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?

 

[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?

 

[Engineer54]

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

There should be no problem using the armouring as a straight CPC, the problem comes if there is any bonding involved at these MCCB panels!!

 

[Spoon]

There should be no problem using the armouring as a straight CPC, the problem comes if there is any bonding involved at these MCCB panels!!

Very true Eng54 but the OP hasn't mentioned that additional bonding is required, so I thought that the armour should be OK for his purpose.

 

[Engineer54]

Unfortunately, most don't even consider the bonding side of things in the first place, so just because it hasn't been mentioned by the OP, doesn't mean it's not a consideration that should be taken into account....

 

[infinity]

There is no bonding required, i will be using armour as CPC and a 25mm fly lead to the board from gland (have 25mm in van to use up)

 

[Buzz-light-bulb]

There should be no problem using the armouring as a straight CPC, the problem comes if there is any bonding involved at these MCCB panels!![/QUOTE]

I am very interested as to why bonding would cause a problem.

 

[davesparks]

There should be no problem using the armouring as a straight CPC, the problem comes if there is any bonding involved at these MCCB panels!!

I am very interested as to why bonding would cause a problem.[/QUOTE]

Because it becomes very difficult to have armouring which complies with the requirements for a bonding conductor.

 

[Electric Yo]

Well if your ZS is accurate then a 10mm2 copper cpc will do the trick with a bit of headroom to spare.

 

[Buzz-light-bulb]

There should be no problem using the armouring as a straight CPC, the problem comes if there is any bonding involved at these MCCB panels!!

I am very interested as to why bonding would cause a problem.

Because it becomes very difficult to have armouring which complies with the requirements for a bonding conductor.[/QUOTE]

Please explain more, do you mean the reason is because bonding has to be bonded from the m.e.t ?? And not from the mccb panel

 

[infinity]

Can you show your calculations for 10mm please? Ive done it a few ways, including working out t instead of assuming 0.1 and I still get 16mm required

 

[davesparks]

Please explain more, do you mean the reason is because bonding has to be bonded from the m.e.t ?? And not from the mccb panel

For example if bonding was required to be 50mm copper at the intake then it would have to have a path back to the MET which is never less than 50mm copper.
Steel is 8 times less conductive than copper so to achieve this with steel armour the actual CSA of the armour would have to be 50x8 = 400sqmm of steel.

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.