Electronic Trip Circuit Breaker Basics: Schneider Electric Micrologic Trip Units

Micrologic Trip Unit | Source: Schneider Electric


What is Electronic Trip Circuit Breakers? 

An electronic trip circuit breaker is a type of circuit breaker that uses electronic components to control the tripping mechanism instead of traditional thermal-magnetic trip elements. Electronic trip circuit breakers are commonly used in industrial and commercial applications where high reliability and selective coordination are required.

Figure 1. Circuit Breaker with Electronic Trip Unit | Source: Schneider Electric

These circuit breakers use a solid-state trip unit to sense and respond to overcurrent conditions. The trip unit monitors the current flowing through the circuit breaker and can be programmed to trip at specific current levels and time delays. This allows for greater precision in protecting against overcurrent conditions and reduces the risk of nuisance trips. Electronic trip circuit breakers can also provide advanced monitoring and diagnostic capabilities. Some models can provide real-time current and voltage measurements, as well as fault event recording and reporting. This information can be used to identify potential issues and optimize system performance.

Related Article: Circuit Breaker Essentials 

Why Use Electronic Trip Circuit Breakers? 

In most cases, the basic overcurrent protection provided by standard thermal-magnetic circuit breakers will meet the requirements of the electrical system design. In some cases, however, basic overcurrent protection might not be enough. Electronic trip circuit breakers can provide the additional features needed in those cases.

Reasons to use electronic trip circuit breakers includes:

  1. Enhanced coordination capabilities
  2. Integral ground-fault detection
  3. Communication capabilities
  4. Future growth potential
  5. Enhanced Coordination Capabilities

Enhanced Coordination Capabilities

In electrical systems where downtime could have critical consequences, electronic trip circuit breakers provide more versatility to achieve coordination. For instance, certain installations serving continuous processes may be required to continue operating during a fault condition because shutting the system down would be more costly than the damage done by the fault itself. Or, in critical care facilities, a
loss of power could result in the loss of life.

These situations require that coordination be optimized at all costs. In order to maximize coordination, downstream branch devices should operate very fast – with no intentional delay – and main devices should delay operation so that the downstream devices have time to clear the fault.

Schneider Electric Micrologic Electronic Trip units

MICROLOGIC electronic trip circuit breakers can help optimize coordination:

  1. Independent adjustments - allow one dial setting to be changed without affecting the rest of the pickup and delay levels. This allows the designer to better define the tripping characteristics needed on the system. 
  2. Interchangeable rating plugs- allow the designer to shift the entire trip characteristic curve (except for ground fault) to improve coordination with other devices. MICROLOGIC rating plugs define the circuit breaker's maximum current rating based on a percentage of the circuit breaker sensor size and can be used on any frame size of circuit breaker within the MICROLOGIC family of circuit breakers. 
  3. Withstand ratings give the designer a larger window of coordination potential - The withstand rating is the level of rms symmetrical current that a circuit breaker can carry with the contacts in the closed position for a certain period of time. At current levels above the withstand rating (and less than or equal to the interrupting rating), the circuit breaker will trip instantaneously. In other words, the withstand rating is the highest current level at which delay can be introduced to maintain coordination with downstream devices. Withstand ratings are available only on full-function trip systems ordered with the adjustable short-time function.
  4. Inverse time delay characteristics - allow for better coordination with fusible switches or thermal-magnetic circuit breakers downstream. Devices that respond to heat generated by current flow (such as fuses and thermal-magnetic circuit breakers) have inverse time tripping characteristics. This means that as current increases, the time that it takes the device to trip will decrease. In order to coordinate better with these types of downstream devices, MICROLOGIC circuit breakers offer inverse time delay characteristics on the long-time, short time and ground-fault functions. 
  5. Ammeter/trip indicator - displays the level of ground-fault leakage current associated with the circuit. The ground-fault pickup level on the circuit breaker may then be adjusted somewhat higher than the amount of leakage current displayed on the ammeter.

Integral Ground Fault Protection

Electronic trip circuit breakers simplify the installation of equipment ground-fault detection into the electrical system. Externally-mounted ground-fault detection systems require the specifying of five different parts – a circuit breaker, a ground-fault relay, a ground-fault sensor, a shunt trip for the circuit breaker, and testing means. Additional wiring is also required to install the system. 

Electronic trip circuit breakers include most of the detection equipment within the circuit breaker housing. The phase current sensors, summing toroid, pickup and delay adjustments, tripping solenoid, and a push-to-test feature are all enclosed within the molded case. 

Communication Capabilities

Communication between circuit breakers at different levels in the system allows the downstream circuit breaker closest to the fault to ignore its preset delay time and trip without any intentional time delay on a short circuit or ground fault. This form of communication is known as zone-selective interlocking (ZSI). Coordination assures that continuity of service is maximized during any type of overcurrent. However, coordination does not eliminate the stress on the system caused by the energy dissipated during a fault. 

Electronic Trip Circuit Breakers can communicate and provide the following information: 
  1. History of last trip
  2. Trip unit pickup and delay levels
  3. Impending trip conditions
  4. Operating currents for each phase
  5. Ground-fault leakage current associated with the circuit
  6. Ground-fault alarm signal

Standard Function Trip Unit Curves

The trip curve below illustrates how the adjustment's made to a standard-function trip unit will affect the circuit breaker's trip characteristics. Adjusting the trip unit switches will shift that area of the trip curve. 

Figure 2. Standard Trip Unit Curve

Long Term Trip Function

  1. LONG-TIME PICKUP Switch — switch value (multiplied by the ampere rating) sets the maximum current level which the circuit breaker will carry continuously. If the current exceeds this value for longer than the set delay time, the circuit breaker will trip. 
  2. LONG-TIME DELAY Switch — sets length of time that the circuit breaker will carry a sustained overload before tripping. Delay bands are labeled in seconds of overcurrent at six times the ampere rating. For maximum coordination, eight delay bands are available.

Short-time Trip Function

  1. SHORT-TIME PICKUP Switch — switch value (multiplied by the ampere rating) sets the short-circuit current level at which the circuit breaker will trip after the set SHORT-TIME DELAY.
  2. SHORT-TIME DELAY Switch — sets length of time the circuit breaker will carry a short circuit within the short-time pickup range. Delay bands are labeled in seconds of short-circuit current at 12 times the ampere rating, P. The short-time delay can be set to one of four I^2t ramp operation positions (I^2t IN).

Instantaneous Trip Function

  1. INSTANTANEOUS PICKUP Switch — switch value (multiplied by the ampere rating) sets the short-circuit current level at which the circuit breaker will trip with no intentional time delay. The instantaneous function will override the short-time function if the INSTANTANEOUS PICKUP is adjusted at the same or lower setting than the SHORT-TIME PICKUP. 

Ground Fault Trip Function
  1. GROUND-FAULT PICKUP Switch — switch value (multiplied by the sensor size) sets the current level at which the circuit breaker will trip after the set GROUND-FAULT DELAY.
  2. GROUND-FAULT DELAY Switch — sets the length of time the circuit breaker will carry ground-fault current which exceeds the GROUND-FAULT PICKUP level before tripping. Delay bands are labeled in seconds of ground-fault current at 1 times the sensor size, S. Ground-fault delay can be adjusted to one of four fixed time delay positions (I^2t OUT). 

In Comparison with Thermal Magnetic Circuit Breakers

Thermal-magnetic circuit breakers are the most common type of circuit breakers used in residential and commercial applications. They operate based on the principles of thermal and magnetic protection. When an overcurrent condition occurs, the thermal element within the circuit breaker heats up and causes the contacts to open, while the magnetic element trips the circuit breaker in response to short-circuit currents.

In terms of advantages, electronic circuit breakers offer several benefits over thermal-magnetic circuit breakers. Electronic circuit breakers provide more precise protection and can be programmed with more advanced settings, making them well-suited for applications that require selective coordination and complex protection schemes. Electronic circuit breakers can also provide advanced diagnostic and communication capabilities, allowing for easier troubleshooting and maintenance.

However, electronic circuit breakers are generally more expensive than thermal-magnetic circuit breakers and may require specialized training and equipment to install and maintain. They are also more complex, which can increase the risk of malfunctions or issues if not properly installed and maintained.

In summary, electronic circuit breakers and thermal-magnetic circuit breakers both offer benefits and drawbacks depending on the specific application and system requirements. The choice between the two will depend on factors such as the level of protection required, the complexity of the system, and the budget available.

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Title: Electronic Trip Circuit Basics
Source: Schneider Electric

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