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Solution to Failed Zs Street Lighting Circuit.

Joe Thornton

Im currently carrying out periodic inspections on a local governments street lighting circuits and many are failing Zs tests. Not just by a little some 20/30 ohms. The circuits are all being protected by BS88-3 Fuses from 10A upto a maximum of 25A depending on the size of the circuits. 

My question is can the circuits have a second fuse inserted at a point where the Zs is acceptable.  I believe not that our only solution is to improve the reading or fit and Rccbo to give the circuits the correct disconnection times.

Thank you for any feed back

Parents
  • 0

    What is the earthing arrangement of the system?

     - If the earthing arrangement is TN-C-S or TN-S, then such high Zs readings might well indicate a serious problem with earthing, or connection of a live conductor, that needs to be corrected.
     - If the earthing arrangement is TT, it is likely you will need to adopt RCDs, such as RCBOs, to achieve disconnection times required by BS 7671.

    How long are the cable runs?

     - Again another issue with such high readings, if this is due to circuit length, is, potentially, excessive voltage drop. Is this conformant to BS 7671?

Reply
  • 0

    What is the earthing arrangement of the system?

     - If the earthing arrangement is TN-C-S or TN-S, then such high Zs readings might well indicate a serious problem with earthing, or connection of a live conductor, that needs to be corrected.
     - If the earthing arrangement is TT, it is likely you will need to adopt RCDs, such as RCBOs, to achieve disconnection times required by BS 7671.

    How long are the cable runs?

     - Again another issue with such high readings, if this is due to circuit length, is, potentially, excessive voltage drop. Is this conformant to BS 7671?

Children
  • 0 in reply to gkenyon

    30 ohms wont really blow any sensible size fuse or MCB reliably and assuming it is through the ground, rather than metal then next month  it might be 50, or 15 if it rains.

    This sounds like the street lamp is acting as its own earth rod and you are in effect reading an 'electrode resistance' for it. As Graham said above verify the earthing method. If it is TT , some sort of earth fault relay or RCD is in order, but the requirement to regularly test such things is hard to meet in practice and the no. of lights that may be lost (and for how long before being discovered) if there is a trip, needs to be considered. 

    Adding an RCD at the lamp base may not be enough if the cable is not capable of protecting itself with ADS either in the event of cable strike.
    Is it a metered private supply or direct DNO supply pillar or cabinet ? 

    Photo or more description may be helpful.


    Mike

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