Motor Control Center Testing: All Details EXPLAINED

A motor control center operates your electric motors. You can find a solid-state motor controller or overload relays to protect the engine. Furthermore, you’ll discover overload protection relays and a circuit breaker that offers short-circuit protection. Also, disconnecting control switches to isolate the motor circuit. Separate control transformers serve for the three-phase power to move into the controller. And wires attach the motor to the terminals.  Additionally, these control centers offer wire ways for power cables and field control.

All in all, each MCC contains multiple components. Depending on the system, it runs one or all motors in the facility. That’s why you need to ensure it works flawlessly at all times, and it’s where motor control center testing comes into play. Now, this guide explains more about the process. 

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The Basics of Motor Control Center Testing

Caption: An electrical panel

MCCs contain multiple motor starters. You’ll find at least one vertical metal cabinet that includes a power bus. Also, it will have a place where you can mount motor controllers, and some controllers have bolts to keep them in position. On the other hand, you might unplug smaller controllers from the cabinet for testing or maintenance.

The main goal of these tests is to check the condition of the motor control center. That includes testing circuit breakers, starters, and other parts of the system. You can do the tests on newly installed and existing motor control centers.

Here are the benefits of MCC tests:

  • Electrical equipment protection from the components breaking down.
  • Ensuring consistent performance of the system for any application of motor control centers.
  • Increasing safety of the system and employees.
  • Minimizing the risk of arc flash and other potential damages. That also reduces costly damage to your system.

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The Procedure for Motor Control Center Testing Explained

The process of testing MCC is complex. If you want the best results, you need to do each step listed below. That way, you’ll be sure everything works the way it should. 

Testing the Current Transformer

Electricians using a tester

Caption: Electricians using a tester

This is a vital test, so it’s essential to be patient while doing it. First, you’ll need to find a megger and a current transformer (CT) analyzer. Here is how to do the test:

  1. You’ll find four leads on the CT analyzer, and two should go to the primary and the others to the CR secondary circuit.
  2. Next, connect the tool to the laptop. Install the software that comes with your CT analyzer.
  3. Run the software to test the performance of the current transformer. It’ll give the ratio, polarity, winding resistance, knee point, and burden. You can print these results if you want.
  4. Find the current transformer factory test. Now, compare the results and make sure the tolerance range is acceptable.
  5. Use one ampere on the CT’s secondary side to calculate the burden. You’ll need a multimeter to find the voltage calculation burden. Finally, compare the CT burden with these results, and this is a crucial step of this test.
  6. You use a megger with a 1KV injection per minute to test insulation. Do the test between secondary to secondary, secondary to primary, and ground to primary.

Additionally, observe the ferrules and overall connection tightness. You want to make sure all connections are optimal. If necessary, do the ground fault test procedure to stay safe from dangers.

Testing the Voltage Transformer

You’ll need a multi-meter and a CPC100 for the voltage transformer (VT) test procedure. But, the first check, check the connections and ferrules

Get the CPC device and connect it to the primary and secondary sides (two leads each). You can start from the VT page to measure the ratio and polarity. On the other hand, if you begin from a different page, you can get winding resistance.

For measuring the loop burden, check the current after injecting the rated secondary voltage. Next, use the megger to do the insulation resistance test. You can inject 1kV for a 1-minute test. Do the test from the ground to the primary and secondary, and between these two sides.

Testing Trip Time for MCCB

Caption: Motor control in electrical engineering

Caption: Motor control in electrical engineering

You need to confirm the trip time is acceptable to ensure short-circuit protection. There’s a broad range of good options, but make sure the trip time is within those limits. Otherwise, you could face problems if a short circuit or overcurrent occurs. You’ll need a megger and the injection tester. Alternatively, you can use a particular test for the circuit breaker.

You’ll find that each molded case circuit breaker has three adjustments – long, short, and instantaneous. It’s vital to test all three options. Additionally, a megger test is necessary for a circuit breaker with a rated circuit current above 100 Amp, and it’s a vital step in the overcurrent protection.

Testing Contact Resistance for the Circuit Breaker and the Bus Bar

The only thing you need for this test is a micro-ohmmeter to measure contact resistance. The goal is to ensure there isn’t too much resistance in the circuit breaker and the bus bar.

Your device will have a couple of leads for voltage and a couple for current. You can connect the leaders to the machine where you want to check the resistance. Now, the voltage leads should be on those current terminals. Once you press the pushbuttons, you’ll see the results in a few seconds. There’s no need to get experts in testing since this is a simple check.

Testing the Potential of the MCC Bus Bar and Contractor

Electric voltage and amperage control room

Caption: Electric voltage and amperage control room

You need a test kit for checking the high potential. The goal is to see how isolators and the insulations withstand high voltage.

First, use a megger for the insulation test for 60 seconds. Check the line to ground, and 3 phases line to line. Next, apply high-potential AC voltage to the contactor line to line or the bus bar. Make sure to note the passing current. Now, use the megger without applying the high-potential voltage. Finally, compare the results to see if they are acceptable.

Testing the Miniature Circuit Breaker

Before beginning, check the ferrules and wire tightness. Furthermore, look for defects or physical damage. If you notice any, you might need new electrical installations.

It would help if you had a multi-meter and a speaker for the test. You do it to check the MCB’s trip time. There is a wide range of options for the excellent time, but make sure it’s within the project requirements.

Here are the steps for this test:

  1. Begin by attaching the sverker to the MCB. See what MCB type you use and the rated current value.
  2. Use the sverker and inject 2x of the rated current. 
  3. Check the trip time and compare it with the one acceptable for that MCB type. It will ensure optimal assembly protection.

Testing the Auxiliary Relay

You’ll need a miniature circuit breaker 2 pole (MCB), ammeter, and speaker. Make sure to check there are no damages on the wires before the test.

The test has four steps:

  1. Check the output contact.
  2. Test the pick-up and drop-off voltage and current.
  3. Check the resetting and operating times.
  4. Test the burden.

Testing the Voltmeter and Ammeter

Testing the voltage

Caption: Testing the voltage

To deliver accurate results, it is necessary to check and calibrate the voltmeter,  ammeter, and selector switch. You’ll need a FREJA300 device for the test. 

Use the ammeter to check the CT ratio and voltmeter for the VT ratio. Next, inject at least 3x of voltage and current. Check the readings and compare them. You need to ensure the lessons have a percentage error within reasonable limits.

Testing the Current Transformer Loop

This test requires FREJA as the current injection, but you’ll also need a mini-clamp and multi-meter. Begin by checking the single earth point, and look for whole loops in ferrules. Next, use FREJA to connect the three-phase power to the transformer’s secondary side. You can use the mini-clamp in each phase. The loop will continue to the last point, so make sure to follow it.

Testing the Voltage Transformer Loop

A control panel in an electronics lab

Caption: A control panel in an electronics lab

You want to analyze the entire VT loop from the first central control panel to all other places. The exact locations will vary on your system. You can use FREJA as the voltage source to the transformer’s secondary side. A multi-meter will help to check the voltage throughout the loop.

Testing the Primary Injection

The goal is to see if the current transformer ratio is accurate. You can consider it a CT loop backup test. The equipment to use includes a power-clamp meter, mini-clamp, multi-meter, and a primary injection tester.

You connect the PI tester to the bus bar with the current transformer. Once you start injecting, you can use the power clamp to check the current. Of course, you want the reading on the screen to be the same as the current you inject.

Motor Control Center Testing– Functional Testing of the MCC

Motor Control Center Testing: Electrical control panel

Caption: Electrical control panel

This is a vital check for the motor control center panel mechanism, and you don’t need any function test items. First, observe the panel to see if there’s any damage. Next, check the motor circuit of the opening command for a contractor. Do the same for the closing one, and then analyze the mechanical interlock. Now, look for the alarm signal from the source to the relay. Finally, you should confirm the relay logic to ensure proper operation.

Motor Control Center Testing– Testing the Protective Relay

The next test is checking the main relay to ensure it works the way it should. You want to check any LED or indicator lights that signal a problem first. Also, identify any damaged components that might have. Finally, don’t forget to check the switch for optimal operation.

Here is what you should do in this test:

  1. Perform the burden test.
  2. Do the binary input/output check.
  3. Finally, check the instantaneous earth fault and time-delayed earth fault (51N).

Testing the Timing for Megger and MV MCC Contractors

This test confirms there are no pole discrepancies between poles. The opening and closing times of the contractors should also be within reasonable limits. You’ll need a speaker to measure these times, and a megger and a 2Pole MCB are also necessary.

Testing the Contact Resistance for Contractors

You do this test after installing the contactor in the facility. This is because the setup process could lead to something that causes high resistance of contactors, and this confirms that doesn’t happen.

You’ll need a device that checks the contactor contact resistance for the unit you want to measure. It’s wise to check the parts for visible damage, and you can hire contact resistance testing service pros for this check.

Checking the RTD Relay

You check the temperature resistance of the MOTOR with this test. Begin by connecting the +ve probe of the resistor and then the –ve examination to the relay’s end. You can set the desired resistance and check the temperature that meets it. The alarm setting and trip time should be optimal.

Motor Control Center Testing–The Steps for Basic Maintenance

Motor Control Center Testing: Contactor wiring work

Caption: Contactor wiring work

If you want the correct operation of your motor control center, you need to maintain it, and maintenance should be regular. Before doing anything, turn off the incoming power and de-energize the motor control.

Here is what to check in the process:

  • Use your palm to check the enclosed sections on the sides and the doors. If you can’t hold the hand for at least three seconds, that’s the sign of a problem. Alternatively, you can use heat detectors for precise measurements.
  • Remove any dust from the vertical bus barriers and insulators. You can vacuum contactors if possible.
  • Make sure the bus, power cable, and control wire connections are tight.
  • Check the wires for any signs of damage. Those could be cuts or wear and tear. If necessary, replace any part of your wiring system.
  • See if the mechanical interlocks and operating handles are easy to handle. Simple and accurate operations are imperative for easy system control.
  • Check the Plating on the Unit Stab Fingers. You want to put focus on the place where these fingers interact with the vertical bus. If there is a film like an oxide on the aluminum and copper, contact might be poor. You should replace any plating that shows wear and tear, and otherwise, you risk a bus flashover.
  • Are all pilot lamps working well? If there’s a defect, replace the lamp.
  • Check the motor starter contacts for erosion. If more than half eroded, replace them immediately. Otherwise, they’ll compromise the functionality of the starters.
  • Also, please don’t lubricate the magnet assembly or armature in a relay or another assembly, and that could compromise the performance since lubricants attract dust.

All in all, you can consult the instruction manual to see what other actions it recommends during the maintenance.


Regular tests are vital to ensure that your motor control center delivers consistent and smooth performance. If you detect any problem with cable assemblies during the tests, don’t hesitate to contact the customer care team of our company. Our experts have years of experience and are skillful enough to help with any wiring!

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