ZS testing which trips rcd's

[Paul Barker]

OK this is hypothetical as I have a megger that doesn't trip rcd's.


So if you don't have that luxury is the official way to measure Ze and measure R1 plus R2 (which you'd already have doneat the start) and arrive at Zs by calculation?

I gather it's a big nono to bridge the rcd for test purposes right?

 

[Fezzie]

I was told at college that "Ze + (R1 + R2)" is the proper way to test for ring final circuit continuity, and calculate Zs.
Having already established R1 + R2 and Ze during testing, there would not seem to be any need to do a separate Zs at each socket outlet.

 

[Lectrician]

You may us R1 + R2 added to the Ze on both EIC's and PIR's

You MUST NOT derive the R1 + R2 by subtracting Ze from Zs when doing an EIC, but may when doing a PIR as the installation is already energised.

I often find it much easier to undertake just the R1 + R2 tests.

On sockets, I only do the r1 + r2 ring end tests (notice lowercase r), and then devide (r1 loop + r2 loop) by 4 to find the R1 + R2 for the circuit (notice uppercase R).

As a side note, the neutral loop is known as rn.

In short - if you never do a Zs test, and always do the R1 +R2, you will comply.

 

[Paul Barker]

Thanks guys.

Out of interest I was watching a well experience Nicky member doing a periodic today and he didn't do a figure of eight on the ring. He also doesn't have C&G 2391. But Nicky allow him to do full scope.

Interesting.

 

[dingbat]

On sockets, I only do the R1 + R2 ring end tests (notice upper case R), and then devide (R1 loop + R2 loop) by 4 to find the r1 + r2 for the circuit (notice lower case r).

Shame you got 'em the wrong way round!

BTW, you should only do that calc as a check that your measured (R1 + R2) is of the expected value. On a new installation especially you should directly measure (R1 + R2) at every point of utilisation.

 

[Paul Barker]

Yes off the top of my head I thought it didn't ring true. Lets get this right.

GN3 pg33

end to end resistance on a ring is little r1 little r2 and little rn. cpc if usual proportions will be 1.67 X r1 or rn.

Then the figure of 8 test should result in resistance of (r1+rn)/4

PTO
Phase and cpc should become (r1+r2)/4 measure at every socket the highest resistance value represents the maximum (R1 +R2) of the circuit and is recorded in the schedule of test results. The value can be used to determine Zs to verify compliance with the loop impedance requirement of the regulations.

 

[Lectrician]

I had it the first way around, and was sure I got it wrong, so changed it!

I will change my post, otherwise it dodgy info!

 

[Paul Barker]

Yes but it was the thought that counts and it answered my question and also now I have found it in the guidance note that it is official you can use R1 +R2 to satisfy the requirents so next time I need to buy an analyser I dont need the expensive fancy pants type.

 

[Jim2287]

Cheaper (time wise) in the long run to use a Tester than calculate values evey time ?

 

[Paul Barker]

If it is challenging do a sum like 0.8 plus 0.21 it must slow you down a lot looking for the calculator.

 

[Jim2287]

oh yeah thats true