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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calle avatar image calle delf67 commented ·

Thank you for the answer!

Is the transfer relay is the one opening L and N on AC-in when the system goes off grid? And it bonds N and PE on AC-out?


Where does the mp2 create a circuit between phase and neutral when in off grid? I don’t understand how it’s possible to complete the circuit without either using the grid or a local earth rod. If the mp2 bonds neutral and PE on AC-out and the PE is connected to the grid’s PE (effectively PEN in our TN-C system) I can’t see how the neutral current will flow any other way than back to the grid if we can’t feed it back down the earth rod.


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klim8skeptic avatar image klim8skeptic ♦ calle commented ·

@Calle You have not looked at Appendix B: block diagram of the manual yet??

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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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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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