R1+R2 and ZS results

[pencilpusher]

I have measured the highest R1+R2 on a lighting circuit as 1.51 ohms at a switch. However, I get 0.86 ohms on a ZS test at the same point. I am on TNCS system and my Ze is 0.22 ohms. Although I expected a lower ZS by measurement than by calculation, I did not expect it to be this low. I understand parallel paths will lower it but by this much?

Views please.

Thanks PP

 

[telectrix]

estimate the cable length. that 1.51ohms for 1.0/1.0mm would indicate a cable run of 40m. if it's 1.5/1.0mm, even longer ( i don't have the figure to hand). seems a bit high.

edit: 1.5/1.0mm a res. of R1+R2 of 1.51ohms would indicate a circuit length of approx 60m.

 

[pencilpusher]

Its 1.5/1.0 and it is no way near 40m. It is an unusual reading compared to the others around the place. I would say 10m tops. Loose connections somewhere?

EDIT: Even so, why the much lower ZS with the 1.51 ohms R1+R2?

Thanks PP

 

[telectrix]

think its your R1+R2 is out. 10m should be 0.3ohms. leads not nulled properly or a high resistance connection, not seen on the Zs test.

 

[Deleted member 9648]

Lots of high R1+R2 readings on lighting circuits are down to poor quality switch contacts...try testing at the end of the feed,missing out any switch contacts and compare.

 

[pencilpusher]

When I tested R1+R2 by removing the conductors from the switch, I still got 1.51 ohms, ish. I don't understand why bad contacts are "not seen" when a ZS test is done either tbh.

 

[telectrix]

maybe the fact that the Zs test is a live test with 230v on the circuit. continuity testing is done at 12v or less.

 

[pencilpusher]

There must be something up with the switch drop. I had about 0.68 ohms at the light point on a two way circuit. Mmm

 

[telectrix]

remember that on a 2 way curcuit, your R1+R2 is through 2 closed switches. this could be as wp says. also, it's common for the 3core between the switches to me 1.0mm, not 1.5mm.

 

[pencilpusher]

There must be something up with the switch drop. I had about 0.68 ohms at the light point on a two way circuit. Mmm

Excellent point. I will try testing without switches and see what I get. Mind you, the ZS is fine and that's the main thing I suppose.

 

[telectrix]

what you could do is link out the switches and read again for R1+R2. if the reading drops, then you've proved it.