Earthing of joined swimming pools

[AusLee]

Hi,

A contractor is building two adjacent houses, each house has a pool, the two pools are formed of one large reinforced concrete pit with a reinforced concrete wall in the middle.

Once the ground works are complete, a fence erected along the mdian wall will separate the two pools. Essentially it is like a double kitchen sink basin.

The electrician is refusing to connect the earthing of the electrical installation of the house to the metal of the pool saying that if he does so he will be joining the electrical installation of the two houses.

Each house has a single phase supply, they are not on the same phase: the network rule state that the houses are connected each to phase for network balancing. So:

House 1 has incoming active A and Neutral N. The earth is made locally by connecting N to Earthg at the meter panel of House 1.
House 2 has incoming active A and (a separate netural cable of course) Neutral N. The earth is made locally by connecting N to Earthg at the meter panel of House 2.

I would therefore say that at the incoming side, the 'joining' of the installation is even worse, because the same neutral is connected to both eathes (the supply authority likes to multipli earth the neutral at each connection).

Inside each house the active, neutral and earth are seaparate and never join (TN-C-S system in european terminology or Multiple Earthed Neutral in Aussie terminology).

What could be so wrong in connecting the earth wires of the two houses to the same combined two-in-one pool structure?

 

[waross]

That is probably a question for the AHJ.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

This is an installation that's meant to be completed under a 'self certified contractor's certificate of compliance', right?

Normally I'd agree with waross, but I suspect that in effect under the certification system in place, the contractor is also the AHJ, rather than having a separate inspection authority.

Are both the houses free standing separate buildings, or are they a duplex installation with a common wall?
Technically, I'm not sure exactly how much difference it would make, the reinforcing of the pool is interconnected regardless, and keeping the electrical installations (at least as far as earthing connections) separate is rather difficult with a large pool in the middle. I also expect its a similar argument for combined HV and LV earthing in AS2067, in effect they mostly end up connected regardless of attempts to keep them separate, thus manage it as a combined system instead.

 

[AusLee]

Hi,

Thanks Freddy, yes you are right in all 3 instances:
1. it is to be cerfied by the contractor.
2. Yes there is a common wall between the two houses.
3. Yes suppose the two pools are not like a double basin sink, rather two basins separated by say half a meter of soil, that soil could most likely be high conductivity, so while the mutual hard connection to the reinforcement bars is paracically nil impedance, the small separation is not that far off.

Do you see any technical issues? any fault being brought onto the other people's premises?

 

[waross]

I expect that any current from a local fault will divide between the grounds and neutrals of both services but not equally.
Consider a fault in dwelling "A";
The current will divide three ways;
1; Through the ground electrodes
2; Through the neutral of dwelling "A"
3; Through the neutral of dwelling "B"
The fault current returning through the ground is what it is and should present no issues.
The remaining current will divide between the neutrals in inverse ratio to their impedances.
In dwelling "A" the neutral will be in close proximity to the conductor supplying the fault current and the reactance will be relatively low.
The neutral in dwelling "B" will be at a distance from the conductor supplying the fault current and will not be affected by the magnetic field of the fault current. The reactance will be relatively higher. If the neutral supplying dwelling "B" passes through ferrous encirclement, and particularly a steel conduit, The reactance will be even higher.
Most of the fault current will flow either in the ground electrode or in the neutral of dwelling "A".
There should be no issues with the relatively small amount of fault current that will flow in the neutral of dwelling "B".

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[AusLee]

Hi Guys,

Just to let you know the AHJ has reviewed the issue and approved it, requiring only that the bonding at the pools be removeable for future fault finding, and a label at the point of supply indicating the additional bonding.

Best Regards.

 

[waross]

Thanks for the feedback.

Bill
--------------------
"Why not the best?"
Jimmy Carter