Street lighting

A specification for street lighting requires an earth electrode at the beginning of the circuit and at the end of the circuit both having an Ra of not more than 20 ohms (Is this a common spec?.) The single phase circuit will be lifted off a DNO PME supply and supply 16 columns of galvanized steel construction.  My view would be that the columns will offer a relatively good connection with earth when considered globally. I intend to measure the arrangement using the two clamp method which we use for lightning system earthing. I would be surprised if it didn't offer something below 10 ohms.

  • 20 ohms pops up in many places in UK for electrodes on small pole pig transformers and supplies to things like bus stops and traffic lights. I suspect some copy and paste at work - but for small loads of an amp or two,  it means that if the full load current went down the earth for a bit (broken PEN like) the rise of earth potential would be below the magical 50V touch voltage and then 'safe' not to operate any ADS and persist for a while so it seems about right sort of figure for a few lights as well..
    As to how well the lamp columns form their own electrode is not so clear, round here they get 'planted' in cement, but before that they have a bitumastic type coating applied to  the below ground section that is not a good conductor. If metal is planted bare or galvanized in your installations I agree it wont need another electrode except to satisfy the form tickers.

    A few years ago I was involved in a camp that had about 10km of festoon lights on vertical scaff poles at 20m intervals, each tall pole clamped to a stub pole driven in about 2-3ft by digger bucket with a multitude of feeder points  By far the best electrode was the poles and connected  catenary wires. not the actual electrodes.

    Mike.

  • It's common to have an earth electrode at the end of the circuit. The columns can also be planted into a sleeve and packed (with sand I think) to make replacement of the column easier, I don't know how this would affect the resistance to ground.

  • 20Ω is a common figure - and an electrode at the end of a run supplied by PME is a common spec. for broken PEN precautions - off the top of my head I'm not sure if it hails from the BS code of practice for Earthing or the DNO GNs).

    The posts themselves aren't always reliable as electrodes - usually the "root" of purpose made posts are normally covered in bitumastic paint and often the posts are 'planted' in bits of twinwall sleeves in urban areas - to minimise ground disturbance (esp to other services) when they eventually need replacement.

        - Andy.

  • It sounds like the specification in BS 7430 for TN-C-S arrangements to street furniture, but there are subtle differences e.g. BS7430 requires an electrode at the point of supply (not necessarily the start of the circuit) and at the far end of the circuit, and the objective is to bring the N-E resistance down to 20Ω at any point in the circuit not just the electrode resistance, with additional electrodes being added as necessary.

    Disclaimer: This isn't my ballpark, just looked it up out of interest. I don't know if there are ENA or DNO specs that supersede this.