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Cable Protection Matching Protection Devices to a Cable.

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Presentation on theme: "Cable Protection Matching Protection Devices to a Cable."— Presentation transcript:

1 Cable Protection Matching Protection Devices to a Cable

2 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

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

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

5 Cable Characteristics If a cable carries high currents it will heat up See AS/NZS :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:

6 Initial temperature of cable = 75 ˚C See AS/NZS :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 : The circuit breaker must trip in less than 48 seconds to protect the cable Therefore: And AS/NZS 3000: K S I t = Constant = CSA of cable in mm 2 = Current = Time

7 An Easier Way A cables current carrying capacity must be equal or larger than the load current See AS/NZS 3000: 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

8 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

9 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

10 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

11 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

12 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

13 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

14 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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