Solar PV earthing earth rod or main earthing terminal in house

[richardg]

Hi installing my first solar PV on my own home before starting the courses next year and need some advice on earthing PV.
System 6kw solar PV - 2 strings
Solis Rhi inverter
20Kwh pure energy battery’s
The solis inverter has its own TT system to run as a floating neutral when in UPS mode which is 6mm.
I’m then looking at earthing the PV panels with 6mm in a loop to a test box next to the inverter for future ease in testing - is it best practice to connect this to the inverter frame earth connection ;to earth rod, or back to main earthing terminal in house. Looked at latest IET guidance but couldn’t see any definitive answer.

 

[davesparks]

Why are you running the inverter as a TT system?

 

[richardg]

Why are you running the inverter as a TT system?

The solis hybrid has a UPS switch which runs in ‘islanding mode’ during power cuts and needs its own earth system - believe it acts as a neutral during the power cut.
Believe the reasoning behind this concerns the possibility that your main earth could be disconnected during a major outage. Prevents any current being fed back into grid and shocking any workers fixing the fault.
Though still re-reading the latest IET guide to glean all the relevant facts

 

[richardg]

Why are you running the inverter as a TT system?

Regulation 551.4.3.2.1 of BS 7671 states that, in TN systems, generators operating as a switched alternative to the public supply cannot continue to rely on the distributor’s means of earthing.

In LV Systems the he neutral of the supply is earthed at the distributor’s transformer. Hence, in systems operating in island mode, the distributor’s neutral-earth link cannot be relied upon, as this is switched out when the live conductors are disconnected.

 

[davesparks]

The solis hybrid has a UPS switch which runs in ‘islanding mode’ during power cuts and needs its own earth system - believe it acts as a neutral during the power cut.
Believe the reasoning behind this concerns the possibility that your main earth could be disconnected during a major outage. Prevents any current being fed back into grid and shocking any workers fixing the fault.
Though still re-reading the latest IET guide to glean all the relevant facts

Regulation 551.4.3.2.1 of BS 7671 states that, in TN systems, generators operating as a switched alternative to the public supply cannot continue to rely on the distributor’s means of earthing.

In LV Systems the he neutral of the supply is earthed at the distributor’s transformer. Hence, in systems operating in island mode, the distributor’s neutral-earth link cannot be relied upon, as this is switched out when the live conductors are disconnected.

Yes, but none of that is a reason to install it as a TT system though. It would be far more sensible to have it as a TN-S system wouldn't it?

 

[richardg]

Not sure if I have explained it well / sorry the system is a TN-S system and remains connected to the main earthing terminal at all times - from what I read the earth rod is only installed if you use battery’s for the UPS function on the Hybrid inverter.

 

[James]

Yes, the earthing system is TN-S when grid connected
to be fair, for most faults, the suppliers earth will still be there but can’t be relied on.
therefore an earth rod is required to provide protection in the event of a fault whilst running in island mode and the distributors earth is not there.

 

[richardg]

Yes, the earthing system is TN-S when grid connected
to be fair, for most faults, the suppliers earth will still be there but can’t be relied on.
therefore an earth rod is required to provide protection in the event of a fault whilst running in island mode and the distributors earth is not there.

Can you clarify my orginal question - will the earth cable for the pv panels be connected directly to the main earthing terminal or the earth rod?
Thanks

 

[James]

Surely the two are connected together so by connecting to the main earth terminal you are also connected to the earth rod?

 

[DickiePhitt]

I'd welcome clarification about whether a TT island is truly an 'island' separate from house supply earth
I have a Zappi 1 and as far as I recall the earth was run by the installer solely to the earth rods. I don't recall the earth from the CU being connected.
Not home so can't check. Maybe someone could clarify whether the TT Island is linked to the house earth?
TIA

 

[James]

An earth rod for a TT car charging point should be connected to the charger ONLY.
no connection between charger earth and main earth terminal should be made as this then negates the safety benefit of the rod.

a separate earth rod should be installed normally near the generator if you are intending to run the house from it when the grid power has failed.

 

[DickiePhitt]

An earth rod for a TT car charging point should be connected to the charger ONLY.
no connection between charger earth and main earth terminal should be made as this then negates the safety benefit of the rod.

a separate earth rod should be installed normally near the generator if you are intending to run the house from it when the grid power has failed.

Thank you, that was my understanding.

My confusion comes from a discussion on a Facebook group discussing how to install a manual EPS switch for an AC battery inverter. The EPS requires a TT island yet it seems people are installing 2 pole switches ie inverter (TT) earth and main earth are connected, not switched.

 

[davesparks]

Thank you, that was my understanding.

My confusion comes from a discussion on a Facebook group discussing how to install a manual EPS switch for an AC battery inverter. The EPS requires a TT island yet it seems people are installing 2 pole switches ie inverter (TT) earth and main earth are connected, not switched.

It does not require a TT system, it requires a TNS system, that is the source of supply is earthed via an earth and seperate Live, Neutral and Earth conductors are used throughout the installation.

For it to be operated as a TT supply you would need to connect the source of supply (inverter in this case) to an earth rod and install a completely seperate earth rod for the installation it is feeding. You would then have just a Live and Neutral conductor feeding the installation with the earth connection being made via the two earth rods and the general mass of earth, which would be a completely daft situation!

Just because an earth electrode is installed it does not make something a TT system. All three common earthing systems (TNS, TNCS and TT) use earth electrodes, it is where and how they are connected that makes the difference.

 

[davesparks]

Not sure if I have explained it well / sorry the system is a TN-S system and remains connected to the main earthing terminal at all times - from what I read the earth rod is only installed if you use battery’s for the UPS function on the Hybrid inverter.

Can you clarify my orginal question - will the earth cable for the pv panels be connected directly to the main earthing terminal or the earth rod?
Thanks

It will be a TNS system regardless of which source of supply is in use. The earth rod is there to connect the inverter neutral to earth to create the earth reference of that TNS system.

All earths should be ultimately be connected back to the MET and all sized so that they are suitable for both supplies. For example if the mains supply requires 10mm bonding and the Bacup supply requires 2.5mm bonding all bonding should be installed as 10mm.