just do not know.

Your earthing system should remain TT.

Reversal of polarity should never occur if testing is carried out in the correct manner.

Hi David.

I have personally seen a TN-(C-S) system in use in North Pattaya. I believe Monty lives nearby the location. The particular transformer that I saw, had the neutral solidly earthed & the neutral was earthed at every pole (or every 2nd pole...I can't quite remember).

I was going to take photos this weekend, of transformers in my local vicinity but I forgot to ask my friend for a lend of her camera.

There is only one big problem with the IT system...it relies upon the electrical insulation to remain intact. As such, monitoring should be used to trip the supply in case of insulation failure. These devices are generally very expensive.

[stgrhe]

There is only one big problem with the IT system...it relies upon the electrical insulation to remain intact. As such, monitoring should be used to trip the supply in case of insulation failure. These devices are generally very expensive.

It's time to start saving then. Compromising with electrical safety is not my thing.

I will actually discuss the matter with PMK when it is time to pick up the cabinet at their office in Nonthaburi. Actually, the specially designed DP is already ready but since the house construction is late it is too early to pick it up. Furthermore, the surge arresters I have ordered for the telephone line and satellite antenna are not available yet.

I am very pleased with PMK's professionalism.

[david96]

The IT distribution system may well be used in Thailand and in many other countries such as South Korea, North Korea, Manchuria (Northern China) Taiwan as a result of Japanese influence in the region prior to 1945.

Thailand has been influenced by the engineering standards of Japan.

Transformer neutral not earthed, all insulated conductors, double pole fuse switches or circuit breakers on switch boards and direct earthing used where earthing is implemented.

(The only time that one has been involved with an IT system is in a hospital environment where it is used in Operating Theatres, ICU units and CCU areas. Line Insulation Monitors (LIMs) are installed to give an alarm only and indication by bar graph LCD).

[stgrhe]

There is one more question that nobody has shine a light on. My system is apparently IT buy my MCCB is a 3-pole only that doesn't switch the neutral and all RCBOs are single pole.

Will this cause a problem?

[elkangorito]

It should not be a problem but Active & Neutral polarity must still be observed & Earth Fault Loop Impedance is very important with regards to cable length of therefore operating time of a circuit breaker (not an RCD).

It's interesting to see what Schneider says about the various distribution systems. See the attached document.

XXXX=Excellent XXX=Good XX=Average X=Caution

It's also interesting to note that Thailand does utilise a relatively safe system but does not implement it correctly as follows:

1] No polarity testing.

2] Wiring colour code is not according to "the rest of the world" (this is a minor problem).

3] Wiring colour code is often ignored or unknown by Thai electricians.

4] No "earth fault loop impedance" testing.

5] No insulation resistance testing.

6] A lack of understanding of earthing principles.

7] Initial inspection of a "new" installation appears to be based upon neatness. To my knowledge, Thai electrical inspectors do not carry out any tests & nor do they require any test certificates from electrical contractors.

8] A lack of knowledge of the appropriate use of RCD's (a maximum 30mA trip current for personal protection).

9] A lack of knowledge of the purchase of appropriate RCD's (RCD's that can be adjusted to currents greater than 30mA are NOT for personal protection).

The above is my "short list".

It's only taken 4 years to realise that the "prevailing" distribution system in Thailand appears to be IT & not any other type, disregarding the TN-(C-S) system, which I have seen in use in Pattaya. For expats who can only use Thai electricians, this could be a big problem.

The "technical education" methods here, in my opinion, are greatly responsible for the current problems experienced by many. Unfortunately, an electrical problem that one may experience in Thailand is sometimes one's final problem, since death can quite often ensue such a problem.

[Crossy]

Sorry Elk. can't open the Excel file, comes up as corrupt

[elkangorito]

I created the file with OpenOffice. If you rename the extension to .ods it might open.

The original file name was Dist_Sys_comparison.ods

[elkangorito]

Errrr, sorry. I made another boo boo. It's a "document" not a spreadsheet. I'll attach it again below & delete the above attachment.

Dist_Sys_comparison.doc

[david96]

Elk, regarding your post number 24 I for one must agree with you 100%.

Very well written.

[david96]

If an IT system is used installing a MEN link changes the characteristics of the LV supply.

Instead of their being a high impedance to earth (no connection) there is now a low impedance to the supply neutral on the installation that has the MEN link and other consumers who may have direct earthing or no earthing at all may have a reduced impedance to earth, which, if RCDs are not installed could be hazardous in the event of an earth fault in other installations supplied by the transformer.

If an adjacent consumer has an earth fault it will travel though the general mass of ground to the main earth up the main earth, though the MEN link though the neutral to the transformer.

And that impedance ( to the adjacent consumer) could be in ohms what a TT system would be. The consumer with the MEN would not be affected.

[stgrhe]

If an IT system is used installing a MEN link changes the characteristics of the LV supply.

Instead of their being a high impedance to earth (no connection) there is now a low impedance to the supply neutral on the installation that has the MEN link and other consumers who may have direct earthing or no earthing at all may have a reduced impedance to earth, which, if RCDs are not installed could be hazardous in the event of an earth fault in other installations supplied by the transformer.

If an adjacent consumer has an earth fault it will travel though the general mass of ground to the main earth up the main earth, though the MEN link though the neutral to the transformer.

And that impedance ( to the adjacent consumer) could be in ohms what a TT system would be. The consumer with the MEN would not be affected.

I am not sure if I fully understand what you are saying David, are you suggesting that I should install a MEN-link despite having an IT system? Crossy suggested not!