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adiabatic equation - 2.5 2c SWA

Stewart Mason
I don't get involved with adiabatic equations all to often, hence I'm a bit rusty! I'm trying to calculate the R1+R2 for a 2 core 2.5mm SWA. Obviously the R1 is the easy bit, I'm struggling with the SWA as the R2.  I saw a link to a table on the old forum but I can't find it now. Any ideas?


Thanks Stewart
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    Chris Pearson:

    Graham, now you are confusing me.


    I can see the point of manufacturing gear to cope with 6, 10, 16 kVA on the basis that the installation may be literally next door to the transformer (so far I have resisted the temptation to knock on the door of our neighbour across the road with my MFT at the ready) but I am not sure why you mention the maximum (PME) external loop impedance.


    The adiabatic tells me that 4 sqmm CPCs will suffice for all circuits given a Ze of 0.2 Ω - I cannot be more than 1150 A. If Ze is anywhere up to 0.35 Ω, the equation holds good.


    However, I have assumed that Ze is 0.35 Ω when it comes to the ends of circuits because I need to be satisfied that Zs will be low enough to satisfy the requirements of ADS. If Ze is lower, the conditions are still met.


    If I have that wrong, please let me know whilst I still have time to change the design.




    We have to use the worst-case conditions for the situation we are looking at.


    • For protection against electric shock, this is the maximum loop impedance (and Ze of 0.35 Ω)

    • For protection against overcurrent, this is not the case, and we need to look at the maximum prospective fault current ... both at the start of the circuit and at the end of it.


    This is well explained in the guidance I have quoted.
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  • 0

    Chris Pearson:

    Graham, now you are confusing me.


    I can see the point of manufacturing gear to cope with 6, 10, 16 kVA on the basis that the installation may be literally next door to the transformer (so far I have resisted the temptation to knock on the door of our neighbour across the road with my MFT at the ready) but I am not sure why you mention the maximum (PME) external loop impedance.


    The adiabatic tells me that 4 sqmm CPCs will suffice for all circuits given a Ze of 0.2 Ω - I cannot be more than 1150 A. If Ze is anywhere up to 0.35 Ω, the equation holds good.


    However, I have assumed that Ze is 0.35 Ω when it comes to the ends of circuits because I need to be satisfied that Zs will be low enough to satisfy the requirements of ADS. If Ze is lower, the conditions are still met.


    If I have that wrong, please let me know whilst I still have time to change the design.




    We have to use the worst-case conditions for the situation we are looking at.


    • For protection against electric shock, this is the maximum loop impedance (and Ze of 0.35 Ω)

    • For protection against overcurrent, this is not the case, and we need to look at the maximum prospective fault current ... both at the start of the circuit and at the end of it.


    This is well explained in the guidance I have quoted.
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