Equipotential grounding… the missing link in bonding & grounding? • David Pilon
October 11, 2016 | By David Pilon
October 11, 2016 – CE Code Section 10 “Grounding and bonding” talks of bonding metallic systems together to the grounding conductor. This reduces the danger of electric shock and property damage by providing a low impedance path for fault current back to the source, and establishes an equipotential plane, such that the possibility of a potential difference between metal parts is minimized.
The objective of grounding the electrical system and non-current-carrying metal parts is to connect the earth to the equipotential plane, minimizing any potential difference to earth.
And that appears to be the end of any talk about equipotential grounding—other than the requirement for raised metallic floors with wiring beneath them, and the appendix note regarding interconnecting and bonding the metallic components around a pool.
While I understand Section 10 is undergoing a major rewrite as we speak, it is still important to discuss it. With the 2015 CE Code change to Rule 10-208, the committee made a substantial step forward but some questions remain.
For example, the Rule states the non-current-carrying metal parts of the electrical equipment in or on a building shall be bonded to ground by a conductor run with the feeder or branch circuit conductors. This is a requirement for buildings housing livestock.
So let’s picture an average farm: a house, garage, shop, machine shed, chicken coop and barn (building housing livestock). The electrical contractor is required as per 10-208(2) to interconnect all the metallic systems in the barn—including stanchions and panels—and run a bond conductor with the feeders to this building back to the system ground.
But where is that system ground? How far back is it? 60 m? 200 m? What about a lightning event? How is this system going to mitigate that energy? What reference does this system ground have with the earth 30 m away?
A goal of this method is to prevent parallel neutrals, eliminate stray currents in the earth—especially around buildings housing livestock—and ensure a fault current path back to the source. Relying on soil resistivity (below 750V would be too high to allow a breaker to operate) has been impractical, as most of the time the soil appears as a highly resistive load.
But what about lightning or the stray currents around the machine shed? The workshop? The garage and the house? Why are we re-grounding our neutrals all over farmyards and other commercial sites? Why are we not mandating the requirement for equipotential grounding to the metallic systems and isolating the neutral at all buildings after the system is grounded at the first point?
I believe once a feeder is used for a second or third building, those buildings should be required by code to be fed by a feeder that includes a bond conductor sized to Table 16. Then, one plate, two rods or a field-installed electrode sized to Table 43 should be installed at each building to establish its own equipotential grounding point.
David Pilon has been an electrical inspector with SaskPower since 2000, and is currently the vice-chair of the Canadian Certified Electrical Inspector (CCEI) committee of the International Association of Electrical Inspectors (IAEI), Canadian Section. David can be reached at firstname.lastname@example.org.
N.B. Always consult your AHJ for more specific interpretations.
* This article also appears in the October 2016 edition of Electrical Business Magazine. Check out our ARCHIVE page for back issues.
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