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lugz89

evening

i am fillling out my logbook and am asked what the purose of doing a Ze test is can anyone tell me please

thanks
 
To ensure that the DNO supply earth system is operating within the correct tolerances for the type of system installed.
 
It depends where the question is coming from, if you are talking initial verification for example the answer is as james correctly said to ensure that there is an external earth present at or less than the stated value for the system where supplied, but also to verify that the value is equal to or less than the designers figures.
 
to ensure that max permitted disconnection time can be achieved under fault conditions
 
I recently was asked what is the most convenient method of obtaining Ze

a. measurement
b. calculation

I thought it depends on situation and who its convenient for ie. client or spark.

I would go with measurement for the above reasons.
 
I recently was asked what is the most convenient method of obtaining Ze

a. measurement
b. calculation

I thought it depends on situation and who its convenient for ie. client or spark.

I would go with measurement for the above reasons.

Surely measurement or enquiry... How would you calculate?
 
always get mine by measurement. it's cheaper and quicker than phoning DNO and listening to stupid automated multiple choices, why don't they include " press * for a human". followed by crappy music, where you get cut off before somebody decides to pick up the phone
 
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Surely measurement or enquiry... How would you calculate?

Well the GN3 states

Ze is determined by Measurement, Enquiry or calculation.

I suppose the remainder from Zs after subtraction.

Not sure if I would ever take that route though.
 
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Would anyone agree that Measurement and Enquiry would be the best option. One to prove there is actually an earth and two, to prove the system is as designed.

I was also thinking how could you measure if it was not convenient to de-energize from the clients point of view and how could you get an accurate Ze from calculation not including parallel paths.
 
yes mate agreed
measurment first option to give a accurate value and also to prove there is actually a earth present , enquiry second and calculation from measuring pefc as des has stated last option

but they are all acceptable as GN3 states
 
I know and agreed, but you would have to disconnect the earth for pefc and if that was not an option was wondering how, if possible, an accurate Ze could be measured under those circumstances as pfc or Zs would include parallel paths from what I know.
 
no mate when measuring Pefc or pscc to find pfc you dont disconnect the earthing conductor you only disconnect the earthing conductor when doing Ze

the earthing conductor has to be connected to carry out a pfc test
 
Ze testing Testing by Megger-18 - EletriciansForums.netZe testing Testing by Megger-17 - EletriciansForums.net
I sit corrected, cheers mate, I don't know what I was thinking there. :eek:


I guess, it would include the parallel paths then.
 
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yes mate for pfc earthing conductor and all bonding in place which is good because the lower the value of the Ze the higher the current that will flow in the event of a fault so will allow enough current to blow the fuse/Cb quicker than the max disconnection times and with the earthing conductor and main protective conductors connected it will lower the value of the Ze because of the paralell paths from the extraneous conductive parts this is what we want more current the better and as long as the pfc is lower or equal to the breaking capacity of the protective device then it will operate correctly and if all your protective devices in the installation have a higher breaking capacity than the pfc then you only need to measure pfc at the origin
Pfc is always higher at the origin so this is the best place to measure and its the highest of the pefc and pscc value which is the PFC for single phase
for three phase measure all lines to Neutral seperatly and double the highest by 2 to get the three phase pfc

When measuring Ze you are checking to see there is actually a earth present and is sufficiently low enough to meet the maximum for the system to allow enough current to flow through to the star point at the transformer and back to your suppliers protective device ie cut out fuse and then through the consumers protective device ie CB to meet max disconnection times for Zs

when measuring Ze by disconnecting the earthing conductor from the MET and disconnecting the main protective bonding conductors will give you the true value of Ze without the paralell paths
and also to check it complies with the designers intentions
 

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