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Statutory voltage limits consultation from Energy Networks Association

The Institution of Engineering and Technology (IET) has responded to a consultation from the Energy Networks Association regarding Statutory Voltage Limits.

The IET welcomes the consultation but has highlighted that reducing the low-voltage (LV) statutory limit should form part of a coordinated, system-wide voltage management strategy rather than a standalone measure.

IET recommendations:

  • Pragmatic actions: Prioritise measures to prevent customer voltages exceeding the upper statutory limit and causing nuisance inverter trips. Low voltage (LV) system voltages should be brought closer to appliance design ratings (220–240V). 
  • Improved visibility: Enhance real-time or regular time-series monitoring of voltage profiles at LV and higher voltages.
  • Avoid unintended system impacts: Actions at higher-voltage substations to reduce LV must not compromise Grid Code compliance, including OC6.5.3 requirements for staged voltage reduction. 
  • Whole-system modelling and monitoring: The impacts of an increasingly inverter-based resource (IBR) system must be studied and monitored to ensure voltage stability, frequency resilience and protection performance are maintained. 

From a technical perspective, several factors require consideration: 

  • Protection performance: Voltage is a factor in short-circuit calculations (e.g. IEC 60909), and earth loop impedance calculations in line with the IET Wiring Regulations (BS7671). 
  • Fault levels and transmission interactions: Declining short-circuit levels are primarily a transmission issue as synchronous machines with inverter-based resources are displaced.
  • Embedded generation and holistic considerations: Batteries and rooftop PV may increase maximum fault levels due to additional infeed during faults.  
  • Inverter-based resource behaviour: LV-connected inverters generally contribute fault current limited to their rating for short transient durations. 
  • Low inertia and stability: Reduced inertia is mainly a transmission concern but can interact with distributed generation ride-through behaviour and voltage stability. 

Overall, future LV networks may experience a broader range of system conditions, with both more generation and lower loads or higher continuous loading at different times of year. The changes expected with the increased adoption of low carbon technologies underlines the need for voltage limit changes to be implemented within a holistic, forward-looking system design framework.