IMO Technical Brochure

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4
PV Installation Isolation
PV installations consist of the DC side, the Inverter and the AC side with isolation required for both the PV-array to the inverter and for the AC
supply from the load, particularly where the system is connected to the Distributed Network, this is a stipulation in G83/1. In some instances
the “Guide to Installation of PV Systems” allows inverter and DC string isolation to be provided by the same device, for example the PV plug
and socket connectors, but this is only deemed suitable for smaller systems and the connectors must be labelled appropriately. Generally
IMO would always recommend the use of a suitably rated DC isolator.
Voltage = N
S
x V
OC
x 1.15 Current = N
P
x I
SC
x 1.25
N
S
- Number of panels connected in series N
P
- Number of strings connected in parallel
V
OC
– Open-Circuit Voltage (from module manufacturer’s data) I
SC
– Short-Circuit Current (from module manufacturer’s data)
The isolator should also be suitable for use in the appropriate application which in PV installations is normally considered to be either
DC-21A, DC-21B, DC-22A or DC-22B. Normally isolation of the DC supply from the inverter would not be a regular occurrence and
therefore generally ratings for DC-21B or DC-22B would, as a minimum, be necessary; although category A types (as previously covered in
Utilisation Categories) would be advantageous due to their capability of a higher number of switching operations, and therefore a longer
guaranteed life.
AC Isolator Selection
AC Isolators are used in both stand-alone grid or network distributed systems. If connected to the distributed network then G83/1 stipulates
the PV system must be connected directly to an isolation switch that is wired so as to isolate both the live and neutral conductors, capable
of being secured in the “OFF” position and in an accessible location within the installation. In a stand-alone system IMO recommend that a
lockable OFF isolation switch is similarly used within the installation. BS 7671 species that isolators that are in compliance with EN 60947-
3 are appropriate for use in PV systems.
Unlike a DC isolator that is required to switch both the positive and negative conductors, an AC isolator should be chosen with regards to
the supply being single phase, which is typically found in domestic installations or three phase, which is typical for commercial or industrial
installations. Ideally for single phase a 2pole isolator should be used to switch the live and neutral line (earth constantly connected) whilst
a 4pole isolator would be used to switch the 3 voltage lines and neutral (earth constantly connected).
The isolator rating should be based on the inverter output which is normally specied per phase, that is line to neutral, and for example
maybe shown as 20A at 230VAC; if this output is from a three phase unit then the AC isolator must be rated to for the line-to-line voltage
which would typically be 415VAC.
With both AC and DC isolators the ambient temperature of the environment in which the switch is mounted must be considered as most
industrial switches are nominally rated for use in 35°C. However, if the isolator is to be used in an area where solar activity is prevalent,
thereby making more efcient use of the installation and greater yield, or in an enclosed space such as a loft or that of an inverter enclosure,
then an isolator capable of handling the elevated temperatures should be selected.
All IMO Solar Isolators are capable of being installed in areas where high ambient temperatures of up to +45°C can be found. In installations
of higher temperatures, our open style product can be used up to +65°C, however, you should ensure safe operating conditions and correct
mounting of the product.
DC Isolator Selection
BS 7671 states that a method of isolation must be provided on the DC side of a PV installation
and this can be provided by a switch-disconnector as classied under EN 60947-3 this is also
covered by “Guide to the installation of PV systems”. The Guide also stipulates that the switch
must isolate all live conductors (typically double pole to isolate PV array positive and negative
conductors).
BS 7671 species that isolators that are in compliance with EN 60947-3 are appropriate for use
in PV systems. The isolator rating must consider the maximum voltage and current of the PV
string being switched and these parameters then adjusted in accordance with the safety factors
stipulated in current standards. This should then be the minimum required rating of the isolator.