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Provision for AC and DC Currents in Electrical Installations under BS 7671:2018+A2:2022

Wednesday 18th September 2024

The BS 7671:2018+A2:2022 standards, known as the 18th Edition of the IET Wiring Regulations, provide guidelines for managing both AC (alternating current) and DC (direct current) in electrical installations. With the increasing use of renewable energy systems and electric vehicle (EV) chargers, these regulations ensure the safe and efficient use of mixed-current systems. Key provisions related to AC and DC currents are found in Part 5 and Section 722 of the standards, addressing systems such as solar photovoltaic (PV) installations and EV charging points.

Key Regulations and Their Importance:

1. Regulation 538.3.2 (Part 5 of BS 7671:2018+A2:2022)

This regulation covers installations using both AC and DC currents, focusing on the safe isolation and protection of such systems. It highlights the importance of using protective devices, such as circuit breakers and residual current devices (RCDs), that can detect both AC and DC faults.

With modern electrical systems often combining AC mains supply with DC components (such as those found in EV chargers or PV installations), it is crucial to ensure that protective devices are capable of responding to faults across both current types. Failure to do so could result in undetected faults, posing a safety hazard.

2. Regulation 722.531.2.101 (Section 722 – Electric Vehicle Charging Installations)

This regulation focuses on the installation of EV charging points, which can introduce DC leakage currents. To address this, it mandates the use of Type B RCDs or equivalent devices that are capable of detecting both AC and DC residual currents.

EV chargers present unique challenges as they can generate DC leakage currents that standard Type AC RCDs cannot detect. Type B RCDs are designed to detect both AC and DC fault currents, ensuring comprehensive protection in installations where DC leakage is a risk.

Importance of Mixed-Current Systems:

The integration of AC and DC circuits is becoming increasingly common with the rise of renewable energy systems and electric vehicles. These mixed-current systems require specialised protective devices to ensure safety and efficiency.

AC and DC Circuit Behaviour

AC and DC currents differ in their behaviour, particularly in fault conditions. AC current alternates direction, while DC current flows in a single direction. This distinction makes DC currents harder to detect with traditional AC-based protection devices. In systems involving both AC and DC, such as solar PV and battery storage, appropriate devices must be in place to handle both current types.

RCD Selection:
One of the critical requirements in the BS 7671:2018+A2:2022 standards is the proper selection of RCDs. While Type AC RCDs work for AC circuits, they are ineffective when DC faults occur. DC leakage currents can impair the function of standard RCDs, leaving installations vulnerable.

The standards require the use of Type B RCDs in installations with potential DC leakage, such as EV chargers and renewable energy systems. Type B RCDs are capable of detecting both AC and DC faults, ensuring comprehensive protection for these modern systems.

Application in Renewable Energy Systems:

The standards’ provisions for DC circuits extend to renewable energy systems like solar PV installations, where DC electricity is generated and then converted to AC for household use or grid supply. Ensuring the safe integration of these systems into the electrical framework requires the use of protective devices that can manage both AC and DC currents. This is particularly important as the UK moves towards more widespread use of renewable energy.

Conclusion:

The BS 7671:2018+A2:2022 regulations provide essential guidelines for safely managing AC and DC currents in modern electrical installations. As technologies such as EV chargers and renewable energy systems become more prevalent, the use of appropriate protective devices, particularly Type B RCDs, is critical for ensuring safety. These standards help support the UK’s push towards more sustainable energy solutions while maintaining the safety and efficiency of electrical systems.