Zs lower than Ze????

[sensible]

Hi,

I am testing my dads house to get a bit of practice and I dont understand the readings?

Its a TT with an Ra of 39ohms, yet when I use the MFT to do a Zs on the sockets I get anywhere from 0.21 - 1.09ohms. I dont understand how this can be lower than the Ra, I must be doing something wrong?

Thankyou.

 

[MDJ]

You are getting readings from the water and gas pipes connected to the Board via bonding.

 

[Darkwood]

TT systems can give fluctuating readings.. Check Ze is stable before, during and after testing circuits.

Meter leads may be faulty did you short test them a few times.

Contact terminals you are testing at may be resistive due to anti-corrosion film on them try down to clean copper...

Parallel paths ... you have reconnected the bonding and reduced the Ze as mentioned above.



Quite a few reasons really but a few to head you in the right direction

 

[Meggerman]

It will be through your main bonds or you may have a N-E fault. Are your neighbours on TT?

 

[SPARTYKUS]

Or.... Your water service pipe is inextricably linked (as in joined) to your neighbours, and they have a TN earthing system that is bonded to the water pipe.

seen it a few times.

Ra 17.8.... Zs at the socket 0.34!?!?! How can this be"..................

 

[sensible]

I did leave the bonding in place when I did the Zs. Would that bring the Zs down so much? Ra was 39.. Zs was say 1ohm which means the gas and water pipes have made it drop by around 38ohm.. how can they do that? Thjats means they are better than the rod!!??

 

[MDJ]

I did leave the bonding in place when I did the Zs. Would that bring the Zs down so much? Ra was 39.. Zs was say 1ohm which means the gas and water pipes have made it drop by around 38ohm.. how can they do that? Thjats means they are better than the rod!!??

so what is your question again, I thought we gave you some Ideas which will get you going in regard to understanding what may be the reason why the reading is so much better than the rod alone? as for being better, you cannot have water or gas pipes as a main earth, you know what your main reading is, you said 39 ohms, thats pretty good, why don't you try to test the readings to the pipes lol, give yourself some more practice.

 

[polo1]

As has been said, water and gas supplies can often give you a much better/lower resistance than a rod, but you are not permitted to use these supplies as your main earth.

 

[HandySparks]

.... Thjats means they are better than the rod!!??

Just think about it for a minute. Which is likely to give the lowest Ra; a 10mm or 16mm rod, say a metre long, or a 30mm metal water pipe, which could be many metres long and connected to a metal water main running down the road?

 

[sensible]

That all makes sense now, sorry for sounding a bit dumb. My head just thought 'main earth' must be better than pipes as thats what its for!

 

[Badged01]

That all makes sense now, sorry for sounding a bit dumb. My head just thought 'main earth' must be better than pipes as thats what its for!

All part of the learning process, ask the question and think about the responses ...... parallel paths in there somewhere!!

 

[Engineer54]

Might be time to ask the DNO if PME is available in your parents area, and if it is, making use of that facility. Is the house supply via an overhead system, if so there's a good chance that the overhead supply has been PME'd. Check the poles, should be a N-E bonding cable to a rod every third or 4th pole... Along with metal/taffo PME warning signage....