MCB (Miniature circuit breaker) is a re-settable device designed to protect a circuit from short circuits and overcurrents. The trip curve of an MCB (B, C, D, K, and Z curves) tells us about the trip current rating of Miniature Circuit breakers. The trip current rating is the minimum current at which the MCB will trip instantaneously. It is required that the trip current must persist for 0.1s.
What is an MCB trip curve?
The MCB trip curves, also known as I-t tripping characteristic consist of two sections viz, overload section and short circuit section. Overload section describes the trip time required for various levels of overload currents and the short circuit section describes the instantaneous trip current level of MCB.
Read More: Miniature Circuit Breaker (MCB) – Principle of operation
The X-axis of the chart represents multiples of the circuit breaker’s operating current. The Y-axis shows the tripping time, utilizing a logarithmic scale to cover a range from 0.001 seconds to 10,000 seconds (2.77 hours) across different multiples of the operating current.
The Trip Curve consists of three main components:
- Thermal Trip area: This area of the curve represents the response of the bi-metallic strip, which is designed to handle slower overcurrents, allowing for inrush or startup currents.
- Magnetic Trip area: This area of the curve illustrates the response of the coil or solenoid, designed to react swiftly to high overcurrents, such as those occurring during a short circuit.
- Ideal Trip area: This area curve represents the theoretical desired response of the bi-metallic strip. Due to the inherent variability of the bi-metallic strip and fluctuating ambient conditions, it is challenging to accurately predict the exact tripping point.
Class B trip curve
The MCB with class B trip characteristics trips instantaneously when the current flowing through it reaches between 3 to 5 times the rated current. These MCBs are suitable for cable protection.
Class C trip curve
MCB with class C trip characteristics trips instantaneously when the current flowing through it reaches between 5 to 10 times the rated current. Suitable Domestic and residential applications and electromagnetic starting loads with medium starting currents.
Class D trip curve
MCB with class D trip characteristics trips instantaneously when the current flowing through it reaches between Above 10(excluding 10) to 20 times the rated current. Suitable for inductive and motor loads with high starting currents.
Class K trip curve
MCB with class K trip characteristics trips instantaneously when the current flowing through it reaches between 8 to 12 times the rated current. Suitable for inductive and motor loads with high inrush currents.
Class Z trip curve
MCB with class Z trip characteristics trips instantaneously when the current flowing through it reaches between 2 to 3 times the rated current. These types of MCBs are highly sensitive to short circuits and are used for the protection of highly sensitive devices such as semiconductor devices.
Class A trip curve
MCB with class A trip characteristics trips instantaneously when the current flowing through it reaches between 2 to 3 times the rated current. Like Class Z MCBs, these are also highly sensitive to short circuits and are used for the protection of semiconductor devices.
Class B and Class C MCBs are the most commonly used ones. Class C MCBs are used in the lighting power distribution boards in residential and commercial buildings. It trips as soon as the current rises between 5 to 10 times its rated current. Class B MCBs are used in the protection of electronic devices such as PLC, DC power supplies, etc. in control panels. It trips as soon as the current rises between 3 to 5 times its rated current.
In some applications, frequent current peaks occur for a very short period (100ms to 2s). For such applications, class Z-type MCBs shall be used. Class Z-type MCBs are used in circuits with semiconductor devices.
Importance of trip curves
It is important to choose an appropriate MCB current rating and trip curve to safeguard the circuit from damage during faults. Hence it is necessary to calculate the short circuit current and i0nrush current before choosing an appropriate MCB rating. If the chosen MCB rating is much higher than required, then it may not trip in the event of a fault. Similarly, if the MCB is underrated, then it may cause nuisance trips, for example, even the starting currents or inrush currents may trip the MCB.
External selection tool: https://new.abb.com/low-voltage/solutions/selectivity/tools-support/curves
Trip curves for other circuit breakers
All circuit breakers, such as MCCB, ACB, VCB, etc have their trip characteristics. The only thing is that may not follow the categorization as that of MCB. Also, the circuit breaker curve types are not the same for all types of circuit breakers. It varies from one circuit breaker type to the other and depends on many design factors.
Learn more about MCB:
- What is an MCB?
- Miniature Circuit Breaker (MCB) – Principle of operation
- What is kA rating of MCB and MCCB?
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3. Difference between Soft Starters and VFDs
4. Difference between MCCB and RCCB
5. Difference between MCB and RCBO
6. Difference between RCCB and RCBO
7. Difference between MPCB and MCCB
Very good explanation. I understood the concept.
Thank you.
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Thankhs google team good explace thanks again
Very good mcb make , what Amps load trip make
Very good mcb make , what Amps load trip make
Thanks
Very good. Nice explain.. Good job
Explanation is good but your second paragraph doesn’t match the charts. It looks like it is the B-curve that trips between 3-5 times its rated current, and C-curve that trips between 5-10 times its rated current.
Very good, thanks
very good …..thanks
Thanks very much
Very good explanation
Is this curves is applicable to Rccb ?
No. These curves are applicable for mcbs only.
Thanks for your information
The information about mcb is very useful and helpful for a technician, many many thanks for sharing your information.
Great information, I got to know a few more details out of what I wanted to know.
Which type is better choice for UPS protection?
The explanations are very good but in the video is a mistake at minute 0.38. The short circuit sections with the overload section are reversed.
Good for selection of MCB’s
On the c type Mcb on the time curves at a short circuit fault current at 220amp it shows dis connection at 6/7seconds are you saying that disconnection will be instant at this current or 6/7 seconds.
I use B-curve in my home when short circuit occured in the appliace MCB tripped but my appliance burned. My appliance lead wires were shorted by a metal piece was lying on it.I thought MCB could have protected but not. And I also headed big noise of it.
Hi Younis,
Sorry to hear that. This could be because the MCB was oversized: Much higher than the rated current of the appliance or the MCB could be faulty. We suggest you replace it with a new one. Make sure that you are choosing the right one.
Thanks for sharing such an informative article about MCB.
sir
Type C is used for average current load. Type B and C are the most commonly used in DBs. Tripping of MCB Type C is 5-10 times higher than normal.
eg: if a 6A mcb put in acircuit , the rated current is 6 A , then how ever the type c mcb with stand 5 to 10 times higherr than normal .
hello, what about the CL curve mcb, because in my home installation I used the cl4 code on the mcb
Perhaps you are referring to product name of the MCB and not its trip curve.
The information is quite educative. Thank you so much
GOOD INFORMATION
best ever