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How to choose a miniature circuit breaker

MCBs or Miniature Circuit breakers are one of the most important pieces of electrical equipment. They provide safety to the end user and can be the difference between life and death. Circuit breakers also protect other electrical equipment from overloads and short circuits. It is vital that each application has the right circuit breaker installed. Chint circuit breakers operate in exactly the same way as other brands such as Merlin Gerin, Siemens, Moeller and Wylex etc. Chint Circuit breakers are one of the most reliable on the market and are approved for worldwide use.

Starting with the essentials: A circuit breaker is an apparatus that is used to protect wiring installations and equipment. In the event of a fault such as a short circuit or surge of electricity, a circuit breaker operates by opening an electrical circuit and stopping the flow of electricity. It basically does the same job as a fuse; however unlike a fuse, a circuit breaker doesn’t have to be replaced, simply reset. The main faults that circuit breakers protect your equipment from are short circuits and overloads; however earth faults can also be protected by residual circuit devices known as RCDs.
Chint is now one of the worlds leading manufacturers of circuit breakers and produces a vast array of current ratings, poles and tripping characteristics to meet your needs. The circuit breakers can be found from the tiniest (MCB’s), which are in distribution boards and consumer units, to the very large vacuum devices filled with oil which are installed for high voltage distribution networks. Below we will discuss the small MCB’s, as electrical contractors rarely have to use the large high-voltage circuit breakers.
The most popular type of MCB circuit breaker is the thermal breaker. It has an integrated bi-metallic strip through which the current passes through. As the current passes through the bi-metallic strip it bends according to the heat strength. In the case that the current is too strong overloading the circuit, the increased heat causes it to bend beyond average which sets off the trip mechanism of the breaker.

The Chint NB1 ranges of thermal circuit breakers are available at a very low cost; however they only provide limited protection against faults such as short circuits. This type of circuit breaker should be used in installations where line protection is the main concern. The NB1 range of circuit breaker starts at 1 amp and goes up to 63 amp and is available in B, C or D type tripping characteristics.
A more versatile MCB that is more frequently used is the thermal-magnetic circuit breaker. Also offering defence from fairly small or moderate overloads it shares similarities with the basic thermal breaker with one small difference. Aside from the circuit breaker described above, the thermal-magnetic circuit breaker also features an electromagnetic mechanism that causes it to trip immediately in the case of a large short circuit or overload.
Thermal-magnetic MCBs can be designed to provide high breaking capacities, and to have various types of trip characteristic. They are ideal for applications where comprehensive protection is required at a moderate price, and where high short circuit levels may be encountered.

For applications were higher currents are involved then the use of a moulded case circuit breaker (MCCB) should be employed. An MCCB such as the Chint NM8 range is available with either thermal-magnetic or electronic protection. Also offering defence from fairly small or moderate overloads it shares similarities with the basic thermal breaker with one small difference. Aside from what the circuit breaker described above, the thermal-magnetic circuit breaker also features an electromagnetic mechanism that causes it to trip immediately in the case of a large short circuit or overload.

Circuit breakers with the thermal-magnetic characteristic such as the Chint NM8 MCCB’s can also be adjusted to have high breaking capacities as well a variety of trip characteristics. For full protection at a modest price they are the perfect choice, especially for protection from high short circuit levels.

How to Choose an MCB
There are four characteristics you need to consider when choosing the right MCB for any application. What is the standard that the MCB matches is the first characteristic. The current rating, tip characteristics as well as the breaking capacities are also very important. Below each will be discussed in further detail.
Most MCB’s are at a basic standard of about BS EN 60898 in the UK, which basically protects low-voltage breakers within domestic and light commercial installations. However, a BS EN 60947-2 would be necessary for an industrial installation. It’s important that any breaker installation in the UK should be done by the standards in order to provide the proper protection.
It’s very important that the MCB chosen is selected to match the current rating as close to the maximum load of current as possible in order to protect the circuit properly. The current rating is the maximum current that an MCB can continuously carry without it tripping.

Tripping Characteristics
MCBs that have a quick trip mechanism when put in flawed circumstances may seem best, but this is not always the case. Short-circuit faults call exactly for that, however, in the case of protection from an overload it’s more complicated. There is a lot of equipment, like transformers, motors, and fluorescent lights, which draw a high current when switched on for a short while.
This means that an MCB that has a quick trip mechanism would be pointless as each time such equipment is turned on the high peak would automatically trip it. This is why MCB manufacturers can actually design and adjust how a breaker reacts to certain peaks and for how long; changing it depending on the necessary current strength and length of time it’s consistent. The MCB’s trip characteristic is a curve in which the relationship of the current and tipping time is represented.
As the curves can be complicated and so users don’t have to deal with them the BS EN 60898 identifies a number of standard characteristics, of which Type B, C, and D are most important. They are all available within the Chint NB1 range. Overall it’s simple to match one of these types to an application being used; in very particular and rare cases will a user have to work with the characteristic curves.

Type B MCB: These circuit breakers are made to react quickly to an overload. They respond quickly and trip at 3 – 4.5 times the usual current at full load. Perfect uses for them is domestically or within an office or commercial environment, as the risk of surges causing the MCB to trip are more limited. It’s important that the lighting is incandescent; if the circuit breaker is being used to protect inductive loads such as fluorescent lighting then a C type MCB would be recommended.
Type C MCB: Circuit breakers that are of type C have a slower reaction time. They are ideal for installations such as fluorescent lighting as they have inductive loads that have high in-rush currents. Their trip capacity is set at 5-10 times the regular current at full load.
Type D MCB: Are the slowest of the three and tip at 10-20 times the usual current at full load. These are made for high in-rush currents ideal for welding machines or transformers.
Breaking Capacity

This is the last factor that needs to be put into consideration when choosing an MCB. It is very important that it is greater than the PSCC, or prospective short-circuit current, where the MCB is to be installed

As part of the testing necessary on all electrical installations it is required to measure the source impedance of the supply, which in turn the PSCC can be calculated easily, it will be simple and quick to make sure that the breaking capacity is higher. Nowadays there are installation testers that actually calculate this information automatically.
Some manufacturers offer MCB’s with ranges of breaking capacities from 15kA with an extension to 25kA for some products, while most other manufacturers offer a range of 6kA or 10kA. The higher the breaking capacity, in some cases, can actually make a substantial reduction in costs as it allows the MCB to be used in place of the more expensive moulded case breakers.

Earth-Fault Protection
With the introduction of the 17th Edition wiring regulations all domestic wiring installations should include earth fault protection. These breakers are usually known as RCD’s or residual current devices as well as RCCB’s or residual current circuit breakers where their main use is to protect should there be an electric shock. These were mentioned briefly earlier in this article.
Though there isn’t any room to discuss them further in this article, it’s important to note that there are MCB’s available now that actually have an earth-fault protection. Usually called RCBO’s or residual current breaker with overload, are great for a large number of applications and can be applied in various locations, not only domestic.

MCB’s today are far more dependable as well as inexpensive and when applied properly offer great protection for both equipment and wiring. It is a simple process and choosing right MCB is made even easier with the information provided above.

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