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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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    Legh Richardson:


    Here's a R1+R2 table used to calculate the earth loop impedance using SWA armour for R2, (Zs), You can then calculate the prospective short circuit current and hence the adiabatic calculation.


    Legh




    Hang on - this isn't the correct method.


    (a) There's no need to calculate Zs for adiabatic for R2 even for fuses. You can simply plot k2S2 on the time-current rating curves. And this is important - see below.


    (b) If you're using a circuit breaker (mcb, RCBO, mccb) you cannot use this method, you must use the let-through energy from either the product standard, or let-through energy provided by the manufacturer, and simply compare against k2S2 - again, no need to calculate the fault current.


    The absolute simplest way for most final circuits with SWA, is to look it up in GN1.


    So, why did I say it's not the correct method?


    Well, the worst-case condition for electric shock is indeed at the remote end of the cable - the furthest point in the circuit.


    This is NOT the case for protection against overcurrent - the adiabatic criterion. The worst-case is often further up the cable, at a point where, if you choose too small a CSA, the line corresponding to k2S2 meets the overcurrent protective device curve, i.e. at higher fault currents / shorter circuit length.


    See Chapter 8 of Electrical Installation Design Guide for a complete explanation.

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  • 0

    Legh Richardson:


    Here's a R1+R2 table used to calculate the earth loop impedance using SWA armour for R2, (Zs), You can then calculate the prospective short circuit current and hence the adiabatic calculation.


    Legh




    Hang on - this isn't the correct method.


    (a) There's no need to calculate Zs for adiabatic for R2 even for fuses. You can simply plot k2S2 on the time-current rating curves. And this is important - see below.


    (b) If you're using a circuit breaker (mcb, RCBO, mccb) you cannot use this method, you must use the let-through energy from either the product standard, or let-through energy provided by the manufacturer, and simply compare against k2S2 - again, no need to calculate the fault current.


    The absolute simplest way for most final circuits with SWA, is to look it up in GN1.


    So, why did I say it's not the correct method?


    Well, the worst-case condition for electric shock is indeed at the remote end of the cable - the furthest point in the circuit.


    This is NOT the case for protection against overcurrent - the adiabatic criterion. The worst-case is often further up the cable, at a point where, if you choose too small a CSA, the line corresponding to k2S2 meets the overcurrent protective device curve, i.e. at higher fault currents / shorter circuit length.


    See Chapter 8 of Electrical Installation Design Guide for a complete explanation.

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