ZS & ZE

[stilllearning]

Hi Folks

I'm part way through my studies and we've just been learning about ELI.
I thought I was begining to understand it until I did a test on my own installation today.

The problem is, is that my ZE reading is HIGHER than ZS. If I'm right ZS=ZE + (R1 + R2). I'm on a TT system and got the following readings :

ZE = 55ohms
R1 + R2 = 0.6ohms
ZS (Measured) is 4.72ohms

If ZS = ZE =(R1 + R2) then surely ZS should be about 55.6ohms??? (Or is ZS simply the R1+R2 measurement?)

To Measure ZE I dissconnected the Main earth cable from the MET and measured between it and the incoming phase.

I also disconnected all MEB's when measuring ZS. I measuerd at all the sockets on the ring but the reading was pretty much the same.

I've got a megger MFT52 and switched it to NO Trip for the ZS test.

Could someone explain where I'm going wrong, please?

Cheers

 

[Spark123]

Zs = Ze+(R1+R2)
Never disconnect the MEBs on a live system for measuring Zs.
The difference in your readings is probably caused by parallel paths.

 

[festa]

Yeah Spark has hit it on the head..
Carrying out the measurement you did your Ze is the resistance to earth
of the supply phase and your earth (rod in your case).
When you measure a Zs on,say a socket outlet you then measurement takes into account parallel paths caused by the main equipotential bonding to pipework which goes to earth.

 

[stilllearning]

Thanks for the quick replies guys - but I'm still confused!!

I know that you're not supposed to disconnect the MEB's but I thought doing so would eliminate all parallel paths.

What other parallel paths can there be other than main Water and Gas supplies??

What sort of reading should I be getting??

Cheers.

 

[Spark123]

Paths can still exist through say the CPC of the circuit feeding the boiler to the water pipes and gas pipes etc. Also any exposed conductive part in contact with a metallic building structure etc.
It is important that the MEBs are in place when you are testing as if they are not, potential may exist between exposed and extraneous conductive parts giving rise to someone coming into contact with two simultaneously and getting a shock.
The incidental connections may give the impression that the pipework is bonded albeit the resistance of the connection will probably be a lot greater than that of a MEB.

 

[stilllearning]

Thanks alot. I think I understand what you mean but just out of curiosity, in an "ideal" situation if there were NO parallel earth paths and the ZE was 50 ohms and R1 + R2 was 0.6ohms then WOULD the ZS measure 50.6??

Thanks again

 

[ricicle]

Yes, because Zs = Ze + (R1+R2)

 

[stilllearning]

Brilliant - thanks again everyone.

 

[plugwash]

Yes, because Zs = Ze + (R1+R2)

if we are being really pedantic shouldn't we take account of the fact that both the Ze and the affects of the cable might not be purely resistive.

though EFLI meters are probablly not accurate enough to tell the difference.

 

[Adam_151]

Not only that, but even if we could measure it very accuratly it would be futile, Ze is going to vary anyway with temperature variations etc

Also the way in which Earth loop is measured isn't fool-proof, see the explanation by docourt in here: http://www.theiet.org/Forums/forum/...=5&threadid=2600&highlight_key=y&keyword1=30%

Its probably just better to think as a loop tester as a think that tells you the earth is actually there, ins't held on by a fine thread and the approx range it lies in, rather than an actual value to 3 places, regardless of what it flashes up on the screen!

 

[ricicle]

Yes, because Zs = Ze + (R1+R2)

if we are being really pedantic shouldn't we take account of the fact that both the Ze and the affects of the cable might not be purely resistive.

though EFLI meters are probablly not accurate enough to tell the difference.

Nothing in Regs says you have to account for reactance of cables when measuring Zs. (We take it into account when working out VD for cables over 25mm²)

I was talking theoretically when the Ze remains constant and Zs is measured at the same temp as when R1+R2 was

 

[plugwash]

Its probably just better to think as a loop tester as a think that tells you the earth is actually there, ins't held on by a fine thread and the approx range it lies in, rather than an actual value to 3 places, regardless of what it flashes up on the screen!

do remember though that just because an earth stands up to an EFLI tester doesn't mean it will stand up to a fault. Remember the picture on here not so long ago of a 3 core SWA reperposed to single phase use but still terminated into a 3 pole switchfuse..................