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?

 

[D Skelton]

Or at least that is my understanding of it, I'd wait for the local earthing expert to share his knowledge before taking it as gospel though.

Where is D.W. Cockburn when you need him?!?

 

[Electric Yo]

Zs=0.04Ω
V= 230V

230/0.04= 5750A

-----------------

I=5750A
T= 0.025s (Read this from you post, also agrees with regs)
K=115


S = √((5750²)x0.025)
-----------------
115

S= 7.906 mm²

So with a 10mm² you will be oversized by 21%.

-----------------------------------------------

If you want to use the Steel armoured of the SWA:

I=5750A
t=0.025s
K=51 (assuming thermoplastic)

S = √((5750²)x0.025)
-----------------
51

S= 17.827 mm²

If you're thinking 'Jesus Christ that seems small' remember that the cpc only has to take this fault current for 0.025 seconds. And 21% is a nice amount of headroom, only if the result was borderline would I move up too the next size.

I presume you got the result:

S= 15.811mm² ????

That's because you used 0.1 as the value of 't' instead of 0.025. t is the actual disconnection time for your PFC (not maximum allowed or anything else). The actual disconnection time can be found by looking at the disconnect time curves in the reg, or better still from the manufacturers data sheet.

Hope that helps. =)

 

[polo1]

At last! Someone has actually provided what the OP was looking for

 

[Geordie Spark]

Where is D.W. Cockburn when you need him?!?

Swapping stories with Old Jag Lover the last I heard.