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High earth leakage current on earth bar from mobile telecommunication's equipment

Ryan Anderson

I'm reviewing a mobile infrastructure power system where we have recorded (using a clamp meter) 16A on the MET earth and similar readings on the various bonds terminated onto a common 8 way copper bar earth bar. The site has a 300mA RCCB installed at the TN-C-S source which operates fine as expected however the high current running through the exposed copper earth bar in the meter cabinet is a concern. 

It's worth mentioning that the primary load is mobile network equipment which includes rectifiers and understandably will produce earth leakage from the DC systems. I suppose this would be considered 'planned' earth leakage, however the potential for electric shock from an exposed copper bar cannot be ignored.

I had assumed the IET Guide to Data Centre Power Systems would have some solution to DC earth leakage issues, however this has not yet been formally published. I have encountered high earth currents in a previous project (rail) which resulting the DNO repairing a cable joint in the street however this site is rural and the DC tech seems to be the prime suspect. Is there an IET guidance on how this should be dealt with?

**UPDATE**

The final outcome to this was a broken neutral conductor in the supply cable. When the DNO removed the LV earth link to separate the LV & HV earthing systems in line with G78 the supply was completely lost which identified the the system had been using the combined earth system as a return path. The supply was rewired back to the nearest joint and problem solved. 

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  • Are you certain that the observed current is from the equipment installed at the site ?  I consider it possible that you are measuring diverted neutral current from the suppliers network.

    A telecommunications site suggests steel masts or towers that are in excellent contact with the general mass of earth. Therefore if the suppliers combined neutral and earth is a few volts away from earth, then a significant current may flow from the suppliers PEN conductor, to true earth via your bonding connections to true earth.

    There might even be an open circuit PEN conductor somewhere upstream resulting in your bonding connections being the mains means of earthing part of the suppliers network.

  • in reply to broadgage

    Thanks for the feedback,  .

    Not certain at all to be fully honest with you, my initial thought was earth leakage due to the extents of DC equipment that's being used. I agree that idea of diverted neutral current may be more plausible as this isn't a common issue across all of our sites with similar equipment installations. It is however a semi-regular occurrence which would point towards DNC given its a major issue for DNO's. 

    This installation is beside and serving equipment on a  HV pylon site therefore we would assume good earthing on the pylon side. As part of these works the supply for the site will be converted to a TT system in line with G78 regulations for LV infrastructure on HV pylons which I understand would remove any DNC issues associated with TN-C-S system ?

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  • in reply to broadgage

    Thanks for the feedback,  .

    Not certain at all to be fully honest with you, my initial thought was earth leakage due to the extents of DC equipment that's being used. I agree that idea of diverted neutral current may be more plausible as this isn't a common issue across all of our sites with similar equipment installations. It is however a semi-regular occurrence which would point towards DNC given its a major issue for DNO's. 

    This installation is beside and serving equipment on a  HV pylon site therefore we would assume good earthing on the pylon side. As part of these works the supply for the site will be converted to a TT system in line with G78 regulations for LV infrastructure on HV pylons which I understand would remove any DNC issues associated with TN-C-S system ?

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