Off Grid - Installing SPD and Earth Correctly

richardimorserichardimorse Registered Users Posts: 98 ✭✭
Please advise if this is correct without going into detail of country specific regulations, TT vs TN etc for an unregulated OFF Grid installation in a third world country where the national grid is TN+C+S but does not connect at all to this off grid site.

1. Use Two Neutral/Negative Bonded Single Earth Points Near to the Inverters and Batteries, one for AC and one for DC and Connect them together Underground with Bare Copper Cable.
2. Do not be tempted to install the Single DC Earth point underneath the solar panels and combiners, install it near the battery Negative
3. For out buildings especially over long distances, Install a secondary unbonded AC Earth Point and ensure that the SWA Armour and/or Earth returns the Earth to the single bonded AC earth point near the inverters to prevent potential difference between the two AC earth points
4. I have seen that many people install a secondary DC earth under the solar panels near to the combiners and I guess the same applies, to ensure the secondary earth point is unbonded and to return the earth to the single bonded DC earth point near the batteries with a separate 16mm Earth wire when using welding cable for the positive/negative.

Most installations I have seen use unshielded 35mm Welding Cable to return the DC from the combiners to the DC panel in the battery room surely it is better to use 25mm SWA 2 core for this purpose which should be rated about the same, If welding cable is used the SPD would become extreemly important on the DC power panel. 

I understand that for off-grid, you can use lower cost AC SPD devices for voltages of 150 V in both the combiner and the DC DB Board as the DC SPD devices are exactly the same just rated at higher voltages of 600, 800, 1000 and 1200 for Grid-Direct installations.
Nearly all of the DC SPD's I have seen apart from ProSurge are 20kA/40kA whereas the AC SPD's are also available as 30KA/60kA and 40kA/80kA, I am assuming using the 40kA/80kA in a very high risk area for both AC and DC is giving maximum T2 protection

Can anyone tell me when you would use a single pole SPD Type 2 Device as opposed to a 1P+N or 2P device - perhaps cascading and 4U mini DB boards, when cascading, do you need a specialised T3 device such as SP202N from Hagar to protect your sensitive equipment or is the following AC cascading just as good:

AC Surge Protection Cascading
 >>> (Inside) Inverter Output = 10kA/20kA 1P+N SPD >>>2 metres away>>> (Inside) Main Load DB Board = 40kA/80kA 1P+N SPD >>>200 metres away>>> (Outside) Sub DB Board for Outbuilding cluster Distribution (Rooms and Chalets) = 40kA/80kA 1P+N SPD >>>15 to 30 metres away>>> (Inside) Room Chalet with MINI DB Board 4U = 10kA/20kA single pole SPD


  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    With DC SPD's the UC voltage is most commonly as you mentioned 600V and up, which I found out the hard way, they do little to provide protection to a much lower voltage system. During my search for lower voltage SPD's I came across a manufacturer who will supply devices tailored to the maximum specified  voltage provided from the data sheet of the controller. As I have two separate systems with  different devices, the  UC voltage  provided were 75VDC and 175VDC respectively.

    With the AC SPD for single phase 230V they supplied a 2 pole L/N with different ratings for each, UC 385V on live and 255V on neutral with a label stating  "Discharge Tube"

    All the above are 20kA/40kA the previous ones were  40kA/80kA but 600V for DC and 475V for AC. Will leave the previous ones in parallel with the new. 

    Not sure about using AC devices for DC, but have seen DC with AC rating, see pdf. AC have no mention od DC.
    Some useful information

    The single pole SPD I have is a Schneider device which takes the place of a circuit breaker in the distribution panel with a pigtail to ground busbar, perhaps designed to provide limited protection, I've opted to improve this with the 2 pole with a separate larger ground conductor as instructions call for.

    The information I've collected has mostly been oriented towards grid tied systems, the assumption would be it's very similar to off grid with regards to fundementals, I'm still in the development stage, so not attempting to provide guidance, but rather sharing information gathered.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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