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What is the advantage of using Thermal Magnetic Circuit Breaker in

motor control?

04:44 Celestino Cole

There are two commonly used methods of motor control system used in the industry. The conventional method is the

3-component system where circuit breaker, magnetic contactor and thermal overload relay are installed to control

and protect the motor. However engineers and technicians found some problems coming out with this method. The

problem is mainly due to incompatibility of the motor protection in the system.

Electrical code such as NEC 430 require that the motor control circuit should be provided with thermal protection and

magnetic protection. Thermal protection protects the circuit against overload and locked rotor condition while

magnetic protection protect the circuit against short circuit condition.

Figure 1.

The provision provided by NEC 430 can be accomplish based on the circuit described in figure 1 where the circuit

breaker, magnetic contactor and thermal overload relay are installed to control and protect the motor. This method is

called 3- component system.

0

Three component system is an ideal interpretation of NEC 430 and it assumes that the circuit breaker is an

instantaneous trip (magnetic trip only) circuit breaker otherwise if it is inverse time (thermal and magnetic trip) it

is redundant. “

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However if it is redundant it does not mean that we can no longer use the inverse time circuit breaker along with

thermal overload relay. If we look at the control circuit of a motor starter there is no other way to integrate the

inverse time circuit breaker (molded case) to that circuit other than installing an overload relay. One of the main

reasons why overload relay is used with inverse time circuit breaker (redundant) is that the overload relay has

contacts that can be connected in series with the control circuit.

The Problem

While the three component system is the best way to follow NEC 430, one should realize the fact that in this case the

thermal protection and the magnetic protection are two separate units. As such it is vulnerable to mismatching of

correct specification when either of the components are replaced. The worst effect when any of these components are

mismatched is non-coordination of the protection devices.

In three component system, redundant or non-redundant does not really matter except one thing--- "coordination" of

the protection system. Coordination means that during

should trip first

control circuit selected the proper ratings of the components

However there is a big possibility that the same shall not be followed during maintenance.

The solution

The solution is to adopt the use of 2-component system such as describe in Figure 2 where only Thermal Magnetic

Circuit Breaker (TMCB) and contactor are used.

Figure 2. Two component system

Based on Figure 2 the molded case circuit breaker is no longer in the motor starter because the Thermal Magnetic

Circuit Breaker has built-in overload relay and magnetic trip mechanism compact in one unit. Meaning to say the

vulnerability of mismatching of specification during replacement has been solved.

How to use Thermal Magnetic Circuit Breaker

Thermal Magnetic Circuit Breaker is capable of protecting the motor against overload and short circuit, viz:

Short Circuit Protection

Simply connect the magnetic contactor right after the TMCB as shown in Figure 3. The line current that will run

through the magnetic contactor which came from the motor will also run through TMCB. So if there is fault

downstream this unit will clear the fault just like what molded case circuit breaker do. And of course in sizing this unit

for short circuit protection one should not forget to conduct coordination study with the upstream protection devices.

Replacing busted components with a specification

any industrial plants. This happen when there are technical troubles that needs

available spare that matches the exact

technician has no idea about the coordination of the protection system.

“

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Figure 3. TMCB protection against short circuit

Overload Protection

Thermal magnetic circuit breakers contain contact platform that is usually concealed and located in the lower part of

the unit. Remove the cover and put the contact block inside the platform and you can already use it just like the

conventional overload relay.

Figure 4. TMCB protection against overload

Conclusion

The advantage of this during maintenance is if we replace the protection of the motor, then we are replacing both

thermal protection and magnetic protection since it comes out as a single unit. The risk of having non-coordinated

protection devices due to human error or any other reasons is eliminated.

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Resources

• Schneider Electric Manuals

• Coordinated Motor Circuit Protection, Allen-Bradley

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