Need Help Learning to Compute Adiabatic Method

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How would I go about calculating it for any given circuit?

Lets say I had a 60 amp feeder, 13.30mm2 phase and 5.261mm2 earth. How long could I run the circuit before the 90*C insulation on the earth wire overheats for a fault? At what point would I need to up size my earth wire?

 

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Above 10ka the wire overheats even if the breaker opens on its magnetic trip? In essence BS7671 addresses this?

 

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So you can use the tabulated values from, for example, UK regs provided the type of conductor and insulation are comparable. Most likely they are as Cu & PVC or XLPE is pretty much common use world-wide.

Sounds doable.

My starting point is either 60*C or 75*C. In the US PVC is rated for 90*C.

 

[pc1966]

Above 10ka the wire overheats even if the breaker opens on its magnetic trip? In essence BS7671 addresses this?

In this case above 10kA and the breaker might explode. That is the MCB's max interrupt capacity.

But for MCCB, etc, you might well reach the 2nd point where it exceeds the cable limits at both very high and meduim-low PFC values.
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My starting point is either 60*C or 75*C. In the US PVC is rated for 90*C.

The max running temp here is also usually 70C or 90C, but that is for prolonged use. You hope that a fault is only a couple of seconds long at most, hence the higher value is tolerated for this method.

 

[pc1966]

If you assume 60A = "1" on the breaker you point to, then the time for 1 & 100 current are:
1 => t = 276.8E3 / (1 * 60)**2 = 77 seconds
100 => t = 276.8E3 / (100 * 60)**2 = 0.008 seconds
So the line of constant I2t looks like:

And is not very satisfactory. It might be safe for PFC values above 3.5*60 = 210A where disconnection might take less than 6 seconds, but really it is only safe for PFC above 20*60 = 1200A when it is definitely in the instantaneous magnetic region and disconnection is under 30ms. But then by around 70*60A 4200A it is re-entering the trip curve so it might fail the adiabatic limits at high PFCs as well!

But really you should be looking for a let-through plot for breakers as the fusing time-current plots are ambiguous in that region.

Assuming your minimum voltage is 110V for now, then your max end-of-cable Zs would be 110 / 1200 = 0.09 ohms, and if you know the supply Ze then you can work out the max R1+R2 to meet Zs, and from the resistance per unit length, therefor the max length.

 

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Thanks. Its just so confusing because Table 250.122 does not specify any limits, exceptions or criteria other than breaker size.