Troubleshooting MCC Bucket Trip Issues: A Systematic Approach

A tripped MCC bucket means a motor is down and production may be affected. The pressure to get it running again is real, but a systematic approach saves time and prevents repeat failures. This guide provides a structured troubleshooting process for the most common MCC bucket trip scenarios.

Before You Start

Safety First

1. Follow your facility's LOTO procedures
2. Wear appropriate PPE per NFPA 70E arc flash requirements
3. Never reset a tripped device without understanding why it tripped
4. Have a qualified electrician perform all troubleshooting on energized equipment

Information Gathering

Before touching anything, gather information:

• What was the motor doing when it tripped? (starting, running, under load change)
• Has this happened before? How often?
• Were there any unusual conditions? (power outage, lightning, equipment changes)
• What tripped: the breaker, the overload, or both?
• Any alarms or error codes on electronic overloads or VFDs?

Scenario 1: Overload Relay Trip

The overload relay (thermal or electronic) has tripped, but the breaker is still closed.

What It Means

The motor drew current exceeding the overload relay setting for long enough to trip. This is thermal protection, not instantaneous. It takes time to trip (seconds to minutes depending on the overcurrent magnitude).

Troubleshooting Steps

Step 1: Check the Motor

• Is the motor shaft free to rotate? A mechanically jammed motor draws locked-rotor current
• Is the driven equipment overloaded? (blocked conveyor, closed valve, binding pump)
• Is the motor bearing seized or failing?
• Has the load changed recently?

Step 2: Verify Overload Settings

• Compare overload relay setting to motor nameplate FLA
• Setting should be at or slightly above motor nameplate FLA
• For electronic overloads: check trip class setting (Class 10, 20, or 30)
• For thermal overloads: verify correct heater element

Step 3: Measure Motor Current Reset the overload and restart the motor. Immediately measure motor current on all three phases:

• Current should be at or below motor nameplate FLA
• All three phases should be balanced within 5%
• If current is high, the motor or load has a problem
• If current is normal, the overload may be defective

Step 4: Check for Single-Phasing Measure voltage at the motor terminals with the motor running:

• All three phases should be balanced within 3%
• A missing or low phase indicates a single-phasing condition
• Single-phasing causes the motor to draw excessive current on remaining phases
• Check for a blown fuse, open contactor pole, or broken conductor

Step 5: Check Ambient Temperature If using thermal overload relays:

• Ambient temperature above 40 degrees C can cause premature tripping
• Apply manufacturer's correction factor or upgrade to electronic overloads

Scenario 2: Circuit Breaker Trip

The bucket's circuit breaker has tripped (handle in the tripped position, midway between ON and OFF).

What It Means

A breaker trip indicates either a short circuit (instantaneous trip) or a sustained overcurrent (time-delay trip). The distinction matters:

Instantaneous Trip: The breaker tripped immediately (within 1-2 cycles). This indicates a short circuit or ground fault. The fault current was high enough to trigger the breaker's instantaneous trip element.

Time-Delay Trip: The breaker tripped after some delay (seconds to minutes). This indicates a sustained overcurrent above the breaker's long-time trip setting but below the instantaneous setting.

Troubleshooting Steps

Step 1: Do NOT Immediately Reset A breaker trip on short circuit means there is a fault in the circuit. Resetting the breaker into a fault will re-establish the arc and may cause equipment damage or arc flash.

Step 2: Inspect the Bucket

• Open the bucket door and visually inspect for signs of a fault
• Look for: burned wiring, melted insulation, carbon deposits, damaged components
• Smell for: burned insulation, ozone, hot metal
• If you find evidence of a fault inside the bucket, do not re-energize until repaired

Step 3: Test the Motor Circuit With the breaker OFF and motor disconnected:

• Measure insulation resistance (megger) on each phase of the motor cable
• Phase to ground: should be 1 megohm minimum (higher for newer installations)
• Phase to phase: should be open (infinite resistance)
• If insulation resistance is low, there is a fault in the cable or motor

Step 4: Test the Motor Disconnect motor leads at the motor junction box:

• Measure winding resistance phase to phase (should be balanced within 5%)
• Megger each winding to ground (should be over 1 megohm)
• If motor tests bad, it needs to be repaired or replaced

Step 5: Check the Breaker If the motor and cable test good:

• The breaker may have tripped on a transient event (voltage surge, switching transient)
• Or the breaker may be defective (weak trip mechanism, worn contacts)
• Reset and restart. If it trips again immediately, there is a fault you have not found
• If it holds, monitor closely

Scenario 3: Control Circuit Failure

The motor does not start when the start button is pressed. No trip condition, just no response.

Troubleshooting Steps

Step 1: Check Control Power

• Is the control transformer energized? Check for 120V on the secondary
• If no control voltage: check control transformer primary fuse and secondary fuse
• If fuses are blown, there may be a short in the control circuit

Step 2: Check the Start/Stop Circuit

• Press START and listen for the contactor closing
• If contactor does not pull in: check control circuit continuity
• Verify stop button is in the correct (closed) position
• Check any remote start/stop devices, selector switches (Hand-Off-Auto position)
• Check any interlocking contacts from other equipment

Step 3: Check the Contactor Coil

• Measure voltage across the contactor coil with START pressed
• If voltage is present but contactor does not pull in: coil is open or mechanically bound
• If no voltage at coil: there is an open in the control circuit between the transformer and coil

**Step 4: Check Auxiliary Contacts and Interlocks

• The seal-in contact (auxiliary NO contact on the contactor) must close to maintain the circuit after the START button is released
• Overload relay auxiliary contact must be closed (not tripped)
• Any safety interlocks (E-stop, door switches, pressure switches) must be in the run-permissive state

Scenario 4: Intermittent Trips

The bucket trips randomly, sometimes runs for hours, sometimes trips within minutes.

Common Causes

Loose Connection: A loose power or control connection creates intermittent contact. Under vibration or thermal expansion, the connection opens momentarily.

Marginal Overload: Motor current is very close to the overload trip setting. Small load variations push it over the trip threshold intermittently.

Voltage Sag: Utility voltage drops cause the motor to draw more current. If the voltage sag is severe enough, the contactor may drop out (typically below 70% of rated coil voltage).

Contactor Chatter: A worn contactor with weak springs or a weak coil does not close fully, causing the contacts to bounce (chatter). This creates intermittent connection and rapid wear.

Troubleshooting Steps

1. Install a current data logger on the motor circuit to capture the current profile leading up to the trip
2. Monitor voltage at the MCC bus for sags or fluctuations
3. Inspect all connections for tightness (perform a complete retorque)
4. Verify overload settings with adequate margin above motor FLA
5. Listen to the contactor during operation for chattering or buzzing

When to Call for Help

If systematic troubleshooting does not identify the root cause, or if the issue involves:

• Repeated breaker trips with no identifiable fault
• Multiple buckets tripping simultaneously (possible bus or utility issue)
• Evidence of arc flash or equipment damage
• VFD faults requiring manufacturer support

Contact a qualified electrical contractor or the MCC manufacturer for assistance. MCC Depot also provides field services for on-site troubleshooting and replacement buckets when repair is not practical.

Call 307-442-0382 or email sales@mccdepot.com for technical assistance and replacement bucket quotes.