Adiabatic equation help

[wade88]

Hi chaps,

Can someone please give me a quick lesson on adiabatics? When to use the equation, detailed description of what it is etc.

Been scouting any literature i have here but not been able to pin something down.

Thanks
B

 

[AP Electrical]

just on way out of office but brief - Determining cpc size using fault current squared x length over k factor all in brb

Alex

 

[AP Electrical]

if I remember rightly

 

[malcolmsanford]

It is basically a calculation that determines the size of the protective conductor for a given installation. distribution/final circuit.

 

[wade88]

Right, so if i was trying to determine what sized CPC to use over a 30 meter run with a PFC of 1256 i would carry out this sum:

1256 x 1256 = 1,577,536 x 30 = 47,326,080 / k factor?

 

[AP Electrical]

Sorry forgot think you square the sum of fault current square x length then over k

Alex

 

[i=p/u]

disconnection time in there too

 

[i=p/u]

read few similar threads underneath i dont know how to do symbols where some can

 

[Deleted member 26818]

It's the square root of the fault current required to operate the CPD within the required time squared then multiplied by the requred time, then that result divided by the k value.

 

[i=p/u]

see! spinlondon can put in words no problem

 

[i=p/u]

<<<<<< ameture

 

[wade88]

Can some one give me an example sum.

 

[wade88]

<<<<<< ameture

Ameteur

 

[i=p/u]

well i was taught like this , the square root of the current squared multiplyed by the current squared multiply by the disconnection time dived by k factor

so example say 6A mcb, 0.4 disconnection time would be as follows square root of 6x6 x 6x6 x 0.4 = dived by k factor in big red book bu say we use 115 for k factor so.... now ive no calculator but you try it

 

[Deleted member 26818]

Ok.
Now as you have used a PFC value, I take it we are checking an existing installation, and determining whether the earth/bonding conductors are correctly sized.
First you determine the type and rating of the CPD, and then determine whether the PFC is high enough to operate the CPD in the required time.
For instance the value you used of 1256A will cause a 100A BS1361 to operate in 5, 1 and 0.4 secs, but not however in 0.1 and 0.2 secs.
You then choose the highest value in which the CPD will operate, in this instance 1200A, square it giving a value of 1440000, multiply it by the time which in this instance is 0.4secs which gives a value of 576000 and then determine the square root, which is 758.94.
This value is then divided by the k value for the type and material of the CPC.
If we use the value for single conductors we would use 143 for k, if however the conductor was in T&E, we would use a value of 115 for k.
So 758.94 divided by 143 gives a minimum CSA for the CPC of 5.3mm², if it was T&E and the value for k 115, we would require a minimum of 6.6mm².

 

[wade88]

4.132?

 

[wade88]

You "thanks" button doesnt appear to be showing Spin but Thanks mate, thats really useful.

 

[wade88]

well i was taught like this , the square root of the current squared multiplyed by the current squared multiply by the disconnection time dived by k factor

so example say 6A mcb, 0.4 disconnection time would be as follows square root of 6x6 x 6x6 x 0.4 = dived by k factor in big red book bu say we use 115 for k factor so.... now ive no calculator but you try it

result is 4.132, but i cant see a 6amp MCB requiring a 4mm CPC?!

 

[Tester Steve]

Have a look at Lennys post here.

Steve.

 

[malcolmsanford]

Use the Adiabatic Equation:

S = sqrt(I^2 x t) / k

where

S = Min CSA of CPC required in mm^2 to satisfy thermal constraints
I = Uo / Zs in Amps (In this I have just picked out a value)
t = Operating time of the protective device in seconds
k = a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor material

so we have:

I = 230 / 0.56 = 410.71 A

t = 0.1 sec - from Fig 3.5 pg 250 in BRB I just used a Type C BS 60898 - the time taken will be less than this in reality - but we must use our "text box" instantaneous time if we do not have specific manufacturer data to hand.

k = 115 - from Table 54.3 for 70 C thermoplastic insulated cable

S = sqrt (410.71 ^ 2 x 0.1) / 115
S = 1.13 mm^2

Don't take note of values or anything as I just made them up but it gives you an idea on how to work the formula