Let through energy - Cable sizing

[NickParker]

When the cables have been already verified using adiabatic equation (case 1), Is it still necessary to check the cables for let through energy (case 2)? Though both the equations are same (just rearrangement) but the I2t value comes from the manufacturer in case 2.

Case 1:
The minimum cross section area of the conductor is found by the equation,
S=(𝐼√𝑡)/𝑘
I - Available fault current
t - time to trip the breaker
k - 143 for XLPE insulated cable

Case 2:
The selected cross section area of the conductor is verified by using the manufacturer published let through curves which is compared against the cable let through curve.
𝐼2𝑡 ≤𝑘2𝑆2
I2t - from the manufacturer published let through curve
k2S2 - Calculated cable let through curves
K = 143 for XLPE insulated cable
S= Cross section area of the selected cable

Also I hear that Case 2 is used in case of current limiting breakers / fuses. If so why?

 

[7anoter4]

It is not the cable you have to check but the breaker. See BS7671 reg.434.5.

 

[NickParker]

Is the let thru energy verification is applied only for current limiting breakers / fuses? Why not for the non current limiting breakers?