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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 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 Value dependant on impedance of cables Bobby pin in power point Cure: Magnetic section of Circuit Breaker
Definitions Overload: Higher than normal current Too many appliances plugged in Cure: Thermal section of Circuit Breaker
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?
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
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
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
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
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
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
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
Fuses 16 Amps 14.5 Amps 1.6 10 Amps IZ IN 9 Amps Maximum Current Cable can supply Current Rating of Protection
Fuses 16 Amps 14.5 Amps 10 Amps 0.9 IZ IN 9 Amps Maximum Current Cable can supply Current Rating of Protection