Grounding a multiplus in semi off grid

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mfred68 asked Mar 29, '24

Grounding a multiplus in semi off grid

My inverter is situated about 25m away from my consumer unit. I have a 25m power cable going to it, and another 25m cable coming from it to the consumer unit.

I have a TT earth system and the rod is near the consumer unit.

The plan is to use grid power to charge batteries (and run the house) at night time cheap rate, then go off grid during the day, which will fully isolate grid power via the "ac in" contactor in the inverter, and then also linking neutral to earth by the relay in the Inverter.

My electrician thinks I need another earth rod by the inverter to ground it, but I think it should be connected to the existing earth rod with a 25m cable. Or else there will be ground resistance between the two rods?

So can anyone advise?

Multiplus-II Grounding

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Answer 1 · 2024-03-29T22:56:30Z

phoenix answered · Mar 29 at 10:56 PM

If the MP was within 1m of the existing rod you wouldn't consider installing a 2nd one. The resistance of 25m of cable is easily calculated and added to the current rod impedance to show that it's within regs.

If you do decide to install a 2nd rod, will this have to be tied to the 1st else you run the risk of a shock in the vicinity of the MP, although this is pretty unlikely but would still need to be proved. You can have any number of rods in parallel and this is used in areas of poor soil eg granite or sandy.

For info my MP is 10m from my rod on 5x1.5mm cable. I run a split board so lights and some sockets are on AC out. All circuits are connected to the rod. During power cuts the AC out neutral will be earthed by the MP earth relay. Bear in mind that the rod resistance in a lot of cases is inadequate to generate fault current to clear the MCB, so the RCD will trip first. The rod keeps the touch voltage to a safe level. I have 30mA RCDs to and from the MP which is poor discrimination but they were to hand.

The MP transfer is not immediate and I sometimes lose my router during changover, but not my NAS. This may become annoying if you're resetting the oven, microwave etc twice a day.

Don't forget that you'll need a transfer switch in case of MP maintenance. What type of batteries are you considering?

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mfred68 commented · Mar 30 at 01:02 PM

Thanks for that valued answer, so basically the MP has to be grounded to the existing rod which is 25m away next to the consumer unit and grid head. But its advisable to place a second rod next to the MP as well, with the 2 rods and MP linked with a 10mm cable? Baring in mind that the swa armor will also parallel as a cpc.

I've already got the batteries and MP, had a bargain, I have 6x 7kwh Rahvolt batteries at £600 each, and a MP 2, 10kva inverter which I got for £1600

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offroadflow commented · Mar 31 at 11:43 AM

The manual changeover switch is recommended.

But connecting both rods? Why? The rods must be in moist ground and at least 1.5 if not 2 meters in the ground. If they are well installed then the ground itself is the connection.

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phoenix offroadflow commented · Mar 31 at 12:31 PM

Rods are in whatever soil they find themselves in. It may be wet, dry or both. There will be a resistance difference between multiple rods and under fault conditions that will result in a voltage difference, which may exceed the touch voltage.

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offroadflow phoenix commented · Mar 31 at 12:45 PM

Called "badly installed" imo

But hey, better safe than sorry

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