Jeffrey Cheney

Interest

Grounding and bonding

Study notes for a skill Jeffrey uses when he terminates panels. Not a grounding study of a named building.

Grounding and bonding get used as if they were the same word. They are not. Grounding connects the system to the earth. Bonding connects metal parts to each other so they stay at the same potential and so a fault has a low-impedance path back to the source. A ground rod, by itself, will not clear a fault. Earth resistance is too high. The equipment grounding conductor is what lets the breaker open.

The electrode system

NEC 250.52 lists grounding electrodes: metal water pipe, metal in-ground support structure, a concrete-encased electrode (the Ufer), a ground ring, rod and pipe electrodes, plate electrodes, and other listed local electrodes. If one of those is present, 250.50 says it is bonded into the grounding electrode system. A single rod, pipe, or plate has to be supplemented by another electrode unless its resistance to earth is 25 ohms or less (250.53). That sentence is not a rule that the whole system must measure 25 ohms. It is a rule about when one rod is not enough.

The grounding electrode conductor ties the system to that electrode system. Its minimum size comes from Table 250.66 in current editions, based on the size of the ungrounded service conductors, not on the breaker feeding a branch circuit.

Bonding jumpers

The main bonding jumper is the connection at the service between the grounded conductor (the neutral, on a grounded system) and the equipment grounding conductors and the enclosure. The system bonding jumper is the same idea at a separately derived system, such as a transformer secondary. Supply-side bonding jumpers land on the load side of that decision. Table 250.102(C)(1) is the usual sizing table. EC&M’s article on the Article 250 tables, linked below, is a clear walk through which table is which.

Equipment grounding conductors

The EGC runs with the circuit conductors and lands on the equipment. Table 250.122 sizes it from the overcurrent device ahead of the circuit, not from the ungrounded conductor size. If the ungrounded conductors are increased for voltage drop, the EGC is increased in proportion. Metal raceway can be the EGC when the fittings are listed for it and are actually tight. A rusty set-screw is not a grounding plan.

One bond, not two

On a grounded system the neutral is bonded to ground at the service, or at the bonding point of a separately derived system, and not again downstream. A subpanel keeps the neutral bar isolated. A second bond lets normal neutral current flow on conduits, locknuts, and equipment grounds. That current is a shock path and a noise path. For a transformer, 250.30 puts the system bonding jumper at the source or at the first disconnect, not at both.

Service bond versus a subpanel At the service, neutral and ground bars are bonded. At the subpanel they are separate. An equipment ground runs between them. A grounding electrode conductor leaves the service. Service Neutral bonded GEC to electrode Subpanel Neutral isolated Ground bar EGC with the feeder
The bond is at the service. The subpanel receives an equipment ground and keeps the neutral floating.

How a ground fault clears

A hot conductor touches a bonded enclosure. Current returns on the EGC to the bond, then on the neutral or the grounded conductor back to the source. The loop impedance is low, the current is high, and the breaker or fuse opens. If the only path were a ground rod and the earth, the current would usually be too small to open the overcurrent device, and the enclosure could sit at a dangerous voltage. Bonding is what clears the fault. The electrode is what holds the system near earth potential and gives lightning and surges a place to go.

Testing

Fall-of-potential testing is how electrode resistance is measured, with the electrode disconnected as the procedure requires so parallel paths do not fake a low reading. Continuity of equipment grounds is a different test, done dead. A clamp meter around a grounding conductor can show current that should not be there, which is a clue that a bond exists in more than one place. None of these tests are done by feel.

Data centers and plants

Racks, tray, and raised-floor pedestals are bonded so the room is closer to one potential. A signal reference grid is a high-frequency bonding mesh used in some computer rooms, described in the IEEE Emerald Book family of practice (IEEE 1100). It does not replace the NEC grounding electrode system or the equipment grounding conductors. Cable tray is bonded whether or not it is used as an EGC. Isolated-ground receptacles are a special case with a separate equipment ground back to the point the designer named. They are not “ungrounded.”

What's happening now

Code section numbers, energy-code rules, and utility practices change by edition and by city. The authority having jurisdiction, and the employer’s written program, decide what applies on a given job.