Adiabatic equation

[infinity]

Evening All,

I have an industrial install this week, installing 200A feeds to MCCB panels in workshop, with 70mm SWA

Im just working out the CPC at the DB, not done this for a while so just wanted to check it..

S= root(I2 X T) / K

The fuses are BS88-2 200A, i have another DB in exactly the same place with same sized cable, and the ZS is 0.04 PFC 5.2KA

so using those figures-

Root ((5200x5200)X0.1= 1644.38/ 115= 14.2mm

So 16mm will be adequate. does this look correct?

 

[Sintra]

Is this install 3 phase. If so your PFC seems to be low.

 

[infinity]

Yes its 3 phase, this is where i am a little confused, should i use the 10.4KA figure?
In which case 28mm is required

 

[Chr!s]

Yes its 3 phase, this is where i am a little confused, should i use the 10.4KA figure?
In which case 28mm is required

Where do you get 10.4 from?

whats the fault loop consist of?

 

[davesparks]

The 10.4 is, I assume, 2 x the original 5.2 he had to give him an approximation of the 3ph fault current based on the line - earth fault current.

The question is where did the 5.2 come from?

 

[Chr!s]

The 10.4 is, I assume, 2 x the original 5.2 he had to give him an approximation of the 3ph fault current based on the line - earth fault current.

The question is where did the 5.2 come from?

2 x a zs value is a very poor way to approximate a three phase fault current.

Chris

 

[davesparks]

I never said it wasn't

 

[Chr!s]

I never said it wasn't

No indeed

I was just saying

Chris

 

[Spark_shark]

I don't believe there's any need to double it (*1.732 to be more precise). Don't forget... It's pefc were concerned with here, not pscc (line to neutral or line to line)

 

[infinity]

The actual 3phase fault current was not calculated I just typed a figure of 10.4 as 5.2 x 2=10.4
Obviously it would be 5.2x root 3
But I don't have that symbol on my phone right now

 

[infinity]

Thanks spark shark that's what I was getting at in first post, it's not the 3 phase fault it's the earth,hence the 5.2 figure, sintra threw me when he sAid about being 3 phase

 

[Spark_shark]

I think the point I'm trying to make is... Doesn't matter how much fault current flows between line to line/line to neutral... It won't effect what size CPC u need to use

 

[Spark_shark]

No worries. U beat me to it with the reply lol

 

[Spark_shark]

I think where he suggested its too low, is because 230/0.04 = 5750A, not 5200...?

 

[Chr!s]

The actual 3phase fault current was not calculated I just typed a figure of 10.4 as 5.2 x 2=10.4
Obviously it would be 5.2x root 3
But I don't have that symbol on my phone right now

No it wouldn't be, think about it?

Chris