MCB
Here is all you wanna know for MCB, including miniature circuit breaker wiki, miniature circuit breaker working principle, MCB types, MCB rating, MCB sizes, miniature circuit breaker function, miniature circuit breaker construction, miniature circuit breaker applications, and also advantages of miniature circuit breaker.
Miniature Circuit Breaker Wiki (Wikipedia):
Miniature circuit breaker (short as MCB) can be also called as mini circuit breaker, micro circuit breaker, etc. It is a widely used appliance for terminal protection, working as protection from short circuit, overload, over voltage and etc. in both single phase and three phase system with rated current up to 125A. It normally includes four types of 1P, 2P, 3P, 4P.
Miniature Circuit Breaker Construction:
Miniature circuit breaker is configured with operating mechanism, contacts, protection devices (all types of releases), arc-extinguishing system, etc. For more details, please check post of Miniature Circuit Breaker Construction.
Miniature Circuit Breaker Working Principle:
1) Overload protection
Overload protection function is realized based on principle that bimetallic strips bend with temperature rises. When miniature circuit breaker is working under normal working condition, inner bimetallic strips heat as electric current flows, also different thermal expansion coefficients of two metal strips lead to bending. The bending angle is so small under normal current (1.13In), therefore the thrust generated is not big enough to make release trip; but bending angle gets larger when overload happens in the circuit (current up to 1.45In), so that bend metal strips touch lever in tripping mechanism, and then make release trip, thus realizing overload protection function.
For miniature circuit breaker, different current flowing through causes different bending degree for bimetallic strips. When normal overload happens in the circuit, the tripping time is a bit longer because of not so large overload current.
2) Short circuit protection
Short circuit protection is realized through instantaneous release. According to analysis of equation of F=IN (suction is directly proportional to product of current and number of winding), and few winding of instantaneous release coil (normally less than 10 turns), the suction generated under normal working current is not big enough to overcome spring’s reactive force, thus the circuit still works. But when short circuit or severe overload happens, large current will flow through induction coil, and a strong magnetic field is generated. Although coil’s winding number is same, the current produced increases by several times and even more than normal working current, thus suction will also increase by same times, so that lever is pushed to make release trip, the tripping time is normally within 0.1s because of large current. Logical choice of springs can realize setting requirements of instantaneous releases of type b, type c, type d.
MCB Types:
MCBs are of following types according to poles:
- Single Pole
- 1 Pole (with N)
- 2 Pole (DP)
- 3 Pole (TP)
- 3 Pole with N (TPN)
- 4 Pole (4P)
But sometimes MCB types refers to MCB characteristic curves, there are MCBs of type b, type c, type
according to characteristic curves.
MCBs of type b, type c, type d are respectively used in different occasion because of different setting short circuit protection current.
Type b MCB: applied in houses and socket circuits. Short circuit protection range is 3In~5In (≤3In not tripping, not tripping time>0.1s; >5In must trip, tripping time<0.1s). Generally speaking, type b MCBs are designed to trip at fault currents of 3-5 times rated current (In). For example a 10A MCB will trip at 30-50A.
Type c MCB: first choice for electrical appliances with large current flowing through, such as lights and electric motors. Short circuit protection range is 5In~10In (≤5In not tripping, not tripping time>0.1s; >10In must trip, tripping time<0.1s). Type c MCBs are designed to trip at 5-10 times In (50-100A for a 10A MCB).
Type d MCB: Suitable for electric appliances with impulse current, such as magnetic valves, capacitors, large battery charging systems, winding motors, transformers, X-ray machines and some types of discharge lighting. Short circuit protection range is 10In~50In (≤10In not tripping, not tripping time>0.1s; >50In must trip, tripping time<0.1s). Type d devices are designed to trip at 10-20 times In (100-200A for a 10A device).
To choose right MCB with suitable characteristic curve is very important for circuit protection.
MCB Rating:
Normally there are ratings 1A, 2A, 3A, 4A, 6A, 10A, 13A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 100A for MCBs.
MCB Sizes:
MCB sizes refers to MCB dimensions. Take DZ47-63 miniature circuit breaker for example, check its outer and mounting dimensions as below:
:Miniature Circuit Breaker Function
Miniature circuit breaker is a kind of low voltage electrical appliance, which is protecting circuits and power source from overload and short-circuit, and also working as infrequent change-overs of electrical apparatus and lighting circuits.
Miniature Circuit Breaker Applications:
1) Selection of rated short circuit breaking capacity
Rated breaking capacity is maximum short circuit current value for breaking without any damage brought to MCB. Nowadays there are several rated breaking capacities for MCB in the market, such as 4kA, 6kA, 10kA. When we choose MCB to use in one place, we should calculate out maximum short circuit capacity in tthis place. If MCB’s rated breaking capacity is smaller than short circuit fault current in the protected range, not only fault circuit can’t be broken, but also explosion will be happened to MCB, thus causing dangers to people and other electrical appliances and circuits. Besides, you must pay great attention to points as below:
a, Because of usage of large capacity’s bus duct and shortening of distance between electrical appliances and power souce, the short circuit current in the terminal of power lines is getting larger, especailly in public buildings, such as office buildings, hotels and shopping malls, etc. So when selecting MCB for such places, we should consider this increase of short circuit current.
b, There are two standards for MCB products, one is IEC898 (GB10963-1999), and the other one is IEC947-2. IEC898 is standard for MCBs used by unprofessional or inexperienced people, but IEC947-2 is for MCBs operated and used by electrical professionals. Rated breaking capacity values are quite different in these two standards, so designers should select MCB according to usage occasion and users.
c, According to characteristic curve, MCB can be classified into type a, type b, type c, type d. Different types of MCBs can be applied in different occasions. For this, please refer to MCB Types above.
2) Working frequency of MCB
MCB is designed and used in AC power system of 50~60Hz. Because electromagnetic force of magnetic release is related with power supply frequency and tripping current, MCB’s tripping current will be different if it is used for protection in DC circuits and occasions with other power supply frequency. Normally we should get a new tripping current value based on variation coefficient of tripping current and power frequency from manufacturers. When MCB is used for protection in DC circuits, because of arc-extinguishing, we should choose MCB exclusively for DC circuits (DC Circuit Breakers).
3) Working temperature of MCB
Normally MCB’s thermal release rated current is set based on basic temperature of 30℃ according to IEC898 standard, so it is recommended that MCB’s working temperature should be -25℃~+55℃. As we know thermal release (constituted of bimetallic strips) is closely related to temperature, thermal release’s operating characteristic will be changed if any change of ambient temperature. Generally MCBs are installed in the distribution box, with each MCB closely linked together, also for most purposes distribution box is embedded in the wall, because of poor heat dissipation, MCB’s actual working temperature is higher by 10℃~15℃ than ambient temperature. When ambient temperature is greater or less than corrected temperature value, we should adjust MCB’s rated current value according to correction curve of temperature and current-carrying capability. Normally speaking, when ambient temperature is greater or less by 10℃ than corrected value, MCB’s rated current should decrease or increase by about 5%.
4) Accessories selection of MCB
MCB can be used in more occasions if used together with some protecting accessories, such as residual current devices (short as RCD), shunt release (short as ST), under voltage release (short as UR).
MCB can work as RCBO (residual current circuit breaker with overload) if working together with RCD. RCBO, installed in the distribution box, can protect people from electric shock and electrical fire when single phase ground fault happens in the circuit.
Undervoltage release (UR) is used to trip when power supply voltage is decreased to under 70%, and to prevent reconnecting when power supply is recovered. Using UR can not only avoid electrical appliances’ damage under low voltage, but also prevent electric motors with large capacity load, starting on its own without receiving any control signal, thus circuit’s security is improved. But UR is not suitable for normal lighting circuits and some special occasions.
Shunt release (ST) is a kind of device controlling MCB’s tripping in remote distance.
Accessories should be selected reasonably according to actual requirements.
Advantages of Miniature Circuit Breaker:
Miniature circuit breaker has several advantages comparing to fuse.
1) Security of the power grid and electrical appliances is improved. When people use fuse, they can adopt larger fuse instead if original fuse is blown, thus fuse is not working as protection again. But if they use miniature circuit breaker, MCB trips when overload happens, and starts to work again only if people decrease electricity consumption, thus potential accident is avoided.
2) If adopting fuse, normally just one phase or two phases’ fuses are blown when fault happens, in this case, there is still one phase supplying power there, and then causing electric shock. But if MCB is used, the other two phases will also trip if one phase trips, making sure all the phases’ power supply is cut down.
3) Users always worry over a suitable fuse after fuse is blown. But they could just switch-on MCB after fault is analysed and eliminated.
4) The shape of 1A fuse and 60A fuse are so different that fuse box’s manufacture is not so easy. But 1A MCB and 60A MCB are completely same in appearance, which brings convenience to distribution box’s design, manufacture and selection.
تعليقات
إرسال تعليق