Ze reading

[Peter Woodford]

Guys

been in to look at a fuse board change and found the following readings on ze reading.

L-PE 0.46 ohms PFC 529A
L-N 0.39 ohms PFC 624A

if i up rate the earth to 16mm this would bring the readings down ?

thanks

 

[Spudnik]

Depending on the supply, which looks like TNS, i would say its fine.

The ME needs to be 16mm anyway.

Why do you want to bring the reading down?

 

[Peter Woodford]

does'nt the reading need to be below 0.35. Or do i just but the height reading on the cert

 

[electricAl]

Because your Ze & Zln readings are different, then we`ve (fairly) assumed the earthing arrangement is TN-S, in which case Ze can be < 0.8 ohms.

Only TN-C-S has to be < 0.35 ohms

In other words, your readings are fine.

 

[Peter Woodford]

Im a bit confused my reading is 0.46 ohms and a tnc should be around 0.8 ohms should 'nt my reading be below 0.8 and not above

 

[Risteard]

Im a bit confused my reading is 0.46 ohms and a tnc should be around 0.8 ohms should 'nt my reading be below 0.8 and not above

TN-S I presume you mean. 0.8 Ohms is 0.80 Ohms (not 0.08 Ohms) which is greater than 0.46 Ohms. As such, it meets the requirement.

 

[acvc]

Im a bit confused my reading is 0.46 ohms and a tnc should be around 0.8 ohms

Pete. It's not the case that the Ze for a TNS "should" be around "0.8". That's the maximum value permissible under the 17th Ed Regs. Check those Regs out!

 

[mechelec]

Pete. It's not the case that the Ze for a TNS "should" be around "0.8". That's the maximum value permissible under the 17th Ed Regs. Check those Regs out!

According to the local DNO max Ze or ELI as they call it is dependant on the cutout fuse size. 0.35 is not the max for a PME supply and can go to almost 1 ohm on a 40 A cutout fuse.

 

[sivoodoo]

According to the local DNO max Ze or ELI as they call it is dependant on the cutout fuse size. 0.35 is not the max for a PME supply and can go to almost 1 ohm on a 40 A cutout fuse.

Whats the claculation to work this out, I am interested

Cheers

Si

 

[xraytek]

According to the local DNO max Ze or ELI as they call it is dependant on the cutout fuse size. 0.35 is not the max for a PME supply and can go to almost 1 ohm on a 40 A cutout fuse.

How are you supposed to design an installation if you cant be sure the DNO will stick to their maximum permissables?

 

[mechelec]

Whats the claculation to work this out, I am interested

Cheers

Si

As far as I can understand it they work on a 5 second disconnection time to bs7671 as the cutout is usually within the property it has to comply with the regs. Check out the max Ze for bs88 and bs1362

 

[CLB]

I was reading a Napit factsheet today regarding Ze readings and on it is a diagram showing the earth disconnected from the MET with the earth lead from the tester attached to it with a croc clip, and the L and N leads/probes on the L and N terminals on the incoming supply. Isn't that testing all three wires to the supply?
I tested mine as shown and got a reading of 0.18ohms. Seems adequate enough to me.

 

[1shortcircuit]

Have you seen the date on the thread you have posted on?

Your Ze sounds fine even if it was to be a TNCS earthing arrangement.

By removing the Main Earth and testing like you explain you are indeed testing the External Earth Fault Loop Impedance (Ze ---- e = external)

Please remember that this is a LIVE test and if you are unsure of the procedure and how to perform the test correctly and what you should be expecting to see on your meter then you should be supervised whilst doing so.

Practicing is all good, killing yourself is not so good... Although considering some of the recent threads some opinions may differ lol (Just kidding )

:thumbsup

 

[Jimmy Boy]

Have I missed something ? Old thread or not, Ze is taken between between Line and the distributors Earth which is disconnected from the board/MET with the CU isolated.
The OP seems to be describing the tests PSSC and PEFC the greater value obtained being the PFC
J

 

[IQ Electrical]

Have I missed something ? Old thread or not, Ze is taken between between Line and the distributors Earth which is disconnected from the board/MET with the CU isolated.
The OP seems to be describing the tests PSSC and PEFC the greater value obtained being the PFC
J

To be fair, he's included the EFLI value at 0.46 Ohms, he just doesn't have a clue what it means or what the recommended upper limits are.
Neither BS7671 nor ESQCR regulations make a maximum limit on Ze values but there was an 'engineering agreement' to attempt where possible to use the commonly known upper limits (0.35 etc etc).