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# Confirmation of earth size is correct, using adiabatic equation.

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#### DanBrown

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Could anyone just confirm my results for adiquate earthing size and calculation using the adiabatic equation...

My test was on a ring main circuit wired in 2.5/1.5 pvc..
My supply voltage is 247v
My Impedance is 0.38 ohms.
The circuit is supplied via a RCBO 32a C type

My results suggest that a min CSA earth is 1.78mm2 and my cable CPC is only 1.5mm2.

Could someone just do a quick confimation of my answer please...
Is it a problem as such or not?? (still learning testing)

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G

#### Guest123

Hey.

Havn't got ol red to hand but it sounds about right.

From memory divide 247 by 0.38 to get (I).
(T) comes from APP 3 of ol red. (K) comes from table 43.1 of ol red. (S) obviously is your C.S.A of the conductor you have.

(S) = the square root of ((I) squared x (T)) divide by (K)

Cheers.

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T

#### Tiger

Could anyone just confirm my results for adiquate earthing size and calculation using the adiabatic equation...

My test was on a ring main circuit wired in 2.5/1.5 pvc..
My supply voltage is 247v
My Impedance is 0.38 ohms.
The circuit is supplied via a RCBO 32a C type

My results suggest that a min CSA earth is 1.78mm2 and my cable CPC is only 1.5mm2.

Could someone just do a quick confimation of my answer please...
Is it a problem as such or not?? (still learning testing)

Max impedance for a type c 32 amp is .72

G

#### Guest123

Hey.

Got my regs book out now.

So where you have measured voltage as 247V, The equation says to use Uo which would be 230V.

I = 230/0.38 = 605.
K = 115
T = 0.01

So I squared = 366,025
366,026 x 0.01 = 3660
square root of 3660 = 60.50

60.50/115 = 0.52.

S should be greater than 0.52 and you have 1.5 so it's ok.

Hope this helps

D

#### DanBrown

Thanks for that guys...
Thants a good point for me to remember lenny, use the Uo - 230v and not the actual voltage 247v..

Also in response to the previous question (why use type c breakers) im not sure, we have just had NIC firm in to rewire entire house, and they seemed to fit all RCBO's in type ''C''....
Does anyone have a reason for this?? becuase i cant fink of a answer!!

J

#### Jono Pashley

just noticed while i was rough pricing a job on the tlc website but the rcbos were all type c

B

#### bahco

i know that my local wholesalers only do c type wylex rcbo's... say they cant get hold of b type.

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#### DanBrown

i know that my local wholesalers only do c type wylex mcb's, say they cant get hold of b type.
Thats probably why.. Our board is Wylex and Wylex RCBO's..

Thanks Guys..

J

#### John In Afghan

Max impedance for a type c 32 amp is .72
Tiger, you are correct in stating that the max Zs for a type C 32amp (MCB), is .72 ohms. However, didn't the OP state a type C, 32 amp RCBO? So the max Zs will be 1667 ohms.

John.

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#### Guest123

Well it was 2 years ago so.......

S

#### Smugley

I've installed Wylex Type B RCBOs recently, but I seem to remember TLC didn't have any and I had to go to CEF... I think. Memory a bit cloudy tonight.

J

#### John In Afghan

Well it was 2 years ago so.......
So RCBO's change to MCB's with age do they?

No? Didn't think so, maybe somebody else was looking for information and stumbled across this.

Please try to be more helpful next time you post.

John.

G

#### Guest123

So RCBO's change to MCB's with age do they?

No? Didn't think so, maybe somebody else was looking for information and stumbled across this.

Please try to be more helpful next time you post.

John.
Hang on a minute, you were replying to the OP by name which is why I replied.

As for being helpful, maybe a quick browse of my 7 and a half thousand odd, other posts may enlighten you!

#### jaresquire

##### -
Arms
Yeah cmon Lenny, you're getting like Millwall Ken!

#### jaresquire

##### -
Arms
..Meant to put smiley after that..

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