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 KnowNote 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!
- Continuous Amps (Ir)
- Long-Time Delay
- Short-Time Pickup
- Short-Time Delay
- Instantaneous Pickup
- Ground Fault Pickup
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.
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]](https://electrical-engineering-portal.com/wp-content/uploads/2016/06/continuous-current-amps.png)
Go back to CB tripping settings ↑
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.
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.
Go back to CB tripping settings ↑
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.
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.
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.
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.
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.
Go back to CB tripping settings ↑
Reference // STEP 2000 – Molded Case Circuit Breakers (MCCB) by SIEMENS
Related electrical guides & articles
Good explanation, clear and succinct
Isn’t it generally not permissible to change the settings of a breaker since these were set based on a commissioning coordination study.
Electricity Section,
University of Cape Coast
Ghana West Africa.
It is very useful.and served as reference about the tie breaker tripping .thanks ,really apreciate it.more power to the author
NEED TO LEARN AND DOING BUSINESS
Just starting to get into ACB is there any other advice or information your could give or point me towards
Great article
Continuos current can be adjusted between 200A to 1000 A isn,t it ? Example says its 800A is that correct ?
I did’nt understand why there’s setting of 6*IR in short term overload release.
Great
Useful information for ACB / MCCB settings.
Thanks a Lot !
Great 👍👍👍🙏🙏🙏
Great presentation. Congratulations
Esta información es muy completa y me ayuda a tener un conocimiento de cómo ajustar los parámetros correctamente.
Thanks the topic is 200%excellent
the information provided is very helpful for practical based and theory based lessons and has been presented in precise manner, it is what i was looking for and i will expect more edvard
Dear Sir,
Does the Direct Acting Series Trip is only build on ACB or both ACB & MCCB?
Thank you
thank a lot for all thing you always provide.
best regards
Ghassan
Just a quick question – what exactly does The adjustment is from 2.2 to 27 seconds “at six times the continuous amps (Ir) setting” mean, i.e. what does the @ 6,000A mean exactly?
Yes. At 6000A or some minor+/- CB will trip at pre set time.
Thanks very helpful information.
Thanks
This information is very complete and helps me to have a knowledge of how to adjust parameters correctly.
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.
Thanks for this technical information
Very useful information, no doubt.
very thanks for detail of this document . very useful
Please explain long time delay setting calculation.
Trip characteristics is adjustable in mcb or not
Thank you Edvard
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 …..
What exactly did you mean here? Based from what I understand, the motor started to trip after 21 seconds it has started to run?
Sir
I want to know about LBB tripping settings for 33kv VCB and 220kv SF6 breaker.
Very helpful article.Thank You for sharing
Hi Edvard,
Thanks for theses technical information of CB. Theses are very helpful for me.
Thank you for detailed information.
sir ,
hope you are fine,
please share difference between lr and lm sitting in mg barker.
Hi Edvard,
Thanks for giving information,
1- what was the difference between Making Capacity and Breaking Capacity?
2- Can we design our systems according to Making Capacity?
3- If Maximum Instantaneous Trip set level 15% of rated Breaker than what happened when Short circuit occurred greater than INS trip Set level.
4- In 3Pole MCCB/ACB without additional C.T Breaker tripped on earth fault what was the reasons?
5- What is the difference between REF and UREF ground fault?
Great information.
I am very thankful to you if you send me the all breaker trip setting level as per different ampare load for data center to fallow the IEC .
How do the terms, “Ir” and “In” relate to one another in an adjustable breaker? I tested an electronic insulated case breaker the other day and the current setting was adjustable from .5 to 1 and labeled ‘XIn” The instantaneous range was adjustable from 3 to 6 and was labeled “XIr” Is one adjustment based on rating plug and the other based on frame size of the breaker?
Good info.
Thank you so much. I was wondering if you let me know IEC/IEEE standards for short time pick up.
That was great. helped me a lot.
Very helpful. Good job.
LSIG BREAKER RELEASE
This site is very helpful to electrical engineers like me …thankyou
Thanks sir……very useful material…….
Thank you so much Edvard, very good job.
I follow the of articles but if is possible to make these articles available in soft copy.
Hi Asan, see the button ‘Get PDF’ on your left? That your answer :) To create PDF out of any technical article.
Great Article
Thank you Edvard, I have been looking for this info for long. I shall inquire about this in detail soon.
What is the standard testing current injection limits for 400A and 800A MCCB?
Muneeb Islam
I like your Job, Very very interesting and very helpfull.
keep better, go on!!
Great article, when you don’t do it every day you forget some of this. Having the curve right next to the explanation helps even more THANK YOU! I’m going to print this and keep it with my field work tools.