Siemens GMI Series Medium Voltage Circuit Breaker Rebuild

The Siemens GMI series traces its lineage through the Allis-Chalmers and Siemens-Allis switchgear lines that Siemens absorbed. These are medium voltage drawout vacuum circuit breakers for GM-class metal-clad switchgear, installed in utility and industrial substations from the 1980s onward. Southern Switch rebuilds GMI-series breakers at our Palm Harbor shop, drawing on parts knowledge accumulated across the Allis-Chalmers, Siemens-Allis, and Siemens nameplate variants of the same switchgear family.

The GMI Design

The GMI interrupts in vacuum. Each phase carries a sealed vacuum interrupter: a ceramic envelope holding a fixed and a moving contact in a hard vacuum. When the contacts part, the arc exists only in metal vapor from the contact surfaces, and at the first current zero the vapor condenses and the gap recovers dielectric strength almost instantly. There are no arc chutes, no blow-out coils, and no arc byproducts to clean; the interruption duty is absorbed entirely inside the sealed interrupter.

The GMI uses drawout construction: the breaker rolls into the switchgear cubicle and engages primary and secondary disconnects in the connected position. The type designation carries the voltage class and continuous current rating. 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

The primary contacts live inside the sealed vacuum interrupter and are not accessible for inspection. What is measurable from outside: the contact erosion indicator on the interrupter stem (most GMI interrupters carry a scribed erosion mark), contact wipe and gap through the external linkage, and the wipe-spring compression that maintains contact force. 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 condition is assessed by the erosion indicator, contact resistance across the closed interrupter, and a vacuum integrity (hi-pot) test across the open gap. An interrupter at its erosion limit, or one that fails the open-gap hi-pot, is replaced as a unit; vacuum interrupters are sealed assemblies and are never opened or resurfaced. Elevated resistance across a closed interrupter, above the manufacturer limit, which is typically well under 100 microohms for these frames, points to contact erosion or loss of wipe. The external stabs, primary disconnect fingers, and bolted joints in the current path are cleaned and remeasured first; if the interrupter itself remains high, it is evaluated for replacement. SSC measures contact resistance after reassembly as part of the test protocol.

Vacuum Interrupters

Vacuum interrupter service is measurement, not refurbishment. Each interrupter is checked four ways: the erosion indicator against the manufacturer limit; contact resistance across the closed contacts; a dielectric withstand (hi-pot) test across the open gap, which is the field test for loss of vacuum; and wipe and gap measured through the operating linkage. An interrupter that fails any of these is replaced as a sealed unit. SSC stocks and sources replacement interrupters for the common GMI frames, machines the linkage and mounting hardware in-house when originals are worn or unavailable, and resets wipe to the Siemens specification on reassembly.

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 interrupter support insulation 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 so the trip-path results can feed the utility’s NERC PRC-005 protection-system documentation where the breaker serves a Protection System function, and so the maintenance record stands on its own for the asset file. SSC's test report provides that documentation directly.