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Automatic disconnection of supply for island mode prosumer electrical installations 

By: Joe Cannon MSc CEng MIET    

Introduction 

Residential prosumer electrical installations (PEIs) are becoming increasingly common in the UK. Solar photovoltaic (PV) installations have seen a surge in recent years, largely due to rising electricity costs combined with a reduction in material costs. Many homeowners are now requesting electrical energy storage systems (EESSs) as part of their prosumer installations. These systems enable users to consume a greater proportion of their locally generated energy, thereby improving the system’s return on investment. 

A crucial part of a solar PV installation is the inverter, which is a form of power conversion equipment (PCE). Some PCE includes functionality that allows a PEI to disconnect from the public network and operate in island mode. This mode of operation presents designers with unique technical challenges, which will be explored in this article. 

This article is dedicated to single residential dwellings with one local PCE. 

Some of the terms and definitions used within this article  

Electrical energy management system (EEMS) 

A system that monitors, manages, operates and controls parts of the prosumer electrical installation. 

Residual current device (RCD) 

A device that operates when a residual current (earth leakage current) is detected. 

Residual current operated circuit-breakers without integral overcurrent protection (RCCB) 

A form of RCD that does not offer overcurrent protection. 

Residual current operated circuit-breakers with integral overcurrent protection (RCBO) 

A form of RCD that also offers overcurrent protection. 

System referencing conductor (SRC) 

A conductor that connects the neutral conductor of the power system to a local earthing arrangement. 

System referencing conductor switching device (SRCSD) 

A device that connects the SRC to the local earthing arrangement. 

Switching device for islanding (SDFI) 

A device for switching an islandable PEI to and from island mode by separating local sources from and to the distribution network. 

BS 7671 

BS 7671:2018+A4:2026 Requirements for Electrical Installations, IET Wiring Regulations, Eighteenth Edition (incorporating Amendment 4). 

IEC 60364-8-82 

IEC 60364-8-82:2022 Functional aspects – Prosumer’s low-voltage electrical installations. 

Earthing arrangement in island mode operation and switching for islanding 

When a PEI operates in island mode, the designer needs to select a suitable earthing arrangement. In a residential installation, IEC 60364-8-82 requires this arrangement to be a TN system. This also requires the installation of a local earth electrode to provide a reference to the general mass of Earth; the electrode may be permanently connected to the earthing system.  

A common earthing arrangement for an islanded PEI is TN-S. To establish a TN-S system, a system referencing conductor (SRC) is required to form the link (system referencing) between the protective conductor and the neutral conductor of the islanded PEI. This link shall only be present when the installation is disconnected from the grid. The SRC provides an earth fault loop path, which can enable the protective measure automatic disconnection of supply (ADS). 

It is important to ensure that no more than one system referencing point exists, at any point in time, in order to: 

  • prevent unwanted operation of residual current devices (RCDs)  
  • prevent circulating currents 
  • reduce the occurrence of differential voltages in the earthing arrangement.  

To connect and disconnect the SRC, a system referencing conductor switching device (SRCSD) is required. This device shall be capable of making and withstanding the current that can flow for the duration of a fault. The sequence of operation of the SRCSD should follow that shown in Figure 1. Operation without reference to the system earth is acceptable for a short duration, generally less than five seconds. This process may be controlled by an EEMS. 

NOTE: A new standard was published in 2025 relating to SRCSDs: BS EN IEC 63445:2025 System referencing conductor switching device. 

In order to allow a PEI to connect to, and subsequently be disconnected from, the grid, a switching device for islanding (SDFI) is required. This can be manually or automatically controlled and shall be interlocked with the SRCSD. The SDFI shall be suitable for isolation (Regulation 826.1.1.5 of BS 7671). 

NOTE: A new standard was published in 2026 relating to SDFIs: BS EN IEC 63552:2026 Switching device for islanding (SDFI). 

The SRCSD and SDFI shall be located so as to prevent unauthorized operation. This may be achieved by installing these within enclosures that require a key or a tool for access by competent persons. 

Figure 1: Sequence of operation from grid-connected[1] mode to island mode and back again 

Figure 2 illustrates the possible locations of the SRCSD and SDFI.  

Figure 2: Simple schematic depicting the possible location of switching devices 

NOTE: The SDFI and SRCSD can be contained within a single item of equipment, such as a ‘gateway’. 

Protective devices when the PEI is in island mode operation 

One of the technical issues associated with islanded PCE, is the limited fault current available.

Unlike traditional systems connected to the public network, which can provide high fault currents until disconnection occurs, residential PCE provides only limited current under fault conditions. Due to this limited fault current capability, IEC 60364-8-82 requires an RCD to be used[2] to meet the disconnection times specified in Regulation Group 411.3.2 of BS 7671. 

Location of RCDs providing fault protection for island mode operation 

Where a conventional RCD, such as a residual current operated circuit-breakers with integral overcurrent protection (RCCB) or residual current operated circuit-breakers with integral overcurrent protection (RCBO), is used for fault protection to achieve the required disconnection times during island mode operation, it is important that the device is located in a suitable position within the installation.

For the RCD to detect a residual current under fault conditions, it must be installed at the load side of the point of system referencing.  

Figure 3 illustrates the earth fault loop path for a fault on load 3. 

Figure 3: Earth fault loop impedance path for a fault on load 3 in island mode operation 

Type of RCD for island mode operation 

The type of RCD required for island mode operation will depend on the characteristics of both the PCE and the connected load. The most onerous of these conditions will determine the type of RCD to be used. 

  • Where the PCE produces a DC component of the output waveform, the manufacturer should state what type of RCD is suitable for use. If no device is specified, a Type B shall be used for non-isolated PCE (Regulation 570.6.2.2 of BS 7671) 
  • The designer of the installation shall also determine the type of RCD that is compatible with the loads of the installation (Regulation 531.3.3 and Figure A53.1 of BS 7671). 

Where an RCD is used for fault protection and the designer wants to achieve selectivity between RCDs, time delayed RCDs may be used upstream of a non-time delayed RCD. 

Classification of RCDs for island mode operation 

RCCBs conforming to BS EN 61008-1 and RCBOs to BS EN 61009-1 are classified based on their operating behaviour with or without voltage, and their response in hazardous conditions. 

Table 1, is based on the information presented in Section 4.1 of Table Z1 of BS EN 61008-1:2012+A12:2017 and BS EN 61009-1:2012+A13:2021; it categorizes RCDs by their voltage dependence. 

  • Voltage-independent RCDs (unmarked) operate purely on residual current and will trip even if no supply voltage is present 
  • Voltage-dependent RCDs are marked with specific classifications:
    • E1: trips in hazardous situations even if supply voltage fails
    • E2: trips upon loss of voltage
    • E3: does not trip if the voltage falls outside its rated range. 

Table 1: RCDs categorized by their voltage dependence 

As E2 and E3 marked devices offer additional protection only, these are not generally suitable for providing fault protection. 

For further information on E markings on RCDs, BEAMA has produced a technical bulletin: Guide to Residual Current Device (RCD) E Marking. 

Compliance with BS 7671 

BS 7671 requires that the inequality formula given in Regulation 411.4.4 of BS 7671 is satisfied when the protective measure of ADS is applied in a TN system. 

Figure 4: Excerpt of Regulation 411.4.4 of BS 7671 

This presents a conundrum where the source of supply is an island mode inverter. The fault current output of PCE under fault conditions is inherently controlled by its internal protection software and is therefore not predictable in the conventional sense. Furthermore, the behaviour of PCE during fault conditions is not standardized and can vary between manufacturers. 

The source impedance, i.e. Ze, of an island mode inverter cannot be accurately measured in a way that meaningfully represents its behaviour under fault conditions. Any measured value would not reliably reflect the current available during an actual fault. 

This limitation is recognized in Regulation 826.7 of BS 7671, which permits the use of an alternative method of verification. 

Figure 5: Excerpt of Regulation 826.7 of BS 7671

An alternative method for determining the earth fault loop impedance is not provided within BS 7671. The manufacturer of the PCE might provide this information; however, where this is not available, a modification of the formula given in Regulation 411.4.4 of BS 7671, based on information contained in IEC 60364-8-82:2022, is presented in Formula 1. 

NOTE: This formula is valid only when the source of supply is an island mode inverter operating with a TN earthing arrangement, and when an RCD is used to provide ADS. 

Formula 1 

Where: 

  • SPCE - is the rated power of the PCE in VA. A rated power quoted in kVA is multiplied by 1,000 to convert to the rated power in VA. 

  • Us - is the line voltage, typically the line to Earth voltage (U0) for earthed single-phase systems and the line-to-line voltage (U) for three-phase systems. 

  • R1 + R2 - is the R1 + R2 of the circuit/s from the inverter to the furthest point of the circuit. 

  • Ia RCD - is the residual operating current in amperes (A) providing disconnection in the time specified in Regulation 411.3.2.2, or Regulation 411.3.2.3, of BS 7671. 

  • U0 - is the nominal AC RMS or ripple-free DC line to earth voltage.
     
  • 1.2 - although not explained in IEC 60364-8-82, this factor increases the impedance of the PCE by 20 %, so can be considered a safety factor. 

An example is included in Equation 1, that is based on the values from Table 2. 

Table 2: Hypothetical values 

SPCE  5 kVA (5,000 VA) 
Us  230 V 
R1 + R2  0.40 Ω 
Ia RCD  100 mA (0.100 A) 
U0  230 V 

Equation 1: Example calculation 

In this scenario, the inequality formula can be considered satisfied. This information should be recorded on the electrical installation certificate. 

It must be noted that the components of the formula are not valid outside the context of this inequality formula.  

Verification 

BS 7671 requires that an installation be inspected and tested before being placed into service (Regulations 134.2.1 and 641.1). The following section focuses on the PCE portion of the installation. 

Inspection of the PCE includes verifying all electrical connections; however, it also involves ensuring that the PCE has been manufactured in accordance with the relevant product standard. The inspector shall confirm that the PCE conforms to the product standard specified by the designer.  

As part of the initial verification process for a system incorporating PCE operating in island mode with a TN-S arrangement, the following minimum test sequence should be carried out: 

  1. Continuity test of protective conductors, including R1 + R2 of all circuits. 
  2. Insulation resistance. 
  3. Polarity. 
  4. Earth electrode resistance (test method E1 or E2 as covered by IET Guidance Note 3: Inspection & Testing). 
  5. Ze or Zs test (minimum one circuit) to prove connection of the SRCSD when the system is in island mode. 
  6. RCD tests for all RCDs. It is important that RCDs are tested according to the mode(s) in which they are designed to operate. For example, if an RCD is required to function in island mode, the test shall be conducted in that mode. This can require repeating tests in different operating modes. The upstream/downstream testing method should only be used when the PCE does not permit an RCD test, for example, due to the PCE’s automatic shutdown.  
  7. ADS can then be verified for island mode circuits where RCDs provide ADS as follows:
    - By RCD test, and 
    - Ensuring Formula 1 is satisfied. 
  8. Functional testing as appropriate – for example (not exhaustive):

    - Switching the installation to island mode operation, ensuring mechanical or electronic interlocks are functional
    - Test button functionality on RCDs. 

NOTE: Some of these tests might have already been undertaken as part of the initial verification process of the grid-connected system[1]. However, some repeated tests, such as RCD testing, are important. This ensures the device(s) function in all intended operating modes. 

Figure 6: Example of an alternative source of supply certificate (courtesy of ECA) 

Summary 

This article examines residential PEIs in the UK, focusing on single dwellings with a single local inverter operating in island mode.

It addresses key technical challenges, including limited fault current from inverters, TN-S earthing arrangements, and the use of RCDs for ADS. Guidance is provided on system referencing, switching devices, verification of earth fault loop impedance, and inspection/testing procedures to aid compliance with BS 7671. 

Further reading 

References 

[1] Grid-connected is ‘connected mode’. 
[2] This is a requirement of IEC 60364-8-82:2022 for TN island mode systems where there is limited fault current. 

Acknowledgements 

  • BEAMA 
  • Calum Mansell IEng MIET 
  • Craig O’Neill BEng(Hons) MIET 
  • Curtis Jones MIET 
  • Darren Crannis MEng(Hons) DIS MIET 
  • Jon Elliott BSc(Hons) PGCert CEng MIET 
  • Mark Coles BEng(Hons) MIET 
  • Michael Peace CEng MIET 
  • Simon Ogborn FIET MSET LCGI.