GE AM-Series Medium Voltage Air Circuit Breaker Rebuild

The GE AM series are air magnetic medium voltage circuit breakers installed in GE type AM and AML metal-clad switchgear from the 1950s through the 1980s. AM-4.16 and AM-13.8 designations indicate the system voltage class. GE no longer supports these breakers and OEM parts are not available; Southern Switch rebuilds AM-series breakers at our Palm Harbor shop using aftermarket parts and in-house fabrication where required.

The AM-Series Design

AM stands for Air Magnetic — the arc interruption method. When AM-series contacts part under fault current, the arc is driven by the magnetic force from a blow-out coil in series with the primary circuit into a ceramic arc chute. The coil produces a field that accelerates the arc upward into the splitter plate stack faster than the arc's own magnetic field would. This produces more consistent arc chute loading across a range of fault current magnitudes compared to designs that rely solely on the arc's self-magnetic force.

The AM series used drawout construction — the breaker rolls into the switchgear cubicle and engages primary stab contacts and secondary disconnects when racked to the connected position. The racking mechanism is part of the switchgear, not the breaker itself, but racking interlock condition affects how the breaker behaves in the cubicle and must be evaluated when a breaker goes back into a specific cubicle after rebuild.

Contact Wear

AM-series main contacts are silver-overlay copper fingers that press against stationary contacts in the closed position. The spring loading on the moving contact finger assembly determines the contact force; reduced spring tension after years of service allows the contact to vibrate slightly under load current, producing fretting corrosion and elevated contact resistance. Contact alignment — the engagement of the moving contact face against the stationary contact face — must be within specification or contact resistance rises regardless of surface condition.

During rebuild, SSC inspects each contact for silver overlay wear, pitting at the contact engagement zone, and evidence of overheating at the contact joint (blue discoloration of the copper base below the silver layer indicates sustained elevated temperature). Contacts with silver overlay worn through to the copper, pitting deeper than the overlay thickness, or signs of thermal damage are replaced. Contact resistance is measured before and after rebuild and compared to the breaker's commissioning value where available.

Arc Chutes

AM-series arc chutes use ceramic plate stacks with fiber side panels. The blow-out coil drives the arc forcefully into the chute entry zone, which concentrates carbon deposition at the lower plates faster than in some competing designs. In the field we consistently find that AM arc chutes develop heavy carbon buildup at the entry zone even when the upper plate stack remains relatively clean. Carbon-packed entry zones reduce the arc-quenching effectiveness of the chute and can produce reignition.

SSC cleans arc chutes with compressed air and appropriate solvents, focusing on the entry zone where carbon accumulates. Each plate is inspected individually for cracking and erosion depth. Entry plates that are eroded beyond cleaning or show through-thickness cracks are replaced. Upper plates that are clean and undamaged are reused. Where the full chute set is degraded, SSC replaces the assembly — aftermarket chute sets are available for common AM configurations; SSC fabricates ceramic plates and fiber side panels for configurations where aftermarket supply is exhausted.

Mechanism and Trip Coil

The AM-series operating mechanism uses an over-center toggle design: the closing spring drives the contact assembly through an over-center toggle that locks the contacts closed until the trip latch releases. The toggle produces a fast, high-force close with good contact pressure in the closed position. The trip mechanism is a shunt coil in the secondary circuit that releases the trip latch when energized. Undervoltage release, where present, trips the breaker when control voltage falls below the UV relay setting.

Mechanism service includes disassembly, cleaning, inspection of toggle pivot pins and latch rollers, spring inspection, and full relubrication. SSC tests trip coil pickup voltage at 50%, 70%, and 100% of rated control voltage to verify reliable tripping at the lower voltage margins that occur during substation battery discharge events. The manual trip interlock is tested to confirm that it operates the breaker correctly without damaging the mechanism.

Test Protocol

Rebuilt AM-series breakers go through contact resistance (microohm), close/trip timing, hi-pot, and insulation resistance tests before leaving the shop. Hi-pot test voltage for AM-4.16 class is 19kV AC (or DC equivalent) across open contacts and phase-to-ground. AM-13.8 class requires 36kV AC. SSC documents as-found and as-left values in a written test report. Timing values are compared to the GE service specification for the specific AM type; close times outside the window indicate a mechanism issue that is corrected before the test is called complete.