MCC

What is an MCC?

An MCC (Motor Control Center) is a low-voltage switchgear assembly that centralises the power supply, protection and control of a large number of electric motors in a line-up of cabinet sections with shared busbars and a dedicated functional unit per motor. Each unit contains a complete motor starter: a disconnector or circuit breaker, a contactor, overload protection and often a soft starter or variable frequency drive. MCCs have been in use since around 1950, starting in the automotive industry, and are now standard in water treatment, chemicals, food processing and power stations. For OT teams, the MCC is where digital control literally turns into switching power.

🧠 How does an MCC work?

An MCC is built from free-standing vertical sections placed side by side in an electrical room. Power enters through an incoming section with a main switch, runs along horizontal main busbars through every section and is brought to the functional units by vertical distribution busbars in each section. Each unit (a bucket in North American usage) feeds and protects one motor or one outgoing circuit.

A typical unit contains:

• Short-circuit protection - a motor protection circuit breaker, moulded-case breaker or fuses that protect the cable and starter during a short circuit
• Switching element - a contactor for direct-on-line, star-delta or reversing starters, or an electronic starter such as a soft starter or VFD
• Overload protection - a thermal or electronic relay that trips the motor when the current stays too high for too long
• Control power - a control transformer or 24 V DC supply, auxiliary relays, pilot lamps and selector switches
• Communication - hardwired signals to the O of a PLC, or a network connection via ProfiNET, Profibus, IP or Modbus TCP

The PLC or DCS decides when a motor should run; the MCC executes that command and reports back whether the motor is running, has stopped or has tripped. The MCC therefore forms the power side of levels 0 and 1 in the Purdue Model.

Most MCCs are low-voltage assemblies for three-phase motors, in Europe typically at 400 or 690 V. For large motors there are also medium-voltage MCCs of roughly 2.3 to 15 kV, which use vacuum contactors instead of conventional contactors. These fall outside IEC 61439 and are covered by medium-voltage standards such as IEC 62271-106 for contactors and motor starters, and UL 347 in North America.

🔧 Which unit types and designs exist?

The way a unit is mounted in its section determines how quickly you can replace it and how safely you can work on it.

A withdrawable unit offers a major advantage for maintenance and commissioning: in the test position the main contacts are separated from the busbar, while control power and communication still work. A technician can test the control of a pump without any risk of the motor starting. A faulty unit is swapped for a spare in minutes while the rest of the MCC stays in service.

MCCs also differ in their form of internal separation under IEC 61439-2: from form 1 (no internal separation) to form 4b (busbars, functional units and terminals fully separated from each other). The higher the form, the better a fault or arc in one unit is contained and the more safely you can work next to a live circuit.

📏 Which standards apply to an MCC?

In Europe an MCC is a power switchgear and controlgear assembly covered by IEC 61439-1 and -2, published in the Netherlands as NEN-EN-IEC 61439. The manufacturer of the enclosure system provides the design verification; the panel builder performs the routine verification on every assembly. The components themselves, such as contactors, circuit breakers and overload relays, fall under IEC 60947, with part 4-1 covering contactors and motor starters.

Two concepts from IEC 60947-4-1 appear in every MCC specification. Type 2 coordination means that after a short circuit the starter can return to service without replacing parts, with at most lightly welded contacts; with type 1 the starter may be damaged and must be repaired. The trip class defines how quickly the overload relay responds at 7.2 times the set current: class 10 within 4 to 10 seconds, class 20 within 6 to 20 seconds and class 30 within 9 to 30 seconds. Class 10 suits pumps and fans with a normal start; heavy starting duty, such as large fans or mills, needs class 20 or 30.

🔄 What is the difference between an MCC and decentralised motor control?

With a central layout, all motor control sits in one electrical room and a power cable runs from there to each motor in the field. With decentralised control, the starter or drive is mounted next to the motor and a shared power feed plus a network cable run along the machines, often combined with O.

In practice many plants combine both: large pumps and compressors on the MCC, small conveyor motors decentralised.

📡 What makes an MCC intelligent?

An intelligent MCC (iMCC or smart MCC) replaces the classic combination of thermal relay and hardwiring with electronic motor management relays that have a network connection. Examples include Siemens SIMOCODE pro, Schneider Electric TeSys T and ABB UMC; Rockwell Automation offers IntelliCENTER software that maps the EtherNet/IP devices in an MCC and shows diagnostics per unit.

Such an MCC provides per-motor data that used to be invisible:

• Phase current, voltage and power - for energy monitoring and for spotting a clogged pump or dry running
• Thermal load and time to trip - so an operator can intervene before the motor stops
• Operating hours and number of starts - the basis for condition monitoring and predictive maintenance
• Cause of trip - overload, phase loss, earth fault or stall, visible straight away on the HMI and in SCADA

It saves wiring: one network connection per unit replaces the individual control and status wires to the PLC’s I/O cards. It also turns the MCC into a network device that has to be secured.

🔐 How do you secure an MCC, electrically and digitally?

Electrical safety comes down to three things. First, protecting motor and cable against overload, short circuit and phase loss. Second, interlocks: a door that only opens when the disconnector is off, and mechanical and electrical interlocking on reversing starters. Third, protection against an internal arc fault, for which the technical specification IEC TS 61641 sets test criteria; manufacturers offer reinforced constructions and arc detection with fast tripping. Work follows NEN 3140 with lock-out tag-out and, where needed, a work permit. Note that an emergency stop that only runs over the network is not acceptable; it must be hardwired or use a certified safety protocol such as PROFIsafe.

Digital security matters as soon as the MCC is on the network. Anyone with write access to a motor management relay can start or stop motors, change overload settings or shut down an entire production line. Practical measures based on IEC 62443:

1. Include the MCC in your asset inventory - every relay and drive with its IP address and firmware version.
2. Segment - put MCC networks in their own zone behind the PLC, in line with network segmentation; no direct route from the office network.
3. Restrict access - disable unused web servers and protocols, change default passwords and apply access control to engineering software.
4. Monitor traffic - passive monitoring and anomaly detection flag unexpected write commands; forward alerts to a SIEM.
5. Plan updates - install firmware updates through patch management during a maintenance stop, after testing on a spare unit.

🏭 How do you approach an MCC project or upgrade?

Many water treatment plants, pumping stations and process sites run on MCCs that are decades old. A replacement or extension usually follows these steps:

1. Inventory the motors with their power rating, starting method, criticality and required availability.
2. Specify the interface characteristics for IEC 61439: prospective short-circuit current on site, ambient temperature (the standard assumes a maximum of 40 °C and an average of 35 °C), IP rating, form of separation and spare space for future motors.
3. Choose the starter per motor: DOL for small motors, a soft starter for a gentle start, a VFD where speed control saves energy.
4. Choose the communication together with the PLC and security teams and define zones and conduits.
5. Test at the factory (FAT) and on site (SAT), including the test position of withdrawable units and alarm handling in SCADA.

In a brownfield renovation, phased replacement section by section is common so the process keeps running. If your organisation falls under the Dutch Cybersecurity Act, in force since 15 August 2026, include a networked MCC in the risk analysis and measures for your OT environment.

❓ Frequently asked questions

What is the difference between an MCC and a control cabinet?

An MCC holds the power components that feed and protect motors, such as circuit breakers, contactors and drives. A control cabinet holds the control equipment, such as the PLC, I/O and network devices. On small machines both share one enclosure; in process plants the MCC and the control cabinet stand side by side.

Which standard applies to an MCC in Europe?

An MCC in Europe falls under IEC 61439-1 and -2 for low-voltage switchgear and controlgear assemblies, in the Netherlands published as NEN-EN-IEC 61439. The built-in components fall under IEC 60947, and safe working on the MCC is governed by NEN 3140 in the Netherlands. In North America, UL 845 and NEMA ICS 18 apply instead.

What is a withdrawable unit in an MCC?

A withdrawable unit is a drawer in an MCC that you can pull out to a test or disconnected position without switching off the rest of the MCC. In the test position the motor supply is isolated, but control power and communication still work. This allows fast replacement and safe testing.

What does type 2 coordination mean for an MCC?

Type 2 coordination under IEC 60947-4-1 means that a motor starter in an MCC can return to service after a short circuit without replacing parts. Only lightly welded contacts are allowed. In the process industry type 2 is the usual requirement because it limits recovery time after a fault.

Is an intelligent MCC a cybersecurity risk?

An intelligent MCC is a risk when it is connected without segmentation or access control, because motors can then be started or stopped remotely. With its own network zone, unused services disabled and monitoring based on IEC 62443, that risk is manageable. The extra diagnostic data then clearly outweighs the risk.

How long does an MCC last?

The enclosure and busbars of an MCC often last thirty years or more. Electronic components such as drives and motor management relays are usually replaced sooner, because vendors discontinue them after a product cycle and spare parts become scarce. Plan spare units and a migration path for the electronics.

📌 In summary

An MCC is a modular low-voltage switchgear assembly that centralises the supply, protection and control of dozens to hundreds of electric motors, designed to IEC 61439 and built from replaceable functional units.

The MCC is the link between control in the PLC or DCS and the motors in the field. Withdrawable units and a high form of separation improve availability and safety during maintenance. An intelligent MCC provides valuable diagnostics for condition monitoring, but like any other OT device it belongs in a secured network zone. A well-designed MCC thus provides oversight, safety and scalability for complex production environments.