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Zs = Ze +(R1 + R2) Where am I going Wrong


gazza96

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Hello all,

I've got a question about Inspection and Testing, I'm a domestic Installer and have done a fair bit of testing and signing off my own work. This is my first time I've had anything to do with TT systems and need some advice.

I have been installing a circuit in a property which is TT, has a Main Switch which is an RCD (100mA) and the circuit I am installing is 32 Amp Type B 60898 MCB with a disconnection time of 0.4s. I'm happy with what I've done, the work is neat & tidy, its not a long circuit so if it was a TNS or TNCS i know the reading would be ok. I am using three leads with the Tester on No Trip, so getting a reading.

The Ze is 11 ohms (very good reading for an earth rod), the R1 + R2 is 0.4 ohms but when I do the Earth Fault Loop Impeadence Test the reading is 13.6 ohms

This is what you would expect if you do the maths, but when I look at the value permitted (1.16 ohms), it obviously fails.

I'm looking at page 103 of 17th Ed OSG.

Many thanks in advance for any replies

Gazza

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could be a loose terminal in the R1 + R2 testing, or a damaged cable..

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Just a few thoughts :-

When you calculated your R1+R2 did you have the earth/cpc to the load disconnected ?. For example if your circuit fed a water heater then your R2 might have been affected/ reduced by a parallel path through pipework.

Did you re-check the Ze again after getting the 13.6 ohm Zs ?

I think you will probably find the 1.16 ohm limit applies only to TN systems and that you are allowed a Zs of up to 1667 ohms on a 30mA protected TT installation.

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gazza.

if ZE = 11 ohms and R1=R2 = 0.4 ohms then ZS =11.4 ohms surely ?

You mention you have installed a 32A circuit .this will almost certainly require a 30mA RCD...you only mention 100mA [Main switch].

With a 30mA RCD the max Zs permissable becomes 1667 ohms.......see Reg. 411.5 and table 41.5

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and if you pay closer attention to the Zs tables, it says its for TNCS and TNS. TT has completely different Zs values, which are upto 1667 ohms where a 30mA RCD is used...

try BRB, table 41.5, and 411.5.3, both page 50

but Zs on TT is generally lower than Ze + R1R2 due to parallel paths, sometimes exactly Ze+R1R2, but if its higher then i think there may be a loose connection somewhere

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Sometimes the "No trip" earth loop test can give you higher results than Zs=Ze+(R1+R2)... it's due to the methods that the testers use to stop the RCD from tripping...

As A1 says... 30mA RCD!!!!!!!!!

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Under the 17th do you not need to bother testing zs? Can't you just add r1+r2 to ze? I would rather do the test to make sure though

Info appreciated.

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Under the 17th do you not need to bother testing zs? Can't you just add r1+r2 to ze? I would rather do the test to make sure though

Info appreciated.

Hi sellars. Under H&S guidlines Calculating Zs is the preferred method of carrying out the test because Live testing is only permitted if all other methods are inpracticable.

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Guys,

Many thanks for your replies, as it was a TT system and as most of my work is on TNS TNC-S (city boy) I looked at the OSG for inspiration about the Zs, Appendix 2 (Pages 100-102), I noted that Tables 2A, 2B and 2C all mention for TN systems, so thought that table 2D must be covering TN & TT as its title doesn't mention either. I can't find table 41.5 from the Regs in the OSG

Anyway, the circuit I installed did have a 30mA RCD (sorry, forgot to mention that) as protection, and I have found that when doing testing my calculations aren't always the exact same as the measured reading, but as long as i'm in the region and not widely off I take it as being ok

Once again thanks for the help

Gazza

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" I've got a question about Inspection and Testing, I'm a domestic Installer and have done a fair bit of testing and signing off my own work. This is my first time I've had anything to do with TT systems and need some advice."

gazza mate based on your statement above you really need to obtain and reference to BS7671 2008 and not rely soley on the on-site guide.

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" I've got a question about Inspection and Testing, I'm a domestic Installer and have done a fair bit of testing and signing off my own work. This is my first time I've had anything to do with TT systems and need some advice."

gazza mate based on your statement above you really need to obtain and reference to BS7671 2008 and not rely soley on the on-site guide.

Having worked on TT installations quite a lot lately myself, I would like to add that I think the OSG should cover TT installations in a bit more detail.

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