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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. - PowerPoint PPT Presentation
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Cable ProtectionCable ProtectionMatching Protection Devices to a CableMatching Protection Devices to a Cable
DefinitionsDefinitionsWhen current flows heat is generated
The longer the current flows the greater the heat build up
RtIH 2H
I
R
t
= Heat
= Current
= Resistance
= Time
DefinitionsDefinitionsShort Circuit: Very high current
Bobby pin in power point
Value dependant on impedance of cables
Cure: Magnetic section of Circuit Breaker
DefinitionsDefinitionsOverload: Higher than normal current
Too many appliances plugged in
Cure: Thermal section of Circuit Breaker
Cable CharacteristicsCable CharacteristicsIf a cable carries high currents it will heat up
See AS/NZS 3008.1.1:1998 Table 52See 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 mm2 copper cable carries a current of 40A. How long will it take to reach 160˚C?
Scenario 1:
Initial temperature of cable = 75 ˚C
2
22
I
SKt
See AS/NZS 3008.1.1:1998 Table 51See AS/NZS 3008.1.1:1998 Table 51
K = 111
If a 2.5 mm2 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 5.3See AS/NZS 3008.1.1:1998 5.3
2
22
40
5.2111 t
The circuit breaker must trip in less than 48 seconds to protect the cable
Therefore:
And AS/NZS 3000:2007 2.5.4.5And AS/NZS 3000:2007 2.5.4.5
K
S
I
t
= Constant
= CSA of cable in mm2
= Current
= Time
An Easier WayAn Easier WayA cable’s current carrying capacity must be equal or larger than
the load current
See AS/NZS 3000:2007 2.5.3.1See AS/NZS 3000:2007 2.5.3.1
Load Current
Circuit protection must be equal to or smaller than the cable’s current carrying capacity
IB ≤ IN ≤ IZ
≤ Protection ≤ Cable Current carrying capacity
Load Current
10 Amps
Maximum Current
Cable can supply
CurrentRating of
Protection
15 Amps
20 Amps
XCable will be
damaged
IB IN IZ
Load Current
10 Amps
Maximum Current
Cable can supply
CurrentRating of
Protection
15 Amps
20 Amps
XCable will be
damaged
IB IN IZ
Protection will nuisance trip
HoweverHoweverA cable can withstand a overload current of 1.45 x it’s rating
before the insulation is damaged
10 Amps
Maximum Current
Cable can supply
IZ
A1045.1 14.5 Amps
14.5 Amps
A Circuit breaker’s trip curve is matched to a cable’s curve
10 Amps
Maximum Current
Cable can supply
IZ A1045.1 14.5 Amps
14.5 Amps
Tripping current is 1.45 x rated current
CurrentRating of
Protection
IN
Circuit BreakersCircuit Breakers
A Fuse’s trip curve is different to a cable’s curve
10 Amps
Maximum Current
Cable can supply
IZ A106.1 16 Amps
14.5 Amps
CurrentRating of
Protection
IN
FusesFuses
Tripping current is 1.6 x rated current16 Amps
Cable will be damaged
X
10 Amps
Maximum Current
Cable can supply
IZ
6.1
5.149 Amps
14.5 Amps
CurrentRating of
Protection
IN
FusesFuses16 Amps
1.6
10 Amps
Maximum Current
Cable can supply
IZ 9.010 9 Amps
14.5 Amps
CurrentRating of
Protection
IN
FusesFuses16 Amps
0.9
