Siemens GMI Series Medium Voltage Circuit Breaker Rebuild

The Siemens GMI series — General Magnetic Interrupter — traces its lineage through the ITE and Gould switchgear lines that Siemens absorbed. These are medium voltage drawout air circuit breakers for metal-clad switchgear, installed in utility and industrial substations from the 1970s through the 1990s. Southern Switch rebuilds GMI-series breakers at our Palm Harbor shop, drawing on parts knowledge accumulated across the ITE, Gould, and Siemens nameplate variants of the same fundamental design.

The GMI Design

GMI stands for General Magnetic Interrupter. The arc interruption method uses a magnetic blow-out coil in series with the primary circuit: as the contacts part, current through the coil produces a magnetic field that drives the arc upward into the ceramic arc chute faster and more forcefully than the arc's own self-field alone. This produces consistent arc chute loading across a range of fault current magnitudes and results in faster arc extinction than in designs without active blow-out assist.

The GMI design inherited the drawout construction from the ITE K-series — the breaker rolls into the switchgear cubicle and engages primary and secondary disconnects in the connected position. Current ratings follow the K-series convention: the number suffix designates continuous current class. The voltage class (5kV or 15kV) determines insulation level, primary stab design, and hi-pot test values. Parts differ between voltage classes; confirm both current and voltage class from the nameplate before staging a rebuild.

Main Contacts

GMI main contacts use a moving contact arm with silver-faced contact tips that engage stationary contacts in the closed position. The moving contact arm is spring-loaded to maintain contact force through the service life of the contact material. Contact gap — the distance between open contacts — and contact wipe — the compression of the moving contact against the stationary at full close — are both specified by Siemens for each current class and must be verified during rebuild assembly.

Contact surface condition is assessed by visual inspection and contact resistance measurement. Silver-faced contacts with pitting at the engagement zone deeper than the silver layer thickness, or with the copper substrate visible through the silver overlay, are replaced. Contacts that are physically intact but show elevated contact resistance in service — above 100–150 microohms for most classes — are cleaned and retested; resistance that remains high after cleaning indicates a surface condition requiring replacement. SSC measures contact resistance after reassembly as part of the test protocol.

Arc Chutes and Blow-Out Coil

The GMI arc chute is a stack of ceramic splitter plates with fiber side cheeks, mounted above the contact assembly. The blow-out coil drives the arc aggressively into the chute entry zone, which concentrates wear at the lower plates faster than in designs where the arc trajectory is more gradual. In our shop we consistently find that GMI chutes develop heavy erosion and carbon buildup at the lower two to three plates even when the upper stack is relatively clean. Replacing only the damaged lower plates is not an option in most GMI configurations — when the lower zone is degraded, the full chute set is replaced.

The blow-out coil itself is inspected for connection corrosion and insulation integrity. A high-resistance connection at the coil terminals reduces the blow-out field and weakens arc guidance into the chute. Coil connections are cleaned and retightened; coils with degraded insulation are replaced. SSC fabricates ceramic chute plates and fiber side cheeks in-house when aftermarket replacement chutes are unavailable for the specific GMI configuration.

Operating Mechanism

The GMI uses a stored-energy spring mechanism with close and trip springs, close and trip latches, latch rollers, and shunt trip and undervoltage release coils. The stored-energy design allows the breaker to close and trip regardless of control power availability at the moment of operation — the spring is charged first, operation is then triggered by a coil signal. Spring mechanisms accumulate wear at latch contact surfaces and roller bearings; wear produces latching failures or sluggish operations that fall outside the timing window.

Mechanism overhaul includes full disassembly, cleaning of dried lubricant from all pivot points, inspection and measurement of latch roller diameter and latch face condition, spring inspection, and full relubrication. Latch rollers that have developed flat spots or bearing looseness are replaced. SSC tests shunt trip coil pickup voltage and undervoltage release dropout voltage before and after mechanism service.

Insulation and Primary Circuit

Phase barriers, bushing insulation, and arc chute housings are inspected for surface tracking and bulk insulation damage. Surface tracking — visible carbon traces across an insulator — indicates sustained leakage current along a surface path. Light surface tracking without penetration to the bulk material can sometimes be removed by sanding or abrading the surface; tracking that penetrates the bulk insulator requires replacement of the affected component. Insulation resistance is measured at 1000V DC phase-to-phase and phase-to-ground. Values below 1000 MΩ are flagged; values below 100 MΩ require investigation before the breaker can be returned to service.

Test Protocol and NERC Documentation

Rebuilt GMI breakers go through contact resistance (microohm per phase), close/trip timing, hi-pot at voltage class rating, and insulation resistance testing before leaving the shop. SSC issues a written test report documenting as-found condition, work performed, and as-left test values. The test report is structured to support NERC PRC-005 maintenance documentation — utilities under PRC-005 compliance requirements need evidence that breaker maintenance was performed and that the breaker met functional performance criteria at the end of the maintenance interval. SSC's test report provides that documentation directly.