McGraw-Edison Type 550 LTC Maintenance Guide

The McGraw-Edison Type 550 is one of the most widely installed load tap changers on distribution and substation transformers in North America. The 550 and 550B use the same drive mechanism and differ only in the drive motor. The 550C uses a different motor and a spur-gear internal drive in place of the geneva drive the earlier two use. The 550, 550B, and 550C share the same basic arcing-tap-switch architecture; the contact materials and the reversing switch changed across the later versions. The 550BLS and 550CS, the current silver-contact builds of the B and C lines, are the kits Southern Switch supplies today and are covered further down this page. All variants require the same maintenance attention, the contacts wear with every operation and the oil degrades with every arc.

Maintenance intervals

McGraw-Edison’s instruction book does not publish a fixed inspection number for the Type 550. It sets the interval by contact erosion, tracked against the operation counter, and states that the interval varies with service conditions, with one firm recommendation: give the unit a thorough inspection at the end of its first year. After that, the operation count and the erosion you find set the pace. On a transformer serving a heavily regulated feeder that cycles voltage all day, the count climbs fast and inspection comes sooner. On a lightly loaded unit that rarely taps, far longer may pass before the contacts justify opening it.

If your SCADA or tap changer counter is recording operations, use the count. If the counter has never been read or was reset at some unknown point, default to calendar interval and inspect. An uninspected LTC on an active feeder transformer is a liability, not a maintenance deferral.

Oil change intervals follow a separate track from contact inspection. McGraw-Edison specifies oil testing at each contact inspection, with a change or reclamation when the dielectric strength falls below the minimum for that unit: 28 kV in standard gap (ASTM D-877) on the 550, 550B, and 550C, and 30 kV for new oil or 26 kV for reused oil on the current 550BLS and 550CS. On active units, the oil typically needs attention before the contacts do, because each arc interruption deposits carbon that accumulates faster than the contacts physically wear.

What the 550B/C contact assembly includes

The Type 550 is a reactance-type arcing tap switch. During a tap change, the contacts bridge the two taps briefly through a preventive autotransformer that limits the circulating current during the bridging interval. The arcing contacts open and close under load current on every operation. The main contacts carry current between operations and see minimal arcing, but they do wear from mechanical cycling. The stationary arcing contacts are arc-resistant sintered copper-tungsten tips on a low-resistance copper base; the movable contacts are sintered silver-alloy, carrying current through a flexible shunt and silver-alloy-faced buttons to a copper collector ring. That arc-resistant tip is the sacrificial wear surface, which is why a worn 550 contact is a candidate for a material-grade rebuild rather than a like-for-like swap.

One ordering trap is specific to the 550. High-recovery-voltage (HRV) and low-recovery-voltage (LRV) contacts can be physically interchangeable while not being electrically interchangeable. The LRV contact is wider, and an LRV contact fitted into an HRV application can cause a tap flashover. Confirm the recovery-voltage class from the transformer serial number and LTC unit number before ordering, do not match on physical fit alone.

A complete contact assembly includes the arcing contacts, main contacts, reversing switch contacts, and the associated springs and hardware. The arcing contacts are the primary wear item. The main contacts are secondary. The reversing switch contacts operate less frequently but should be inspected at every maintenance visit, a stuck or worn reversing switch is a common cause of LTC failure.

Contact inspection: what to look for

With the LTC compartment drained and the contact assembly accessible, inspect each arcing contact tip for erosion depth. McGraw-Edison sets the replacement point at 3/16 inch of smooth rubbing surface remaining on the movable arcing contact. Below that, the contact can go thermally unstable at the interface and weld or burn the copper, so a contact worn to that limit must come out. The stationary arcing contact comes out before its arcing insert erodes to where burning would reach the copper base underneath. Running past minimum increases the likelihood of a weld or a failed interruption on the next operation.

Look for contact pitting, scoring, and uneven wear across the contact face. Minor pitting on arcing contacts is normal and acceptable within wear limits. Deep craters, a contact surface that has begun to cup or bell-mouth, or any sign of material transfer between the fixed and moving contacts means the contact is at end of life regardless of remaining thickness.

Check main contacts for evidence of overheating: discoloration of the contact tips, darkened or carbonized oil in the area immediately around the main contact, or fretting wear on the contact interface. Main contact overheating indicates either high contact resistance from a worn or misaligned contact, or a history of higher-than-normal load current.

Inspect the reversing switch contacts for pitting, and verify the reversing switch operates freely through its full range of motion. A reversing switch that binds or hesitates in the mechanism is a problem even if the contact surfaces look acceptable.

Check all springs for correct seating, visible fatigue cracking, or corrosion. A contact spring that has lost tension will cause the contact to bounce on make or not hold adequate contact pressure in the closed position, both of which accelerate wear. Spring replacement at each major inspection is inexpensive insurance.

Drive mechanism inspection

The Type 550 drive is a motor-driven gear train, not a stored-energy spring mechanism. On the 550 and 550B the motor turns a geneva pinion; on the 550C it drives through a spur-gear train. The universal drive shaft rotates 180 degrees per tap change in three distinct 60-degree movements, and a complete tap change takes roughly one to one and two-thirds seconds depending on the model. At the end of travel the drive is stopped positively by d-c braking of the motor. Because the motor drives the tap change continuously rather than a spring snapping it over, drive-train condition and motor torque directly set how cleanly the switch completes each operation.

Check drive mechanism gear teeth for wear, chipping, and adequate lubrication. The factory specifies grease type and points, do not substitute a different lubricant. Incompatible grease can attract contaminants or change viscosity in a way that affects drive-train timing.

Inspect the motor for correct operation and correct voltage at the motor terminals during operation. The 550 motor is a single-phase capacitor-start capacitor-run induction motor, so there are no brushes to service; a start or run capacitor that has drifted or failed shows up as slow or failed starting. Slow or sluggish operation that drags out the tap-change stroke is a warning sign. A drive train that is operating on the edge of its torque margin will eventually fail to complete a tap change mid-operation, leaving the LTC in the bridging position under load, a condition that must be resolved immediately.

Time the tap change operation. The complete operation from motor start to tap change completion should fall within the manufacturer’s specification. A tap change that is running slow is wearing contacts faster because the arcing contacts spend more time in the bridging position with circulating current flowing through the preventive autotransformer.

Oil testing and condition

Sample the LTC oil before draining the compartment at each inspection. The sample tells you what the oil has been doing since the last change. High carbon content, low dielectric breakdown voltage, or elevated moisture all tell different stories about the operating history of the contacts.

LTC oil is kept separate from the main tank oil in the Type 550. Do not mix them and do not use the main tank oil specification interchangeably with the LTC compartment. The LTC compartment uses the same type of mineral insulating oil as the main tank, but it is managed as a separate system: it sees arc byproducts and is evaluated against different condition criteria than main tank oil.

If the oil is darkened significantly or the carbon content is high, filter the compartment walls and contact assembly before refilling with new oil. Leaving heavy carbon deposits behind means the new oil begins degrading immediately from suspended contamination rather than starting clean.

DGA sampling of LTC oil is also worth including in your maintenance program. Dissolved gas analysis on the LTC compartment detects arc-related gases that indicate abnormal arcing beyond normal tap change activity, a sign of contact problems, timing issues, or a stuck mechanism that caused an extended arc. A DGA result showing high acetylene in the LTC oil alongside rising ethylene is a red flag that warrants immediate inspection regardless of where the unit is in its maintenance schedule.

When to replace vs. refurbish

Contact replacement is the most common maintenance action on the Type 550. A complete contact kit, arcing contacts, main contacts, reversing contacts, springs, and hardware, replaces all wear items at once. Replacing only the arcing contacts while leaving worn main contacts or fatigued springs creates an unbalanced assembly that is likely to need another partial replacement sooner than a full-kit replacement would.

The drive mechanism rarely needs replacement if it has been properly lubricated, but worn geneva or spur gears, a failed motor, or a drifted motor capacitor should be addressed rather than deferred. A drive mechanism problem that causes an incomplete tap change is more disruptive than a planned replacement during a scheduled outage.

A Type 550 that has never been opened and has an unknown operation count is a candidate for a complete internal inspection, contact replacement, and fresh oil regardless of calendar age. Unknown maintenance history is not a reason to defer, it is a reason to establish a known baseline and work forward from there.

Current kits: the 550BLS and 550CS

McGraw-Edison evolved the 550 in two lines, and the contacts are what changed. The B line began as the 550B with copper-tungsten moving contacts, moved to the 550BL and the rare 550BH with silver-tungsten moving contacts, and then to the 550BLS, which added silver plating to the stationary contacts. The C line ran the same course and ends at the 550CS. The 550BLS and the 550CS are the current builds and the only two Type 550 kits Southern Switch makes. The earlier 550, 550B, 550BL, 550BH, and 550C are obsolete, and when one comes in we rebuild it to current specification: silver-tungsten moving contacts and silver-plated stationary contacts that outlast the originals. That is a material upgrade, not a like-for-like swap.

The 550CS is McGraw-Edison's improved version of the 550C (the 550-series arcing-tap-selector was introduced in 1964 for medium-size transformers); it runs the same 550C-style spur-gear drive but a revised arcing-tap-selector and reversing switch. CS parts changed across builds, most notably the pre-1975 and post-1975 reversing parts, so confirm the model and build date from the LTC nameplate before ordering. One exception worth knowing: the silver-plated main dial stationary contacts on the CS are directly interchangeable with all previous 550C units.

The 550 BLS uses a three-shaft drive: one input shaft from the motor drive unit and separate LH and RH output shafts that operate the tap change mechanism in the raise and lower directions. Inspect all three shafts for wear and confirm they transmit motion freely through their full range, a shaft that binds or has developed play at its coupling produces inconsistent tap changes and can leave the LTC in an intermediate position between taps. The main moving and stationary contacts on the BLS use locking tabs to secure the contact in its mounting. A disengaged locking tab lets the contact shift under operating loads, which accelerates wear and can eventually cause a partial tap change.

The complete 550BLS kit is a full three-phase change: 6 main moving and 3 reversing moving contact assemblies, 54 main stationary contacts, 6 reversing stationary and 3 neutral stationary contacts, and 36 locking tabs (6 reversing-switch and 30 selector-switch). The 550BLS full kit adds the three drive shafts, the input shaft plus the left-hand and right-hand output shafts, for a complete drive rebuild. The 550CS kit carries the equivalent CS-geometry contacts. Confirm the model from the LTC nameplate before ordering, since 550BLS and 550CS parts do not interchange.

The CS reversing switch changed design in February 1975: the reversing segment gained an offset slot that lets the main dial contacts interrupt the current first, which stopped the arcing that older units saw on reversing stationary contact No. 1 and let McGraw-Edison move to silver-plated stationary and coin-silver movable reversing contacts. Pre-1975 and post-1975 reversing parts are not the same, so confirm the build date before ordering. The CS reversing moving contact comes in both two-finger and three-finger configurations, and the two are not interchangeable, so confirm which your unit uses before ordering. The CS reversing movable contacts wear mechanically rather than by arc erosion, and the manual replaces them when the initial gap between the movable contacts opens to 9/32 inch. The movable arcing contacts come out at 3/16 inch of rubbing surface remaining, and the silver-plated stationary arcing contacts come out before erosion burns through the plating. CS contact assemblies are held by shouldered mounting bolts, locknuts, and lockstrips that must be correctly seated on reassembly; a bent or damaged lockstrip should be replaced, not reinstalled.

S210-40-3: the current Type 550 service bulletin

The current McGraw-Edison documentation for the Type 550 is Service Information S210-40-3, issued by the McGraw-Edison Power Systems Division in Canonsburg, Pennsylvania. It covers the 550, 550B, and 550C together. The replacement-parts data lives in its Supplements 1, 2, and 3, one per variant, and the contact-erosion patterns and replacement criteria are in Supplement 5. The drive controls are documented separately in PTI-224. S210-40-3 supersedes the older PTI-195-1 instruction book, so a shop still working from PTI-195-1 is a revision behind on part references. The wear limits that matter in the field: replace the movable arcing contact when about 3/16 inch of smooth rubbing surface remains on the arcing tip, replace the stationary contact before its sintered insert erodes to the point of burning on the copper base, and take the compartment oil out of service below the 28 kV minimum in standard gap (ASTM D-877). The mechanism was built in 32-step and 16-step versions; the scroll cam rotates 180 degrees per tap change on a 32-step unit and 360 degrees on a 16-step unit, which is one way to confirm which you have. We keep the current OEM documentation and factory part references for these units on file. Send a nameplate photo through our contact form and we will confirm the model and quote the correct kit.