Update to BS 7909

BS 7909:2023 is now live and in effect

What's New?

Major changes in BS 7909:2023 include:

  • Information and use of RCDs in Annex E has been updated

  • “Bypass” and “disable” in relation to the operation of RCDs have been changed to “override” to reflect better the technical meaning

  • A greater emphasis has been placed on planning and provision of supplies for an event

  • A new Annex K gives more background information on power quality symptoms and causes

  • The facility to override RCDs in FDUs has been removed from Annex B

  • The use of thermoplastic ‘SY’ and ‘YY’ cables has been clarified

  • A new section gives requirements for battery storage systems

IN-S system

The IN-S system has been properly introduced to label section - Generator earthing not required.

IN-S system is defined as:

"system having separate neutral and protective conductors throughout the system and no intentional connection to Earth"


To align with the recent changes in BS 7671:2018(2022) the selection, and testing, requirements of RCDs selected to achieve the requirements of additoinal protection under CHapter 41 of BS 7671 is been adjust to just 1xIΔn.

Cable routing

To align with BS 7671 the erection of wiring systems overhead need to take into consideration the risk of entaglement from premature collapse in a firefighting activity.

For considerations are also necessary to minimisize the risk of danger and trip hazards.


Due to the different wiring configurations for multi-circuit cable assemblies from different suppliers, the pin configuration and pin compatibility should be verified prior to connecting equipment.

requirements for SPDs

The designer of the temporary system should assess the risk presented by switching and environmental overvoltages when taking account of the application of SPDs within the temporary distribution.

BS 7671:2018+A2:2022 provides a framework for evaluating when SPDs are required, where they are to be installed and the type of SPDs at a particular point in the temporary distribution, taking into account:

  1. the duration of the event;
  2. the origin of the supply (e.g. generator, overhead line, underground cable); and
  3. the type of the equipment connected to the temporary distribution and any associated installation.

Where SPDs are installed

Where SPDs are installed, they should be correctly selected to enable coordination between devices.
SPD protection should be coordinated as follows:

  1. choose the correct type of SPD for the temporary distribution and the location;
  2. refer to Regulation 443.6.2 and Table 443.2 of BS 7671:2018+A2:2022 (impulse withstand voltage);
  3. choose SPDs with a protection level (Up) sufficiently lower than the impulse withstand voltage orlower than the impulse immunity of the equipment to be protected;
  4. choose SPDs of the same make or manufacture;
  5. type 1 SPDs should be installed at the origin of the electrical supply, e.g. the generator;
  6. type 2 SPDs should be installed at each distribution unit; and
  7. type 3 SPDs are required at each item of sensitive electronic equipment that cannot be disconnected from the supply at the time of evacuation.

Testing of systems with SPDs

When carrying out insulation resistance tests at 500 V DC, SPDs in circuit could influence the results of the test; three options to mitigate the problem are recognized:

  1. obtain data from SPD manufacturer regarding 500 V testing compatibility;
  2. disconnect the SPDs prior to testing and test the temporary distribution at 500 V DC; or
  3. where disconnection of the SPDs is not possible or the manufacturer’s information is not available, the insulation resistance test can be carried out at 250 V DC.

Regardless of the test voltage, the insulation resistance should have a measured value of not less than 1 MΩ (see Table 64 of BS 7671:2018+A2:2022).
NOTE The insulation resistance testing is typically.

Battery storage (EEMS)

With the increasing used of renewable energy and battery storage technologies, battery storage systems are now a regular consideration for event power.

  • Charging of Electric Vehicles
  • Ruding generator hours
  • Reducing emissions
  • Enabling silent operation
  • To increasse peak demand

The designer should define the purpose of an EESS or hybrid generator in the design and agree the operating principles with the client.