The McGraw-Edison 550 BLS and 550 CS are load tap changers used in McGraw-Edison power transformers. The BLS and CS are distinct models with different internal configurations and contact geometries. The BLS uses a three-shaft drive arrangement with a main input drive shaft and separate LH and RH output drive shafts. The CS uses a two-finger and three-finger reversing moving contact configuration that must be identified correctly before ordering. Both models use locking tabs on contact assemblies that must be correctly seated during reassembly. Parts between the BLS and CS do not interchange — confirm the model from the LTC nameplate before ordering any contact component.
The BLS uses a three-shaft drive system: one input drive shaft from the motor drive unit and separate LH and RH output drive shafts that operate the tap change mechanism in the respective raise and lower directions. Inspect the drive shaft linkage for wear and verify that all three shafts operate freely and transmit motion correctly through their full range. A drive shaft that binds or has developed play at its coupling produces inconsistent tap change operation and can result in an incomplete tap change that leaves the LTC in an intermediate position between taps.
The main moving contact assembly on the BLS is the current-carrying contact that engages the stationary contacts at each tap position. Inspect the main moving contact for surface wear and verify that the locking tabs are correctly seated. The locking tabs on BLS contact assemblies secure the contact in its mounting and prevent movement under the mechanical loads of tap change operation. A contact with a disengaged locking tab can shift position during an operation, producing inconsistent contact engagement that may not cause an immediate failure but will accelerate surface wear and eventually result in a partial tap change.
The main stationary contacts on the BLS engage the main moving contact at each tap position. Inspect each stationary contact for surface condition, oxidation, and wear depth at the contact engagement zone. Measure contact resistance at each tap position and record for trending between maintenance intervals.
The 550 CS uses a reversing moving contact that comes in two configurations: a two-finger variant and a three-finger variant. The two configurations are not interchangeable — the correct variant must be identified from the existing contact before ordering. To identify which variant is installed, inspect the reversing moving contact directly before disassembly. Count the contact fingers on the moving arm: two fingers or three fingers. Order the replacement contact assembly to match the count. Installing a two-finger contact in a unit designed for three-finger geometry, or vice versa, will produce incorrect contact engagement with the reversing stationary contacts.
As with the BLS, the CS contact assemblies use locking tabs that must be correctly seated during reinstallation. Inspect locking tab engagement on each contact assembly during reassembly and verify that all tabs are fully engaged before installing the oil compartment cover. A locking tab that is bent or damaged should be replaced rather than reinstalled — a damaged locking tab that appears to be seated but releases under operating loads will allow the contact to shift during the first few tap changes after return to service.
Maintenance intervals for the 550 BLS and CS follow operation-count guidance from McGraw-Edison service documentation, withNERC PRC-005Calendar limits applying for utility programs. Sample LTC oil at each maintenance interval for dielectric strength and dissolved gas. Check the three-shaft drive linkage on the BLS for wear and play at each maintenance inspection — drive linkage wear is a mechanical failure mode separate from contact wear and does not show up in contact resistance measurements or oil sampling.
Confirm the model (BLS or CS) and — for the CS — the reversing contact finger count (2 or 3) before ordering. Southern Switch stocks both variants. Contact us with your nameplate details for a quote.