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

 

[infinity]

5750 your right. I didn't work all the figures out it was mainly the equation I wanted to clarify, I can confirm the actual pefc once on site this week
So it's only ever the single phase fault current to earth basically. Cheers

 

[D Skelton]

I'd also go as far as to check with the fuse manufacturer's instructions what it's actual disconnection time is as opposed to using the nominal figure of 0.1. You will probably find it is even lower, thus having a more favourable bearing on your final cpc size

You win some you lose some, but it's always worth doing if you have a roll of Xmm in the van which you think might do, but using 0.1 as your disconnection time throws up Ymm.

 

[Richard Burns]

I have just had a quick check on BS88-2 and even using the I²t value I can only get down to 6.1mm￾² and as the disconnection time at 5750A is 0.025s and that gives about 7.9mm² so it looks like you are stuck with at least 10mm².

 

[Engineer54]

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?

Stick with your original CPC size of 16mm, buggered if i'd install a 10mm or less CPC for a 200A feed supplying MCCB panels!! By the way, what is/are the Max Zs values on the MCCB's??

 

[Spoon]

Why not see if you can use the armour as the CPC?