Search

Premium Membership ♕

Limited Time Offer: Save 15% on PRO Plan with discount code: LRN15 and study specialized LV/MV/HV technical articles and studies.

Home / Download Center / Electrical Engineering Books and Technical Guides / ABB’s Power System Protection Practices / Working with trip characteristic curves of low voltage circuit breakers

Low Voltage Circuit Breakers

An electric system shall be protected by the damages which may result from overcurrents. An overcurrent can be divided into short circuit and overload. In both cases (short circuit as well as overcurrent), the cables overheat and, if there is not adequate protection, the temperature rise damages the electrical system and the connected equipment, with the risk of causing fires and subsequent serious damage to people and things.

Working with trip characteristic curves of low voltage circuit breakers
Working with trip characteristic curves of low voltage circuit breakers (photo credit: Юрий Цыпин via Youtube)

Among overcurrent protective devices used as protection against overcurrents, there are circuit breakers which, in case of fault, open the circuit based on their tripping characteristics and on the overcurrent value.

The circuit breaker is a mechanical switching device, capable of making, carrying, and breaking current under normal circuit conditions and also, making and carrying for a specified time and breaking current under specified abnormal circuit conditions such as those of short circuit.

Each circuit breaker is equipped with a trip unit which actuates the circuit breaker release mechanism and allows opening on the basis of the current flowing through it.

Two types of trip units are used in low voltage circuit breakers: Thermal magnetic (thermal magnetic trip unit) and Electronic (electronic trip unit).

Thermal magnetic trip unit
Thermal magnetic trip unit

The thermal magnetic trip unit consists of two parts:

The thermal trip unit – Made up of a bimetallic thermal device which actuates the opening of a circuit breaker with a delay depending on the overcurrent value; for overload protection.

The magnetic trip unit – Made up of an electromagnetic device, with a 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) for short circuit protection.

The electronic trip units instead use a microprocessor to process the current signal and opening in case of fault.

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

  • The long time-delay trip function (ANSI code: 51, AC time overcurrent relay);
  • The short time-delay trip function (ANSI code: 51, AC time overcurrent relay);
  • The instantaneous trip function (ANSI code: 50, instantaneous overcurrent relay);
  • The ground-fault trip function (ANSI code: 51 N, AC time ground fault overcurrent relay).
Title:Low voltage circuit breakers – Working with trip characteristic curves by ABB
Format:PDF
Size:948 KB
Pages:40
Download:Right here | Video Courses | Membership | Download Updates
Working with trip characteristic curves of low voltage circuit breakers
Working with trip characteristic curves of low voltage circuit breakers by ABB

Premium Membership

Get access to premium HV/MV/LV technical articles, advanced electrical engineering guides, papers, and much more! It will help you to shape up your technical skills in your everyday life as an electrical engineer.
50% Discount 💥 - Save 50% on all 90+ video courses with Enterprise Membership plan.

More Information

6 Comments


  1. Tonderai
    Oct 03, 2023

    Thanks for information


  2. Hipolito Talucod
    Aug 27, 2023

    Please explain for Ground Settings of PR222DS the formula of K=I^2*t. Thanks


  3. saravanakumar
    May 03, 2020

    I am expecting about relays like ref 542 615 functional depends on the protection how it manipulates and notify the alarms due to interrupt


  4. saravanakumar
    May 03, 2020

    I want to more about relay REF 542


  5. JOSE LUIS FALCON
    Apr 27, 2019

    GRACIAS¡¡¡¡ EXCELENTE INFORMACION… GRACIAS POR COMPARTIR…SALUDOS DESDE MEXICO¡¡¡


  6. Barrie Skinner
    Jan 31, 2017

    A good manual and while acknowledging the good content and presentation these features are available on other manufacturers breakers. The explanations associated with the curves and showing the Maximum and minim settings on one graph adds value to the information.

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!

  −  1  =  one

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