Measuring Zs

[glenmac]

Hi
When measuring maximum earth fault loop impendance Zs on a light circuit, do you measure on a 3 plate pendant between loop and earth or live and earth
Thanks for you assistance.

 

[industryspark]

measure zs at the furthest point of the circuit.

 

[GaryMo]

If it's a standard loop in system then you'll probably find the highest R1+R2 reading will be at the switch in the last room on the circuit. Obviously this will also be the place where you should measure the highest Zs figure too.

 

[17thman]

I have been tought, switch live to earth at last ceing rose

 

[GaryMo]

I have been tought, switch live to earth at last ceing rose

As I say, you'll find the higher value at the switch assuming it's cabled in 1.5/1.0 t&e.
I normally measure Zs at the place I record the highest R1+R2 reading.

 

[pompeygit]

Test both then........the highest reading is Zs........more likely to be found at switch as G suggests...........

 

[17thman]

if you take at the last ceiling rose, does that not take into account the length of switch live cable which would be ommited if taken at the switch.??

 

[GaryMo]

Think resistance, you'll have an extra cpc which is only 1mm.
If you don't understand then draw it out (not meant to be offensive).

 

[pompeygit]

Just as an aside. On my 17th update course I was told the following: The new Zs tables in chapter 41 have been updated and the rule of thumb no longer applies.............Now we have to give credit to tutors who are as new to the 17th as us, so it is encumbent on us to investigate and challenge stufff daily. Anyway after purhasing GN3 for the 17th which came out very recently it is clear that the Zs readings still need correcting down 80%. Expect the OSG to give rule of thumb at 75% or 80%......lets wait and see. The point is that you should listen and go home and read. I learnt tons on this site and am happy to ask Q's because evidently there are some really top notch guys out there. I have a lot of trouble understanding the regs because they are not written in a user friendly way, that is why is forum is so good because everyone can put their spin on it and usually some sort of consensus evolves . It is very clear to me who the expert posters are, and they know who they are.........so thanks guys for all the help you give to me and others.

 

[17thman]

non taken, i have drawn it out and the length of wire is the same but the earth has a smaller mm2 so once again Gmo is correct, i bow to u but wonder why GN3 suggest the way i said. I need some practical experience!

 

[Lectrician]

I have been tought, switch live to earth at last ceing rose

As I say, you'll find the higher value at the switch assuming it's cabled in 1.5/1.0 t&e.
I normally measure Zs at the place I record the highest R1+R2 reading.

Testing at the switch and the sw live at the pendent should give a similar reading.

The switch has a live and earth of equal distance in the reading.

The pendent has two lives of equal distance in the reading.

 

[GaryMo]

Testing at the switch and the sw live at the pendent should give a similar reading.

The switch has a live and earth of equal distance in the reading.

The pendent has two lives of equal distance in the reading.

Two conductors of equal length that have a 1.5/1.0 combination would have a higher resistance than a 1.5/1.5 combination.
It's very minor and they will be similar but it's still a fact.

 

[Lectrician]

And still a fact that I said "similar"

 

[GaryMo]

 

[securespark]

Surely if you test at the switched live terminal of the rose or fitting, that takes into account the return from the switch. Also, by doing it this way, you're automatically confirming correct polarity too.