9.4 Switchgear Bus IR, MCC Inspection & Bonding Jumpers

Switchgear Bus Insulation Resistance

The bus is the backbone of the switchgear — bare or insulated copper/aluminum bars bolted to insulators, running phase-to-phase and phase-to-ground through the line-up. A bus insulation failure is a phase fault inside the gear, with no easy repair. The NETA MTS insulation resistance test is the routine screen.

Test Setup

Before applying any test voltage, the bus must be isolated so the reading reflects the bus, not the connected load:

1. Open and rack out every breaker / disconnect in the section under test.
2. Disconnect or short surge protective devices (SPDs) and any capacitor or electronic gear that the DC test voltage could damage.
3. Verify LOTO and confirm de-energized (live-dead-live).
4. Test phase-to-phase (A-B, B-C, A-C) and phase-to-ground (each phase to the grounded enclosure), one minute each.

A common error is leaving a breaker closed or an SPD connected — the reading is then dominated by the downstream load or by the SPD leakage, and a good bus can look bad (or worse, the SPD is destroyed by 2500 V DC).

Test Voltage and Acceptance

Per ANSI/NETA MTS Table 100.1, the test voltage and minimum insulation resistance scale with the equipment's nominal rating:

These are minimums when manufacturer data is unavailable; the manufacturer's published value governs. Trend results over time and temperature-correct for comparison. A dielectric withstand (hipot) test must not be run until the IR is above the minimum.

MCC Visual Inspection and IR

A motor control center (MCC) is a lineup of starters, feeders, and drives feeding motors. The NETA inspection is mostly visual and mechanical, supplemented by IR:

Follow with an IR test on the MCC bus, the same way as switchgear — breakers/bucket starters open or withdrawn, SPDs disconnected, phase-to-phase and phase-to-ground at the Table 100.1 voltage. On a 480 V (600 V class) MCC the test voltage is 1,000 V DC with a 100 megohm minimum. Record phase-to-ground and phase-to-phase readings; a healthy MCC bus should read well above the minimum (often hundreds of megohms) when clean and dry. A low reading on one phase only points to a specific bucket — withdraw buckets one at a time to isolate the bad section.

Common MCC Defects Found on Inspection

• Mismatched overload heaters — the single most common MCC defect; a heater sized for the previous smaller motor causes nuisance trips or, worse, a heater too large for the new motor fails to protect the winding.
• Burned contact tips — from repeated jogging or undervoltage pickup; replace contacts and check the coil voltage.
• Loose stab fingers — high Ductor reading on the line-to-load path through the bucket; re-torque or replace the stab block.
• Dead anti-condensation heaters — moisture accumulates in buckets installed in unconditioned spaces, dropping IR and corroding contacts. Verify heaters are energized, not just present.
• Wrong bucket in the wrong cell — verify the bucket nameplate matches the cell ID and the loaded motor; mislabeled buckets are a frequent cause of the wrong motor being de-energized during troubleshooting.

Bonding Jumper Test

The equipment grounding and bonding path is what makes a fault clearable: when a phase faults to a motor frame or enclosure, the fault current returns through the bonding path to the source neutral/ground, and the upstream overcurrent device operates. A high-resistance bond raises touch voltage (shock hazard) and lowers fault current (the device may not operate, or may take long enough that an arc flash develops).

NETA MTS requires verifying the continuity and low impedance of the bonding path, including bonding jumpers across expansion joints, flexible conduit, separations, and around any non-continuous enclosure joint:

• Test with a low-resistance ohmmeter (Ductor / DLRO), not a standard ohmmeter — milliohms matter, and a 2-wire ohmmeter cannot resolve them.
• Acceptance: the bond between two enclosures should be no greater than a few milliohms — high enough to read on a Ductor, low enough to carry full fault current. (This is very different from a ground electrode resistance, which is typically <= 5 ohms for a single rod.)
• Verify the jumper is the correct size (NEC Table 250.122 / 250.66 as applicable), mechanically secure, and free of corrosion.

Why It Matters

A switchgear line-up with a broken bonding jumper across an expansion joint is effectively two ungrounded enclosures. A ground fault on the far section has no low-impedance return path — the upstream breaker sees a low-magnitude, high-impedance fault and may never trip, while the enclosure stays energized at a dangerous voltage. The Ductor test across the jumper catches this in seconds.

Worked Example

A 480 V MCC has been in service 10 years. Visual inspection finds two buckets with overload heaters sized for a 25 HP motor where 40 HP motors are now installed — the heaters are undersized and nuisance-tripping. The IR test on the MCC bus reads 350 megohms phase-to-ground (above the 100 megohm minimum for a 600 V class bus). A Ductor across the bonding jumper between the MCC and the adjacent feeder panel reads 2.1 milliohms — acceptable. The heaters are replaced, the bucket labels are corrected, and the MCC is returned to service.