question

There are two purposes for the safety ground (aka PE) connection.

First is a path for fault currents. If load develops a short between the hot leg and the chassis, the chassis will be elevated to the voltage of the hot leg unless there is a path from the chassis to the neutral leg at the source. This is different from the neutral conductor run to all loads and normally carries the return current. In normal electrical systems, there is a connection between the safety grounds from the branch circuits to the neutral conductor where the service enters the building. So if there is a fault in a piece of equipment, the current flows back to the service entrance safely and trips the branch circuit breaker or fuse.

There can also be fault paths for the DC side of a system like you are building. A floating chassis could reach the battery voltage and is less lethal but there could still be opportunities for system damage due to the high currents involved. So the same safety ground mechanism should be in place.

The safety ground circuitry doesn't need to be as large as the normal current carrying conductors. It's there only to trip the breaker or blow the fuse and to keep equipment cabinets at a safe voltage relative to "earth". Typically, 1/4 the cross-sectional area is sufficient but check local codes.

The "earth" connection is a continuation of the safety ground to the earth by way of a ground rod. This is a relatively high resistance path and is not a substitute to the safety ground connection to neutral at the service entrance. In fact, a ground rod may not even trip a 20 amp breaker. The purpose of this earth connection is to drain any static charge off the electrical system. Without an earth connection to the safety ground system, a nasty static charge could build up on the chassis of the vehicle or metal parts of a building. Earth connections carry very little current and can be on the small side.

Most Victron inverters and inverter/chargers include two important relays: an AC input relay that disconnects the grid from the inverter/charger core and the AC output; a ground relay that makes a neutral/safety ground connection. The ground relay is open when the AC input relay is closed because the incoming AC already has a neutral/safety ground connection up stream (at the service entrance). But when the inverter disconnects from the AC input there is no neutral/safety ground connection, so the ground relay closes. The ground relay then allows fault current to return to the neutral side of the inverter.

Some generators "bond" its neutral to the generator chassis internally. Some (most?) to not. The reason a generator does not have a neutral/safety ground connection is to avoid issues when the generator is connected to a transfer switch feeding a panel. Transfer switches typically do not switch the neutral connection. So the service entrance provides the neutral/safety ground path (there can only be ONE such connection).

When generators are used in an off grid situation, it's essential the installer creates an appropriate neutral/safety ground connection for the generator.

I'm not sure why but portable generators often do not have an internal neutral/safety ground connection. So its AC output floats relative to it chassis and the chassis of anything plugged into it. If a fault occurs, there is not return current path and the equipment chassis will be connected to the hot side of the AC from the generator. But this is a little less dangerous since the neutral will simply float allowing the hot leg to rest at the chassis ground voltage. That is unless another device has a neutral/safety ground fault!

@Kevin Windrem Thank you for your detailed reply.

I'll try and dissect your reply to see if I can turn it into practical wisdom applicable to my system, like a 5 year old would do.

If I do something wrong it's an opportunity to fix it and allow other people not to make the same conceptual/practical mistakes.

First is a path for fault currents. If load develops a short between the hot leg and the chassis, the chassis will be elevated to the voltage of the hot leg unless there is a path from the chassis to the neutral leg at the source. This is different from the neutral conductor run to all loads and normally carries the return current.

So if something catastrophically wrong happens inside one of the components, let's say inside the Quattro inverter/charger, the positive wire can touch the chassis. At this point, the new path of current could be through the chassis, if someone like me ignores this stuff and touch the chassis together with (let's say) the negative busbar. Now the circuit is close, I'm part of the new path and I'm fried.

For this reason, a ground connection is needed, which is a cable connected to the chassis of the components, back to the negative busbar (which is source, being connected to the battery bank). In my case that would be the central negative connection inside the Lynx Distributor.

When such connection is established, and something goes wrong inside one of the components, the new path of the current will be through this new ground wiring, because it offers way less resistance than my body.

So...lesson number 1: all chassis of all the components should be wired together and connected to the negative busbar.

Am I right?

In normal electrical systems, there is a connection between the safety grounds from the branch circuits to the neutral conductor where the service enters the building. So if there is a fault in a piece of equipment, the current flows back to the service entrance safely and trips the branch circuit breaker or fuse.

This would be the AC side of the system, where a safe neutral/ground connection is established inside the AC distribution (the panel with breakers)?

If something goes wrong in one of the branches (i.e. hot wires touches the metal casing of a lamp), the current has a way back to the source (which is the breaker) through the green ground connection that is (hopefully) provided with the lamp ---> the breaker opens the circuit (and shuts down the current).

Lesson number 2: all branch circuits in AC side, should have a ground from equipment/appliance back to the breakers panel. Also, make sure that the breaker panel has neutral/ground connected together (ground busbar connected to neutral busbar, inside the panel).

There can also be fault paths for the DC side of a system like you are building. A floating chassis could reach the battery voltage and is less lethal but there could still be opportunities for system damage due to the high currents involved. So the same safety ground mechanism should be in place.

I guess "floating chassis" == no ground connection between chassis and negative busbar?

This is already covered in lesson number 1, right?

The safety ground circuitry doesn't need to be as large as the normal current carrying conductors. It's there only to trip the breaker or blow the fuse and to keep equipment cabinets at a safe voltage relative to "earth". Typically, 1/4 the cross-sectional area is sufficient but check local codes.

In DC side, that would be 1/0 AWG, because I'm using 4/0 AWG from +/- busbar to inverters.

So it's not 4/0 AWG as I said, but neither is 6 AWG. Correct?

The "earth" connection is a continuation of the safety ground to the earth by way of a ground rod. This is a relatively high resistance path and is not a substitute to the safety ground connection to neutral at the service entrance. In fact, a ground rod may not even trip a 20 amp breaker. The purpose of this earth connection is to drain any static charge off the electrical system. Without an earth connection to the safety ground system, a nasty static charge could build up on the chassis of the vehicle or metal parts of a building. Earth connections carry very little current and can be on the small side.

So the ground rod is needed to drain the static charge, and being no substitute for a safety ground connection between all the chassis' components (DC side) and all circuit branches (AC side), it should be implemented together with said safety ground connection.

Lesson number 3: a ground rod is a needed addition to the safety of the system. I should connect the whole ground wiring (DC and AC) to a ground rod.

Probably my best bet would be to wire the DC ground (from Lynx Distributor) and the AC ground (from breakers panel) to a dedicated ground busbar, and then to a dedicated rod.

Most Victron inverters and inverter/chargers include two important relays: an AC input relay that disconnects the grid from the inverter/charger core and the AC output; a ground relay that makes a neutral/safety ground connection. The ground relay is open when the AC input relay is closed because the incoming AC already has a neutral/safety ground connection up stream (at the service entrance). But when the inverter disconnects from the AC input there is no neutral/safety ground connection, so the ground relay closes. The ground relay then allows fault current to return to the neutral side of the inverter.

I'm pretty sure that my Quattros do have such neutral/ground internal connection.

And because what I have is an off-grid system, with absolutely no AC inputs for the inverter (no grid, no generator, no nothing), the neutral/ground connection is automatically established by the inverter, inside the inverter.

(I believe there's a checkbox inside the inverter configuration, I just have to make sure that it's checked so that the connection is established).

Which means that if a ground fault occurs, a safe path for the current is automatically established (provided the ground wiring). And the 250 amp fuses are in charge of shutting down the current if something bad happens (same as the breakers for the AC side).

Lesson number 4: if an inverter/charger is powered by both a battery bank AND the grid (or a generator), this whole ground safety connection scenario is a lot more complicated, and is highly dependent of the specs of the components.

Such as:

• does the inverter have a neutral/ground connection inside?
• does the generator have a neutral/ground connection inside?
• are you installing the right ground connections without creating ground loops?

But all of this would not change what happens for the ground connections for the AC output side of the system: the breakers panel and circuit branches (see lesson number 2 and 3).

---

I hope my attempt will be useful for other people.

Let me know if something went wrong in my analysis!

UPDATE: some stuff shown below is wrong. I'll update the diagram when everything has been fixed!

Ok I made a potato quality digram to summarize the whole grounding of my system, apart from the PV array which I guess deserves its own grounding rod.

I have a couple of questions:

• is it correct that I do not have to run a ground cable from the breakers panels to the main ground busbar, as I wrote in my previous reply? Reasoning being that the ground protection for the AC circuit branches is provided by inverters themselves, thanks to their internal neutral/ground connection. Which means that any ground fault in AC distribution will be redirected to the source, meaning the inverters, through the neutral wire.

• what happened to the RDC/GFCI? Wiring Unlimited says that in my case, I should install it just before the loads, which in my case would be between inverters and breakers panel. So that would mean to install a RDC/GFCI in the L1/L2/N wiring. But I've got two inverters, so should I install two RDCs/GFCIs? That's not clear to me.

Consider that with this system I'm powering both 120V and 240V loads, but it was impossible to find any 240V Victron inverter where I live, so I decided to buy two 120v Quattros and use them in split-phase.

All of the Victron system diagrams that show ground wiring detail has the Multi/Quatro chassis connected to the battery negative busbar. This will connect the battery negative to the AC safety ground through the internal wiring inside the Multi/Quatro. Also an earth connection is shown to this negative busbar.

E.g.,:

https://www.victronenergy.com/upload/documents/MultiPlus-system-example-3KW-24V-120V-AC.pdf

For Land based PV installs:-

A connection to earth of any of the current carrying d.c. conductors is not recommended. However,
earthing of one of the live conductors of the d.c. side is permitted, if there is at least simple
separation between the a.c. and the d.c. side. Where a functional earth is required, it is preferable
that where possible this be done through high impedance (rather than directly).

Connecting to earth is a very complex duscussion and many have opposing opinions. I have pasted the guidelines from our "PV Land" installs here in the UK. Please feel free to digest and comment.

Guide to the Installation of Photovoltaic Systems
38
2.2 Design Part 2 – Earthing, Protective Equipotential Bonding and Lightning
Protection
2.2.1 Lightning Protection
Whilst this installation guide does not cover specific guidance on selection, or application of
lightning protection, it was felt that a brief overview was required as given below. Where further
information is required, this can be referenced from BS EN 62305.
In most cases the ceraunic value (number of thunderstorm days per year for a given installation
location in the UK) does not reach a level at which particular protective measures need to be
applied. However where buildings or structures are considered to be at greater risk, for example
very tall, or in an exposed location, the designer of the a.c. electrical system may have chosen to
design or apply protective measures such as installation of conductive air rods or tapes.
If the building or dwelling is fitted with a lightning protection system (LPS), a suitably qualified
person should be consulted as to whether, in this particular case, the array frame should be
connected to the LPS, and if so what size conductor should be used.
Where an LPS is fitted, PV system components should be mounted away from lightning rods and
associated conductors (see BS EN 62305). For example, an inverter should not be mounted on an
inside wall that has a lightning protection system down conductor running just the other side of the
brickwork on the outside of the building.
Where there is a perceived increase in risk of direct lightning strike as a consequence of the
installation of the PV system, specialists in lightning protection should be consulted with a view to
installing a separate lightning protection system in accordance with BS EN 62305.
Note: It is generally accepted that the installation of a typical roof-mounted PV system presents
a very small increased risk of a direct lightning strike. However, this may not necessarily be the
case where the PV system is particularly large, where the PV system is installed on the top of a tall
building, where the PV system becomes the tallest structure in the vicinity, or where the PV system is
installed in an open area such as a field.
2.2.2 Earthing
Earthing is a means of connecting the exposed conductive parts to the main earthing terminal,
typically this definition means the connection of metallic casings of fixtures and fittings to the main
earthing terminal via a circuit protective conductor (cpc).
Importantly, it must be noted that we only make this connection when the accessory or appliance
requires it. This connection is required when it is considered to be a class I appliance or accessory
and is reliant on a connection with earth for safety using ‘automatic disconnection of supply’ (ADS)
as the fault protective measure.
Guide to the Installation of Photovoltaic Systems
39
As the d.c. side of PV systems is a current limiting generating set, the protective measure ADS is
almost never used and is outside of the scope of this guidance. In these circumstances, where the
d.c. side of the installation is constructed to meet the requirements of an installation using double or
reinforced insulation, no connection to earth between the PV Modules or frame and main earthing
terminal would be required.
Earthing of the inverter at the a.c. terminations will still be necessary where the inverter is a
Class I piece of equipment and must be applied where necessary. Where class I inverters are used
externally (ie field mount systems) careful consideration must be given to the requirements for
earthing.
2.2.3 Protective Equipotential Bonding
Protective equipotential bonding is a measure applied to parts of the electrical installation which,
under fault conditions may otherwise have a different potential to earth. By applying this measure
the risk of electric shock is limited as there should be little or no difference in voltages (potential
difference) between the parts that may otherwise become live. These parts are categorised as either
Exposed-Conductive-Parts or Extraneous-Conductive-Parts
In most PV systems there are no parts that are considered to be an exposed-conductive-part or
extraneous-conductive-part, therefore protective equipotential bonding is not usually required. For
guidance on when to consider protective equipotential bonding please see the decision tree on the
next page.
On the d.c. side of the PV installation the designer will have usually already selected double
or reinforced insulation as the protective measure and therefore the component parts of the
installation will be isolated and will not require protective equipotential bonding.
Guide to the Installation of Photovoltaic Systems
40
Earthing and or Bonding Decision Tree:
5 1
Earthing and/ or bonding of PV array frames
Is the d.c. side of the installation constructed to meet the requirements for an
installation using double or reinforced insulation as a protective measure?
Is the PV array frame an extraneous
conductive part
Is the array frame an exposed
conductive part ?
No protective
equipotential
bonding required
Protective equipotential
bonding as defined in
BS7671 should be applied
Earthing should be
applied if required
by BS7671
YES
YES YES
NO
NO
NO
Fig 10
Guide to the Installation of Photovoltaic Systems
41
2.2.4 Determining an Extraneous-Conductive-Part
The frame of the array has to be assessed as to whether it is likely to introduce a potential into the
installation. This aim of this assessment is to find out if the frame has any direct contact with ground
that would make it introduce a potential.
The details on carrying out these tests are best given in the IET BS 7671 Guidance Note 8 Earthing
& Bonding and this should be referred to before undertaking a test. The principle behind the test is
to ascertain whether or not there is a low enough conductivity between the part under test and the
Main earthing terminal (MET) to say that it could introduce an earth potential.
To find this out a resistance test should be carried out between the part in question (the array
frame) and the MET of the building. Where the value recorded is greater than 22kΩ (most cases)
the part can be considered to be isolated from earth and NOT an extraneous conductive part. If
however the reading is less than 22kΩ, then the part is considered to be extraneous and protective
equipotential bonding, as required by BS 7671, should be applied.
Where the array frame is mounted on a domestic roof or similar, the likelihood of the frame being
an extraneous-conductive-part is very low - due to the type and amount of material used between
the ground and the roof structure (which will mainly be non-conductive). Even in the case of an
array frame being mounted on a commercial building where mostly steelwork is used, it is likely that
the frame will be either isolated, and therefore not required to be bonded, or will be bolted to the
framework or steelwork of the building which will often be sufficient to maintain bonding continuity
and a sufficiently low enough resistance to consider it to be bonded through the structure itself.
Careful consideration needs to be given to systems that are ground mounted as they may initially
appear to be an extraneous-conductive-part. However, as they are usually a good distance away
from the earthed equipotential zone, by bonding them you may well be introducing a shock risk that
wasn’t there initially, and in the case of an installation supplied by a TN-C-S (PME) supply you may
be contravening the supply authority’s regulations (ESCQR 2002). In most cases these installations
wouldn’t require bonding – in such cases the designer must make an informed decision based on the
electrical design of the entire installation, not just the PV system in isolation.
2.2.5 System Earthing (d.c. Conductor Earthing)
There are a variety of possible PV array system d.c. earthing scenarios which can be broadly
summarised as follows:
No earth connection•
Hardwired connection of positive or negative conductor to earth•
Centre tapped array – with / without earth connection•
High impedance connection of positive or negative conductor to earth (for functional reasons)•
Guide to the Installation of Photovoltaic Systems
42
The manufacturer’s instructions for both the PV modules and the equipment to which the PV array is
connected must be taken into account in determining the most appropriate earthing arrangement.
A connection to earth of any of the current carrying d.c. conductors is not recommended. However,
earthing of one of the live conductors of the d.c. side is permitted, if there is at least simple
separation between the a.c. and the d.c. side. Where a functional earth is required, it is preferable
that where possible this be done through high impedance (rather than directly).
The designer must confirm whether the inverter is suitable for earthing of a d.c. conductor.
Transformerless inverters will not be suitable, and an earthed conductor may interfere with the
inverter’s built-in d.c. insulation monitoring. Hence, if an earthed d.c. conductor is required, this is
ideally done in the inverter in accordance with guidance from the inverter manufacturer.
NOTE: In the case of PV systems connected to an inverter, IEC62109-2 (Safety of Power convertors
for use in photovoltaic power systems – Part 2: Particular requirements for inverters), includes
requirements according to the type of earthing arrangement (and inverter topology). These
include minimum inverter isolation requirements, array ground insulation resistance measurement
requirements and array residual current detection and earth fault alarm requirements.
2.2.5.1 Systems with High Impedance Connection to Earth
A high impedance connection to earth of one of the current carrying conductors may be specified
where the earth connection is required for functional reasons. The high impedance connection
fulfils the functional requirements while limiting fault currents.
Where a functional earth is required, it is preferred practice that systems be functionally earthed
through high impedance rather than a direct low impedance connection (where possible).
2.2.5.2 Systems with Direct Connection to Earth
Where there is a hardwired connection to earth, there is the potential for significant fault currents
to flow if an earth fault occurs somewhere in the system. A ground fault (earth fault) interrupter
and alarm system can interrupt the fault current and signal that there has been a problem. The
interrupter (such as a fuse) is installed in series with the ground connection and selected according
to array size. It is important that the alarm is sufficient to initiate action, as any such earth fault
needs to be immediately investigated and action taken to correct the cause.
An earth fault interrupter shall be installed in series with the earth connection of the PV array
such that if an earth fault occurs the fault current is interrupted. When the earth fault interrupter
operates, an alarm shall be initiated. The nominal overcurrent rating of the interrupter shall be as
follows:
Guide to the Installation of Photovoltaic Systems
43
Array size Overcurrent rating
≤3kWp ≤1A
3- 100KWp ≤3A
>100kWp ≤5A
The earth fault alarm shall be of a form that ensures that the system operator or owner of the
system becomes immediately aware of the fault. For example, the alarm system may be a visible or
audible signal placed in an area where operational staff or system owners will be aware of the signal
or another form of fault communication like Email, SMS or similar
NOTE: In grid connected systems, an earth fault alarm may be a feature of the inverter. In such
systems and where the inverter is located in a remote location, the system should be configured
so that a secondary alarm is triggered that will be immediately seen by the system operator. For
systems in accordance with BS 7671 conductors used for earth fault detection are usually cream in
colour.
2.2.6 Surge Protection Measures
All d.c. cables should be installed to provide as short runs as possible and positive and negative
cables of the same string or main d.c. supply should be installed together, avoiding the creation of
loops in the system. This requirement includes any associated earth/bonding conductors.
Long cables (e.g. PV main d.c. cables over about 50 m) should be installed in earthed metal conduit
or trunking, or be screened cables such as armoured.
Note: These measures will act to both shield the cables from inductive surges and, by increasing
inductance, attenuate surge transmission. Be aware of the need to allow any water or condensation
that may accumulate in the conduit or trunking to escape through properly designed and installed
vents.
Most grid connect inverters have some form of in-built surge suppression; however discrete devices
may also be specified.
Note: Surge protection devices built into an inverter may only be type D and a designer may wish
to add additional (type B or C) devices on the d.c. or a.c. side. To protect the a.c. system, surge
suppression devices may be fitted at the main incoming point of a.c. supply (at the consumer’s cut-
out). To protect the d.c. system, surge suppression devices can be fitted at the inverter end of the d.c.
cabling and at the array. To protect specific equipment, surge suppression devices may be fitted as
close as is practical to the device.