Ask the Expert: EESS – protection against electric shock

Correctly identifying the method of protection against electric shock when using an electrical energy storage system (EESS) is crucial to maintaining safety and compliance – what do I need to know?

Electrical energy storage systems (EESS), such as battery storage solutions, are increasingly used to enhance energy efficiency, provide backup power, and integrate with renewable energy sources. However, ensuring the correct method of protection against electric shock is crucial to maintaining safety and compliance with electrical safety standards/regulations.

Incorrect identification or improper implementation of protective measures can lead to significant electrical dangers, including the risk of electric shock due to the failure of automatic disconnection of supply (ADS) in the event of a fault.

Understanding the method of protection against electric shock

When using a supply from an EESS, the primary method of protection against electric shock must be correctly determined based on the system’s earthing arrangement and operating mode. The protective measures should align with BS 7671, which sets out specific requirements for electrical installations, including those involving energy storage. Further guidance can be found in the IET Code of Practice for Electrical Energy Storage Systems.

In a typical grid-supplied installation, earthing and ADS rely on a solid and continuous connection to the distribution network. However, when an EESS operates in island mode (i.e., when disconnected from the grid and supplying power independently), the standard earthing reference point may be lost. This can compromise the protective measures designed to prevent electric shock, making it essential to establish a reliable means of maintaining earthing and ADS.

The role of a neutral-bond relay in ensuring safety

A neutral-bond relay (NBR) plays a critical role in maintaining electrical safety when an EESS is in operation, particularly in island mode. Its primary function is to create a defined earthing arrangement by bonding the neutral of the output supply to earth when the system is operating independently. This ensures that ADS functions correctly in the event of a fault.

Without an NBR, the system could operate with an undefined or floating neutral, leading to:
• Failure of ADS: Protective devices such as circuit breakers, RCDs and RCBOs may not detect faults correctly, increasing the risk of electric shock.
• High touch voltages: A floating system can result in hazardous voltages appearing on conductive parts, posing a significant risk.
• Non-compliance with regulations: BS 7671 requires a properly referenced earthing system for electrical safety.

Installing an NBR ensures that in the event of a fault, the protective devices disconnect the supply as intended, maintaining compliance and protecting individuals from electric shock.

Conclusion

Many of the EESS have an emergency power supply (EPS) output that will remain energised when a grid connection is lost via the stored energy within the batteries. In the instructions, some manufacturers recommend this EPS is only used to feed a single item of double insulated equipment. Using the EPS to supply a number of maintained final circuits is possible, but only once an NBR has been installed to ensure the protective devices will operate in the event of a fault, and to allow the designer and installer to use class 1 equipment and products when in islanding mode.

It is highly recommended that anyone wanting to offer a maintained supply from an EESS should ensure the system is capable of having this essential work carried out. Not all EESS have the ability to be changed easily.

cef.co.uk

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This article appeared in Electrotechnical News May/June 2025 edition – read it here