mcb types
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The type is referring to the magnettic setting of the MCB. For type B the magnetic setting is 3-5x its rating, for type C it is 5-10x its rating and type D it is 10-20x its rating.
This operates to switch the MCB off very fast, usually within 0.1seconds.
The type is important when you select an MCB as if you have a few heavy loads such as wire wound transformers or a few fluorescent fittings on a type B it may switch off when in normal use. This is when we use a type C. Other applications which have a higher inrush current (when switched on) such as welders may need a type D.
Going back to above, to ensure the MCB trips quickly to remove the fault we need to ensure the current flowing in the circuit exceeds the higher of the values. This is done ensuring the efli (earth fault loop impedance) for the circuit is low enough to ensure enough current flows. The maximum permittable efli for a type B can be given by 230/(5xrating), so for a 6A the maximum permittable efli is 230/30 = 7.66 ohms. For type C this halves i.e. 230/(10x6) = 3.83 ohms and for type D it halves again i.e. 230/(20x6) = 1.91 ohms.
These are max tabulated values i.e. when the cable is operating at its maximum operating temp, you need to multiply the answers by 0.8 to give max measured values for a normal twin and earth, conductor temp 10ºC.
This operates to switch the MCB off very fast, usually within 0.1seconds.
The type is important when you select an MCB as if you have a few heavy loads such as wire wound transformers or a few fluorescent fittings on a type B it may switch off when in normal use. This is when we use a type C. Other applications which have a higher inrush current (when switched on) such as welders may need a type D.
Going back to above, to ensure the MCB trips quickly to remove the fault we need to ensure the current flowing in the circuit exceeds the higher of the values. This is done ensuring the efli (earth fault loop impedance) for the circuit is low enough to ensure enough current flows. The maximum permittable efli for a type B can be given by 230/(5xrating), so for a 6A the maximum permittable efli is 230/30 = 7.66 ohms. For type C this halves i.e. 230/(10x6) = 3.83 ohms and for type D it halves again i.e. 230/(20x6) = 1.91 ohms.
These are max tabulated values i.e. when the cable is operating at its maximum operating temp, you need to multiply the answers by 0.8 to give max measured values for a normal twin and earth, conductor temp 10ºC.
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The MCB is two devices in one.
The magnet part ensures very fast disconnection in the event of a short circuit.
The thermal part is much slower acting and works where we have an overload.
The 1,2,3 or 4 and on latter B, C, and D refers to the magnet part and are multipliers so B=5, C=10 and D=20 therefore a B6 MCB with trip with a 30 amp short circuit or 6 amp overload.
Wikipedia is good for explaining these sort of things.
The magnet part ensures very fast disconnection in the event of a short circuit.
The thermal part is much slower acting and works where we have an overload.
The 1,2,3 or 4 and on latter B, C, and D refers to the magnet part and are multipliers so B=5, C=10 and D=20 therefore a B6 MCB with trip with a 30 amp short circuit or 6 amp overload.
Wikipedia is good for explaining these sort of things.
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//www.diynot.com/forums/viewtopic.php?t=93406
This will help for starters.
Basically, the different types of CB are designed for different jobs.
The higher the designation, the more tolerant they are of surge or start-up currents, but the lower the maximum earth fault loop impedance that circuit must have.
This is the resistance of the (L-E) earth fault path taken during a fault situation.
The resistance (or impedance) should be mimimal so that enough fault current can flow very quickly to earth, thus operating the protective device, disconnecting the supply and making the faulty part of the installation safe.
Circuits protected by C type breakers must have a maximum earth fault loop impedance half that of B types. Similarly, D type protected circuits (rarely used outside industry or medical environments) must have a maximum EFLI half that of C types.
Pah and double pah! Stop to think about my reply & you's lot beat me to it!
This will help for starters.
Basically, the different types of CB are designed for different jobs.
The higher the designation, the more tolerant they are of surge or start-up currents, but the lower the maximum earth fault loop impedance that circuit must have.
This is the resistance of the (L-E) earth fault path taken during a fault situation.
The resistance (or impedance) should be mimimal so that enough fault current can flow very quickly to earth, thus operating the protective device, disconnecting the supply and making the faulty part of the installation safe.
Circuits protected by C type breakers must have a maximum earth fault loop impedance half that of B types. Similarly, D type protected circuits (rarely used outside industry or medical environments) must have a maximum EFLI half that of C types.
Pah and double pah! Stop to think about my reply & you's lot beat me to it!
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