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Cable Protection Matching Protection Devices to a Cable

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Definitions When current flows heat is generated The longer the current flows the greater the heat build up H I R t = Heat = Current = Resistance = Time

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Definitions Short Circuit: Very high current Bobby pin in power point Value dependant on impedance of cables Cure: Magnetic section of Circuit Breaker

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Definitions Overload: Higher than normal current Too many appliances plugged in Cure: Thermal section of Circuit Breaker

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Cable Characteristics If a cable carries high currents it will heat up See AS/NZS 3008.1.1:1998 Table 52 V-75 = 160˚C If the temperature goes too high the insulation will be damaged If a 2.5 mm 2 copper cable carries a current of 40A. How long will it take to reach 160˚C? Scenario 1:

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Initial temperature of cable = 75 ˚C See AS/NZS 3008.1.1:1998 Table 51 K = 111 If a 2.5 mm 2 copper cable carries a current of 40A. How long will it take to reach 160˚C? t = 48 seconds See AS/NZS 3008.1.1:1998 5.3 The circuit breaker must trip in less than 48 seconds to protect the cable Therefore: And AS/NZS 3000:2007 2.5.4.5 K S I t = Constant = CSA of cable in mm 2 = Current = Time

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An Easier Way A cables current carrying capacity must be equal or larger than the load current See AS/NZS 3000:2007 2.5.3.1 Load Current Circuit protection must be equal to or smaller than the cables current carrying capacity I B I N I Z Protection Cable Current carrying capacity

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Load Current 10 Amps Maximum Current Cable can supply Current Rating of Protection 15 Amps 20 Amps X Cable will be damaged I B IN IN IZ IZ

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Load Current 10 Amps Maximum Current Cable can supply Current Rating of Protection 15 Amps 20 Amps X Cable will be damaged IB IB IN IN IZ IZ Protection will nuisance trip

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However A cable can withstand a overload current of 1.45 x its rating before the insulation is damaged 10 Amps Maximum Current Cable can supply IZ IZ 14.5 Amps

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A Circuit breakers trip curve is matched to a cables curve 10 Amps Maximum Current Cable can supply IZ IZ 14.5 Amps Tripping current is 1.45 x rated current Current Rating of Protection IN IN Circuit Breakers

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A Fuses trip curve is different to a cables curve 10 Amps Maximum Current Cable can supply IZ IZ 16 Amps 14.5 Amps Current Rating of Protection IN IN Fuses Tripping current is 1.6 x rated current 16 Amps Cable will be damaged X

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10 Amps Maximum Current Cable can supply IZ IZ 9 Amps 14.5 Amps Current Rating of Protection IN IN Fuses 16 Amps 1.6

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10 Amps Maximum Current Cable can supply IZ IZ 9 Amps 14.5 Amps Current Rating of Protection IN IN Fuses 16 Amps 0.9

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