Let Through Energy

That may be a measured value rather than a calculated value.

Try reading the manufacturers literature. Commonly referred to as RTFM

I agree with you, except the fact , in my opinion, according to BS it has to be RTBM [ instead of RTFM in USA].

However, a Swiss/Switzerland explanation may be taken into consideration. See what ABB try to explain:

https://library.e.abb.com/public/9567591b95e3345b48257b05001b78d8/Protection%20of%20LV%20Cables.pdf

I've spent too long on here.

I think that "Let trough energy" of a cable would only apply if you think of the cable as a fuse...

I think the first step is to learn what you are talking about.

JRaef #3 is correct..."Let through energy" applies only to a protective device.."Energy withstand" applies to a cable.

The protective device manufacturer will supply a current/time curve which can be used to calculate the maximum allowable temperature rise of the cable under fault conditions.

This maximum temperature is its temperature before the fault and its maximum allowable insulation temperature

The temp. rise is proportional to the square of the fault current times the duration of the flow (I²t).

Obviously the energy let through of the device needs to be no more, and preferably less, than the energy withstand of the cable in use.

Cable manufacturers also provide a current/time curve for each of their cables.

The two curves can be used to determine the acceptable energy withstand for your cable selection.

Mr.Spade thanks for detail,

Meanwhile What u explained is nice informative

and i am asking is how to calculte Let through energy of circuit breaker and cables,

So that theoritically we can ensure the design.

The protective device will have a current/time curve applicable to that particular device. Generally only the 5 second operating time at the maximum available fault current would be considered. This maximum available current would take into consideration factors such as the supply energy availability, the impedance of the upstream circuit conductors etc.

You can then either read the operating time for said device straight from the curve and multiply it by the square of the corresponding current to get the let through energy and then compare that figure to the cable manufacturer's published energy withstand figures.

Or...You can plot both the cable withstand time/current curve and the device time/current curves on the same graph, choose an operating time, divide the I²t by that figure and then take the square root of that answer to arrive at the current related to that chosen time.

The let through energy MUST NEVER be greater than the energy withstand value of the cable under the particular conditions of installation involved. A cable protected by a fuse must be de-rated to 90% of that if protected by a circuit breaker.

I suggest you do some further research on the subject to gain a better understanding.

Er......"can be found in British Standard 7671" - is that the answer?

No, you need the CB manufacturer's data and then use the adiabatic equation for the cable. Nether of which are in BS7671, it skirts very neatly around the matter.

Regulation 434.5.2 of BS7671 Amnd 3 : 2015 does give maximum disconnection time as t = k²S²/I² , S is sq.mm, I amps and k values in Table 43.1. That's the adiabatic equation. And it does have lots of amp/time curves for standard fuses & MCBs.