Multiplus II grounding - again

[cdsolar]

Yes, yet another grounding question.

I am designing a DC coupled system that will have a Victron inverter and SCC, and the inverter and batteries power a critical loads panel for the house. Said critical loads panel can be switched by a transfer switch to be powered by the grid/main panel or the inverter. There will be no grid feedback.

PV will be ground mounted.

I am in the USA and NEC compliance is required.

1. PV EGC will come into a grounding busbar, where all DC cases and enclosures, are connected. Said busbar will connect to the main panel ground where the neutral/ground bond is formed, then out to the rods at the side of the house via existing wiring.

2. Battery floats with no +/- ground, only the case is grounded to the ground busbar.

3. The AC-in of the Victron will come from the main panel and *only* be used to charge the batteries and power loads when batteries are low, otherwise ignored or disconnected.

My question concerns what to do with the chassis ground and the ground relay.

Victron recommends (in the Multiplus II manual), to not use the chassis ground in a fixed (as opposed to a mobile (RV)) installation where AC-in is always present. But in my case that is not true (AC-in won't always present). I am aware that the ground relay in the inverter allows the AC-in to provide grounding when it is used.

In a mobile installation, Victron says to connect the chassis ground to DC-negative busbar, and then the DC negative to ground or the vehicle chassis.

But In my situation, what should I do?

A. Connect Victron chassis ground lug to aforementioned ground busbar?
B. Connect to DC negative bar and DC negative bar to ground busbar?
C. Something else?

Implicit in these choices is what to do with the ground relay, but if I connect the chassis ground, I suspect I need the relay.



Thanks.

 

[timselectric]

A , yes.
B , no.

Victron recommends (in the Multiplus II manual), to not use the chassis ground in a fixed (as opposed to a mobile (RV)) installation where AC-in is always present. But in my case that is not true (AC-in won't always present).

You are only switching the line conductors with the transfer switch. So the AC in neutral and ground (N/G bond) is still always present. And this is what matters.

 

[Q-Dog]

Victron recommends (in the Multiplus II manual), to not use the chassis ground in a fixed (as opposed to a mobile (RV)) installation where AC-in is always present.

That's not what it says.

• In a fixed installation, an uninterruptable grounding can be secured by means of the grounding wire of the AC input. Otherwise, the casing must be grounded.

 

[ricardocello]

If the critical loads panel neutral is not wired to the house already,
I would enable the ground relay when inverting in the Multiplus, because of this Victron block diagram (Multiplus-ii).
I’ve circled the input relay and ground relay.



Any circuits on the AC outs will not be connected to the house neutral when inverting.

On the other hand, if the critical loads panel is wired to the house neutral already through the transfer switch (as Tim said), then that switched neutral on the Multiplus input relay is meaningless because it is bypassed, and the ground relay should be disabled to avoid two G-N bonds.
Confusing, eh?

Your ground busbar must still connect to the house ground lines, there should only be one grounding system.
All metal chassis should be grounded.

Also, A yes, B hell no.

What kind of transfer switch? Manual? Auto? Panel interlock breaker?

 

[cdsolar]

Thanks everyone. @ricardocello I haven't decided yet whether to use a manual or automatic transfer switch or interlock. For now I am assuming manual, but that may change. Just concentrating on one piece at time for the moment. I've done my barn and got it approved, but taking it slowly for the house.

The critical loads panel does not exist yet.

Based on information provided here, I will use the chassis ground. I did a quick drawing. Here it is.

Edit: replaced drawing

 

[ricardocello]

Thanks everyone. @ricardocello I haven't decided yet whether to use a manual or automatic transfer switch or interlock. For now I am assuming manual, but that may change. Just concentrating on one piece at time for the moment. I've done my barn and got it approved, but taking it slowly for the house.

The critical loads panel does not exist yet.

Based on information provided here, I will use the chassis ground. I did a quick drawing. Here it is.

You have decided to only run L1, so is the transfer switch single-pole or double-pole?
If double-pole, are you switching the neutral? I assume yes because you are enabling the ground relay.
Also, as shown, the main panel breaker for the AC-In connection would be single-pole.

----

No other questions about the drawing, but I do have a few thoughts.

In general, if you are going to run power from one location to another, it is usually best to run 4 wires {L1, L2, N, G},
for future expansion mostly, and to avoid the added expense of retrenching/burying/running new wires later.
Similarly, running wire larger than you need now is also good to handle future power needs.

The downside of both of these comments is of course the initial added cost, and most people have budgets to meet.
Maybe you'll never need 120/240V split-phase from your inverters. It's your call.

 

[cdsolar]

You have decided to only run L1, so is the transfer switch single-pole or double-pole?
If double-pole, are you switching the neutral? I assume yes because you are enabling the ground relay.
Also, as shown, the main panel breaker for the AC-In connection would be single-pole.

----

No other questions about the drawing, but I do have a few thoughts.

In general, if you are going to run power from one location to another, it is usually best to run 4 wires {L1, L2, N, G},
for future expansion mostly, and to avoid the added expense of retrenching/burying/running new wires later.
Similarly, running wire larger than you need now is also good to handle future power needs.

The downside of both of these comments is of course the initial added cost, and most people have budgets to meet.
Maybe you'll never need 120/240V split-phase from your inverters. It's your call.

Note:;I just updated the drawing....

I am just trying to understand the single phase at this time. I may try to design split phase, but want to understand fully before I complicate things, particularly grounding. It is just my way of learning. I'm not buying anything yet.

 

[ricardocello]

Note:;I just updated the drawing....

I am just trying to understand the single phase at this time. I may try to design split phase, but want to understand fully before I complicate things, particularly grounding. It is just my way of learning. I'm not buying anything yet.

You are doing great, keep asking questions. You'll know when you are ready to buy.

 

[cdsolar]

You are doing great, keep asking questions. You'll know when you are ready to buy.

My barn system, also single phase, was simple.

 

[cdsolar]

I woke up at 3am two days ago, and I think I have had an epiphany with regard to grounding for my proposed system.

In my proposed system above, I will have

A. a critical loads panel that is fed either by my main panel or my inverter(s), which are themselves fed from batteries and solar.

B. The inverters will have their AC-in fed from main panel for battery charging when/if necessary.

When in scenario A, the inverters are inverting, and are for all intents are operating as a separate system, with the Victron ground relay providing the N/G ground, as the house AC neutral is not involved or connected to the circuit provided by the inverter(s). As such all that is needed is for the chassis ground to have a place to go, thus it is connected to the house ground. But again, since no house neutral is involved in the inverter AC circuit, it needs to provide its own N/G bond with the ground relay closed.

When in scenario B, the AC-in provides house neutral and the ground, and the ground relay opens, as the house system itself provides the N/G bond, as the inverter should not do it on its own.

 

[timselectric]

I woke up at 3am two days ago, and I think I have had an epiphany with regard to grounding for my proposed system.

In my proposed system above, I will have

A. a critical loads panel that is fed either by my main panel or my inverter(s), which are themselves fed from batteries and solar.

B. The inverters will have their AC-in fed from main panel for battery charging when/if necessary.

When in scenario A, the inverters are inverting, and are for all intents are operating as a separate system, with the Victron ground relay providing the N/G ground, as the house AC neutral is not involved or connected to the circuit provided by the inverter(s). As such all that is needed is for the chassis ground to have a place to go, thus it is connected to the house ground. But again, since no house neutral is involved in the inverter AC circuit, it needs to provide its own N/G bond with the ground relay closed.

When in scenario B, the AC-in provides house neutral and the ground, and the ground relay opens, as the house system itself provides the N/G bond, as the inverter should not do it on its own.

It depends on the rest of the wiring.
I assume that the bypass around the inverter, between the main panel and sub panel, will still provide the N/G bond, in either scenario.
If so, then the inverter should never create a second N/G bond.

 

[SolarReaper]

In the UK, a lot of houses have a TN-C-S earthing arrangement, where the Earth and Neutral conductors are joined (or split off depending on which way you're looking at it) at the point the supply cable enters the building. Only two conductors go back to the DNO's transformer. Less metal in the ground similar to why a lot of our houses have ring mains rather than individual spurs to sockets, saved on copper in the war years and carried on for a long time afterwards.
The IET has these handy guides on how they suggest an islanding inverter should be earthed, where an earth rod is installed alongside the supply cable earth in case the combined neutral earth conductor is cut beyond the building potentially removing that earth connection back at the transformer.
The Consumers Earth Electrode at the bottom is the Earth Rod installed and permanently connected alongside the Neutral Combined Earth just to the left where the supply cable exits the building.
The Island Mode Isolator is the relay arrangement inside the Multiplus, and the Combined Neutral/Earth is reapplied by the N-E Bond relay (circled to the right in ricardocello's diagram above) within the Multiplus to keep the earthing arrangement as it would have been had the supply cable been connected, though it is now using the earth rod as the earth locally rather than at the transformer down the road (assuming the cable has been cut) If the cable hasn't been cut then it uses it anyway alongside the earth rod.

 

[aonangj]

I’m having a really hard time understanding this concept as well as there seems to be so little information and what is seems to sometimes conflict even when the scenarios seem the same. So to make it simple, as I think after reading everything this is, all items with EGC case connections (Quattros, MPPTs, EG4 Batteries and case) should run to the Electrical panel N bar that connects in some way (depends on setup) back to the main grounding rod. This can happen via a negative bus bar, or individual wires large enough to carry the loads that could be carried through them (seems like generally 6awg copper should suffice in a split phase scenario. Any major concerns with this?

I’ve seen some other non-mobile scenario diagrams showing the inverter/MPPTs grounding back at the battery itself but it seems based on these discussions that the case grounds should ultimately be grounded back to earth via the ground rod which grounding them back to the batteries does not accomplish