Okay, this is a classic motor starting problem, and it's great you're trying to understand it! Here's a breakdown of what to investigate and how to approach the troubleshooting

1. Verify the 310 Digitrip Settings
Most Important

The first thing to do is verify the settings on the 310 Digitrip. Are they what you think they are? This is the most likely culprit. Here's what to look for:

Long Time Pickup (LT Pickup)

This is the continuous current setting. It's likely set correctly, but double-check. It should be above the full load amps of all the motors that can be running simultaneously (including service factor).

Long Time Delay (LT Delay)

This setting determines how long the breaker will tolerate currents above the LT Pickup before tripping. You may need to increase this to give the motors time to start. It's often adjustable in seconds (e.g., 3, 6, 9, 12 seconds).

This is the MOST LIKELY setting that needs adjustment. Increasing this will allow the high inrush current from starting the 150HP or 200HP motor to subside without tripping the breaker. Short Time Pickup (ST Pickup)

This is a multiple of the LT Pickup, and it's designed for short-circuit and ground-fault protection. It allows higher currents for a shorter duration than the LT Pickup. Ensure it is not set too low.

Short Time Delay (ST Delay)

Similar to LT Delay, but for short-time faults. Adjust if needed, but focus on the LT Delay first.

Instantaneous Pickup (INST Pickup)

This setting trips the breaker instantly when the current exceeds this value. It's generally set high for motor starting applications to avoid nuisance tripping. Make sure it's high enough to allow inrush.

Ground Fault Settings (GF Pickup/GF Delay)

Ensure these settings are appropriate for your system and aren't contributing to the issue.

Record the Settings

Before making any changes, carefully record the current settings of the Digitrip. Take photos, write them down, document everything. This is essential for reverting if your changes don't solve the problem or make things worse.

Understand the Curves

Familiarize yourself with the Time-Current Characteristic (TCC) curves for your Digitrip unit. These curves show the time it takes for the breaker to trip at different current levels. This helps you visualize how the settings affect the tripping behavior.

2. Calculate Motor Full Load Amps (FLA) and Locked Rotor Amps (LRA) Nameplate Data

Get the FLA and LRA values from the actual motor nameplates. Don't rely solely on lookup tables, as they can vary. The LRA is the current the motor draws when it's initially started (locked rotor condition).

Service Factor

Check if the motors have a service factor. The service factor multiplier should be applied to FLA.

Example Calculation

Let's say (for example only, look at your nameplates) 100 HP Motor: FLA = 125A, LRA = 750A 150 HP Motor: FLA = 180A, LRA = 1080A 200 HP Motor: FLA = 240A, LRA = 1440A

3. Analyze the Current Draw During Starting Worst-Case Scenarios

Calculate the current draw for these scenarios: Scenario 1 (Tripping): 100 HP running (125A) + 150 HP starting (1080A) = 1205A Scenario 2 (Tripping): 100 HP running (125A) + 200 HP starting (1440A) = 1565A Scenario 3 (Working): 100 HP running (125A) + 200 HP starting (1440A) = 1565A Scenario 4 (Working): 100 HP running (125A) + 150 HP starting (1080A) = 1205A

Compare to Digitrip

The LT Pickup must be greater than the continuous current draw of all motors that could be running simultaneously. The LT Delay must be long enough to allow the starting motor's inrush current to subside. The INST Pickup must be higher than the maximum possible inrush current.

4. Investigate System Issues (Less Likely, but Important) Voltage Drop

During motor starting, there will be a significant voltage drop. Measure the voltage at the MCC bus during the starting of the 150 HP and 200 HP motors. Excessive voltage drop can cause the motors to take longer to start, drawing LRA for a longer period, which can trip the breaker. Use a recording voltmeter if possible.

Motor Condition

A motor with degraded insulation or winding faults will draw excessive current. Have the motors meggered to check insulation resistance.

Mechanical Load

If the driven equipment (pump, fan, etc.) is unusually difficult to start (e.g., binding, excessive static head), it will increase the starting time and current draw.

Wiring

Check the connections at the MCCB, the MCC bus, and at the motors themselves. Loose or corroded connections can increase resistance, causing voltage drop and potentially affecting breaker performance. Make sure all the connections are tight and clean.

Power Factor Correction

If you have power factor correction capacitors, make sure they are not switching on during motor starting.

Breaker Health

While less common, the breaker itself could be faulty. Have it tested and inspected by a qualified technician. It may not be tripping to the right curve or it might be old and the trip unit is acting up.

Troubleshooting Steps1. Safety First

Always follow proper lockout/tagout procedures when working on electrical equipment. This is critical.2.

Verify Digitrip Settings (as detailed above). This is almost always the issue.3. Measure Voltage Drop

Use a recording voltmeter or a multimeter with Min/Max recording capability to measure the voltage at the MCC bus during motor starting.4.

Motor Testing

Have the motors meggered.5.

Connection Inspection

Inspect all connections.6.

Single Motor Starts

Try starting each motor individually to see if you can isolate the problem.7.

Slightly Adjust LT Delay

If the settings seem reasonable and you suspect inrush duration is the problem, make a small increase to the LT Delay. Document everything!8.

Breaker Testing

If all else fails, have the breaker tested.

Important Considerations



Coordination Study

Ideally, you should have a complete coordination study for your electrical system. This study will help you select the proper breaker settings to ensure selective coordination (the breaker closest to the fault trips first, minimizing disruption).

Qualified Personnel

If you're not comfortable working with electrical equipment, do not attempt to troubleshoot this yourself. Call a qualified electrician or electrical engineer. Electricity is dangerous, and mistakes can be fatal.

Documentation: Keep detailed records of all your troubleshooting steps, measurements, and changes.By systematically investigating these areas, you should be able to identify the cause of the breaker tripping and implement a solution. Good luck! Let me know if you have any other questions. Flag for review