Zs on TT

[i=p/u]

What is the point in carrying out zs on TT system other than verifying a loop.
We got an Ra of 14.8 but when all bonding is in place it comes down to 1.9 ohms for most circuits zs results. I just don't see the point if rcd protected .

 

[MDJ]

again true 612.9 loop impedance can be determined by an alternative method other than measurement, still worth doing and a grey area when verification is required IMO

 

[Zebra Stripe]

So, back to original post, is a Zs measurement of any significance on final circuits within a TT installation which requires RCD protection to meet disconnection times, when an Ra has been measured and an R1+R2 of each circuit has been measured?
IMO I think it is an academic test which as the OP puts it, is probably not worth doing, but Im still a little unsure!

 

[MDJ]

The thread has moved away from where it started' even I am lost now LOL

 

[Zebra Stripe]

What is the point in carrying out zs on TT system other than verifying a loop.
We got an Ra of 14.8 but when all bonding is in place it comes down to 1.9 ohms for most circuits zs results. I just don't see the point if rcd protected .

So what is the point or significance of carring out a Zs test on final circuits within a TT install when the Ra has been measured and the R1+R2 (presumably) has also been measured and an RCD is used for earth fault protection........still the same point???

Sorry Im not trying to be arguementitive, Im just interested to know what other electricians think about this.

 

[Hawkmoon]

So what is the point or significance of carring out a Zs test on final circuits within a TT install when the Ra has been measured and the R1+R2 (presumably) has also been measured and an RCD is used for earth fault protection........still the same point???

Sorry Im not trying to be arguementitive, Im just interested to know what other electricians think about this.

Zs tests the full circuit (think about what you are testing when you carry out ze and R1 + R2 - also let's say you made a dog's dinner of reconnecting R2 after your testing (broken/loose term), only Zs will id this.

 

[Zebra Stripe]

Ahhh Hawkmoon the eternal champion....the old "you might of damaged or pulled out a conductor" argument. But what if it was a radial with a FCU on the end? Wouldn't you have to take it off the wall to measure Zs?
I know Zs is the whole circuit or loop impedance path, read post No 58..oh here it is

"Agreed, I like to test it myself too, but an Ra plus R1+R2 of each final circuit will give a more accurate/stable assessment of the earth fault loop impedance and would not include parallel paths which may be removed at some point.
I've had Zs values on TT installs that fall easily within the TN bracket and would suggest that an RCD would not be required to meet disconnection times, but if these parallel paths were to be removed then obviously the tiny earth fault currents would not disconnect at any time. So how significant is a measured Zs in this instance?

Remembering Im talking about RCD protected TT install here

 

[Sintra]

Ok just to throw in another scenario for doing a Zs test on all circuits. As stated this is the only test that measures the "complete" circuit so what if you have a high resistance across a mcb or rcd?

 

[MDJ]

A good answer By Sintra and a proper way to verify, Sorry I have been away, just got back from the Dentist

 

[Zebra Stripe]

Ok, now that's a very good point and I think one that I should of thought of. But its the only point thats swaying me away from Zs on RCD TT install being academic.

 

[Hawkmoon]

Ahhh Hawkmoon the eternal champion....the old "you might of damaged or pulled out a conductor" argument. But what if it was a radial with a FCU on the end? Wouldn't you have to take it off the wall to measure Zs?
I know Zs is the whole circuit or loop impedance path, read post No 58..oh here it is

"Agreed, I like to test it myself too, but an Ra plus R1+R2 of each final circuit will give a more accurate/stable assessment of the earth fault loop impedance and would not include parallel paths which may be removed at some point.
I've had Zs values on TT installs that fall easily within the TN bracket and would suggest that an RCD would not be required to meet disconnection times, but if these parallel paths were to be removed then obviously the tiny earth fault currents would not disconnect at any time. So how significant is a measured Zs in this instance?

Remembering Im talking about RCD protected TT install here

When I say zs tests the full circuit I am not talking about parallel paths -
With your FCU example - yes you would have to unscrew the front plate for the Zs test...but you would not have to disconnect anything at any point in the circuit.

 

[telectrix]

A good answer By Sintra and a proper way to verify, Sorry I have been away, just got back from the Dentist

you had them sharpened ready to savage the next Electrical Trainee that asks a stupid question?

 

[MDJ]

Just need the hygienist otherwise all good

 

[i=p/u]

We are getting there slowly. But I think you see what I mean now all ---

 

[MDJ]

We are getting there slowly. But I think you see what I mean now all ---

You still need to consider overcurrent protection as well as an RCD M8, which is something I think you mentioned somewhere. To be honest we are so far off the original question now I may even be wrong about that HAHA

 

[Zebra Stripe]

Ok, I'm becoming slightly more convinced, that perhaps while the actual value recorded on the certificate is somewhat academic, the interpretation of the measured value in relation to the Ra and perhaps R1+R2 and possible high resistances across MCBs etc would only be found doing the actual Zs test.
Just to point out though, that I do Zs test circuits whether TT or TN installs.