TT System 400 Volts To Earth

[ck_shekel]

Hi all.
First time on the forum, but have enjoyed reading many ofthe threads, thanks. I am looking for some advice on a fault from a TT systemand would greatly appreciate any input.
History, my wife and I are volunteering in Africa for a fewyears. The charity I am helping does not have an electrician and we have been askedus to stay for as long as possible. The support I have here for this type ofproblem is limited. Our site is fed from our own 2 X 200KVA Transformers with abackup 265KVA Generator (With load shedding). We have 2 apartments off site,but within 60 to 80 Meters, which is fed by a separate Transformer owned by thesupply authority. This Transformer feeds around 40 other homes within thecomplex.
When I arrive a cable had been ran from our site to the apartmentsand 2 auto change over units installed with Over & Under Voltage Relays togive the apartments a supply via us when the city power goes down. They used a4 Core Cable.
First thing I did when we arrived was to undertake anInspection of all our properties. These apartments had no earth, none, no earthrod and only RCD protection on the Fridge Sockets. We replaced the DB’s withinthe apartments (4 of including 300m Amp Time Delay & 30m Amp RCD’s) andinstalled 4 of 2.4 Meter earth rods per apartment. When supplied via ourTransformer the reading of Ze is 2.24 Ohms, which is the best reading I havehad on a TT system (I am using my Meggar Multi Tester which I bought in the UK,this does not have the Earth Rod option on it so readings are taken using theEarth Loop Function). When supplied via the Supply Authority the reading fluctuatesbetween 90 and 100 Ohms at the time of installation. I contact them and askedthem to check their earthly arrangements, not sure if they did this though. Iam checking the earth rods every 3 months and on Friday the following voltages weretaken on the system: -
Supply Authority=
L1-L2=404, L1-L3=403, L2-L3=404
L1-N=237, L2-N=235, L3-N=235
L1-E=394, L2-E=398, L3-E=8
N-E=225
Our Transformers=
L1-L2=406, L1-L3=407, L2-L3=411
L1-N=231, L2-N=233, L3-N=234
L1-E=233, L2-E=233, L3-E=232
N-E=1
Ze=
L1= 2.24 Ohms
L2= 1.93 Ohms
L3= 1.91 Ohms

Could the supply neutral maybe burnt out within thetransformer or their Main Panel and the fact that the loads maybe be close to balancedacross the 40 to 50 homes fed from the Transformer has stop anyone noticing thefault? I am sure that the other homes do not have earth rods.

This still does not explain the readings from Line to Earth,unless they have an earth rod connected to the star point in the Transformer?

Any ideas would be greatly appreciated. Thank you.

 

[Sintra]

Looking at those readings from the supply authority the earth rod is at the same or close to the same potential as L3.

 

[Marvo]

Looks like the supply authority neutral is just floating toward the potential of the phases with the highest load. The supply authority needs to fix their broken neutral or reinstate their neutral-earth bond. Not sure why nobody has noticed, this is the kind of fault that usually results in skips full of blown appliances.

 

[Knobhead]

L3 is to earth and the neutral floating on the utility transformer or intake panel. Looks like L3 and N have been swapped.
If you get a N → E fault you have no O/C protection.

Run on your transformers until it is sorted out. to say it’s dangerous is an understatement.

 

[ck_shekel]

Hi. Thank you for taking the time to respond.

I had a look in one of the other apartments and its the same as ours was before I arrived here, there are earths to all the sockets (None to the lights) but there is no earth rod and no RCDs installed.
I have seen this so many times since I arrived here, homes have Class 1 equipment into sockets with earths back to an MET but no Earth Rod and never an RCD. 1 item gets an earth fault and the whole earthly system lives up. Its frighning.
With no earth in these homes here I think it has only been pick up because I installed the earth rods. Hopefully we should get the company to attend site this week although I am not confident. I have disconnect the change over switch and are feeding direct from our Transformer.
Thank you once again for the info. Regards

 

[ck_shekel]

Hi All.

Just to give you an update.

On Thursday and Friday last week I got access to the Main Switch Room for the complex as it turns out the company looking after the apartments have been getting complaints from some of the other homes, but don't have an electrician to investigate the problem. After isolating 1 block at a time the fault cleared once I isolated one of the blocks. I got access to around 15 of the homes on that block and at the last home (Had to be the last) the fault cleared once I isolated the main switch. Traced the fault in the home to an A/C Unit which compressor was faulting to earth. The Ze at the board was giving around 294 Ohms, since no RCD, only around 0.6 of an Amp was flowing to earth.

I did a little experiment. Went to our apartment and using a 400 Volt lamp set (The old supply authority type) I tested between line to earth. With the A/C Unit in the other apartment disconnected I got the low glow from the lamps, as soon as the A/C Unit was turned on I got the bright 400 Volt glow from the lamp set. This home is about 125 meters from our apartment but the fault on their system presents on our earth bar. In the 20 years I have been working as an electrician I have never seen this.

Just a thought, if someone made contact between line and earth in our apartment, they would receive 400 Volts. We have 300m Amp Main RCD and 30m Amp on all final circuits, but would the RCD afford sufficient protection?

Thank you once again.

Regards

 

[Darkwood]

RCD's do not monitor external fault conditions they look for an imbalance between current down the L and current returned down the N on the circuit/supply they monitor, current flow piggy backing your earthing would not be seen if from a different unmonitored supply ... the other faulty appartment block supply would require its own monitoring and protection.

 

[ck_shekel]

Hi darkwood.

Yes agree with that, we will be sending a letter the all the owners urging them to install RCDs, but I doubt this will happen. If someone in our apartment made contact with L1 or L2 to Earth they would basically, with this fault, be making contact across 2 phase and the potential would be 400 Volts. Our RCD would disconnect when it sees the imbalance, if its greater than 30m Amp, but since under normal conditions the RCD is designed to protect with Line to Earth voltage of 230 Volts what damage would be done to a human under this situation when earth is now become phase. Could the shock received be fatal?

 

[Darkwood]

If your seeing 400v to earth you have a high impedance earth set-up and the floating voltage present is on another phase to which your checking your supply, Its uncommon but ive seen external faults piggy back onto an adjacent property before making all the bonded metalwork live even when the cutout fuse was not prese, are these apartment blocks private owned or tenancy set- up?

If the latter then the landlord or council has a role of responsibility to ensure the safety of the occupants if private the the DNO may need to be contacted but they themselves do not have to provide an earth for a TT system that's down to the user, but they may consider converting to a pme set up given the danger to life.