R1 plus r2 reading

[IQ Electrical]

It's often much easier to get a quick Zs measurement on a circuit, and not have cause to isolate it and the DB in order to start linking out for R1+R2 testing.

The problem arises when you take your Zs and subtract Ze thinking you're left with R1+R2.
You are not because of parallel paths on the Zs tests, often resulting in a lower value than you would derive through calculation.

Try this when you get out in the field, you end up with some impossibly small R1+R2 values as a consequence!

 

[SPARTYKUS]

It's often much easier to get a quick Zs measurement on a circuit, and not have cause to isolate it and the DB in order to start linking out for R1+R2 testing.

The problem arises when you take your Zs and subtract Ze thinking you're left with R1+R2.
You are not because of parallel paths on the Zs tests, often resulting in a lower value than you would derive through calculation.

Try this when you get out in the field, you end up with some impossibly small R1+R2 values as a consequence!

So right - got a good example that might also help

At a house, it is TT.

We found the earth rod, tested it. 17.8 ohms. Fine.

Connected everything up, plugged meter into nearest socket, Zs : 0.47 Ohms!

Now that is one heck of a parallel path. How could it drop to such a small value when its got an earth rod at 17.8 Ohms (you may ask) ?

Answer is, turns out the water board tried (and failed) to fit a water meter to the house, was going to cost too much, so just rated them. And when this was going on they mentioned to the owner that the water pipes are connected to next doors ..... sussed it yet?

Thats right, the main bond to the water service was parallel-ing the earth rod value and guess what earthing system they had next door. TNCS

 

[IQ Electrical]

Lol and that's why I wouldn't want to be the only guy in the street on TN-C-S !!

 

[Mark.W]

Thanks for the reply,

I still seem to get a loop when i draw it out even though i know your right,

im obviously doing something wrong,

regards

locket

Here you go Locket I have drawn the scenario for you!