ITE K-Don Low-Voltage Power Circuit Breaker Rebuild

The ITE K-Don is a K-frame drawout low-voltage power circuit breaker rated at 600V, found in ITE switchgear in utility substations and industrial facilities from the 1960s through the 1980s. Common continuous ratings are 800A, 1600A, 2000A, and 3000A; the K-Don was the ITE equivalent of the GE AKR and Westinghouse DB, used for bus tie, bus sectionalizing, and feeder protection applications. ITE passed through Gould, ABB, and Siemens, and OEM parts for the K-Don are no longer manufactured — shop rebuild is the practical path to returning these breakers to service without replacing the switchgear around them. Southern Switch rebuilds K-Don breakers at our Palm Harbor shop, the same shop that handles ITE K-series medium-voltage work and GE AKR rebuilds.

Contact Assembly

K-Don main contacts are silver-faced copper, engaging with defined wipe distance between the moving arm contact and the stationary contact. Contact wipe — the compression travel after initial touch — determines contact force and is a function of contact arm position, spring condition, and contact face thickness. As the contact faces erode, effective wipe decreases even if the spring is intact; reduced wipe produces elevated contact resistance and heating at rated load. SSC measures wipe against the ITE specification for the frame class and replaces contacts when the moving arm position indicates insufficient remaining wipe travel.

Arcing contacts on the K-Don are separate dedicated tips mounted on the moving and stationary assemblies that part last on opening and make first on closing, absorbing the interruption arc at their contact surface. Arcing tip erosion is the primary wear mechanism in LVPCBs: the tip is consumed with each interruption, and when depleted, arc energy transfers to the main contact face. SSC measures arcing tip remaining depth against the ITE wear limit; contacts at or past the limit are replaced regardless of main contact appearance. The arcing contact interface is also inspected for side-to-side misalignment that concentrates arc erosion on one edge of the tip.

Arc Chutes and Blow-Out Coils

K-Don arc chutes use magnetic blow-out coils wired in series with the main circuit to generate a magnetic field that drives the arc into the splitter plate stack for rapid extinction. The blow-out field is proportional to fault current — the higher the fault, the stronger the arc guidance — which is why the 600V LVPCB duty cycle, where fault currents reach tens of kiloamperes, produces more aggressive arc chute wear than the medium-voltage K-series despite lower voltage. K-Don arc chutes from breakers that have interrupted heavy fault duty typically show significant carbon packing between the ceramic splitter plates and erosion of the arc entry zone at the base of the chute.

SSC removes and disassembles each arc chute, cleans the splitter plate stack and chute housing with compressed air and solvent, and inspects the ceramic plates for cracking, erosion, and carbon build-up in the plate gaps. Plates that are cracked, eroded through, or packed with carbon that cannot be cleaned out are replaced. The blow-out coil connections at the chute entry are cleaned and inspected for corrosion — a high-resistance blow-out coil connection reduces the magnetic field strength and degrades arc guidance into the chute at partial fault currents. Each arc chute is reassembled and verified for correct plate count and alignment before reinstallation.

Electromechanical Trip Unit

K-Don breakers use electromechanical overcurrent trip units with thermal-magnetic construction: bimetallic elements provide time-overcurrent (long-time) protection through a heat-deflection mechanism, and magnetic elements provide instantaneous or short-time tripping through direct electromagnetic force on the trip latch. In long-service breakers, bimetallic elements drift from their factory calibration — a bimetal that has seen heavy load cycling or elevated ambient temperatures for decades will not trip at the same current-time relationship as when it was manufactured. The direction of drift is unpredictable; the breaker may nuisance-trip on moderate overcurrents or fail to trip within the intended time at high overcurrents, both of which are protection failures.

SSC verifies trip unit calibration by injecting primary current through each phase individually at multiples of the long-time setpoint — typically 150%, 300%, and 600% — and recording the trip time at each level against the published time-current curve for the trip unit type and rating. Instantaneous pickup is tested by ramping current from below pickup to above and recording the trip point. Units that fall outside the tolerance band at any test point are adjusted where adjustment provisions exist; those beyond adjustable range are replaced with a new or reconditioned unit. Ground fault trip elements, where present, are verified separately by injecting residual current through the neutral element and confirming trip at the adjusted pickup.

Operating Mechanism

The K-Don operating mechanism stores energy in a close spring that is charged either by the manual charging handle or the motor operator. On a close command, the spring energy drives the contact arm from open to closed in milliseconds; the trip latch holds the mechanism closed until a trip signal releases it. After decades in service, the mechanism pivot pins develop wear that produces play in the close and trip linkage — play that translates to variability in close time, trip time, and contact velocity. Mechanism lubrication in LVPCBs is critical: the original grease hardens over time and produces sticky, slow operation that appears normal on manual exercise but fails to meet timing specifications under power.

SSC disassembles and cleans the mechanism, replaces worn pivot pins and roll pins, lubricates with the appropriate low-temperature grease for the operating environment, and verifies close spring tension and trip latch engagement force. The anti-pump relay, where fitted, is tested to confirm the breaker will not re-close after a trip with a continuous close signal present. Motor operator function — where the breaker has an electrically charged spring — is verified for reliable spring charging within the specified time window.

Secondary Stabs and Drawout Function

K-Don breakers connect the control circuit through a secondary stab assembly that mates with a corresponding female connector block in the switchgear cubicle when the breaker is racked to the connected position. The secondary stab fingers are spring-loaded; after long service the springs lose tension and the contact force at the mating surface drops. Low contact force at the secondary stab is the most common source of intermittent control circuit problems in aging LVPCBs — trip coil alarms, fail-to-close, or spurious undervoltage trips — because the secondary resistance fluctuates with vibration and thermal cycling. SSC measures secondary stab contact resistance and replaces finger assemblies that fall outside the acceptable resistance range.

The drawout racking mechanism and its interlocks are verified as part of every rebuild. The racking interlock must prevent the breaker from being racked from test to connected with the breaker in the closed state, and must prevent re-closing in the test or disconnected position unless specifically required by the protection scheme. Worn racking threads or interlock fingers that allow the breaker to settle between positions are repaired or replaced; a K-Don that can drift from connected to test position under vibration will cause protection failures that are not traceable without careful inspection.

Test Protocol

Every rebuilt K-Don is tested for contact resistance (microohm across each phase, main current path, compared to new-condition acceptance values), insulation resistance (phase-to-phase and phase-to-ground at 1000V DC), trip unit calibration by primary current injection, close time and trip time measurement, and hi-pot at 2.2kV AC across open contacts and 2.2kV AC phase-to-ground. Each phase of the main current path is tested individually for contact resistance; a single elevated phase flags a contact alignment or wipe problem that requires correction before the breaker leaves the shop. SSC issues a written test report documenting pre-rebuild as-found values, all work performed, and post-rebuild acceptance test results. The report is formatted to support NERC PRC-005 breaker maintenance documentation for utilities that maintain a compliance program.