The McGraw-Edison Type 550 is one of the most widely installed load tap changers on distribution and substation transformers in North America. The 550B and 550C variants share the same basic arcing contact and drive mechanism design, with detail differences in the reversing switch and selector configurations. Both require the same maintenance attention — the contacts wear with every operation and the oil degrades with every arc.
McGraw-Edison’s published interval for contact inspection is 50,000 tap change operations or every 5 years, whichever comes first. On a transformer serving a heavily regulated feeder — one that cycles voltage continuously throughout the day — 50,000 operations can arrive in under two years. On a lightly loaded substation unit that rarely taps, five years may pass with a fraction of that count.
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 acceptable levels. 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.
The Type 550 uses a resistor transition design. During a tap change, the arcing contacts bridge the two taps briefly through a transition resistor 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.
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.
With the LTC compartment drained and the contact assembly accessible, inspect each arcing contact tip for erosion depth. McGraw-Edison specifies the minimum remaining contact material before replacement is required — this dimension is in the instruction book for the specific unit. A worn contact that has reached minimum thickness must come out. 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.
The Type 550 drive mechanism is a spring-energy-storage system. The motor winds the spring; the spring releases to drive the tap change. This design means the tap change itself is fast and consistent regardless of motor speed, which is one of the reasons the Type 550 is a reliable unit with proper maintenance.
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 spring-release timing.
Inspect the motor for correct operation, brush condition if it is a brush-type motor, and correct voltage at the motor terminals during operation. Slow or sluggish motor operation that allows the spring to partially charge before the mechanism trips is a warning sign. A drive mechanism 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 current flowing through the transition resistor.
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 oil is specifically formulated for arc quenching performance and has different additives than standard transformer 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.
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 gears, a failed motor, or a damaged spring 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.
We manufacture McGraw-Edison 550 contact kits and provide field maintenance services for LTCs across the Southeast. Send us your unit details and we’ll respond within one business day.