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Home / Technical Articles / 6 Adjustable Tripping Settings of a Circuit Breaker You MUST Understand

Basic tripping settings of CB

There are (at least) six basic adjustable tripping settings (functions) you really should understand in order to fully understand how circuit breaker actually works. All these adjustable functions actually shape the time-current curve of a circuit breaker and allows proper tripping according to the network parameters and also the proper coordination between upstream and downstream devices.

6 Adjustable Tripping Settings of a Circuit Breaker You MUST Know
6 Adjustable Tripping Settings of a Circuit Breaker You MUST Know

Note that modern circuit breakers (MCCB, ACB) mostly have an electronic tripping unit which is much more advanced comparing to these explained here, but the basics are the same, very same.

So, let’s start with explanation!

  1. Continuous Amps (Ir)
  2. Long-Time Delay
  3. Short-Time Pickup
  4. Short-Time Delay
  5. Instantaneous Pickup
  6. Ground Fault Pickup
Circuit breaker adjustable tripping functions
Circuit breaker adjustable tripping functions

The below time-current curve reflects one possible setup for a 1200 ampere circuit breaker with a nominal (maximum continuous ampere) rating of 1000 amps. This time-current curve will be the basis for discussing adjustable tripping settings of the circuit breakers.

Time-current curve of a circuit breaker
Time-current curve of a circuit breaker


1. Continuous Amps (Ir)

Continuous Amps (Ir) varies the level of current the circuit breaker will carry without tripping. Ir is a percentage of the circuit breaker’s nominal rating (In). Continuous amps can be adjusted from 20 to 100 percent of the circuit breaker’s nominal rating.

For example, a 1000 amp breaker can be changed from 1000 amps to 800 amps by adjusting the breaker continuous amps setting to 80%.

Continuous current [Amps]
Continuous current [Amps]

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2. Long-Time Delay

Long-time delay causes the breaker to wait a certain amount of time to allow temporary inrush currents, such as those encountered when starting a motor, to pass without tripping.

The adjustment is from 2.2 to 27 seconds at six times the continuous amps (Ir) setting.

As shown below, the long-time delay effects the position of an I2T slope. This means that lower levels of current will allow the breaker to remain online for longer periods of time.

Long time delay tripping settings
Long time delay tripping settings

Go back to CB tripping settings ↑


3. Short-Time Pickup

Short-time pickup is used for selective tripping. The short-time pickup function determines the amount of current the breaker will carry for a short period of time, allowing downstream protective devices to clear short-circuits without tripping the upstream device.

Short-time pickup is adjustable from 1.5 to 10 times the trip unit ampere setting (Ir).

For example, a 1000 ampere frame can be adjusted to trip anywhere from 1500 to 10,000 amps. The switch also has an “OFF” position to eliminate short-time pickup and short-time delay.

Short-time pickup used for selective tripping
Short-time pickup used for selective tripping

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4. Short-Time Delay

Short-time delay, used in conjunction with short-time pickup, controls the time involved in postponing a short-time pickup trip.

There are two modes: fixed time, or I2T ramp. Fixed time is adjustable from .05 to .5 seconds. The I2T ramp mode is adjustable from .18 seconds to .45 seconds, providing a short inverse time ramp.

This allows better coordination with downstream thermal-magnetic circuit breakers and fuses. A fixed instantaneous trip point of 10,000 amps trips the breaker automatically and overrides any pre-programmed instructions.

Short-time delay, used in conjunction with short-time pickup
Short-time delay, used in conjunction with short-time pickup

Go back to CB tripping settings ↑


5. Instantaneous Pickup

Instantaneous pickup is used to trip the circuit breaker with no intentional delay at any current between 2 and 40 times the breaker’s continuous ampere setting (Ir).

In this example instantaneous pickup has been set to 10 times the continuous amp setting, or 10,000 amps (10 x 1000) with a continuous amp setting of 1000 amps. In this case a higher setting would still trip at 10,000 amps due to a fixed instantaneous override of 10,000 amps which automatically trips the breaker regardless of the instantaneous pickup setting.

If the continuous amp setting had been 300 amps, setting the instantaneous pickup at 10 would make the instantaneous setting equal to 3000 amps, well below the fixed instantaneous override.

Instantaneous pickup - Used to trip the circuit breaker with no intentional delay at any current between 2 and 40 times the breaker’s continuous ampere setting (Ir)
Instantaneous pickup – Used to trip the circuit breaker with no intentional delay at any current between 2 and 40 times the breaker’s continuous ampere setting (Ir)

Go back to CB tripping settings ↑


6. Ground Fault Pickup

Ground fault pickup controls the amount of ground fault current that will cause the breaker to interrupt the circuit. The adjustment can be set from 20 to 70% of the maximum breaker rating. In compliance with NEC® 230-95 (A), no trip point setting exceeds 1200 amps.

The ground fault pickup is divided into three sections; .1s, .2s, and .4s. This feature adds a time delay of .1, .2, or .4 seconds to the breaker’s trip when a ground fault occurs.

Ground fault pickup controls the amount of ground fault current that will cause the breaker to interrupt the circuit
Ground fault pickup controls the amount of ground fault current that will cause the breaker to interrupt the circuit

Circuit breaker coordination

The ground fault pickup time delay feature is useful for circuit breaker coordination. In the following scheme, upstream breaker “A” has been set to .2s and downstream breakers “B” and “C” have been set to .1s.

The ground fault pickup time delay feature is useful for circuit breaker coordination
The ground fault pickup time delay feature is useful for circuit breaker coordination

A ground fault occurring in the circuit supplied by “B” will trip the “B” breaker without disturbing “A” or “C”.

Go back to CB tripping settings ↑

Reference // STEP 2000 – Molded Case Circuit Breakers (MCCB) by SIEMENS

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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.

45 Comments


  1. Shridhar
    Sep 05, 2019

    Thanks very helpful information.


  2. roberto soto
    Aug 20, 2019

    Thanks

    This information is very complete and helps me to have a knowledge of how to adjust parameters correctly.


  3. Lay Lwin
    Jul 19, 2019

    The information is given is very useful for us. Thank you. Please also post similar technical information detail. Truly appreciate your time spending to share knowledge.


  4. Hari kusnomo
    Jul 08, 2019

    Thanks for this technical information


  5. Abdul khaliq
    Jun 06, 2019

    Very useful information, no doubt.


  6. mohamad ramezani
    Jan 15, 2019

    very thanks for detail of this document . very useful


  7. Vishal
    Jan 03, 2019

    Please explain long time delay setting calculation.


  8. SEEN BAYAS AHAMED
    Nov 28, 2018

    Trip characteristics is adjustable in mcb or not


  9. ali taheri
    Nov 17, 2018

    Thank you Edvard


  10. Thiagarajan
    Oct 20, 2018

    Hi Sir ,

    I have problem to start 3.3kv motor . The motor start trip 21 secs due Thermal Pickup
    Starting Current 1640Amps , The protection relay we use GE multilin 869 . As per Mechanical documnet the Motor Alignment everything looks ok . Please advice …..


    • Jimuel
      Jul 28, 2020

      What exactly did you mean here? Based from what I understand, the motor started to trip after 21 seconds it has started to run?


  11. Vivek
    Oct 06, 2018

    Sir
    I want to know about LBB tripping settings for 33kv VCB and 220kv SF6 breaker.


  12. NITIN BHAGWAN MHASKE
    Oct 03, 2018

    Very helpful article.Thank You for sharing


  13. OM SINGH
    Sep 30, 2018

    Hi Edvard,
    Thanks for theses technical information of CB. Theses are very helpful for me.


  14. Rustam Gasimov
    Sep 29, 2018

    Thank you for detailed information.


  15. Khuram shahzad
    Aug 10, 2018

    sir ,
    hope you are fine,
    please share difference between lr and lm sitting in mg barker.

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