Adiabatic calculation confirmation

[DanBrown]

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Could anyone just confirm my results for adiquate earthing size and calculation using the adiabatic equation...

My test was on a ring main circuit wired in 2.5/1.5 pvc..
My supply voltage is 247v
My Impedance is 0.38 ohms.
The circuit is supplied via a RCBO 32a C type

My results suggest that a min CSA earth is 1.78mm2 and my cable CPC is only 1.5mm2.

Could someone just do a quick confimation of my answer please...
Is it a problem as such or not?? (still learning testing)

 

[Jono Pashley]

assuming the rcbo is going to operate in 10ms then i get the minimum CPC size of 0.57mm2 dont have big red book with me to cross reference breaker trip times. Fault Current = 247/0.38=650A so Sa = the Square root of (650 squared X 0.01) then divided by 115 = 0.565 edit. just checked my design calcs books type C 32A minimum cpc size of 1.5 so it should be fine.

 

[kermit1202]

as you have a ring you have 2 cpc's in parraral making your CSA of the CPC approx 3mm2.

 

[super spark]

Firstly I am unsure why an adiabatic calculation is required. Is the floor area covered above 100m2 ?

The voltage used must always be 230v , all the equations in BS7671 are based on this voltage. also voltages will fluctuate dependent on demand etc...

need a bit more details, I deal more with design and understand why I would need to use the adiabatic equation, but I do not see why you are using it here.

perhaps I am missing something???