Grounding and Bonding Systems in the Denver Metro
Grounding connects a system to earth; bonding ties metal together so fault current has a low-impedance path back to the source and opens the device.
The grounding electrode system under NEC 250.50 uses whatever qualifying electrodes exist at the building: a concrete-encased electrode in the footing, metal underground water pipe, building steel, driven rods, and any electrode installed for the purpose. The grounding electrode conductor is sized from Table 250.66 and the equipment grounding conductors that run with the circuits are sized from Table 250.122. The main bonding jumper at the service is what ties the grounded conductor to the equipment grounding system, and that connection happens at exactly one place on the service.
Downstream of a transformer the same logic repeats. NEC 250.30 requires a system bonding jumper at the transformer or the first disconnect and a grounding electrode conductor landed at that same point, and adding a second neutral-to-ground bond anywhere else puts normal neutral current onto conduit, structural steel, and equipment frames. Metal raceway, enclosures, structural steel, and metal piping likely to be energized get bonded under 250.104, and connections get torqued to the listed values because a loose bond is a high-impedance bond. Where equipment is sensitive or where ground faults are hard to clear, electrode resistance testing and continuity checks on the bonding path tell a lot more than a visual inspection.
What is the difference between grounding and bonding?
Grounding is the connection to earth, which stabilizes voltage relative to ground and handles lightning and line surges. Bonding is the connection of all the metal parts to each other and back to the source so that a fault has a low-impedance path that lets enough current flow to open the breaker or fuse. Earth itself is a poor fault-current path, so bonding, not the ground rod, is what clears a fault.
Why does a transformer need its own grounding electrode?
Because the secondary is a separately derived system with no direct connection back to the source. NEC 250.30 requires a new neutral-to-ground bond at that transformer or its first disconnect, plus a grounding electrode conductor to a qualifying electrode near that same point. The equipment grounding conductor running back to the primary panel does not satisfy that requirement, and the electrode does not replace the bonding path either.
What causes current on a plant's grounding system?
The most common cause is more than one neutral-to-ground bond, often a bonding screw left in a subpanel or a second bond added at a transformer secondary disconnect. Neutral current then splits between the neutral conductor and every metal path in parallel with it. Shared or mis-wired neutrals and high-frequency common-mode current from drive output cable that lacks proper shield termination produce the same symptom, and tracing it means finding and removing the extra path rather than adding grounds.
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