This engineering brief explores the inner workings of the Phoenix Contact model 1019972 circuit breaker as shown in Figure 1. This miniature circuit breaker is part of Phoenix Contact’s TMC 7 family which includes single as well as three-phase breakers ranging in size from 1 A to 63 A. The featured breaker is rated for up to 277 VAC or up to 60 VDC. This circuit breaker is included in DigiKey’s PLC trainer kit.
Figure 1: Side-by-side image of two Phoenix Contact TMC 71C 01A circuit breakers. One has been opened to reveal the internal construction.
Tech Tip: Circuit breakers are designed for code compliance in specific applications. As engineers, we must design for compliance with all applicable safety regulations. For instance, the featured breaker is UL 1077 compliant. It would be inappropriate and potentially hazardous to use a TMC 7 breaker in branch circuit when a UL 489 breaker is required. In that situation, a Phoenix Contact TMC 8 circuit breaker should be used. The Phoenix Contact TMC 7 and TMC 8 are easy to tell apart as the TMC 8 has a larger body with dividers that extend beyond the screw terminal connections. These “fins” increase the distance a phase-to-phase arc must travel, thereby providing additional protection for the branch circuit.
How is current detected?
Current detection and response are the primary functions performed by all circuit breakers. Current may be detected in several different ways. Direct methods can provide an actual value for the current while indirect methods provide a response proportional to the current.
For instance, we could:
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Direct: Measure the voltage drop across a low resistance shunt resistor. An analog comparator circuit or a microcontroller may then be used to fire a solenoid that causes the circuit breaker to trip.
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Direct: use a Hall sensor placed near a current-carrying wire.
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Direct: Use inductive coupling to measure the current such as when a current-carrying wire is passed through a toroidal Current Transformer (CT).
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Indirect: Pass the current-carrying wire through a solenoid. Excessive current causes the plunger to move. This presses against a mechanical latch, causing the breaker to trip.
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Indirect: Pass the current through a heating element wound around a bimetallic strip. Excessive current bends the strip causing it to press against a mechanical latch, causing the breaker to trip.
Given the variety of circuit breakers, we can safely assume that each method has been used at one time or another. However, we recognize that the Phoenix Contact TMC 7 is a low-cost device. It uses the indirect methods, which have a proven track record for reliable operation in a small package as shown in Figure 2.
Tech Tip: Current Transformers (CTs) must be handled with care to prevent electrocution. Recognize that a CT must always have a low resistance burden resistor across its output terminals. If this resistor is missing or if a wire breaks, there will be a dangerous high voltage on the CT’s terminals.
Be safe and keep those burden resistors in place.
Figure 2: Internal component for the TMC 7 circuit breaker with labels for solenoid, heater, contacts, and the arc chute.
Tech Tip: Without the supporting side cover, the elements shown in Figure 2 are slightly out of position. Also, the lack of integral support makes it very difficult to set the breaker. A strong spring, hidden in the upper trip mechanism, is used to force the contacts into the open position. Without the cover, the components want to jump out of the assembly.
What are the thermal and magnetic protection elements in a circuit breaker?
Figure 2 shows the internal construction of the TMC 7 circuit breaker. We direct our attention to the thermal and magnetic components:
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Thermal: The metal wire for the heater is visible on the right-hand side of Figure 2. The heater is wound around a bimetallic strip with a white insulating material separating the heater from the inner strip. A thermal image of the heater is included as Figure 3.
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Magnetic: The solenoid is visible in the upper section of Figure 2. Close inspection reveals that the current flows though the solenoid on its way (right to left) from the breaker contact to the output terminal.
In both cases, an overcurrent condition causes a mechanical change. The thermal mechanism bends the bimetallic strip while the magnetic portion (solenoid) extends a plunger. These mechanical motions then push against the mechanical spring-loaded mechanism causing the breaker to trip. This heavy spring forces the contacts to open as fast as possible.
Figure 3: Thermal image of the circuit breakers heater under load.
Tech Tip: Close inspection of Figure 3 reveals thermal reflection. Just like your face in a mirror, the shiny metal behind the heater shows a reflection. Also, this image demonstrates a common problem with temperature measurement and emissivity. In this image, the heater element appears to be cold relative to the underlying white insulating material. We could fix this problem by changing the internal setting for the Fluke Ti32 thermal imager used to record the image.
What is the difference between a thermal and a magnetic trip?
The primary difference between the thermal and the magnetic trip is time. In relative terms, the magnetic trip is fast while the thermal response is slow. This is reflected in the tripping curve as shown in Figure 4.
This slow response is seen in the upper left-hand corner of the tripping curve, where the gray band of variance asymptotically approaches 1 A as time approaches infinity. This implies that the breaker will not trip when conducting a 1 A (RMS) steady-state current flow.
In another example, it takes time for the heater to respond when the circuit breaker is conducting 4 A. From Figure 4, we estimate that any individual breaker will trip between 1 and 10 seconds.
The mechanical trip time constant is observable at the base of the tripping curve. From this curve we can assume that it takes 10 ms for the solenoid to strike the trip mechanism and for the breaker to open the contacts. Beyond this activation threshold, the response time is largely independent of current as the response time is dominated by the mechanism.
Figure 4: Tripping curves for the Phoenix Contact TMC 71C 01A circuit breaker.
Tech Tip: Figure 4 presents data for a circuit breaker with type C tripping characteristics. This is ideal for general purpose applications. The type B breaker is preferred for sensitive loads while type D breakers are preferred for loads with higher inrush or momentary overloads.
Arc quenching
No circuit breaker introduction is complete without exploring the hazards of electrical arcs. This is an underappreciated function of the circuit breaker. Normally, we view a circuit breaker as a simple benign “switch.”
Absolutely not!
In truth, the circuit breaker is cleverly designed to open the circuit under brutal conditions. Here the term “brutal” involves high fault currents associated with inductive sources and loads. Under all circumstances, the circuit breaker must detect and then quickly open its primary contacts. It must do so when inductive circuit elements attempt to sustain the arc. In some ways, this is like handling the turn-off flyback voltage from a relay but on a much larger scale.
It can be a violent operation with the potential to burst the relay if protective mechanisms are not employed. This is one of the reasons we differentiate between the Phoenix Contact TMC 7 vs TMC 8 circuit breaker families. Each is designed for use in a specific location with stringent code requirements to appropriately handle the different energy ratings found within an industrial control panel.
With that said, we can explore the arc chute.
What is an arc chute?
The arc chute (arc chamber) is the largest and heaviest component located inside the Phoenix Contact TMC 7 circuit breaker, as shown in Figure 5. It serves to divide and conquer the arc that forms within the circuit breaker.
An arc (plasma) is extraordinarily hot. Once established, it will conduct through air and like a Jacob’s ladder, the arc is always in motion seeking a path of lower resistance while being blown about by air currents generated from its tremendous heat.
As a thought experiment, consider what would happen if the conventional “V” type Jacob’s ladder were turned upside down. The normal upward thermal buoyance would be lost. The arc would remain at the top (narrow end) of the inverted V.
With that vision of the Jacob’s ladder in mind, consider what happens when the contacts attempt to open under an inductive load. The arc will tend to grow under the influence of thermal buoyancy. Looking carefully at Figure 2, we see an extended (curved) portion on the left-hand contact. The arc is guided by this curved horn directly into the arc chamber. Once the arc enters the arc chute it finds a path of lower resistance as the chamber with its many plates is directly between the input and output contacts of the circuit breaker.
Once the arc enters the chamber, it is trapped. Two things immediately happen, first, what was a large arc is broken into many small arcs that naturally form across the metal plates. Each plate absorbs heat which robs the arc of energy. Taken together, the smaller and cooler arcs are easier to extinguish than the arc that would have otherwise sustained itself across the primary contacts.
Figure 5: Image of the TMC 7 breaker’s arc chute.
Parting thoughts
Electrical safety in general and circuit breakers in particular, provides a rich field worthy of study. In this document, we explored the operation of a simple circuit breaker commonly found in industrial control panels. Larger breakers such as those used for commercial buildings, industrial environments, and electrical power generation, build upon these concepts. In all cases we are challenged to open the breaker’s contacts and quickly extinguish the arc before the internally dissipated energy grows and destroys the breaker.
As a next step, I encourage you to read about arc flash and blast and best practices to keep you safe in the event of a catastrophic breaker explosion.
Be safe.
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.