Premium Membership ♕

Save 50% on all Video Courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles and guides.

Home / Technical Articles / The Basics Of Circuit Breaker Tripping Units

Circuit breaker releases / Tripping units

The main objective of circuit breaker tripping units and protective functions in general is to detect faults and to selectively isolate faulted parts of the system. It must also permit short clearance times to limit the fault power and the effect of arcing faults.

The Basics Of Circuit Breaker Tripping Units
The Basics Of Circuit Breaker Tripping Units (on photo: Siemens molded case circuit breaker 'Sentron' Series; 400 Amp Frame - 400 Amp Trip)

The protective function of the circuit breaker in the power distribution system is determined by the selection of the appropriate release (see Figure 1). Releases can be divided into:

Variants of circuit breaker tripping curves
Figure 1 – Variants of circuit breaker tripping curves

Thermal-magnetic tripping units //

The thermomagnetic trip unit consists of two parts:

The thermal trip unit – Made up by a bimetal thermal device which actuates the opening of a circuit breaker with a delay depending on the overcurrent value. This trip unit is intended for the protection against overloads.

The magnetic trip unit – Made up by an electromagnetic device, with fixed (fixed instantaneous trip) or adjustable (adjustable instantaneous trip) threshold, which actuates the instantaneous trip of the circuit breaker on a pre-determined overcurrent value (multiple of the In) with a constant trip time (about some tens of milliseconds). This trip unit is intended for the protection against short circuit.

TM thermal-magnetic and MA magnetic trip units
Figure 2 – TM thermal-magnetic and MA magnetic tripping units

Go back to CB tripping units ↑

Electronic tripping units //

The electronic trip units instead use a microprocessor to process the current signal and operate the circuit breaker opening in case of fault. In addition to this, electronic tripping units offer more tripping criteria which are not feasible with electromechanical releases.

By digital processing of the signal, they provide the following protection functions:

  1. Long time-delay trip function (ANSI code: 51, AC time overcurrent relay);
  2. Short time-delay trip function (ANSI code: 51, AC time overcurrent relay);
  3. Instantaneous trip function (ANSI code: 50, instantaneous overcurrent relay);
  4. Ground-fault trip function (ANSI code: 51 N, AC time earth fault overcurrent relay).
Microprocessor tripping unit
Figure 3 – Microprocessor tripping unit (Overloads: Long time protection (Ir); Short-circuits: Short-time protection (Isd); Short-circuits: Instantaneous protection (Ii), Additional ground fault protection (Ig) and Neutral protection)

1. Overload protection

Designation: L (LT: long-time delay), previously a-release.

Depending on the type of release, inverse-time-delay overload releases are also available with optional characteristic curves.

This adjustable function simulates the effect of a bimetal conductor in a thermal-magnetic circuit breaker. It reacts to overload conditions and determines how much current the circuit breaker will carry continuously.

The nominal pickup point where a circuit breaker trip unit detects an overload is at 1.075 times the selected ampere rating. After the circuit breaker has picked up, it will not trip until the delay determined by the long-time delay adjustment has been achieved.

2. Neutral conductor protection

Inverse-time-delay overload releases for neutral conductors are available in a 50% or 100% ratio of the overload releaseThe neutral must have specific protection if:

  • It is reduced in size compared to the phases
  • Nonlinear loads generating third order harmonics are installed

It may be necessary to cut off the neutral for functional reasons (multiple source diagram) or safety reasons (working with power off).

3. Short-circuit protection, instantaneous

Designation: I (INST: instantaneous), previously n-release

Depending on the application, I-releases can either be used with a fixed or an adjustable release current Ii as well as with a switch-off or non-switch-off function.

The instantaneous pickup function simulates the magnetic characteristic of a thermal-magnetic circuit breaker. This function trips the circuit breaker with no intentional time delay.

In circuit breakers with both short-time and instantaneous pickup, the instantaneous pickup will override the short-time pickup if the instantaneous pickup is set at the same or lower setting than the short-time pickup.

4. Short-circuit protection, delayed

Designation: S (ST: short-time delay), previously z-release.

To be used for a time adjustment of protective functions in series. Besides the standard curves and settings, there are also optional functions for special applications:

Definite-time overcurrent releases
For this “standard S function”, the desired delay time (tsd) is defined as of a set current value (threshold Isd) (definite time, similar to the function of “definite-time overcurrent-time protection (DMT)” at the medium voltage level).

Inverse-time overcurrent releases
In this optional S function, the product of I2t is always constant. In general, this function is used to improve the selectivity response (inverse time, similar to the function of “inverse-time overcurrent-time protection” at the medium voltage level.

5. Earth-fault protection

Designation: G (GF: ground fault), previously g-release.

Besides the standard function (definite-time) an optional function (I2t = current-dependent delay) is also available.

Ground-fault Characteristic Curve
Figure 4 – Ground-fault Characteristic Curve

The ground-fault function is divided into pickup and delay components (see Figure 4). The pickup portion determines at what point the circuit breaker will begin detecting a ground fault.

The delay adjustment determines how long the circuit breaker will delay tripping after a ground-fault has been detected. It is supplied with both an “I²t IN” and an “I²t OUT” function on the circuit breakers.

In a circuit breaker with the ground-fault function, there is a maximum unrestrained ground-fault delay provided. This delay determines the maximum amount of time the circuit breaker will delay during a ground-fault condition when not restrained by a downstream breaker. The maximum delay is shown by a single line and stays constant for all ground-fault delay switch settings.

6. Fault-current protection

Designation: RCD (residual current device), previously also DI (differential current interrupter).

It detects differential fault currents up to 3 A, similar to the FI function for personal protection (max. 500 mA).

Go back to CB tripping units ↑

References //

  • Planning of Electric Power Distribution – Technical Principles by SIEMENS
  • Circuit Breaker Characteristic Trip Curves and Coordination by Schneider Electric
  • Low Voltage Circuit Breakers complying with the Standards UL 489 and UL 1066 by ABB

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Bill Cowhig
    Jan 11, 2022

    Suggestion: work a few examples using Figure 1 to teach the subscripts

    Question: GE standard trip breakers, in 24 yo residential setting. What does “standard trip mean” in terms of trip current and time to trip, in a 45 amp 2-pole 240 V breaker?

    Thank you for your informative website.

  2. Engr. Nasir
    Sep 25, 2021

    It’s my pleasure to meet you on this platform. I look forward to asking questions regarding Electrical Engineering Installation soon.

  3. suraj
    Sep 09, 2019


    i appreciate your work ,

    u cover all thing in a single with easily understanding .

    now i dont need to open any other pages .

    thank sir

    Aug 17, 2019

    It was very interesting and keen to read more and ti understand more about air circuit breaker technology, safeties.

  5. Alicious Alpha Kamara
    Aug 17, 2019

    I so much appreciate this web site for the vital message given about engineering

  6. Saleh Farman
    Dec 22, 2018

    Hello, does series tripping is also incorporated in MV & HV Switch Gear?

    Thanks and regards

  7. Sarvajeet yadav
    Feb 16, 2018

    My question is that how to set trip timing of ACB .what data take to set timing of ACB for setting time short circuit ,earth fault etc

  8. Yoni
    Jul 25, 2017

    I have 60A 480V 3PH 50HZ Square D breaker.
    main (400/230, 50hr 3phase + pen) feeds two 3phase 5hp motors and with 25a mccb breaker feeds the sub panel 400/230a, 50hz.
    the sub panel (400/230, 50hr 3phase + pen) feeds two AC with 1.7kva each , 20a single phase breakers and 1hp .4kava wtih 15a single phase breaker.
    i have an issue with main breaker tripping when i start nay of the single phase equipment from the sub panel.
    i don’t have main breaker trip when i start any equipment from the main panel.
    any suggestion?

  9. Waseem
    Apr 01, 2017

    I have 400 A breaker, please advises what its setting if it will connect the 250 kva 3ph transformer, and it LT Side circuit run 355 Amp each phase. Expect to have your valued reply soon.

  10. Vladio
    Nov 20, 2016

    A clear explanation for non-specialist, excellent language.

    Jul 19, 2016

    Nice article. If possible give an example ,how the selectivity is achieved.

  12. Manuel Bolotinha
    Dec 21, 2015

    What is writen is only valid for LV circuit breakers. MV and HV circuit breakers trips through na input signal of the protection relays to the tripping coil of the circuit breakers.
    Common rated values for LV residual current devices are:
    • High sensibility: 6; 10; 30 mA.
    • Medium sensibility: 100; 300; 500; 1000 mA.
    • Low sensibility: 3; 10; 30 A.

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

2  ×    =  six

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge