Solar photovoltaic DC switch-disconnector selection and configuration

Introduction

Solar photovoltaic (PV) system installations are increasing at a substantial rate. There is currently more than 5 GWp [1] of domestic solar PV installed within the UK.

Inverters are a fundamental part of these domestic PV installations; their role is to convert DC from the solar PV array into usable AC for use within the installation. Inverters require a means of isolation from both the AC and DC side of the installation to facilitate safe maintenance.

Where a designer opts to select one or more suitable external switch-disconnectors¹  for isolating the DC side of the system, it is crucial that the device is correctly rated for the specific solar PV system. Incorrectly rated or poorly installed DC switch-disconnectors are a major contributor to PV-related fires in the UK [2] [3].

This article aims to aid the designer in correctly selecting and erecting a DC switch-disconnector for a solar PV system. It will also aid electricians when maintaining or inspecting existing solar PV systems.

AC vs DC switch-disconnectors

AC and DC switch disconnectors have different rating criteria. Some devices are capable of switching both AC and DC, although the device will likely have two totally different ratings based on the type of current being switched.

Many electricians are likely familiar with selecting an AC-rated device, basing the ratings on the system’s voltage and current. For example, a switch-disconnector in a typical domestic consumer unit is often rated for 230 V and 100 A. In the AC side of the installation, the voltage rating of equipment, such as switch-disconnectors, typically remains fixed, although they are dependent on being either single or three phase.

However, the current rating of devices depends on the nature of the load being isolated. For example, a switch-disconnector for a 40 A heat pump might be rated at 230 V and 40 A.

In contrast, the DC voltage of a solar PV system depends on the PV module’s rating (which varies by make and model) and the number of PV modules connected in series. Adding more modules in series increases the system’s voltage.

The DC current of the system is determined by the module’s rating (also varying by manufacturer and product range) and the number of parallel strings, with more parallel strings resulting in higher current.

The values of voltage and current obtained from a PV module manufacturer first require correcting to account for a worst-case scenario, before this is used to specify equipment.

This is due to the manufacturer using what is called standard test conditions (STC) to determine the PV module’s output.

Since both the voltage and current of the DC system can vary significantly between installations, and the manufacturers’ values require correcting, selecting a DC switch-disconnector is often more complex than choosing an AC-rated device

Standard test conditions (STC) and maximum values

Solar PV module manufacturers are required to give their ratings based on STC to conform to the relative product standard.

STC values are used to provide a consistent and standardized benchmark for the PV module’s performance rating. A PV module might have a maximum rating of 450 W, although this is the maximum rating based on STC conditions.

In real world conditions, when irradiance levels are high, the output power can be higher than this.

The STC values are included in Table 1. These values are explained as follows:

• irradiance is how intense the sunlight is, measured as power received per square metre of the module in the unit W/m²
• temperature represents the temperature of the PV module’s cells, measured in °C
• air mass is a measure of how much atmosphere sunlight must pass through to reach the module. An air mass of 1.0 represents the sun directly overhead (zenith angle of 0°). An air mass of 1.5 corresponds to a zenith angle of approximately 48.2°.

Table 1: Standard test condition from PD IEC/TS 61836:2016

It is worth noting, some PV module manufacturers also include nominal operating cell temperature (NOCT) values. These values are not representative of the worst-case conditions and shall not be used when determining the voltage and current of the system for selection of equipment.

Figure 1: Current measurement in high irradiance. North-West England 7 July 2015

AC vs DC interruption

Where a means of isolation is required for the inverter, this should be rated for on-load interruption. Interrupting a DC circuit is more difficult than interrupting an equivalent AC circuit. A 50 Hz AC supply has a zero-point crossing every 10 milliseconds, which helps extinguish the arc within the switching device during on-load operation (breaking). In contrast, a DC system has no zero-point crossing; instead, the contacts of the switching device must withstand a much stronger and more sustained electrical arc.

To disconnect the equivalent current as an AC system, DC switch-disconnectors often employ spring-loaded mechanisms and specially designed contacts. In addition, multiple contacts or poles may be used, helping to elongate and dissipate the DC arc as it ionizes the air between the switching contacts.

Future standards are being developed to address some of the challenges associated with DC switching. Some of these standards use technologies that differ from conventional electromechanical devices, such as semiconductor circuit-breakers (IEC 60947-10). 

Figure 2: AC vs DC voltage as a function of time

Determining the DC voltage of a PV system

Solar PV arrays are divided into strings. Smaller domestic PV systems can have just one string, while larger systems can have many.

A string is essentially a circuit of PV modules connected in series. With each additional module in a string, the voltage increases. The number of modules in a string depends on several factors, including the:

• voltage rating of the PV module
• available roof space
• inverter’s minimum voltage rating (start-up voltage)
• inverter’s maximum voltage rating.

Figure 3: Simplistic PV array block diagram

To determine the voltage rating for selecting appropriate equipment, Formula 1 is used².

Formula 1: Determining design voltage of a PV string

U_{OC MAX} = No. of modules × U_{OC STC} × 1.2

Where:

• U_OC MAX is the maximum open circuit voltage.
• No. of modules is the total number of modules on the string.
• U_{OC STC} is the open circuit voltage of the module under STC as declared by the manufacturer.
• 1.2  is a multiplier to take account of the voltage increase due to temperatures lower than that of STC.

As an example, if a PV module was selected, as shown in Figure 4, and the designer choses a ten module string, the maximum open-circuit voltage (U_{oc MAX}) is calculated as per Equation 1 using Formula 1 from earlier.

Equation 1 Determining design voltage

U_{OC MAX} = No. of modules × U_{OC STC} × 1.2

U_{OC MAX} = 10 × 40 V × 1.2

U_{OC MAX} = 480 V

Figure 4: Example PV module data

Determining the DC current of a PV system

As stated earlier, PV systems can consist of multiple strings, and the current of the system increases proportionally with the number of strings connected in parallel.

A PV array can have multiple strings, but the current increases only when they are connected in parallel. Many inverters have multiple maximum power point trackers (MPPTs), and each MPPT can have multiple inputs. When connecting to an MPPT, the system’s current increases only when PV strings are connected to the same MPPT and only at this point onwards.

If strings are combined in the wiring system, any conductor from this point on shall be based on the combined current of the strings³.

In larger installations, strings may be combined in string combiner boxes, from which larger conductors carry the combined current to the inverter. Figure 5 shows a PV string combiner box with an integral switch-disconnector. The rating of the switch-disconnector is based on the combined current of all the strings combined. 

Figure 5: PV combiner box (identification of conductors to a previous version of BS 7671)

Formula 2 is used to determine the design current of parallel strings. This formula is also used to correct the current from STC to maximum in a single string.

Formula 2: Determining the design current of parallel strings

I_{SC MAX} = No. of strings × I_{SC STC} × 1.25

Where:

• I_{SC MAX} is the short-circuit maximum current. 
• No. of strings is the total number of strings in parallel, where there are no parallel strings. This value is 1.
• I_{SC STC} is the short-circuit current of the module under STC as declared by the manufacturer.
• 1.25 is a multiplier to take account of the higher irradiance than that of STC.

As an example, if a PV module was selected, as per Figure 4, and the designer had chosen two strings to be installed in parallel, the short-circuit maximum current (I_SC MAX) is calculated as per Equation 2 using Formula 2 from earlier.

Equation 2: Determining design current

I_{SC MAX} = No. of strings × I_{SC STC} × 1.25

I_{SC MAX} = 2 × 14 A × 1.25

I_{SC MAX} = 35 A

Selecting a DC switch-disconnector

Once the voltage and current for selection purposes have been determined, the designer can select a suitable device. It is important to select a device that conforms to the correct product standard, which for a switch-disconnector is BS EN IEC 60947-3. The device shall be suitable for making, breaking and isolating, which is indicated by a marking, as shown in Figure 6. 

Figure 6: Switch-disconnector making, breaking and isolating symbol from BS EN IEC 60947-3:2021+A1:2025

BS EN IEC 60947-3 includes utilization categories that are specific to PV systems. These are summarized in Table 2. Devices marked DC-PV0 are not suitable for isolating an inverter where breaking capabilities are required - they are disconnectors rather than switch-disconnectors. This category is not rated for making or breaking current. DC-PV1 can be suitable where there are no parallel strings, while DC-PV2 is appropriate when multiple strings are connected in parallel.

Table 2: Utilization categories from Table D.1 of BS EN IEC 60947-3:2021+A1:2025

Once the product line has been chosen, a suitably rated device must be selected. The breaking current of a device depends on the system voltage. Generally, the higher the voltage, the lower the current the device can safely break. 

From the example earlier, although based on a single string without any parallel connections, the voltage and current are as below:

U_{OC MAX} = 480 V (10 × 40 V × 1.2) 

I_{SC MAX} = 17.5 A (1 × 14 A × 1.25)

The device shall be selected based on these values. The following section aids the designer in the correct configuration of the device.

Configuration of a DC switch-disconnector

Figure 7 shows an example of the data that might be obtained from a manufacturer of a switch-disconnector. To determine the correct configuration, the steps are as follows:

• Step 1: U_{OC MAX}, as determined, is selected from the top row.
• Step 2: the corresponding current from that column is selected that is greater than or equal to the I_{SC MAX}.
• Step 3: the corresponding configuration from the left-hand column is utilized.

It is important the installation instructions are referred to when configuring the device. These instructions can vary between manufacturers.

Figure 7: Hypothetical DC switch-disconnector ratings

DC switch-disconnectors generally come in two-pole or four-pole configurations. The number of poles corresponds to the number of internal switching contacts within the switch-disconnector. A greater number of poles results in a larger arc gap and hence, a longer arc when breaking current, which in turn, allows for a higher breaking current rating with higher DC voltages.

Figure 8 illustrates an example of two-pole and four-pole switching. Figure 9 illustrates the configurations from the first column of Figure 7.

Figure 8 and  Figure 9 illustrate bridging links to enable certain configurations. These links are supplied by the manufacturer, although might require ordering separate to the switch-disconnector. It is important to follow manufacturers’ instructions on the use of the bridging links.

Figure 8: Two-pole vs four-pole switching

Figure 9: Switching configurations illustrated

Using the data from Figure 7, the correct configuration for the device can be determined. The voltage determined in the previous section was 480 V so therefore, the 500 V column is selected. The current determined in the previous section was 17.5 A so therefore, the only suitable configuration based on this scenario is the ‘two-pole series + two-pole parallel’ configuration, which gives a breaking current of 28 A. Other configurations at this voltage are only suitable for 16 A, which is not sufficient. The correct value is highlighted in green in Figure 10.

If the STC, rather than the MAX values of current and voltage values were used, a two-pole configuration might appear sufficient, as highlighted in red within Figure 10.  However, this could cause the device to fail when switched under load, which could be catastrophic.

Figure 10: Selected configuration in green compared to STC in red

Figure 11: Incorrectly configured DC switch-disconnector that was switched under load

Further considerations

Other considerations that require attention include ensuring the device is suitable for the external influences that it will encounter.

For devices used outdoors, bottom entry glands might limit the chance of water ingress. Additionally, care should be taken to ensure the enclosure is not compromised by the fixings used; some external switch-disconnectors have fixing points located on the outside of the enclosure.

When mounting equipment outdoors, the potential increase in ambient temperature inside the enclosure should be considered. A switch-disconnector mounted on a south-facing wall will experience higher temperatures and increased UV exposure when placed in direct sunlight.

It is also important that, when using flexible conductors such as Class 5 conductors, the terminals are either suitable for this class of conductor or the conductors are properly treated. Some manufacturers might specify that fine-stranded conductors are to be fitted with a sleeve or ferrule.

Figure 12: DC switch-disconnector installed without consideration for the potential external influences

Maintenance

Solar PV systems are not fit-and-forget systems. They rely on regular maintenance to ensure they continue to operate safely.

The BS EN [IEC] 62446 series recommends that inspection and testing intervals are no greater than those required for the AC system to which they are connected.

IET Guidance Note 3 Inspection & Testing details maximum recommended intervals between initial periodic inspection and testing, as well as routine checks.

Lack of maintenance is considered a contributing factor to PV fires in the UK. When checking the DC switch-disconnector, the inspector must consider the possibility that the device was not correctly installed from the outset.

The following checks should form part of a minimum inspection (suitable safe working systems shall be in place):

• the device is suitable and configured for the maximum voltage and current of the system/string
• the device is suitably rated and installed for the potential external influences
• the terminals are tightened to the correct torque
• the conductors are suitable for the terminals (for example, treated with ferrules where required)
• cycle the device off-load (with the inverter off, there is no current flow). Ensure there is no contact resistance when the contacts are closed.

Further checks that are recommended include the use of infra-red thermography. Where this is carried out, it is important the system has been operating with a minimum ideal irradiance of 600 W/m². 

Summary

This article outlines how to safely select and install DC switch-disconnectors for solar PV systems. It covers calculating maximum voltage and current for PV strings, choosing devices that conform to BS EN IEC 60947-3, and configuring them correctly to handle switching DC loads.

Additional guidance addresses environmental suitability, proper installation, and conductor compatibility to prevent device failure and reduce fire risk. Maintenance checks are also included, which are important for systems that are already operational.

Footnotes

1. A circuit-breaker suitable for isolation may also be used as per Regulation 712.537.2.2.102 of BS 7671:2018+A2:2022+A3:2024+A4:2026
2. An alternative method to this is available that considers the lowest temperature of the location and the PV module’s temperature coefficient, although this is out of the scope of this article.
3. String fuses might be required where there are more than two strings, although protection against overcurrent is out of the scope of this article.

References

[1]     Department for Energy Security and Net Zero, Solar Photovoltaics deployment in the UK, May 2025. 

[2]     RC62: Recommendations for fire safety with PV panel installations, Fire Protection Association, 2023. 

[3]     BRE, Fire and Solar PV Systems - Investigations and Evidence, BRE National Solar Centre, 2018.

Further reading

• IET Code of Practice for Grid-connected Solar Photovoltaic Systems, 2nd Edition.
• BS EN [IEC] 62446 series:
    - BS EN 62446-1 Photovoltaic (PV) systems. Requirements for testing, documentation and maintenance. Grid connected systems. Documentation, commissioning tests and inspection
    - BS EN IEC 62446-2 Photovoltaic (PV) systems. Requirements for testing, documentation and maintenance. Grid connected systems. Maintenance of PV systems
    - PD IEC TS 62446-3 Photovoltaic (PV) systems. Requirements for testing, documentation and maintenance. Photovoltaic modules and plants. Outdoor infrared thermography
• Video: Why AC switches are dangerous in DC circuits

Acknowledgements

• BEAMA 
• Calum Mansell 
• Craig O’Neill 
• Darren Crannis MEng (Hons) DIS MIET
• Luke Gonzales MIET EngTech ACIBSE AIFireE 
• Matt Owens
• Michael Peace CEng MIET
• Simon Ogborn FIET MSET LCGI