TN-C-S Distrubution Fault Symptoms?

[ElectricsRanger]

Hello to everyone,

A house i am checking has a large amount of Electro Magnetic Fields around the main cables feeding from the electric cupboard.

Naturally i expected it was just normal, but when i checked with a micro Tesla meter it was 5.0 uT right next to the wires, even when we had no current flowing in the system, with all the breakers & main breaker off. Also, the water main copper pipe in our house disappears in to the ground is also showing s higher value uT reading near the ground.

When we have less than 3000w flowing in the whole house we are in the area of 14.00uT.

Typically the field should drop off quickly, with the middle of the room being 0.04uT, but the field is seems to climb above 0.04 two foot from the cables than it should for such little power usage.

So i safely disconnected the main earth bond from the electricity distribution PE, and the fields dropped to 0.01uT as expected, and afterwards only increased as power usage increased. There was a second PE Earth bonding going to a unused Economy7 breaker box, so i know we were not completely disconnected from the PE.

Could the neutral return resistance to the substation be higher than the house earth, or their is a neutral fault even? Resulting in current flowing from our neighbours installation through our earth and back to the substation?

I checked for obvious short between Earth and Neutral, but nothing obvious 0.1R on my standard meter. I just got my MIT320 meter so my resistance readings will be more accurate so im going to perform a full check.

ElectricsRanger

 

[ban-all-sheds]

 

[ElectricsRanger]

Hehe... I recommend no one use a foil hat, they will turn their head in to antenna, and make the problem worse.

Okay Ban-all-sheds, are you a trained electrician?

Should a TN-C-S have its PME earth connected to the outer sheath of the mains cable coming from the substation supply. Seems the house was possibly upgraded, and has a PME and an earth to the outer part of the main supply cable and form of TT earthing by the water main. Would that not screw up the return neutral to the substation, where it should be earthed to the ground?

ElectricsRanger

 

[bernardgreen]

So i safely disconnected the main earth bond from the electricity distribution PE,

Not an easy thing to do and still be safe.

Did you measure the current flowing in the bond wires ? A clamp meter would the best way to avoid disconnecting them.

What is probably happening is that the local 3 phase distribution is not well balanced ( an impossible task at the best of times ) and the supply Neutral has current flowing to the substation. If your water supply pipe is low impednace to ground then some of the current that should be flowing along the Neutral bcak to the substation will be flowing through the bond wires and returning via the wate supply pipe to ground. If the local water supply network is all metal in the street mains then surprisingly high currents can flow along bond wires.

 

[securespark]

If there's an earth connection to the outer sheath, it's not TN-C-S.

TN-C-S has a combined neutral and earth conductor coming into the property.

 

[bernardgreen]

Should a TN-C-S have its PME earth connected to the outer sheath of the mains cable coming from the substation supply.

The supply cable is likely to be in the form of a coaxial cable with the Live as the centre conductor and the combined Earth and Neutral as the outer layer. The separation of "Earth" and Neutral occurs in the cut out next to the main fuse and before the meter.

Older systems did have an earthed outer layer, often lead, with two conductors, Live and Neutral running parallel in the cable. But hat would not be TN-C-S but TN-S as Earth and Neutral are not combined in the network

 

[ban-all-sheds]

Hehe... I recommend no one use a foil hat, they will turn their head in to antenna, and make the problem worse.

Nonsense.

Seems the house was possibly upgraded, and has a PME and an earth to the outer part of the main supply cable and form of TT earthing by the water main.

That will be the main equipotential bonding.

 

[ElectricsRanger]

Not an easy thing to do and still be safe.

Did you measure the current flowing in the bond wires ? A clamp meter would the best way to avoid disconnecting them.

What is probably happening is that the local 3 phase distribution is not well balanced ( an impossible task at the best of times ) and the supply Neutral has current flowing to the substation. If your water supply pipe is low impednace to ground then some of the current that should be flowing along the Neutral bcak to the substation will be flowing through the bond wires and returning via the wate supply pipe to ground. If the local water supply network is all metal in the street mains then surprisingly high currents can flow along bond wires.

If there's an earth connection to the outer sheath, it's not TN-C-S.

TN-C-S has a combined neutral and earth conductor coming into the property.

Thank you for your replies. I did disconnected from the main earthing bar in the MCU, but there is a second earthing bond to a second MCU so to speak fitted for a previous Economy7 system, i didnt disconnect that. I had reconnect the main earth bond from the PME to the MCU 24 hours later, the house felt considerable worse. We currently have the downstairs ring main off, as when its on we feel considerably worse than when its off.

Here is an image of the actual PME install... Your thoughts are most welcome on this issue.

The installation shown here is as if there is both an TN-C-S and an TN-S... based on the information displayed in the new on-site guide it should not have that metal brade connecting the earth of the PME to the earth of the cable. Could the installation have been upgraded?

 

[stillp]

Why are you checking the EMF, and what with?

 

[PBC_1966]

Here is an image of the actual PME install... Your thoughts are most welcome on this issue.

The installation shown here is as if there is both an TN-C-S and an TN-S... based on the information displayed in the new on-site guide it should not have that metal brade connecting the earth of the PME to the earth of the cable. Could the installation have been upgraded?

That's an older cable head and looks as though you had a supply which started out as TN-S and which has been converted to TN-C-S at some point.

 

[ElectricsRanger]

Why are you checking the EMF, and what with?

I have actually been investigating the electrical installation, as everyone has found by trial and error that when we have the downstairs ring main and immersion heater off at the breaker, we feel and sleep amazingly better in the house. So i have been investigating the installation to conclude if something is incorrect with the installation. Its a family home with two generations of family currently resident, and we all feel noticeable difference, and we are not paranoid bunch.

Im using a ELF extra low frequency meter, its a a Magnetic Field Meter, TM-191. Its probably not the best meter i could purchase, and i see it as a guide than a full proof detection kit. I also have a bosch GMS 120 professional multi-detector, which amazingly picks up cable fields half a meter more on the ring from cables with high fields, and warns me that drilling in that area would hit a cable. If i turn the mains off it, the meter detects nothing. Its fascinating detection tool, but im now looking more for conclusive reasons why, and since we already feel horrid in the same areas the meter detects strange readings, there is more evidence than just a odd meter.

This is not mind over matter, noticeable physical differences are felt went entering or leaving the home, regardless if we think about it or not.

That's an older cable head and looks as though you had a supply which started out as TN-S and which has been converted to TN-C-S at some point.

Yes i started to think the same after reviewing the installation. Shouldn't the old TN-S earth be disconnected when it was upgraded? and if a TT earthing water main was present, would that change the way a TN-C-S combined earth functions?

I sore a video by John Ward called
TN-C-S Danger - Broken PEN Conductor (Combined Earth & Neutral)
He said if the pen was broken it would potentially make the earth live, but if the current was discharging to a type of TT earth, would that make it less likely to cause shock hazard, specially if the person was indoors?

Thank you for your replies so far.

 

[JohnW2]

Yes i started to think the same after reviewing the installation. Shouldn't the old TN-S earth be disconnected when it was upgraded?

Even if the original TN-S sheath were disconnected at the transformer end (or if it were broken somewhere), if it still entered the building it would count as an "extraneous-conductive-part" which had to be 'bonded' (i.e. connected to the earthing system of the installation - now derived from the TN-C-S earth) - so, however you look at it, it has to remain 'connected'.

... and if a TT earthing water main was present, would that change the way a TN-C-S combined earth functions?

Similarly, if a metal water pipe entered the building, that would count as an extraneous-conductive-part, and therefore would have to be bonded (connected) to the TN-C-S earth.

Kind Regards, John

 

[PBC_1966]

Yes i started to think the same after reviewing the installation. Shouldn't the old TN-S earth be disconnected when it was upgraded?

If the distribution main in the street has been replaced, it's likely that the other end of the sheath and the neutral contained within it are connected together to the neutral of the main, effectively making the old sheath which used to be the separate earth path part of the PME system's neutral to the house.

and if a TT earthing water main was present, would that change the way a TN-C-S combined earth functions?

It's not TT earthing. TT means that the sole earth fault path for the house is by way of the earth, which is not what you have. The connection to the water system in this case is to ensure that under fault conditions you don't develop a potential between that water piping and the earthing system within the home.

In practice, however, an underground metallic water line which is so bonded will also act as an effective earth electrode, so even with a TN system under fault conditions some of the current will flow through that bonding. With TN-C-S, as there's also a link from the incoming neutral to that electrode, a small portion of the normal neutral current will also find its way through the bond connection and water system.

He said if the pen was broken it would potentially make the earth live, but if the current was discharging to a type of TT earth, would that make it less likely to cause shock hazard, specially if the person was indoors?

With TN-C-S the risk is that should the neutral be broken at some point on the supply network, then the incoming neutral to your house could rise in potential. Since the earthing system in the house is connected to the supply neutral, that would cause everything connected to it to rise in potential as well, including all the water piping, radiators, metal casings of appliances, etc. The aim is to have everything bonded together so that in the (rare) event of this happening, there cannot exist a potential difference between different things within the home which should normally be at earth potential.

Certainly any form of earth electrode within your own installation (be it a dedicated electrode installed specifically for the purpose or an incidental one by way of bonded pipework etc.) will help to keep the potential down in the event of a broken supply neutral. Here in the U.S. where all supplies are the equivalent of TN-C-S it's actually required that an earth electrode be provided at each house and connected to the incoming neutral (it's not necessarily a dedicated electrode and may actually be a bonded water line or similar). In the U.K. there is no such requirement, but in practice there will often be an effective electrode by way of the bonded services anyway.

 

[JohnW2]

If the distribution main in the street has been replaced, it's likely that the other end of the sheath and the neutral contained within it are connected together to the neutral of the main, effectively making the old sheath which used to be the separate earth path part of the PME system's neutral to the house.

Perhaps - but a common reason for converting TN-S to TN-C-S is because the sheath ('TN-S earth') has broken, or at least deteriorated, somewhere. Whatever, as I said, if the sheath enters the building, I think it has to be considered to at least potentially being an extraneous-c-p, and therefore probably should be main bonded - which really makes the questions go away!

Kind Regards, John

 

[stillp]

I also have a bosch GMS 120 professional multi-detector, which amazingly picks up cable fields half a meter more on the ring from cables with high fields, and warns me that drilling in that area would hit a cable.

I don't think that's amazing at all. However, I suggest that they are not 100% reliable, so don't assume that if it doesn't detect a cable there isn't one.