Zs-Ze = R1+R2 - doesn't it?!

[Marcus Vaughan]

Hi all. The title is a slight red-herring to provoke a response....

Looking through previous threads here and elsewhere tells me that the formula in the title just isn't true - that you cannot just measure Zs at a socket outlet (live test) and subtract a measured Ze to get an R1+R2 value for your minor works certificate.

Fair enough - I thought - Ze requires the earthing conductor to be temporarily removed, parallel paths etc - so I can see why it wouldn't work....

So - it seems odd then that my MFT seems to encourage doing just exactly that - entering a Zref value (Ze or Zbd) then doing your live Zs as normal and it will actually subtract the Zref and give an "R1+R2".

Am I missing something here?
I don't get it?

 

[Andy78]

If your MFT is encouraging you to do anything, you should stop talking to it and get a holiday.

 

[Deleted member 9648]

My megger ELI meter has the same function. Just because it's there doesn't mean you have to use it.

 

[LankyWill]

I use it I can't see what all the fuss is about, it has its uses.

 

[Zdb]

From what I've read, Zs minus Zdb will sometimes give you an accurate R1+R2 (provided that there are no parallel paths), but it's really bad practice because you can't guarantee that you're getting true readings.

 

[Wilko]

The 3 things are slightly different so the best method is to perform 3 measurements, IMHO

 

[davesparks]

My MFT has the same function, it seems like it could be used as a quick reference whilst carrying out testing to check that a measured Zs correlates to the R1+R2 previously measured.

 

[Marcus Vaughan]

If your MFT is encouraging you to do anything, you should stop talking to it and get a holiday.

Great idea - but can I take it with me?

 

[Andy78]

Great idea - but can I take it with me?

Not even if it asks nicely.

 

[Marcus Vaughan]

My MFT has the same function, it seems like it could be used as a quick reference whilst carrying out testing to check that a measured Zs correlates to the R1+R2 previously measured.

The 3 things are slightly different so the best method is to perform 3 measurements, IMHO

 

[Marcus Vaughan]

Thanks for the replies. Great stuff.

I'll just carry on as I think I should - if I add a socket to a Ring Final then I'll go for the full cross connection R1+R2 test (and Ze/polarity). Remove the doubt. It takes a while though. It's just that background feeling of not wanting to disconnect the circuit from the consumer unit unless I have to. But that's what I'll do!

 

[Marcus Vaughan]

Not even if it asks nicely.

I've been told.

 

[Andy78]

Thanks for the replies. Great stuff.

I'll just carry on as I think I should - if I add a socket to a Ring Final then I'll go for the full cross connection R1+R2 test (and Ze/polarity). Remove the doubt. It takes a while though. It's just that background feeling of not wanting to disconnect the circuit from the consumer unit unless I have to. But that's what I'll do!

Nothing stopping you dead testing at an available socket.

 

[Dave OCD]

Thanks for the replies. Great stuff.

I'll just carry on as I think I should - if I add a socket to a Ring Final then I'll go for the full cross connection R1+R2 test (and Ze/polarity). Remove the doubt. It takes a while though. It's just that background feeling of not wanting to disconnect the circuit from the consumer unit unless I have to. But that's what I'll do!

Marcus you don't have to disconnect at the DB to do the cross connection/figure of 8 test - just do it at a socket with both ring legs in it.

 

[Zdb]

It's just that background feeling of not wanting to disconnect the circuit from the consumer unit unless I have to

If you're extending the ring and not adding a spur you could always do end to end tests at the socket outlet and calculate your R1+R2. That way you don't have to disconnect anything at the D.B.

I think it's okay to use this method?

 

[Dave OCD]

Andy beat me to it - but I explained it better.

 

[Dave OCD]

If you're extending the ring and not adding a spur you could always do end to end tests at the socket outlet and calculate your R1+R2. That way you don't have to disconnect anything at the D.B.

I think it's okay to use this method?

No you have to do the cross connecting as well to check it's actually a ring , then each socket should have pretty much the same reading apart from spurs which will obviously be higher.

 

[Marcus Vaughan]

Marcus you don't have to disconnect at the DB to do the cross connection/figure of 8 test - just do it at a socket with both ring legs in it.

Dave - sometimes I can't see the wood for the trees. Brilliant and simple at the same time. Thankyou. To me it makes sense to cross connect at the last new socket going in - and test at the consumer unit. I could always do a quick live Zs test on this last socket once it's in to ensure 'all is good'.

 

[Dave OCD]

Happy to help mate.

 

[buzzlightyear]

op have got a onsite guide!