Earthing question

[jaa312]

Hi all….I have encountered a problem which I cannot easily solve! I hope someone can help me.

The question is :
How can 20 apartments supplied through 20 different electricity meters (single phase) situated in a small common area be earthed in a TT system? A TT system requires an independent connection to earth for earthing purposes for each apartment.
A dedicated earth electrode for each apartment cannot be used since the distance between them must be a at least equivalent to the depth of the earth electrode. In this case the common area is small and cannot house 20 different earth electrodes. Connecting the earthing conductors of a number of apartments to a single earth electrode is also not recommended for obvious reasons.

How can this situation be sorted?

Thanks much!

 

[Jay Sparks]

Tns, tncs????

 

[Engineer54]

Hi all….I have encountered a problem which I cannot easily solve! I hope someone can help me.

The question is :
How can 20 apartments supplied through 20 different electricity meters (single phase) situated in a small common area be earthed in a TT system? A TT system requires an independent connection to earth for earthing purposes for each apartment.
A dedicated earth electrode for each apartment cannot be used since the distance between them must be a at least equivalent to the depth of the earth electrode. In this case the common area is small and cannot house 20 different earth electrodes. Connecting the earthing conductors of a number of apartments to a single earth electrode is also not recommended for obvious reasons.

How can this situation be sorted?

Thanks much!

No It Doesn't. But you will need to install an earth field. If there is no chance of a DNO supplied PME connection, then it may be prudent to have a contractor come in to bore out some deep holes to accommodate the installation of a series of earth rods and then set the rods in a electrode enhancer such as Bentonite or Marconite. Further, if the building sits on a substantial concrete rebar reinforced foundation, then you could also break out a section to reach the rebar and make a sound earth connection (then repair breakout) and include this as part of your TT system. Another solution would be to lay a deep buried copper tape around the building with a Bentonite/Marconite covering, or even using all three methods above. Where there is a will, there is always a way!!

The main aim/point being to get a stable as low a Ra value as possible, ideally sub 1 ohm.

 

[TJ Anderson]

The main aim/point being to get a stable as low a Ra value as possible, ideally sub 1 ohm.

Or, according to the IET's on-site guide, who coincidently, also publish the BS7671 regulations that we all work to. They say "A value exceeding 200 Ohms may not be stable"

 

[jaa312]

No It Doesn't. But you will need to install an earth field. If there is no chance of a DNO supplied PME connection, then it may be prudent to have a contractor come in to bore out some deep holes to accommodate the installation of a series of earth rods and then set the rods in a electrode enhancer such as Bentonite or Marconite. Further, if the building sits on a substantial concrete rebar reinforced foundation, then you could also break out a section to reach the rebar and make a sound earth connection (then repair breakout) and include this as part of your TT system. Another solution would be to lay a deep buried copper tape around the building with a Bentonite/Marconite covering, or even using all three methods above. Where there is a will, there is always a way!!

The main aim/point being to get a stable as low a Ra value as possible, ideally sub 1 ohm.

Thanks for your reply.....but I have some considerations for you!:thinking:

1. If the series of earth rods are too near each other and there is an electrical fault in one of the apartments, wouldn't a dangerous voltage appear in the earthing system of other apartments?

2. Your other two suggestions (using the copper tape and rebar connection) do make a lot of sense and might help to reduce the resistance to less than 1 ohm, but then again what happens in adjacent/other connected apartments when a fault occurs? Will a dangerous voltage appears in the earthing system of the other apartments?

My main concern is that earthing is not something static but quite dynamic in that soil/earth humidity, corrosion and also the soundness of the electrical connections do change over time!

Regards
Ja

 

[jaa312]

"Or, according to the IET's on-site guide, who coincidently, also publish the BS7671 regulations that we all work to. They say "A value exceeding 200 Ohms may not be stable"

200 ohm is the maximum accepted for for a single TT installation.....earthing for multiple TT installations in a restricted area might be a bit more tricky. I am trying to see what is the best approach.

 

[Engineer54]

"Or, according to the IET's on-site guide, who coincidently, also publish the BS7671 regulations that we all work to. They say "A value exceeding 200 Ohms may not be stable"

200 ohm is the maximum accepted for for a single TT installation.....earthing for multiple TT installations in a restricted area might be a bit more tricky. I am trying to see what is the best approach.

And if you accept that totally numbty value, more fool you. BTW, no it's ''NOT'' a maximum value, the BS7671 do not give a maximum value...

 

[jaa312]

And if you accept that totally numbty value, more fool you. BTW, no it's ''NOT'' a maximum value, the BS7671 do not give a maximum value...

Yes you are perfectly right.....

 

[Engineer54]

Thanks for your reply.....but I have some considerations for you!:thinking:

1. If the series of earth rods are too near each other and there is an electrical fault in one of the apartments, wouldn't a dangerous voltage appear in the earthing system of other apartments?

No.....

2. Your other two suggestions (using the copper tape and rebar connection) do make a lot of sense and might help to reduce the resistance to less than 1 ohm, but then again what happens in adjacent/other connected apartments when a fault occurs? Will a dangerous voltage appears in the earthing system of the other apartments?

No....

My main concern is that earthing is not something static but quite dynamic in that soil/earth humidity, corrosion and also the soundness of the electrical connections do change over time!

That's why you go deep to keep the system stable. Any mechanical connections (bolted) between conductor and earth rod/electrode needs to be protected with say 2/3 or more layers of amalgamating tape or other similar covering, to seal the connection. All earth rods need a suitably sized robust flush to ground earth inspection pit (not one of those soft plastic things that look like a standard adaptable box) so that inspections can easily be carried out. Only exothermic and Brazed weld connections do not require an earth pit to be installed...
Regards
Ja

Depending on the available space and soil condition/conductivity you may well get away with 3 no of earth rod positions, then if you utilise/incorporate one or both of the other methods mentioned (Ufer and perimeter tape) you may well find that you have a decent sub 1 ohm working TT system that will last in excess of 30/50 years without any further maintenance....

 

[Knobhead]

You seem to be at cross purposes.

Jaa, each flat doesn’t have its own electrode. Each flat is connected via a main earth bar to a ground network of multiple electrodes and as E54 suggested use the slab rebar to form an Ufer earth.

Your location shows EU that doesn’t help us due to differing regulations across the EU.

 

[Knobhead]

This shows what E54 was talking about. All 20 flats should be connected to the common earth. I’ve shown six electrodes, depending on the size of the building more can and should be used. Again as E54 said connect the rebar to the earth bar. The rebar becomes an Ufer earth electrode.

What I find worrying is you asking something like this for such a large installation. It should have been taken care of at the design stage.

 

[jaa312]

This shows what E54 was talking about. All 20 flats should be connected to the common earth. I’ve shown six electrodes, depending on the size of the building more can and should be used. Again as E54 said connect the rebar to the earth bar. The rebar becomes an Ufer earth electrode.

What I find worrying is you asking something like this for such a large installation. It should have been taken care of at the design stage.

Thank you soo much tony for your extremely helpfull answer! Appreciated!