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John cullen

TT system

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John cullen

Hi guys is it ok to do the Zs=Ze + R1 R2 calculation on a TT system. I got a reading of 20.3 ohms on the Ze and .55 on a radial socket circuit. So how does the equation work.

john

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NozSpark

The calculation doesn't take into account any parallel paths through the bonding conductors... remember that the same services could be bonded next-door and be part of a TNS or TNCS installation

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Onoff

Just don't forget to reconnect any bonding conductors removed for testing!

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John cullen

So essentially you can’t add the Ze and the R1 & R2 together?? 

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Rob.
4 hours ago, John cullen said:

So essentially you can’t add the Ze and the R1 & R2 together?? 

 

This will give you the worst case scenario. 

 

An if there wasn't any extraneous parts it would work out. Remember if it has metal water an gas services. An they are bonded, they are in effect 2 giant additional earth rods.  

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binky

In theory on a 30mA RCD max Zs could be 1667 Ohms. However to me that means a cct / earrh rod is buggered somewhere! So on TT I work on the basis that R1+R2 values should be as per house with suppliers earth. Rod should be less than 200 Ohms - recomended figure for a stable earth, so as you have done above add your Rod to your R1+R2 figures to give you Zs.

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John cullen

Cheers mate

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Andy™

you can calculate Zs by Ze + R1R2 (you can also do the same on TN systems)

 

 

Edited by Andy™

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