Miniature circuit breaker working principle
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Miniature circuit breaker working principle

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Miniature circuit breaker working principle Miniature circuit breaker working principle Document Transcript

  • Miniature Circuit Breaker Working PrincipleThere are two arrangement of operation of miniature circuit breaker. One due tothermal effect of over current and other due to electromagnetic effect of over current.The thermal operation of miniature circuit breaker is achieved with a bimetallic stripwhenever continuous over current flows through MCB, the bimetallic strip is heatedand deflects by bending. This deflection of bimetallic strip releases mechanical latch.As this mechanical latch is attached with operating mechanism, it causes to open theminiature circuit breaker contacts. But during short circuit condition, sudden risingof electric current, causes electromechanical displacement of plunger associated withtripping coil or solenoid of MCB. The plunger strikes the trip lever causing immediaterelease of latch mechanism consequently open the circuit breaker contacts. This wasa simple explanation of miniature circuit breaker working principle.The MCB has some advantages compared to fuse.1. It automatically switches off the electrical circuit during abnormal condition of thenetwork means in over load condition as well as faulty condition. The fuse does notsense but Miniature Circuit Breaker does it in more reliable way. MCB is much moresensitive to over current than fuse.2. Another advantage is, as the switch operating knob comes at its off position duringtripping, the faulty zone of the electrical circuit can easily be identified. But in case offuse, fuse wire should be checked by opening fuse grip or cutout from fuse base, forconfirming the blow of fuse wire.3. Quick restoration of supply can not be possible in case of fuse as because fuseshave to be rewirable or replaced for restoring the supply. But in the case of MCB,quick restoration is possible by just switching on operation.4. Handling MCB is more electrically safe than fuse.Because of to many advantages of MCB over fuse units, in modern low voltageelectrical network, Miniature Circuit Breaker is mostly used instead of backdated fuseunit.
  • Only one disadvantage of MCB over fuse is that this system is more costlier than fuseunit system.Operating Mechanism of Miniature Circuit BreakerThe Operating Mechanism of Miniature Circuit Breaker provides the means of manualopening and closing operation of miniature circuit breaker. It has three-positions “ON,”“OFF,” and “TRIPPED”. The external switching latch can be in the “TRIPPED”position, if the MCB is tripped due to over-current. When manually switch off the MCB,the switching latch will be in “OFF” position. In close condition of MCB, the switch ispositioned at “ON”. By observing the positions of the switching latch one candetermine the condition of MCB whether it is closed, tripped or manually switched off.What is the difference between MCB, MCCB, ELCB, and RCCBPosted OCT 25 2011 by JIGUPARMAR in ENERGY AND POWER with 10 COMMENTSTranslate »Get PDF »MCB (Miniature Circuit Breaker)
  • CHARACTERISTICSRated current not more than 100 A.Trip characteristics normally not adjustable.Thermal or thermal-magnetic operation.TopMCCB (Moulded Case Circuit Breaker)CHARACTERISTICSRated current up to 1000 A.Trip current may be adjustable.Thermal or thermal-magnetic operation.RCCBPhase (line) and Neutral both wires connected through RCD.It trips the circuit when there is earth fault current.The amount of current flows through the phase (line) should return through neutral .It detects by RCD. any mismatch between two currents flowing through phase and neutral detectby -RCD and trip the circuit within 30Miliseconed.If a house has an earth system connected to an earth rod and not the main incoming cable, thenit must have all circuits protected by an RCD (because u mite not be able to get enough faultcurrent to trip a MCB)RCDs are an extremely effective form of shock protection
  • The most widely used are 30 mA (milliamp) and 100 mA devices. A current flow of 30 mA (or 0.03 amps)is sufficiently small that it makes it very difficult to receive a dangerous shock. Even 100 mA is a relativelysmall figure when compared to the current that may flow in an earth fault without such protection (hundredof amps)A 300/500 mA RCCB may be used where only fire protection is required. eg., on lighting circuits, wherethe risk of electric shock is small.TopLimitation of RCCBStandard electromechanical RCCBs are designed to operate on normalsupply waveformsand cannot be guaranteed to operate where none standard waveforms aregenerated by loads. The most common is the half wave rectified waveform sometimes calledpulsating dc generated by speed control devices, semi conductors, computers and evendimmers.Specially modified RCCBs are available which will operate on normal ac and pulsating dc.RCDs don’t offer protection against current overloads: RCDs detect an imbalance in the liveand neutral currents. A current overload, however large, cannot be detected. It is a frequentcause of problems with novices to replace an MCB in a fuse box with an RCD. This may be donein an attempt to increase shock protection. If a live-neutral fault occurs (a short circuit, or anoverload), the RCD won’t trip, and may be damaged. In practice, the main MCB for the premiseswill probably trip, or the service fuse, so the situation is unlikely to lead to catastrophe; but it maybe inconvenient.It is now possible to get an MCB and and RCD in a single unit, called an RCBO (see below).Replacing an MCB with an RCBO of the same rating is generally safe.Nuisance tripping of RCCB: Sudden changes in electrical load can cause a small, brief currentflow to earth, especially in old appliances. RCDs are very sensitive and operate very quickly;they may well trip when the motor of an old freezer switches off. Some equipment is notoriously`leaky’, that is, generate a small, constant current flow to earth. Some types of computerequipment, and large television sets, are widely reported to cause problems.RCD will not protect against a socket outlet being wired with its live and neutralterminalsthe wrong way round.RCD will not protect against the overheating that results when conductors are not properlyscrewed into their terminals.RCD will not protect against live-neutral shocks, because the current in the live and neutralis balanced. So if you touch live and neutral conductors at the same time (e.g., both terminals ofa light fitting), you may still get a nasty shock.
  • What is the difference between MCB, MCCB, ELCB, and RCCBPosted OCT 25 2011 by JIGUPARMAR in ENERGY AND POWER with 10 COMMENTSTranslate »Get PDF »MCB (Miniature Circuit Breaker)CHARACTERISTICSRated current not more than 100 A.Trip characteristics normally not adjustable.Thermal or thermal-magnetic operation.TopMCCB (Moulded Case Circuit Breaker)
  • CHARACTERISTICSRated current up to 1000 A.Trip current may be adjustable.Thermal or thermal-magnetic operation.TopAir Circuit BreakerCHARACTERISTICSRated current up to 10,000 A.Trip characteristics often fully adjustable including configurable trip thresholds and delays.Usually electronically controlled—some models are microprocessor controlled.Often used for main power distribution in large industrial plant, where the breakers are arrangedin draw-out enclosures for ease of maintenance.TopVacuum Circuit BreakerCHARACTERISTICSWith rated current up to 3000 A,These breakers interrupt the arc in a vacuum bottle.These can also be applied at up to 35,000 V. Vacuum circuit breakers tend to have longer lifeexpectancies between overhaul than do air circuit breakers.Top
  • RCD (Residual Current Device / RCCB(Residual CurrentCircuit Breaker)CHARACTERISTICSPhase (line) and Neutral both wires connected through RCD.It trips the circuit when there is earth fault current.The amount of current flows through the phase (line) should return through neutral .It detects by RCD. any mismatch between two currents flowing through phase and neutral detectby -RCD and trip the circuit within 30Miliseconed.If a house has an earth system connected to an earth rod and not the main incoming cable, thenit must have all circuits protected by an RCD (because u mite not be able to get enough faultcurrent to trip a MCB)RCDs are an extremely effective form of shock protectionThe most widely used are 30 mA (milliamp) and 100 mA devices. A current flow of 30 mA (or 0.03 amps)is sufficiently small that it makes it very difficult to receive a dangerous shock. Even 100 mA is a relativelysmall figure when compared to the current that may flow in an earth fault without such protection (hundredof amps)A 300/500 mA RCCB may be used where only fire protection is required. eg., on lighting circuits, wherethe risk of electric shock is small.TopLimitation of RCCBStandard electromechanical RCCBs are designed to operate on normalsupply waveformsand cannot be guaranteed to operate where none standard waveforms aregenerated by loads. The most common is the half wave rectified waveform sometimes calledpulsating dc generated by speed control devices, semi conductors, computers and evendimmers.Specially modified RCCBs are available which will operate on normal ac and pulsating dc.
  • RCDs don’t offer protection against current overloads: RCDs detect an imbalance in the liveand neutral currents. A current overload, however large, cannot be detected. It is a frequentcause of problems with novices to replace an MCB in a fuse box with an RCD. This may be donein an attempt to increase shock protection. If a live-neutral fault occurs (a short circuit, or anoverload), the RCD won’t trip, and may be damaged. In practice, the main MCB for the premiseswill probably trip, or the service fuse, so the situation is unlikely to lead to catastrophe; but it maybe inconvenient.It is now possible to get an MCB and and RCD in a single unit, called an RCBO (see below).Replacing an MCB with an RCBO of the same rating is generally safe.Nuisance tripping of RCCB: Sudden changes in electrical load can cause a small, brief currentflow to earth, especially in old appliances. RCDs are very sensitive and operate very quickly;they may well trip when the motor of an old freezer switches off. Some equipment is notoriously`leaky’, that is, generate a small, constant current flow to earth. Some types of computerequipment, and large television sets, are widely reported to cause problems.RCD will not protect against a socket outlet being wired with its live and neutralterminalsthe wrong way round.RCD will not protect against the overheating that results when conductors are not properlyscrewed into their terminals.RCD will not protect against live-neutral shocks, because the current in the live and neutralis balanced. So if you touch live and neutral conductors at the same time (e.g., both terminals ofa light fitting), you may still get a nasty shock.TopELCB (Earth Leakage Circuit Breaker)CHARACTERISTICSPhase (line), Neutral and Earth wire connected through ELCB.
  • ELCB is working based on Earth leakage current.Operating Time of ELCB:The safest limit of Current which Human Body can withstand is 30ma sec.Suppose Human Body Resistance is 500Ω and Voltage to ground is 230 Volt.The Body current will be 500/230=460mA.Hence ELCB must be operated in 30maSec/460mA = 0.65msec