1. The document discusses earthing and protection against indirect contact in photovoltaic systems. It addresses earthing of exposed conductive parts and the power generation system, as well as requirements for earthing resistance. 2. Plants with and without transformers are considered. For plants with transformers, the document distinguishes between exposed conductive parts upstream and downstream of the transformer. 3. Requirements are provided for different earthing systems including TN, IT, and TT, and the need for residual current devices is discussed in relation to single faults or double faults.

1. 5 Earthing and protection against indirect contact
5.1 Earthing In this case the earthing resistance Re of the exposed
5 Earthing and protection against indirect contact
conductive parts shall meet the condition (CEI 64-8):
The concept of earthing applied to a photovoltaic (PV)
system may involve both the exposed conductive parts 120
(e.g. metal frame of the panels) as well as the genera- Re [5.1]
Id
tion power system (live parts of the PV system e.g. the
cells). where Id is the current of first fault to earth, which is not
A PV system can be earthed only if it is galvanically sepa- known in advance, but which is generally very low in
rated (e.g. by means of a transformer) from the electrical small-sized plants. As a consequence, the earthing re-
network by means of a transformer. A PV insulated sys- sistance Re of the consumer plant, which is defined for a
tem could seem apparently safer for the people touching fault in the network, usually satisfies only the relation [5.1].
a live part; as a matter of fact, the insulation resistance In case of a double earth fault, since the PV generator
to earth of the live parts is not infinite and then a person is a current generator, the voltage of the interconnected
may be passed through by a current returning through exposed conductive parts shall be lower than:
such resistance. This current rises when the voltage to
earth of the plant and the plant size increase since the Isc . Reqp ≤ 120V [5.2]
insulation resistance to earth decreases. Besides, the
physiological decay of the insulators, due to the pas- where Isc is the short-circuit current of the cells involved,
sage of time and the presence of humidity, reduces the whereas Reqp is the resistance of the conductor inter-
insulation resistance itself. Consequently, in very big connecting the exposed conductive parts affected by
plants, the current passing through a person in touch
fault. For instance, if Reqp = 1Ω (value approximated by
with the live part may cause electrocution and therefore
the advantage over the earthed systems is present only excess), the relation [5.2] is fulfilled for Isc not exceeding
in case of small plants. 120A, which is usual in small-sized plants; therefore the
effective touch voltage in case of a second earth fault
5.2 Plants with transformer does not result hazardous. On the contrary, in large-sized
plants it is necessary to reduce to acceptable limits the
In the plants with transformer, in addition to the analysis chance that a second earth fault occurs by eliminating
of the PV system either insulated or earthed, for the the first earth fault detected by the insulation controller
protection against indirect contacts it is necessary to (either inside the inverter or external).
make a difference between the exposed conductive parts
upstream and downstream the transformer1. 5.2.1.2 Plant with TN system
In this type of plant the live parts and the exposed con-
5.2.1 Exposed conductive parts on the load side ductive parts are connected to the same earthing system
of the transformer (earthing system of the consumer’s plant). Thus a TN
system on the DC side is obtained (Figure 5.2).
5.2.1.1 Plant with IT system
In this type of plant the live parts result insulated from Figure 5.2
A
earth, whereas the exposed conductive parts are earthed2
(Figure 5.1).
+ - + - + - B
Figure 5.1
A
Id
+ - + - + - B
Load
Id Re
Load 1
In this case upstream and downstream are referred to the direction of the electric power
produced by the PV plant.
2
For safety reasons the earthing system of the PV plant results to be in common with the
consumer’s one. However, to make the insulation controller of the inverter operate prop-
Re
erly and monitor the PV generator it is necessary that the frames and/or the supporting
structures of the panels (even if of class II) are earthed.
Photovoltaic plants 39

2. Technical Application Papers
In the presence of an earth fault, a short-circuit occurs conductive part of the transformer or of the inverter
as in the usual TN systems, but such current cannot when the transformer is incorporated, a residual current
5 Earthing and protection against indirect contact
be detected by the maximum current devices since the device4 shall be interposed as Figure 5.4 shows; this re-
characteristic of the PV plants is the generation of fault sidual current device detects the leakage currents coming
currents with values not much higher than the rated cur- both from the network as well as from the PV generator.
rent. Therefore, as regards the dangerousness of this fault, When the residual current device trips due to an earth
the considerations made in the previous paragraph3 on fault current, the inverter goes in stand by due to lack of
the second fault for an IT system are valid. network voltage.
Figure 5.4
5.2.2 Exposed conductive parts on the supply
side of the transformer A
Take into consideration the network-consumer system of
TT type. The exposed conductive parts belonging to the + - Id Grid
Idr
consumer’s plant protected by a residual current circuit- B
breakers positioned at the beginning of the consumer’s
plant (Figure 5.3) result protected both towards the
network as well as towards the PV generator.
Figure 5.3 Rn
A
Re Id
+ - IdPV Idr Grid
IdPV
B
Rn Load
On the contrary, if the network-consumer system is type
Id
TN, for both the supply possibilities, either from the net-
work or from the PV generator, residual current circuit-
breakers are not needed provided that the fault current on
Load the AC side causes the tripping of the overcurrent devices
by the times prescribed in the Std. (Figure 5.5).
Re Figure 5.5
A
There must not be an exposed conductive part between
+ - IdPV Grid
the parallel point A-B and the network because, in such Idr
B
case, the normative requirement that all the exposed con-
ductive parts of a consumer’s plant in a TT system must
be protected by a residual current circuit-breaker fails.
As regards the exposed conductive parts upstream the
parallel point A-B, such as for instance the exposed
Rn
3
The Std. IEC 60364-7 recommends that the whole installation on the DC side (switch-
Load
boards, cables, and terminal boards) is erected by use of class II devices or equivalent
insulation. However, to make the insulation controller of the inverter operate properly and
monitor the PV generator it is necessary that the frames and/or the supporting structures
of the panels (even if of class II) are earthed.
4
The rated residual current shall be coordinated with the earth resistance Re, in compliance
with the usual relation of the TT systems:
50
Re ≤
Idn
40 Photovoltaic plants

3. 5.3 Plants without transformer For earth faults on the DC side and on the exposed
5 Earthing and protection against indirect contact
conductive parts upstream the parallel point A-B, the
In case of absence of the separation transformer between residual current circuit-breaker on the load side of the
the PV installation and the network, the PV installation inverter is passed through by a residual current which is
itself shall be insulated from earth in its active parts be- not alternating. Therefore such device must be of type
coming an extension of the supply network, generally with
B5, unless the inverter is by construction such as not to
a point connected to earth (TT or TN system).
inject DC earth fault currents (IEC 60364-7)6.
As regards the exposed conductive parts of the con-
sumer’s plant and upstream the parallel point A-B, from
a conceptual point of view, what described in clause
5.2.2 is still valid.
On the DC side an earth fault on the exposed conduc-
tive parts determines the tripping of the residual current 5
The residual current device of type B detects the following typologies of earth fault
circuit-breaker positioned downstream the inverter (Fig- currents:
ure 5.6). After the tripping of the residual current device,
• alternating (also at frequency exceeding the network one, e.g. up to 1000 Hz);
• pulsating unidirectional;
the inverter goes in stand by due to the lack of network • direct.
voltage, but the fault is supplied by the PV generator. 6
The Std. CEI EN 62040-1 prescribes that the protection of the UPS (including an inverter)
Since the PV system is type IT, the considerations made against earth faults is realized by using residual current devices type B (for three-phase UPS)
and type A (for single-phase UPS), whenever an earth fault current with DC components
in clause 5.2.2.1 are valid. may be possible according to the UPS design.
Figure 5.6
IdPV A
Idr
Id Grid
type B
B
+ - + - + -
Rn
Load
Re
Photovoltaic plants 41