PV earthing UK

Hi,

I'm based in the UK doing an off grid system and cannot help but notice the lack of equipment and information regarding PV earthing... Like grounding bolts (Tyco), etc. There's BS7671 as a NEC equivalent but offers little information about pv earthing ...

So I paid attention when going out and about in town, none of the PV installations that I could spot were grounded...

I know it's not compulsory, but still I think it is something very good to have, right? It's not common to have thunderstorms in this area though.

Cheers

It's a good idea to do it to bleed off static buildup. also for lightning (supposedly) and to give fault currents a path to take.

With regards to lack of availability for the specific things needed, Improvise, Adapt, Overcome applies I believe.

That's what I have found anyway. Get creative (within reason)

Well said, that's what I'm trying to do

I think its down to weather, building material differences and perception of risk.

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.

Continued:-


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.