possible to have zs lower than ze?

[Sidewinder]

^Eh?

 

[SLIPSHOD & SLAPDASH]

Are you mixing up the calculations of parallel and series resistances ?

 

[Manator]

I think some people are mixing impedance with resistance. Impedance is represented by Z, which is the first symbol in Ze, and Zs for example.

 

[Lee321]

^Eh?

you add to the loop with the circuit your doing your zs at

 

[kme]

As in Zs=Ze+(R1+R2)?

But back to the O/P.

Lee: What is the issue? Why bring this up at all? If you`ve got the TX adjacent; then ask the DNO what the supply is. They`ll tell you.

 

[apprentice87]

Hi Lee,

Ze is exactly that, earth loop impedance completely EXTERNAL to the installation, so when you measure it, you must isolate the ENTIRE installation COMPLETELY and DISCONNECT THE EARTHING CONDUCTOR FROM THE MET and measure between the incoming phase conductor on the "live" side of the "switched off" incomer and the disconnected earthing conductor itself.

When you measure Zs, the installation is energised, and of course, this means that ALL EARTHING AND BONDING CONDUCTORS MUST BE SECURELY BACK IN PLACE.

The practical effect of this, is that, as parts such as the gas and water pipes are now ALSO connected [bonded] to the MET, as well as the "proper" earthing conductor, the pipes etc, act as "extra" "sooper dooper" earths, [see it as having the water pipes acting as earth rods 10 miles long] so it is quite likely that you may very well have a value for Zs [even though you are measuring at the furthest point of whichever circuit you are testing, which obviously includes the impedance of the actual cables of that circuit] of LOWER than the "original" value for Ze was to start with, purely because of the "extra" "sooper dooper" earths that you now have, due to the MET being bonded to water pipes etc.

john...

 

[tonyyoung]

As in Zs=Ze+(R1+R2)?But back to the O/P.

Lee: What is the issue? Why bring this up at all? If you`ve got the TX adjacent; then ask the DNO what the supply is. They`ll tell you.

Exactly !!

Tony

---------- Post Auto-Merged at 03:39 ---------- Previous post was made at 03:35 ----------

NO, Zs can indeed be lower than Ze,not normally by much though,

are we talking about a normal TN situation here?

in TT it will often be much lower, if you read Ze or Zs and not Ra, this is why an Ra test should be carried out.

How? Zs = Ze + R1+R2

 

[Sidewinder]

Tony,

I know of an install that has an external earth fault loop impedance of around 160 ohms

The final circuit earth fault loop impedances are around 8 to 9 ohms

 

[Andy™]

Tony,I know of an install that has an external earth fault loop impedance of around 160 ohms

The final circuit earth fault loop impedances are around 8 to 9 ohms

pretty much describes most TT installations

 

[batty]

Well i had a good TT install the other day external garage. Whole installation is TT but i have used 2 coe swa to supply building so have also TT'd Garage no sevices so rod is only earth. Ground is clay and wet i did go through a hard bit probably could have been a car or something as site was agricultural. RA was 1.5 ohms pretty damn good for TT i thought. Shame there not always like that.

 

[batty]

Don't be silly he weighs them in.

 

[Andy™]

Don't be silly he weighs them in.

in pieces though. his horse & cart cant carry the whole thing in one go

 

[hifly]

I did a TT job a couple of years ago the rod was giving about 30 ohms, when i did the Zs i got around 0.8 ohms on a RFC.

next door was pme and the bonding was using thier earth path, probably on both gas and water.

 

[batty]

Your on shakey ground here with Steptoe you really should have rodded next door.

 

[Kama]

Tony,I know of an install that has an external earth fault loop impedance of around 160 ohms

The final circuit earth fault loop impedances are around 8 to 9 ohms

Is that Ze & Zs

 

[kme]

Yes, in that order.

Actually, ignore that. No - technically, they`re Ra and Zs