Regulation 434.5.2 - Time a conductor can withstand fault current

[Gigsy]

Can anyone help me clarify the formula in regulation 434.5.2, the time a conductor can withstand a fault current without sustaining damage.

t=k2 x S2 / I2

The regulation states I is the effective fault current.

Does effective fault current mean the prospective short circuit current/ prospective earth fault current or does it mean the maximum operating time / current characteristics (In x 5 for a type B MCB)

 

[Pretty Mouth]

PSCC or PEFC, depending on which conductor you're calculating for

 

[Ian1981]

The formula relates to the live conductors only so pefc is not applicable as that formula for csa of a protective conductor is transposed to S= I2t square root over K

In the formula t=K2S2/I2 then t is the maximum time in seconds that the live conductors can withstand the fault current until damage to the conductors and the surrounding insulation

 

[Gigsy]

So you are saying S is the PSSC

Not the current at which the breaker / fuse will break the current.

 

[Ian1981]

S is the size of the conductor required to satisfy the equation I2t square root /K
I is the fault current squared, K is the conductor material generally 115 for copper conductors bunched or within a cable or 143 for not bunched or incorporated in a cable.
K can be different values depending on the material of the protective conductor ie 52 or maybe 51 for the armouring of an swa cable

 

[Pretty Mouth]

The formula relates to the live conductors only so pefc is not applicable as that formula for csa of a protective conductor is transposed to S= I2t square root over K

Hi Ian, the 1st sentence of the reg says it's to prevent overheating of any conductor or cable, so it must include the CPC. Also, PEFC may be higher than PSSC but would still affect the line conductor.

 

[Ian1981]

Hi Ian, the 1st sentence of the reg says it's to prevent overheating of any conductor or cable, so it must include the CPC. Also, PEFC may be higher than PSSC but would still affect the line conductor.

Regulation 434.5.2 says a given fault current that will raise the live conductors from the highest permissible temperature in normal duty to the limiting temperature can be calculated from the formula t=K2S2/I2
No mention of the cpc other than The I2t value for a fault below 0.1 seconds must be equal to or less than the energy withstand of the conductors.
The above sentence of regulation 434.5.2 states that any given fault that shall not raise the temperature of any conductor so includes the cpc but that is a given for basic protection of the conductors for the characteristics for fault protection by a protective device. the below formula is for live conductors (t=K2S2/I2) as stated in the regulation
For a protective conductor it is the adiabatic equation as given in 543.1.3

 

[Pretty Mouth]

Regulation 434.5.2 says a given fault current that will raise the live conductors from the highest permissible temperature in normal duty to the limiting temperature can be calculated from the formula t=K2S2/I2
No mention of the cpc other than The I2t value for a fault below 0.1 seconds must be equal to or less than the energy withstand of the conductors.
The above sentence of regulation 434.5.2 states that any given fault that shall not raise the temperature of any conductor so includes the cpc but that is a given for basic protection of the conductors for the characteristics for fault protection by a protective device. the below formula is for live conductors (t=K2S2/I2) as stated in the regulation
For a protective conductor it is the adiabatic equation as given in 543.1.3

It does indeed say that, thanks for the explanation.