Two Coils, Two Different Jobs
A vacuum circuit breaker's spring-operated mechanism is charged by a motor, but the actual open and close commands are triggered electrically through two separate solenoid coils. The closing coil releases the charged closing spring to close the breaker on command. The trip coil (also called a shunt trip coil) releases the trip latch to open the breaker, either on a manual open command or automatically from a protection relay signal during a fault. They are electrically and mechanically independent — a breaker can fail to trip while still closing normally, and vice versa, which is part of why routine testing checks both separately.
How the Trip Path Actually Works
On a fault, the protection relay (numerical or electromechanical) detects an abnormal condition and energizes the trip coil. The coil's plunger moves to strike the trip latch, releasing the stored spring energy that drives the contacts open — typically within a few cycles. Because this path has to work reliably even after the breaker has sat closed and untouched for months, trip circuits are usually supervised with a trip circuit supervision relay or continuity indication, precisely because an undetected open trip circuit means the protection scheme has no way to actually clear a fault when it matters.
Common Failure Modes
- Burnt or open coil winding — from prolonged energization (coils are rated for momentary duty, not continuous), often caused by an auxiliary contact failing to interrupt the coil circuit after the operation completes.
- Mechanical binding of the plunger — dirt, corrosion, or lack of lubrication on the plunger can prevent it from striking the latch with enough force, especially in breakers installed in dusty or humid environments.
- Latch misalignment — even a functioning coil won't trip the breaker if the mechanical latch geometry has shifted, which is why mechanism timing tests check both electrical and mechanical performance together.
- Undervoltage at the coil — trip coils have a rated voltage range; a weak station battery or long control cable run with voltage drop can leave the coil under-energized even though it "looks" like it's getting a signal.
Testing and Replacement
A trip/close coil test typically checks coil resistance against the manufacturer's rated value, confirms pickup at the minimum rated voltage (often as low as 70% of nominal for trip coils, by design — a trip signal has to work even during a fault-induced voltage dip), and verifies the mechanism actually completes full travel. If resistance is significantly off spec or the coil fails to pick up within its rated voltage window, it's a straightforward like-for-like replacement rather than a repair — coils are sealed, wound components.
When ordering a replacement, the voltage rating (commonly 24V, 30V, 110V, or 220V DC depending on your station battery) and the physical mounting/plunger interface both need to match your specific breaker model exactly. Check current stock for trip coils and closing coils, or if you're not sure which rating your panel uses, send us your nameplate details via request a quote and we'll confirm before you order.