The Westinghouse UTTB is an in-tank, oil-immersed load tap changer found on power transformers throughout the utility fleet. Its transfer-switch design uses arcing contacts to interrupt current during each tap change, making those contacts the primary wear component and the focal point of every maintenance visit. Contact condition, oil quality, and drive mechanism function determine whether a UTTB continues to operate reliably or begins to create problems on the system.
The UTTB uses a transfer-switch mechanism to move the circuit between adjacent tap positions on the transformer winding. During a tap change, the arcing contacts bridge the two taps momentarily, carrying the circulating current that flows between them during the transition. The arc that forms when the arcing contacts separate is extinguished in oil — the arc energy is absorbed by the contact material and by the surrounding oil, which is why contact erosion and oil degradation are the two maintenance drivers that run in parallel.
The UTTB compartment is sealed from the main transformer tank. The LTC oil and the main tank oil are independent systems. They must be sampled, tested, and serviced separately — and they will degrade at different rates, since the LTC oil is exposed to arc products from every tap change while the main tank oil is not.
UTTB variants differ in current rating, tap range, and specific contact dimensions. The nameplate data and the Westinghouse instruction book for the specific model govern the maintenance intervals, wear limits, and torque specifications. If the instruction book is not on site, locate it before performing internal work — general guidance cannot substitute for the model-specific data when it comes to acceptable wear dimensions.
Westinghouse specified contact inspection intervals in terms of operations — typically in the range of 50,000 to 75,000 tap change operations, depending on the UTTB model and the load current being interrupted at each operation. Units on active voltage regulation duty in distribution feeders accumulate operations quickly and may reach their inspection interval in three to five years. Units on infrequently tapped transmission or substation transformers may go seven to ten years between inspections on the calendar, while still remaining within their operation-count limit.
Oil testing runs on a faster schedule than contact inspection. Sample the UTTB compartment oil at least annually and compare dielectric strength, moisture, and acid number to Westinghouse specifications. DGA of the LTC oil provides early warning of abnormal arcing beyond the normal tap change signature. A unit that is tapping at high frequency under heavy load may need oil service well before its contacts reach their wear limit.
UnderNERC PRC-005, LTC maintenance activities on bulk electric system assets must be performed and documented within the standard's maximum intervals. Coordinate the UTTB maintenance schedule with the protection system maintenance records for the transformer to ensure both are in compliance.
The arcing contacts — both stationary and moving — are the first item to measure at every internal inspection. Drain the LTC compartment, remove the contact assembly, and measure the remaining contact material against the minimum thickness specified in the instruction book. Contact material worn to or past the minimum must be replaced immediately, regardless of where the unit stands in its maintenance calendar. Running arcing contacts below minimum thickness increases the risk of a failed arc interruption, which can weld the contacts together and leave the LTC unable to complete a tap change.
Look beyond the thickness measurement. Normal arcing contact wear on a healthy UTTB produces a relatively uniform erosion pattern across the contact face. Localized deep pitting, a severely cupped or irregular surface, or evidence of contact material ejected onto surrounding parts indicates abnormal arcing. Abnormal contact erosion patterns are usually caused by a problem upstream — a timing issue in the transfer mechanism, a drive that is slow through the critical bridging interval, or a high-resistance joint in the current path that shifts more arc energy onto the contacts than they were designed to handle. Replacing the contacts without investigating the cause will result in the same abnormal wear pattern on the next set.
Inspect the contact fingers and their spring loading. The contact fingers must maintain adequate pressure against the stationary contacts throughout the full range of tap positions. A finger that has lost spring tension from fatigue or overheating will have elevated contact resistance on the affected tap — visible as a milliohm reading significantly above baseline after reassembly. Measure contact resistance at each tap position before closing the compartment and compare to the previous maintenance record.
The stationary arcing contacts receive arc erosion on the same face that the moving contact bridges during transition. Inspect each stationary contact for erosion depth, surface condition, and secure mounting. A stationary contact that has been eroded unevenly — more deeply on one side of its face than the other — indicates that the moving contact is not centering correctly on transfer, which may be a drive alignment or linkage wear issue.
Southern Switch manufactures replacement stationary arcing contacts for the UTTB to OEM specification. Both standard replacement contacts and fully refurbished contact assemblies are available, including complete refurbished assemblies where all wear components are replaced and the assembly is tested before shipment. When the stationary contacts are worn, replacing both stationary and moving arcing contacts together at the same service interval is the most cost-effective approach — installing one new contact against a worn mating surface accelerates wear on the new part.
The UTTB drive mechanism must complete each tap change within the time specified in the instruction book. A slow transfer extends the bridging interval — the period during which the arcing contacts are carrying circulating current between two taps — which directly increases the energy the contacts must interrupt and the arc damage they sustain per operation. A mechanism that is within specification at last inspection can slow down significantly over the following maintenance interval if lubrication dries or wear accumulates in the linkage.
Time the tap change from initiation to completion and compare to the specification. If timing is slow, work through the mechanism systematically before assuming the drive motor is the cause. Clean and re-lubricate all pivot points, verify latch engagement, check for worn bushings in the linkage, and confirm that no external drag exists in the wiring harness or control connections that are routed through or near the drive mechanism. Dry lubricant is the most common cause of slow mechanism timing, and it is the easiest to fix.
Inspect the drive motor, motor connections, and the gear train between the motor and the operating mechanism. Verify that the motor is drawing current within the specified range during a complete tap change cycle and that it comes to rest correctly at each tap position. A motor that is hunting — cycling on and off repeatedly after an operation — indicates a position sensing problem, a worn limit switch, or a contact that is not completing cleanly on the tap position indicator circuit.
Drain the UTTB compartment oil before internal inspection and take a sample for laboratory analysis. The pre-drain sample documents the condition of the oil going out and is the correct comparison point for the incoming oil analysis at the next service. Do not let the sample sit in the container for an extended period before analysis — DGA results change as dissolved gases slowly come out of solution.
Clean the compartment interior before refilling. Carbon deposits from normal arc interruption accumulate on the compartment walls and on the contact assembly framework. Introducing fresh oil into a carbon-fouled compartment shortens the service life of the new oil immediately. Wipe down all surfaces, remove settled carbon from the bottom of the tank, and inspect the compartment seals before refilling. A seal that is compressed flat or has begun to extrude indicates it is past its service life and should be replaced during this outage rather than deferred.
Use only the oil specified for the UTTB compartment. LTC compartment oil is formulated for arc interruption service. Main tank transformer oil and LTC oil are not interchangeable — using the wrong oil type in the LTC compartment changes the dielectric and arc-interruption characteristics that the contact assembly was designed around.
For UTTB units that have accumulated significant wear or that have been out of service for an extended period, a complete refurbished contact assembly is often more practical than replacing individual components. Southern Switch rebuilds UTTB contact assemblies — disassembling, inspecting, replacing all arcing contact material and any worn current-carrying parts, and assembling to OEM specification. A refurbished assembly ships ready to install, with all contact faces in like-new condition, which eliminates the field time required to evaluate and selectively replace individual worn components.
The refurbished assembly approach also provides a documented baseline for the next maintenance cycle — the new contacts, their as-shipped dimensions, and the assembly inspection record go into the transformer file and become the reference for the next internal inspection.
We manufacture and refurbish Westinghouse UTTB arcing contacts and support LTC maintenance across Florida and the Southeast. Send us your unit details — nameplate data and approximate operation count if available — and we’ll come back with options.