A miniature circuit breaker or MCB trip curves are one of the most important characteristics of an MCB. They define the operating current and time delay at which the MCB will open its contacts and disrupt the circuit. That, in turn, provides buyers with an indication of the level of protection an MCB offers. Read for more information on the types of MCB curves and what they mean.
MCB Trip Curves Explained
The MCB operation is based on two functions: overload and short circuit protection. The MCB trips when either of these functions is activated, usually after designated time durations.
These two parameters can be represented on a graph with the x-axis being the current (in amperes) and the y-axis is the time (in seconds). This graph is commonly called the MCB trip curve or MCB current time graph.
The two protection functions are usually shown as separate lines on the graph. The upper section of the graph is dedicated to overload protection while the lower section depicts short circuit protection.
Because overload protection is normally sensed by the MCB thermal element, this section of the curve is sometimes called the “thermal” curve.
The MCB short circuit protection, on the other hand, is usually based on magnetic principles and is therefore also known as the “magnetic” curve.
Types of MCB Curves
MCB trip curves are defined by IEC standards. The MCB curve code is, on the other hand, based on the letters of the alphabet. So we have MCB curve types B, C, and D as the most common. Other less frequent MCB curves are types K and Z.
Type B MCB Curve
The type B MCB trip curve is the most common one. It’s characterized by a quick response to short-circuit conditions. The time delay for the short circuit is usually around 0.04 seconds, tripping for currents between 3-5 times the rated current.
The type B MCB curve is best suited for most domestic and light commercial applications where the inrush currents aren’t too high. These include lighting, small motors, and similar loads, but mostly for the protection of cables.
Type C MCB Curve
The type C MCB curve is characterized by a somewhat slower response to short circuit currents than the type B curve. In addition to the longer time delay, the C curve also indicates a higher capacity for overcurrents, usually 5-10 times the maximum rated current.
The type C MCB trip curve is, therefore, most suitable for installations with relatively higher inrush currents, such as motors. These include pumps, air conditioners, and similar loads.
Type D MCB Curve
The type D MCB curve has the slowest response to overcurrent. Miniature circuit breakers with this tripping characteristic trip at 10-20 times the breaker’s rated current.
The MCB trip curve type D is used in installations with the highest inrush currents, such as from very large motors. These include cranes, elevators, and other loads that produce a starting inrush current.
Type K MCB Curve
These MCB curve types represent tripping characteristics that will cause the device to trip on current 10 to 14 times its rated current.
Just like the type D MCB curve, the type K MCB curve is mostly used for heavy industrial equipment that causes high inrush currents.
Type Z MCB Curve
The type Z MCB curve provides tripping that’s characterized by a high sensitivity to overcurrent situations. As a result, selecting this type of miniature circuit breaker curve provides a device that can be used with delicate circuits such as those used for computers and other semiconductor-based equipment.
The breakers designed to use the type Z MCB curve will trip at currents between 2 to 3 times their rated current. As you can see, this is much lower than the other MCB curve types that have been mentioned.
MCB Curve Selection
With the vast amount of miniature circuit breakers produced today, proper MCB curve selection is a crucial step for anyone choosing a circuit breaker for their application. The wrong selection could result in any one of these scenarios:
- The circuit breaker trips too frequently, interrupting the power to the load and causing downtime.
- The circuit breaker doesn’t trip when it should and allows an overcurrent condition to persist, damaging equipment or causing a fire.
For this reason, it’s important to have a basic understanding of how miniature circuit breaker curves work before making a selection. Consider these tips:
MCB Curve for High Current Circuits
When choosing MCB curve types for high-current circuits one of the most important factors to consider is the tripping time and tolerance to inrush current. This is because the inrush current can cause tripping in some MCBs and not others.
To avoid this situation, you would need to select an MCB with a curve that can handle the inrush current of the load without tripping. In this case, a type K or D MCB curve would be the best choice.
MCB Curve for Low Current Circuits
For low-energy circuits, the most important factor to consider is the sensitivity to short circuit current. For these circuits, a high sensitivity will adequately guard against overcurrent conditions before any damage can occur.
To achieve this, a type B curve is typically selected. However, if the application is particularly sensitive, a type Z curve may be the best choice.
In conclusion, buyers must understand MCB tripping characteristics when buying miniature circuit breakers for different applications. That’s because these show the behavior of specific breakers in different conditions. The incorrect choice may result in inadequate circuit protection and dangers that could see damage to cables or appliances and other equipment. It could also cause unnecessary tripping of the breaker due to an incorrect current rating.
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