The Westinghouse URT is a resistor-transition load tap changer found on power transformers throughout the utility fleet. Like all in-tank LTCs, it operates in oil and its contacts wear with every tap change. The arcing contacts absorb the brunt of that wear, but the reversing switch, drive mechanism, and oil condition all require attention on the same maintenance cycle.
Westinghouse specified contact inspection intervals based on operation count — typically in the range of 50,000 to 100,000 operations depending on the specific URT variant and the load current being switched. Calendar-based intervals of five to seven years apply when operation counts are unavailable or the counter has not been consistently read.
The instruction book for the specific URT model governs. URT variants differ in contact geometry and wear limits, and the nameplate or instruction book number is the correct starting point for any maintenance interval or specification question. If the instruction book is not on site, it should be located before any internal inspection is attempted.
Oil testing runs on a separate track. Sample the LTC compartment oil annually on active units and compare dielectric strength, moisture content, and acid number to Westinghouse specifications. A unit that taps frequently may need oil service well before the contacts reach their wear limit.
The URT uses a resistor transition design: the arcing contacts bridge two adjacent taps briefly through a current-limiting resistor during each tap change. This limits the circulating current during the bridging interval and transfers the arc interruption duty to the arcing contacts rather than the main contacts. The main contacts close onto an already-bridged circuit and open after the arcing contacts have cleared — they carry load current but see minimal arcing under normal conditions.
The transition resistor itself is a wear item in the sense that it must maintain its specified resistance value across its service life. An open or high-resistance transition resistor changes the current interruption duty on the arcing contacts in ways that accelerate wear. If contact wear appears inconsistent with the known operation count, check the transition resistor before assuming the contacts alone are the cause.
With the LTC compartment drained, inspect arcing contacts for erosion depth against the minimum thickness specified in the instruction book. Contact material that has worn to or past minimum must be replaced regardless of where the unit falls in its maintenance calendar. Running worn arcing contacts increases the risk of a failed interruption or contact weld on the next operation.
Look at the contact surface morphology beyond just thickness. Normal arcing contact wear produces a relatively uniform erosion crater. Uneven wear across the contact face, deep localized pitting, or evidence of material splash onto adjacent surfaces indicates abnormal arcing — possibly caused by a timing problem in the drive sequence or a transition resistor issue. These conditions warrant investigation before simply replacing contacts and returning the unit to service.
Inspect main contacts for surface condition and spring-loaded contact pressure. A main contact that is not seating with adequate pressure shows elevated resistance on a milliohm measurement after reassembly. Measure contact resistance on each phase before closing the compartment; a reading significantly above baseline for that contact assembly indicates an alignment or spring problem that needs to be resolved before the unit goes back into service.
The reversing switch changes tap direction and operates far less frequently than the arcing contacts, but it must be inspected at every maintenance visit. Reversing switch contacts that are pitted or have insufficient contact pressure will cause high resistance or arcing on the relatively infrequent operations they do perform — conditions that are harder to catch because they happen so rarely.
The URT drive mechanism stores energy in a spring charged by the drive motor. Inspect the mechanism for adequate lubrication at all pivot points and the spring guide, gear tooth condition, and correct latch engagement. Old lubricant dries into a varnish that adds friction throughout the mechanism. Clean all surfaces thoroughly before applying fresh lubricant of the type specified in the instruction book.
Time the tap change operation from motor start to completion and verify it falls within specification. A slow tap change extends the bridging interval, which increases the energy the arcing contacts must absorb on each operation. If timing is slow after cleaning and lubrication, the spring has likely taken a permanent set and should be replaced.
Verify motor operation, brush condition on brush-type motors, and correct control voltage at the motor terminals. A motor that struggles to charge the spring or takes longer than specified to complete the charge cycle is a maintenance item, not a monitoring item — deferred motor work translates directly into mechanism timing problems.
The URT LTC compartment is isolated from the main transformer tank. Do not mix the oils and do not use the main tank oil specification interchangeably with the LTC compartment. LTC oil is formulated for arc interruption duty; main tank oil is not.
Sample oil before draining at each inspection. The sample documents the condition of the oil going out and gives you trending data across maintenance cycles. Dark or heavily carbon-loaded oil means the compartment walls and contact assembly should be cleaned before the new charge of oil goes in. Introducing fresh oil into a carbon-fouled compartment shortens the service life of the new oil immediately.
DGA sampling of LTC oil alongside the routine dielectric and moisture tests provides early warning of abnormal arcing beyond the normal tap change signature. Elevated acetylene in the LTC compartment oil, particularly when it is rising between samples, is a sign of an arc problem that warrants inspection before the next scheduled maintenance cycle.
The URT was applied across a range of transformer ratings, including high-current applications where the contact assembly carries substantially higher load current than standard ratings. On high-ampere units, contact wear accelerates and oil degradation from I²R heating at the contact interface is more pronounced. High-ampere contact kits use heavier contact material and larger current-carrying cross-sections to manage the additional thermal load.
Verify which contact kit applies to the specific unit before ordering replacement contacts. A standard-ampere contact kit installed on a high-ampere application will wear faster than specified and may not provide adequate current-carrying capacity under full load.
We manufacture Westinghouse URT contact kits and support LTC maintenance across the Southeast. Send us your unit details and we’ll respond within one business day.