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Solar-Battery Installation to Metal Garage

John R.

I am proposing to build a new garage-workshop.  Based on current cost estimates, this will be metal-framed and clad, on a concrete base.

The proposal is to install 12 to 14 solar panels on the roof and a Tesla Powerwall in the garage.  And yes, this installation will need a G99.  The panels are DC-coupled to the Powerwall.

The garage will have two separate circuits from the meter area in the house  (as recommended by the installer)

  • Cicuit feeding an EV charger, 3-core SWA
  • Circuit connecting the Powerwall, 3-core SWA

There will also be a local consumer unit in the garage, connected to (b) to feed lights, sockets, power tools etc.

The plan is for the builders to install the cabling to leave tails, and then the installer will install and connect the electrics.  Even though SWA cables will be used, the plan is to install these in a duct for future proofing where this crosses the garden

I’m also likely to install a fibre data cable.  This may also be SWA but I wasn’t proposing to connect the armour, it’s only for mechanical protection.  This is mostly to avoid the latency of a WiFi link.

The supply to the house is TN-S with Zₑ 0.24 Ω 

My only query about this installation is the earth bonding of the garage structure & cladding metalwork.   I’m assuming that the structure’s frame components will need to be earth bonded and perhaps also the cladding panels – does this sound about right?

Would the garage likely need a local earth rod – or is it preferred to use the extended supply earth?

One of the reasons that the earthing is of interest is that the garage may have some ham radio equipment.  My practice is to power such radios from a 12V battery charged by a charger where mains earth is not bonded to the 12V negative rail.  A local clean RF earth is then used, and it’s undesirable to have this close to a mains protective earth rod.

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  • +1
    John R. said:
    The supply to the house is TN-S with Zₑ 0.24 Ω 

    You'll probably be obliged to treat that as PME (Protective Multiple Earth), even if it doesn't look like PME at the cut-out (as most DNO's policy is to combine N and PE in modifications or repairs that they do "in the street", either already done or in the future).

    Presuming the metal structure of the garage is accessible on the inside, it'll almost certainly need bonding. There are pros and cons to the choice whether to use the DNO's earthing facility for that, or to create your own independent earthing system (TT).

    An independent RF earth is an "interesting" one - others here will have more practical advice, but certainly from a safety point of view not only do the electrodes need to be kept apart but anything accessible connected to the two different systems (including class I appliances on flexes or extension leads) need to be kept well out of reach of each other (2.5m+). Under rare situations where the DNO earth has a significant voltage on it, and the RF earth is still close to zero volts, a lethal voltage can appear between the two systems.

    Ducts are certainly a good idea. Fibre is preferred over copper between buildings (especially where the buildings have differing earthing systems), but won't work for everything of course. Note that BS 7671 (and I suspect BS 6701) demand that mains and communication cables are kept at least 100mm apart underground (or have a fire resistant partition between) - so not only is the combined TP/mains SWA cables a bit questionable underground, you might well need separate ducts as well. CTs can be an interesting one - depending in the design details they might be classed as SELV/PELV circuits or might be "functional extra low voltage" (FELV). CT's peculiar nature of generating high voltages if they become disconnected from the measuring circuit while current is still flowing in monitored circuit can complicate that distinction. FELV circuits, although normally operating at "safe" voltages need to be treated much more like mains circuits from a safety point of view - i.e. generally mains voltage rated cables would be needed.

      - Andy.

  • 0 in reply to AJJewsbury

    Thanks Andy

    That's a very interesting question re the combined data and power cables for EV and Powerwall.  I'll look into that one further.  I wonder whether there's a definition of 'communication cable' that needs to be considered.  

    I can easily pop in a duct to run the fibre - I was expecting separate ducts from the house for the (a) and (b) cables anyway since they go to different ends of the garage.  The fibre I'm considering is simply to extend a LAN for use with the radio equipment but will also need to provide a data connection for the Powerwall with the metal building option.  I'd really much prefer breeze block construction, but a metal building is significantly cheaper.

    Noting the potentially high o/c voltages from CTs is another reason IMHO to avoid the use of CAT5 cables for connecting CTs although I wonder whether CTs may incorporate some voltage limiting components ...

    The building cladding has an insulated layer on the inside, but screws, brackets etc will inevitably created exposed metalwork.  If this needs to become TT with its own earth then so be it.  It's not too much of a pain to earth radio chassis to 'mains earth' and transformer couple to RF earth as an alternative technique for avoiding a direct connection and de-risking PDs.

  • +1 in reply to John R.
    John R. said:
    I wonder whether CTs may incorporate some voltage limiting components ...

    Some "consumer grade" ones certainly do - either a built-in burden resistor or back-to-back zeners or similar just to clamp the voltage, others don't (you often have to read the small print). Whether you can depend on such things to guarantee safety (e.g. from shock) rather than just reduce the risk of damage to equipment to tolerable levels is another question though. The other side of the question is how well the receiving end is separated from mains - again that might place things in the FELV bracket even if the CT itself is OK. Again small print.

       - Andy.

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  • +1 in reply to John R.
    John R. said:
    I wonder whether CTs may incorporate some voltage limiting components ...

    Some "consumer grade" ones certainly do - either a built-in burden resistor or back-to-back zeners or similar just to clamp the voltage, others don't (you often have to read the small print). Whether you can depend on such things to guarantee safety (e.g. from shock) rather than just reduce the risk of damage to equipment to tolerable levels is another question though. The other side of the question is how well the receiving end is separated from mains - again that might place things in the FELV bracket even if the CT itself is OK. Again small print.

       - Andy.

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