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Cable Protection

Cable Protection. Matching Protection Devices to a Cable. 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. Definitions. Short Circuit:. Very high current.

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Cable Protection

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  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 Value dependant on impedance of cables Bobby pin in power point 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 If the temperature goes too high the insulation will be damaged See AS/NZS 3008.1.1:1998 Table 52 V-75 = 160˚C Scenario 1: If a 2.5 mm2 copper cable carries a current of 40A. How long will it take to reach 160˚C?

  6. If a 2.5 mm2 copper cable carries a current of 40A. How long will it take to reach 160˚C? See AS/NZS 3008.1.1:1998 5.3 Initial temperature of cable = 75 ˚C And AS/NZS 3000:2007 2.5.4.5 K S I t = Constant = CSA of cable in mm2 = Current = Time See AS/NZS 3008.1.1:1998 Table 51 K = 111 t = 48 seconds Therefore: The circuit breaker must trip in less than 48 seconds to protect the cable

  7. An Easier Way A cable’s current carrying capacity must be equal or larger than the load current Circuit protection must be equal to or smaller than the cable’s current carrying capacity See AS/NZS 3000:2007 2.5.3.1 IB ≤ IN ≤ IZ Load Current ≤ Protection ≤ Cable Current carrying capacity

  8. 20 Amps Cable will be damaged X 15 Amps 10 Amps IZ IB IN Maximum Current Cable can supply Current Rating of Protection Load Current

  9. 20 Amps Cable will be damaged X 15 Amps Protection will nuisance trip 10 Amps IZ IB IN Maximum Current Cable can supply Current Rating of Protection Load Current

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

  11. Circuit Breakers A Circuit breaker’s trip curve is matched to a cable’s curve Tripping current is 1.45 x rated current 14.5 Amps 10 Amps IZ IN 14.5 Amps Maximum Current Cable can supply Current Rating of Protection

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

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

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

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