question

calle avatar image
calle asked

Relying on earth rod in off grid?

Hello!

Some thoughts about building a three phase system with multiplus 2 with off grid capability. Is there some way of confirming the connection to earth when in off grid (the multi disconnects L and N on AC in, correct?)


Seeing as I don’t want to risk sending a current back to the grid from the multi when using battery power, I cannot connect the AC out to the grids neutral or PEN. When in off grid mode the idea is to use a grounding rod as the neutral point and earth (effectively creating a TN-C from the multis when in off grid)?


How do I eliminate the risk of losing my neutral point (connection to earthrod is lost somehow, or the resistance to earth is to high) and thereby risking damaging electronics connected if the loads are uneven over the three phases (0-400V on regular outlets)?


Has anyone experienced problems with this? Is there some kind of module that could be installed that measures the ground resistance continuously and breaks the AC out if ground is lost?

Multiplus-IIoffgridGrounding
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4 Answers
Kevin Windrem avatar image
Kevin Windrem answered ·

There is a common misconception that the earth rod is there to carry fault currents. This is not the case. Rather it is there to drain off static energy that might built up on the system safety ground so the safety ground voltage is at earth potential.

Separate to the earth rod, the safety ground is connected to the system neutral AT ONE POINT in the system. The purpose of this connection is so that if there is a hot to chassis short in some piece of equipment, the circuit breaker for that branch circuit will trip in stead of elevating the chassis to hot line voltage. Line voltage on a chassis could kill a person.

There is never a connection between neutral and the earth rod. The connection is made from the safety ground wiring and the earth rod. This does make an indirect connection between earth rod and neutral through the above mentioned connection.

The neutral to safety ground connection is made at the service entrance if there is one. In off grid with generator this bonding is often made inside or close to the generator. When the Multi is inverting, it disconnects both neutral and hot legs from the grid and makes a local safety ground to neutral connection internally at the AC output.

Another misconception is that the neutral must be at zero volts relative to the safety ground. I practice, this is seldom the case due to current flow in the neutral wiring. Safety ground to neutral voltage is typically under a few volts and does not indicate a fault or safety issue of any kind.

You should never make any connections from the Multi AC output neutrals to anything up stream (e.g., the grid or generator). The Multi isolates the AC output from the AC input when inverting and it must remain isolated, including the neutral. As mentioned above, the safety ground to neutral bond is made inside the Multi in this case.


ALL PE (safety ground connections) ARE connected together. Always. While the Multi has PE connections for each AC input and output, they are all connected internally. External connections to the same point in the system is expected and does not create any ground loop or safety problems.

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calle avatar image calle commented ·
Thank you for the detailed and informative answer!


I understand the point of the protective earth (not sure about the lingo here, earth for protection against humans, pets and property). I am however still unsure about the function here, imagine if we have a fault between neutral and phase in our system when in off grid, the current will surely mostly flow back to the grid seeing as we most likely have a lower resistance back to the transformer station neutral point than to our earth rod. If a fault in our system occurs during the time work is being done on the grid or transformer the possibility of a dangerous current flowing back through someone working seems unsafe.


My main question is however how it’s possible to create a circuit between the neutral and earth when in off grid? Does it simply return to the multis neutral AC-out and then bond to the earth rod? Where do we create the neutral point in the three phase system created by the multis if we cannot rely on the grid neutral or the earth rod?


Thank you!

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Kevin Windrem avatar image Kevin Windrem calle commented ·
The earth connection as nothing to do with handling fault currents.

A hot to neutral short will trip the breaker in the hot lead because the fault current returns through the neutral connection. This is the same path as operating current but significantly more current flows because of the short circuit.

A hot to chassis shop will trip the breaker because the neutral and safety ground are connected at some point in the system.

Neither of these situations involves the earth connection and in fact will remain safe without any earth connection.

The earth connection comes into play when the chassis (safety ground) of the system is subjected to electrostatic stimulus (wind blowing, near by lightening) or other mechanisms to introduce a voltage on the safety ground network. In this case, without an earth connection, a voltage will build up on the chassis relative to earth and a person could discharge this voltage if in contact with the chassis and earth (standing barefoot on wet ground). With the earth connection, this charge buildup safely bleads off and there is no voltage difference between chassis and earth.

In an off grid system, an earth rod should connect the safety ground to earth to prevent this buildup. When connected to the grid, that connection is made in the service entrance, but it is generally safe to make another safety to earth connection locally if desired and is recommended if the grid can be disconnected (e.g., unplugged) or if there is no grid connection.

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delf67 avatar image delf67 calle commented ·
There is zero possibility of current flowing back down the earth to the transformer. When the transfer relay is open, you only have 1 wire connected to the grid, the earth. There is no circuit for current to flow through, just a single wire.


If you disconnect the supply neutral on any 3-phase system with unbalanced loads, yes the voltages will rise to near phase-phase voltage, but disconnecting the neutral is not the same thing as having a high earth-loop impedance through your earth rod.

Disconnecting your earth rod has no difference on voltages in your system.

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Robert Boerescu avatar image Robert Boerescu commented ·
@Kevin Windrem Kevin, do the ac out 1 and ac out 2 in multiplus 2 use a common neutral wire?
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Kevin Windrem avatar image Kevin Windrem Robert Boerescu commented ·
From the block diagram it appears they are common. What isn't clear from that diagram is if there is anything in the neutral paths of either output like current transformers.

Victron is probably the only one who knows for sure.

In any case, I'm not sure it would be safe to use a common neutral as the internal wiring may not support the combined current of both outputs.

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phoenix avatar image
phoenix answered ·

Answer based on UK regs.

The production of 3 phase 400/220 with a neutral in an unbalanced, earthed or unearthed system will not cause voltages on each phase to go out of specification. Earthing has no affect on voltage. Earthing is there for personal protection.

If you wire the mp2 to the grid with an Earth connection, that will be adequate for the output circuits. The mp2 has a relay to make the earth to neutral connection when grid is not available, giving tnc. This will not push current back to the grid.

Whilst you could install a rod and go TT, unless you have the test gear, it could be worse resistance than that at your incomer. You would also need to install a 100mA time delay RCD to meet regs.


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calle avatar image calle commented ·
Thank you! I still do not understand where the circuit between phase and neutral is completed if we cannot rely on the grid or the earth rod.


I’m quite certain that I would get 0-400V on my single phase asymmetric loads if I disconnect the neutral from the mp2 ac output.


Consider that L1 carries 10A, L2 10A and L3 20A. I will have 10A returning to neutral, where does that current go if not back to the transformer earth via cable or via ground through the earthrod.

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ejrossouw avatar image ejrossouw commented ·

"If the Power Generating Module is intended to operate independently of the DNO’s supply, the Power Generating Module shall include an earthing system which does not rely upon the DNO’s earthing terminal." This introduced the requirement for an earth rod if the system will power critical loads during a grid outage and is best to consult with an electrician which earthing system you have.

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delf67 avatar image
delf67 answered ·

The transformer coil is the circuit between phase and neutral in the MP2. The current is generated in the coil, flows out thru the phase, through the loads, and back down the neutral. The earth is bonded to the neutral in the MP2. The idea of earthing is to make an alternative, low-resistance path back to the star point of your power source for a fault current so it doesn't flow through you!

An earth rod in a completely off grid setup is to ensure your system ground (from the MP2 E-N bond) and all your grounded appliances are at the same 0V reference point as the general mass of earth. This is so if you are standing in bare feet in a damp basement touching a metal-cased electrical appliance, there is no voltage difference between you and the case.

If you lose your earth rod connection or even if you lose your E-N bond, you still have phase and neutral circuits intact. The only implication is that the safety of your system would be compromised with regard to earth faults.

In your system a local earth rod is probably unnecessary as you get your 0V reference from your earth connection to the grid.

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calle avatar image calle commented ·
Thank you!
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phoenix avatar image phoenix commented ·

@delf67 Just to clarify, earth is only bonded to neutral on the MP2 AC OUT 1 when you have loss of grid (also called island mode)

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delf67 avatar image delf67 phoenix commented ·

Yes, that is correct.

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sharpener avatar image sharpener commented ·

"In your system a local earth rod is probably unnecessary as you get your 0V reference from your earth connection to the grid. "

This is not allowed in the UK, you must have a separate earth rod with resistance less than 200 ohms.

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delf67 avatar image delf67 sharpener commented ·

A TT system requires an earth rod with <200 ohm resistance to give correct disconnection times for your protective devices. But this isn't a TT system.


Your star point in "island mode" is within the MP2, so no matter what the resistance of your earth rod, your earth-loop readings will be the same, and your protective devices will operate correctly.

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sharpener avatar image sharpener delf67 commented ·

OP talks about "off grid capability". Without a grid connection (and you cannot rely on their earth connection in island mode) you need a separate earth connection to complete the circuit, so that if you touch a live fitting a current will flow and trip the protective RCD. Otherwise you may not experience the current but you will not detect the fault either

I agree this is not a TT connection, in island mode it becomes TN-S. But an earth electrode <200 ohms is called for by the IET CoP nevertheless. This is for stability concerns, the maximum that will keep the voltage to <50V with 30mA flowing is 1667 ohms but they consider a resistance that high already may go even higher in e.g. unusually dry weather.

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delf67 avatar image delf67 sharpener commented ·
I stand corrected.
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Craig Chamberlain avatar image Craig Chamberlain delf67 commented ·
I concur with @sharpener an earth rod must be installed in order to use island mode in the UK according to current regs. It might seem unlikely that you would lose the earth on your grid feed but for example on a TN-C-S (PME) system, you only need to lose the neutral connection on the service cable and you've lost earth as well because the earth is derived from the neutral at the service head.


In my own situation where I live in the country and have my own 11kV transformer with an earth rod/mat at the base of the pole and then a concentric service cable where the earth and neutral surround the line conductor, it's very unlikely that I would lose the neutral and earth without also losing the line conductor (ie a PEN fault) and it's also very unlikely that I would lose the earth without noticing a JCB digging in my driveway. But regardless of this, I bought and earth pit and rod to comply with regs rather than take the risk. It's not expensive so why not.

The Ze earth impedance I get with my grid connection is 0.13ohms which is way lower than I've got any chance of getting with the earth rod or even several of them, so I will be keeping the earth rod and grid earth connected at the MET (which is allowed) and in most cases the earth rod will never be needed.

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Craig Chamberlain avatar image Craig Chamberlain Craig Chamberlain commented ·

FYI, I installed my earth rods today and was very pleased to achieve a 20.0 ohm earth fault loop impedance. I installed two 1.2m rods attached end to end to form a single 2.4m rod. I used 5/8 inch rods because I knew my ground conditions were very stoney and I’m glad I did because it was indeed hard going beyond the first metre or so. Ended up using a full size sledge hammer to drive the last metre but got there in the end.

I installed the rod into a Furse PT205 heavy duty earth pit from TLC which I think was great value. I didn’t really need a heavy duty pit but this one works nicely with my paving slab sizes and lets me just lift a 300x300mm slab and replace it with the pit and a bit of mortar pointing.

I used 10mm earth cable protected by a short length of kopex because I had the 10mm cable already and my PFC is only 12A which is way below what the 10mm cable can handle. It’s now wired back to my MET. Attached a “Saftey Earth Connection - Do Not Remove” label inside the pit and labelled the cable at the MET accordingly, I’ve got 30mA RCBOs fitted to all island mode circuits so all are protected.

Anything I’ve missed? Hopefully safe and compliant now for island mode.

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daza avatar image daza Craig Chamberlain commented ·

Lol got the same from TLC but I’m hoping for six 5/8 rods deep just waiting for the tool for my drill and I’ve run 25mm in conduit back to the MET, OTT I know just hoping to get the lowest reading I can and it’s clay soil so deeper for when the soil drys out, like you all RCBO’s in both boards

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Craig Chamberlain avatar image Craig Chamberlain daza commented ·

I'm curious why you feel you need such heavy cable or so many rods? Are you sure you even have enough depth of soil before you hit bedrock? My soil type is also heavy clay once you get below the surface layer of rubble and stones. I don't have a problem with it drying out though - it is always soaking wet beyond 2 feet deep even in the middle of summer.

10mm2 cable has a resistance of 0.00183 ohms per metre and in my case I run about 5 metres so that's 0.00915 ohms total. 25mm2 cable has a resistance of 0.000727 ohms per meter so that would be 0.003635 ohms total for 5 metres. These cable resistances are insignificant compared to the resistance of the overall earth loop. According to the OSG, even 2.5mm2 cable is fine as long as it's protected against corrosion and mechanical damage.

As for the use of 6 rods compared to 2, I have read somewhere that each doubling of rod length reduces resistance by 40% which, if true means that I could get maybe 20 * 60% * 60% = 7.2 ohms if I tripled my overall rod length which would yield around a 32A PFC which is half the current capacity of 10mm2 cable clipped direct. From the topology of the land around me here I suspect I don't have enough soil depth for 6 rods and 2 was hard enough to drive in.

Don't get me wrong, there is no harm in over-engineering things other than the harm to your wallet, but have you considered maybe distributing your 6 rods around the property, perhaps as 3 x 2.4m rods linked back to the MET using smaller cross section cable? Or maybe 2 x 3.6m rods? Or have you considered a copper mesh earth mat buried 600mm deep? I understand these achieve good results in many cases.

Let us know how you get on in any case. I kinda wish I had done a loop test with just rod one installed before I installed rod two just out of curiosity. It'd be really interesting to see how additional rods reduce loop impedance as you add 1-6 of them.

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daza avatar image daza Craig Chamberlain commented ·

Not at bedrock it’s still going only 4rods in at the mo my chisle drill is struggling now, just borrowed a titan to see if all will go down may only get 5 before it stops but should give really low readings.

I thought about distributing them around face it that is a lot more more and offers multi point failure ie would I ever be able to tell if one earth rod connection to ground wasn’t as good thus mucking up what I originally had easier just to test with one connection point.

I thought about the matts and the coiled disc, but sticking to the more traditional methods as they have worked for years. My run is 10 to 15meters long in order to avoid all the noise in the ground. With the distance the online chart for the length and diameter showed resistance decreased and knocked 600 to 900 off of the .000 figure, yeah I could have gone with 16mm which would have been a lot easier to go through the conduit as that was a pig. I’m only looking to do it once with something that will last / stand the test of time. I’ve not got the equipment to test it the sparks are coming round this weekend so just trying to do as much as I can to cause minimal downtime, as we still got to run the 5 core 25mm SWA to the garage that is going to be hard enough plus the duplex CU and another box in the house to house all the 25mm connections will let you know what reading I get when the boys test it

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Craig Chamberlain avatar image Craig Chamberlain daza commented ·

I actually have a Titan but didn't bother trying to use it because I assumed a sledge hammer would be more effective. Maybe that was a bad assumption. Perhaps I could have saved myself a lot of sweat and cursing!

In a multi location setup, I imagine you would test each earth separately before combining them at the MET but I take your point. I'm fortunate that I do have a Megger MFT so I can test the loop impedance myself, and retest it at different times of the year to gauge the effect of ground drying etc.

My TN-C-S ground is 0.15 ohms and it's extremely unlikely that I would ever lose my DNO earth except if someone was digging deeper than about 3 feet with a JCB in my driveway. So for me, the earth rods are just to comply with regs and provide a way to discharge any static potential when running in island mode.

Yeah, I bet the 25mm 5 core isn't much fun to work with. I've got two SWA cables going between house and garage. Both are 10mm2 CSA with a 3 core for the sub main (line, neutral, bonding for garage steel structure) and a 4 core for the solar PV (2 strings) because I'm about to install 6.8kWp of panels on my garage roof but I want the MPPTs to be next to the Multiplus and batteries which are in the house. Those cables were hard enough to work with and not cheap either! I hate to think what 25mm2 cables are like, especially with 5 cores.

Good luck getting your remaining rods sunk, even if you end up with 5 it'll be fine I'm sure.

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daza avatar image daza Craig Chamberlain commented ·
Yeah cables are not cheap at all, but just want to do it once and done, the 25mm 5core was so I could run a EV charger from the duplex CU in the garage and I need to take the main tails into a junction box with two DP isolation switches so I can join all the wires and make it look nice at the sametime,


I’ve not run the 5 core yet as need to take up the block paving and I need help as it’s not easy to manipulate and at 60Kg on a massive real not a one man job lol plus need to get the old 10mm swa that I ran last time to replace it with this one, yeah I dread to think how hard it would have been with a sledgehammer, the bit that screws to the earth rod isn’t cheap for what it is but I would probably still be whaking down the 3rd rod still. I’m buying stuff month by month as regards to all in one go which would pro be cheaper getting a deal but can’t do it like that just have to work all the hours that god sends lol.


I’ll let you know how I get on. Good luck with yours mate that’s a big array that your installing.
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daza avatar image daza Craig Chamberlain commented ·

Only just over 4.5meters that was hard work, took like 2hours to sink the 4th all the way then over half of the 5th

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daza avatar image
daza answered ·

Just finished wiring achieved earth loop impedance from the board furthest from the MET of 2.15ohms so I’m happy with that

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