Article summary
This engineering brief demonstrates how a traditional circuit breaker is augmented with an auxiliary contact block. It then describes how a PLC may be used to monitor the circuit breaker system.
Estimated reading time: 3 minutes
Introduction to the circuit breaker’s auxiliary contact block
The circuit breaker is a critical safety device, found in industrial control panels. It’s a binary device that is either actively providing power to a load or is interrupted due to an overcurrent trip or deliberately being turned off by the technician.
There are applications where the state of the circuit breaker should be known. For example, a Programmable Logic Controller (PLC) should be able to identify a tripped condition and take appropriate action.
As a crude example, consider a pump system with a tripped circuit breaker. The PLC could identify the fault, log the time, alert the technician, and take appropriate backup action, knowing that the primary system has been compromised.
Larger circuit breakers often feature onboard auxiliary contacts. Smaller circuit breakers, such as the Phoenix Contact 2907558 TMC81C01A as shown in Figures 1 and 2, include provisions for side-mounted auxiliary contact.
Figure 1: Image of the side-mounted auxiliary contact mated to the Phoenix Contact TMC 81C circuit breaker.
Author’s personal experience: The auxiliary contact is equipped with a test slide mechanism as shown in Figure 1. When I first saw this switch, I assumed it was used to trip the circuit breaker. This assumption was incorrect, as the slide switch is used to independently activate the SPST switch. From a maintenance perspective, this would allow a technician to verify the PLC circuitry without activating the circuit breaker.
Figure 2: Image of the Phoenix Contact UL 489 circuit breaker and associated auxiliary contact, shown prior to installation.
Tech Tip: The UL 489 TMC 81C circuit breaker is equipped with clip-in side pieces that surround the input and output power cables. These are essential for electrical safety as they extend the distance an electrical arc must travel. Instead of a short terminal to terminal distance, the would-be arc is forced to take the longer up-and-over path around the obstacle. The longer distance generally prevents the arc from forming.
PLC monitoring of a circuit breaker
We are likely to encounter situations where a branch circuit interrupter has tripped, yet the PLC remains active. This is a recognition that industrial controls often include multiple circuit breakers for the branch circuits. The PLC itself is powered from an independent yet related control circuit. For instance, a 208Y/120 panel may have multiple three-phase branches. The PLC may be driven from a power supply originating from the same feeder, or through an interposing UPS. Either way, the PLC may be active while one of more circuit breaker(s) are tripped.
The PLC can detect the circuit breaker status via the auxiliary contact. It can then take appropriate action, such as:
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sound a local alarm
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signal an alarm to the Supervisory Control and Data Acquisition (SCADA) system
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log the event
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send an email to alert the service technician
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perform an orderly system shutdown
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activate redundant systems
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coordinate time-dependent process restoration for chemical or thermal processes
Parting thoughts
The control system actions are highly dependent on the application. However, circuit breaker monitoring via an auxiliary contact is a tool that every designer should have in their toolkit.
Best wishes,
APDahlen
Related articles by this author
If you enjoyed this article, you may also find these related articles helpful:
- Introduction to the Shunt Trip Circuit Breaker
- Electronic Circuit Protector for Industrial Control
- Circuit Breaker Branch Protection vs Supplementary Protector in a UL 508A Listed Control Panel
TL;DR
- Auxiliary contact allows remote monitoring of the circuit breaker’s status
- Demonstration using real-world hardware TCM81C01A
- A slide switch is provided to test the auxiliary contact independently of the circuit breaker
- Article provides recommendations for PLC response strategies to improve plant stability
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.