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

[gww96]

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 OSG.

Many thanks in advance for any replies

Gazza

[Ohmslaw]

Hi.

The max Zs values in BS7671 do not apply to 'TT' systems, if you turn over the page you will se the the max permissable Zs is 1667ohms providing that you have a 30mA RCD installed which really there should be nowadays. If your circuit however does not fall under the requirements for 30mA RCD protection i.e surface run and not a socket outlet etc etc then the max for a 100mA RCD is 500ohms so again your readings are fine.

It also states that the max allowable Ra is 200ohms but as you have 11ohms then you are totally safe.

[gww96]

Ohmslaw,

Thanks, I've been using the On Site Guide, Appendix 2 where pages 100 - 102 all state TN systems so thought as it doesn't mention either TT or TN on page 103 it covers them both.

I have now got out the big red book and found the figures on page 50, but even then it only mentions up to 32 Amps, what about 40 - 45 amps

Any ideas

Thanks

Gazza

[Central Scrutinizer]

when you measured Ze did you have the main earth disconnected?

and presumably you had it reconnected when checking the Zs?

it is unusual to have a higher Zs

i did a TT a while ago

34 ohms Ze - no problems

however Zs had dropped to 5 ohms, becuase of the parallel paths through the extraneous services

i presume your 100mA is an S type?

You will still need a 30mA for your sockets, your bathroom, and buried cables etc etc, so as said, as long as your under 1667 ohms, your fault voltage will be less than 50V and it is ok