adiabatic equation

[lofty84]

so you use the adiabatic equation to work out the cross sectional area of a conductor

the fomula is obviously S=(sq root) of I2xt/K

can someone give me some numbers so I can work out the answer useing this equation as I want to make sure i know how to do it.

 

[Peter Stanley]

I squared - is the PSC value u can calculate this 230v divided by Ze or look at the PSC graphs in the reg book
t - is the time generally 0.4 sec
K - is the value given from table 54 bs 7671 for copper at 70 degree C is 115

 

[IQ Electrical]

so you use the adiabatic equation to work out the cross sectional area of a conductor

the fomula is obviously S=(sq root) of I2xt/K

can someone give me some numbers so I can work out the answer useing this equation as I want to make sure i know how to do it.

Hi 'Lofty' Have a read of this excellent thread with worked examples: http://www.electriciansforums.net/i...ons/28128-example-use-adiabatic-equation.html

 

[i=p/u]

example.... 80A 1361 fuse has 1100A ticked agasinst it to disconnect in 0.1 secs so square root of 1100 x 1100 x 0.1 /115 = 3mm earthing conductor

 

[telectrix]

the value of k is 115 for copper if the conductor is part of a multicore cable or bunched with other cables. in the case of an earthing conductor, on it's own, the value is 143.

 

[i=p/u]

<<<<<<<<<< traineee with load sof infor and no more filein cabinets to store it

 

[lofty84]

thanks alot guys

 

[Deleted member 26818]

If you are using the tables in Appendix 3, remember to use the current values in the table, rather than the measured value.

 

[telectrix]

If you are using the tables in Appendix 3, remember to use the current values in the table, rather than the measured value.

why is that, spin? i generally use the actual pfc. tell me why i'm wrong.

 

[Deleted member 26818]

The tables show the values of current, which will achieve the corresponding disconnection times.
Measured values will achieve different disconnection times.
The greater the current, the faster the disconnection time.
If your measured value is not shown with a corresponding time value, then you should be using the graph.
However the graphs are not accurate enough to allow this with any degree of accuaracy.
Take for instance an 80A BS1361 fuse:
The disconnection time at 460A is shown as 5s, however the disconnection time for a value of 550A, would be 2s.
When using the values of 460A and 5s, the adiabatic equation produces a result of: 8.94.
When using 550A instead, and the disconnection time of 5s the adiabatic equation produces the result of: 10.7.
Wheras the correct value using 2s would be: 6.8. (I've used 115 for k in each calculation.)
As you can see, using a slower disconnection time produces a larger value.