Adiabatic Equation

[steve100]

I am trying to get my head around the Adiabatic equation and was hoping that someone could check my understanding?

I know the equation is S = √I² x t/k

S = the CSA of the CPC
I = the PFC which is calculated from Uo/Zs
t = now this is where I get a little confused, my understanding is that you use the PFC to find the disconnection time from the time curve tables in BS7671 and this is used in the equation, is this correct? I also read somewhere that the PFC must be higher than the current required to disconnect the protective device, is this correct? Is this part of the equation process, as I thought this was established through meeting the required Zs values in the regs?
k = is taken from the tables in the regs

I would really appreciate it if someone would help me and maybe give me an example?

Thanks
Steve

 

[topquark]

Have a read of this: http://www.electriciansforums.net/i...ons/28128-example-use-adiabatic-equation.html

 

[Guest123]

We have a long thread on this very topic. Just type 'adiabatic' into the search box.

 

[malcolmsanford]

Yes that is how you calculate your CPC size.

Your PFC is the value you use. You have to remember that fuses will have various values for various times upto 5 seconds, but MCBs will have just the one value upto 5 secs.

So if you had a Ze/Zs of 0.56 ohms it would be 230/0.56 = 410 amps. If you chose say a 25amp BS 88-2 Fuse type E or G for your protection device for a final circuit you will see that the device will trip at 170 amps for 0.4 secs and 220amp for 0.1 sec as you fault current will be 410 then your t value would then be 0.1

 

[steveberry11]

You need to find your PFC and square it when you have this go to the BGB and find the time current curve for your protective device
Using the chart work out t
K is normally 115 for copper
dont forget to do the square root part first prior to finishing the calculation.

 

[steve100]

Thanks, this makes sense now.

Can i just ask one more thing, I also read somewhere that the PFC must be higher than the current required to disconnect the protective device, is this correct? Is this part of the equation process, as I thought this was established through meeting the required Zs values in the regs?

Steve


Read more: http://www.electriciansforums.net/e...s/59695-adiabatic-equation.html#ixzz1zYfeM029
Can I ask just one more thing,

 

[Richard Burns]

Your PFC would normally be much higher than the required current to cause disconnection in the time required.
However you are correct that this is determined by the measurement of Zs, and it is possible that for a supply using a high current OCPD and with a high Ze that you would be unable to meet the requirements.

PFC is more usually measured to ensure that it is not too high for the maximum breaking capacity of the device.