TT SYSTEM. Zs readings confusing me.

[Nath88]

Howdy all.
A friend of mine has recently bought a house and asked me to pop round and do a few tests just for piece of mind everything's ok.
It's a TT system with 30mA split DB
Ze(Ra) - 10.59 with all bonding disconnected
When I got some Zs readings they were all coming up at around the 15 ohm mark
I know All readings are pretty good (under 1667 - regs and 100 - niceic)
but I can't see how there's such a difference between the Ze and Zs
Ze = Zs + R1+R2
i never took any R1+R2 readings but can't see them being that high. Everything looks good n the DB.
Am I missing something obvious??
Cheers

 

[somersetsparks]

When testing the rod, I assume you are doing a full current loop test on it?

You then test the final circuits with the Loop tester on D-LOK, which will put a small current through, thus struggling to force its way through the poorer path to earth,
It shows alittle when testing a PME installation, but is greatly exagereted with the High Resistances of TT systems.
In this case I would just use R1+R2 to calculate Zs.

p.s. full current is about 10A i think where as D-Lok is low current, maybe just 15ma.


You could prove the theory by testing the rod using the D-lok setting then comparing it with the test results from the high setting.

 

[Nath88]

I thought that was the issue so I tested the rod on d lock and it was higher but not by too much.
But as you say the difference is greatly exaggerated so that would cause it.
Cheers

 

[Engineer54]

I thought that was the issue so I tested the rod on d lock and it was higher but not by too much.
But as you say the difference is greatly exaggerated so that would cause it.
Cheers

Doesn't say much for the water/gas service pipes (if they are metallic) contribution to your Zs values!! lol!!

 

[somersetsparks]

Doesn't say much for the water/gas service pipes (if they are metallic) contribution to your Zs values!! lol!!

But an Ra even you would be happy with to compensate! lol

problem is now there are less and less metallic utility pipes left in service i suppose, my street still has steel services into houses for water and gas (bulit 1952). however they were replaced upto the stops taps in the road a few years ago.

 

[Engineer54]

But an Ra even you would be happy with to compensate! lol

problem is now there are less and less metallic utility pipes left in service i suppose, my street still has steel services into houses for water and gas (bulit 1952). however they were replaced upto the stops taps in the road a few years ago.

Mores the pity as regards the steel/metal service pipes being replaced with plastic...though lol!!


Yep, that value is certainly a dam sight better than the numbty 100/200 ohms of the usual present day TT systems!!

I'd still advise on the installation of an up-front 100mA S type RCD though, i just wouldn't trust a single RCD looking after several circuits on each side of the CU!! lol!!

 

[Guitarist]

I'm assuming you first carried out the ELI test as 2-wire, then Zs as 3-wire no-trip?

 

[Sintra]

Howdy all.
A friend of mine has recently bought a house and asked me to pop round and do a few tests just for piece of mind everything's ok.
It's a TT system with 30mA split DB
Ze(Ra) - 10.59 with all bonding disconnected
When I got some Zs readings they were all coming up at around the 15 ohm mark
I know All readings are pretty good (under 1667 - regs and 100 - niceic)
but I can't see how there's such a difference between the Ze and Zs
Ze = Zs + R1+R2
i never took any R1+R2 readings but can't see them being that high. Everything looks good n the DB.
Am I missing something obvious??
Cheers

Think you mean Zs = Ze + (R1+R2)

I think that the R1+R2 tests would be the most important tests you would do.