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:
- Enhanced coordination capabilities
- Integral ground-fault detection
- Communication capabilities
- Future growth potential
- Enhanced Coordination Capabilities
Enhanced Coordination Capabilities
Schneider Electric Micrologic Electronic Trip units
- 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.
- 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.
- 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.
- 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.
- 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
Communication Capabilities
- History of last trip
- Trip unit pickup and delay levels
- Impending trip conditions
- Operating currents for each phase
- Ground-fault leakage current associated with the circuit
- Ground-fault alarm signal
Standard Function Trip Unit Curves
Figure 2. Standard Trip Unit Curve |
Long Term Trip Function
- 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.
- 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
- 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.
- 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
- 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 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.
- 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).
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