TT Testing advice.

[beats]

Evening all,
Today was my first time testing a TT system.(Hooray)

I have a couple of questions about my results if anyone can help.

1.Is the Ze reading the same as the RA reading?.My was 119.2ohms and this is ok yes because its between 21-200 ohms(ideally below 100ohms)

2.After testing the PEFC and PSCC my result was 0.01KA.this seems very low?i have calculated ohnms law and i came to 1.9A.

3.what would be the maximum zs for a 100mA RCD board? 500ohms? All my zs results where around the 16.80 ohms mark.

thanks chris

 

[IQ Electrical]

Evening all,
Today was my first time testing a TT system.(Hooray)

I have a couple of questions about my results if anyone can help.

1.Is the Ze reading the same as the RA reading?.My was 119.2ohms and this is ok yes because its between 21-200 ohms(ideally below 100ohms)

2.After testing the PEFC and PSCC my result was 0.01KA.this seems very low?i have calculated ohnms law and i came to 1.9A.

3.what would be the maximum zs for a 100mA RCD board? 500ohms? All my zs results where around the 16.80 ohms mark.

thanks chris

1 The reading is fine but forget about the 21 Ohms figure-it's for the suppliers electrode.

2 Normally on TT the PEFC is very low because of Ra but PSSC is quite a bit higher for obvious reasons.

3 Yes, 500 Ohms is the theoretical maximum for protection by a 100mA RCD

The system sounds spot on with the exception of that 0.01kA value-did you do a neutral loop impedance to double check this value?

 

[beats]

Thanks IQ
I did do a PSCC test and a PEFC and they were both 0.01KA.?

 

[IQ Electrical]

Thanks IQ
I did do a PSCC test and a PEFC and they were both 0.01KA.?

Strange, even at the end of a long run on a TT the neutral loop impedance rarely seems to exceed 0.5 Ohms which would equate roughly to 460 Amps.

 

[beats]

is it worth noting the low result on the PIR?

Also on a PIR would the 119.2 go in both the Ra and Ze boxes?
ta

 

[IQ Electrical]

Just record the PFC value as normal, it is what it is.

Yes, you can enter that value in both boxes if you wish.

 

[beats]

very true.
Thanks again IQ,thats what i like about this forum people are always willing to help.ta

 

[telectrix]

very true.
Thanks again IQ,thats what i like about this forum people are always willing to help.ta

that's when we're not busy creating silly , inane threads, like my DIY Rewire one. LOL.

 

[moorman84]

Your PSCC result is wrong. On a TT system you need to test between the live and the neutral or between the live to live and live to neutral on a 3 phase installation and record the highest.

 

[platty1005]

Hi beats it seems you have some conflicting results as you have posted them
RA is the electrode resistance measured by using an electrode resistance tester
Ze is the external loop impedance which will be slightly higher than the RA but if the installation is protected by RCD then this measurement can be used as the RA
Now you say Ze 119.2 but Zs 16.8 this cannot be right Zs = Ze + (R1+R2) so Zs should be higher than 119.2
If Ze is correct the PEFC will be approx 230/119.2 = 1.9 amps
The PSCC is measured between Line and neutral and should easily be above 200amp make sure you are testing correctly check OSG or GN3
Formula for max Zs for RCD = 50V/100mA = 50/0.1 = 500 ohms

I stand corrected by IQ re: bonding : so back to PEFC this should be measured with bonding connected so PEFC should be approx 230/16.8 approx 13.6 Amp

 

[IQ Electrical]

Hi beats it seems you have some conflicting results as you have posted them
RA is the electrode resistance measured by using an electrode resistance tester
Ze is the external loop impedance which will be slightly higher than the RA but if the installation is protected by RCD then this measurement can be used as the RA
Now you say Ze 119.2 but Zs 16.8 this cannot be right Zs = Ze + (R1+R2) so Zs should be higher than 119.2
If Ze is correct the PEFC will be approx 230/119.2 = 1.9 amps
The PSCC is measured between Line and neutral and should easily be above 200amp make sure you are testing correctly check OSG or GN3
Formula for max Zs for RCD = 50V/100mA = 50/0.1 = 500 ohms

This is quite common on TT systems, Ze or Ra is measured without bonding conductors connected, Zs is measured with so is almost always lower on TT.

 

[platty1005]

Good point IQ forgot about the bonding !!

Will edit original

 

[SPARTYKUS]

platty - especially evident when you have a TT and the water mains are shared with another house thats TNCS .....

 

[aw258025]

what is the likely result on an r1+r2 on a tt system I got a low result is that correct ?

 

[malcolmsanford]

The earthing system is irrelevant when your doing the r1 + r2 or the R1 + R2 test.