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Highway CCTV, Traffic Signals and Bus Shelter Earthing Arrangements (TT vs PME)

Rafael Granados

Hi all,

I'm interested to understand what approach other local authorities are taking for the earthing of highway assets such as:

  • CCTV and enforcement cameras
  • Traffic signal feeder pillars and associated street furniture
  • Bus shelters (illuminated and RTI-equipped)
  • Communication cabinets and other roadside equipment

ENA EREC G12 appears to permit the use of PME for certain street electrical fixtures subject to the load and earth electrode resistance requirements being met, whereas the IET Guide to Highway Electrical Street Furniture generally suggests TT + RCD arrangements for many items of equipment in the public realm.

In practice, what are authorities currently adopting for new installations?

Specifically:

  1. Are you generally specifying TT as the default for CCTV, bus shelters and other roadside electrical equipment?
  2. Are any authorities still accepting PME arrangements for these assets under G12?
  3. If using PME, are you requiring proof that the earth electrode resistance complies with G12 Table 6.2.15? not exceeding 100 Ω for loads up to 500 W?
  4. Are your DNOs supportive of PME for public-facing highway equipment, or is TT now effectively the standard approach?
  5. Have any authorities adopted a formal policy or design standard covering earthing arrangements for highway electrical street furniture?

I'm particularly interested in hearing from local authority/consultancies highway electrical designers.

Parents

  • I'm not one of the above mentioned persons, but I might add that the choice isn't necessarily a stand-alone one - in many cases, because of the requirement that simultaneously accessible exposed-conductive-parts must be connected to the same earthing system, it's not a decision that can be taken without taking into account of what already exists in the area in each particular case. E.g. if you're installing a new bus shelter and there's a lighting column less than 2.5m away, then either you're obliged to adopt the same system as the column, or change the column's earthing system to share yours. In many cases existing lighting columns will be PME and owned/operated by quite different organisations to the owner of the bus shelter. Ditto for comms cabinets.

    It can all get very messy when EV charge points are introduced (the 2.5m is then from the car, whose exact position isn't necessarily guaranteed).

       - Andy.

Reply

  • I'm not one of the above mentioned persons, but I might add that the choice isn't necessarily a stand-alone one - in many cases, because of the requirement that simultaneously accessible exposed-conductive-parts must be connected to the same earthing system, it's not a decision that can be taken without taking into account of what already exists in the area in each particular case. E.g. if you're installing a new bus shelter and there's a lighting column less than 2.5m away, then either you're obliged to adopt the same system as the column, or change the column's earthing system to share yours. In many cases existing lighting columns will be PME and owned/operated by quite different organisations to the owner of the bus shelter. Ditto for comms cabinets.

    It can all get very messy when EV charge points are introduced (the 2.5m is then from the car, whose exact position isn't necessarily guaranteed).

       - Andy.

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