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Posted By Topic: Advice or knowledge on earth testing

OctaneOutlaw
Oct 20 2018 10:47

Greetings

I\'ve just been reading through section 8 of 3000 and 3017 to further my knowledge of these subjects

\"The resistance of the main earthing conductor and any equipotential bonding conductor shall be
not more than 0.5 Ω.
The resistance of protective earthing conductors shall be low enough to permit the passage of
current necessary to operate the circuit protective device. This condition is satisfied when the
fault-loop impedance is low enough to allow sufficient current to flow in the earth fault-loop to
cause the protective device to operate within a specified disconnection time.\"

Questions:
(1) MEC value is simply from the disconnect start of the MEC to the electrode connection as per 3017 firgure 3.1? Correct? What about with buried Earth\'s where from what I\'ve seen they normally bring part up into a wall, what if there was a break downstream or am I just over thinking it?

(2) I understand that in 3017 table 3.2 may be used as a guide to the PEC value for 0.4s disconnection time, and I understand these values are probably just another cause of making things super simple using them for all conductors but just for my understanding on things where the disconnection time is say the 5s time could value of continuity be higher than in table 3.2 and still be deemed as compliant provided they would still cause \"protective device to operate within a specified disconnection time\" which for said item is 5s?

(3) Do we need to check all protective earthing conductors or just those used for the equipment that needs them such as exposed metal or earth pin on sockets? Because in figure 3.2 at the top it has a picture or a lamp holder earth terminal being tested?

I believe that\'s all for now, thanks
   

OctaneOutlaw
Oct 20 2018 12:41

Just a little further to make sense on question 3, what I\'m meaning is for example in 3000 where 5.4.3 requires an earth conductor at every lighting point do we still have to ensure continuity of these despite if they are used or not? Is more what I\'m trying to get at
   

AlecK
Oct 20 2018 12:46

Overview
Section 8 is about what tests are required when doing PEW. It does include some mehodology but on;ly where a particular method is required to be used.

\"3017\" is about how to do tests. It doesn\'t say anything about when the tests it describes are re4quired.

Q1A
\" MEC value is simply from the disconnect start of the MEC to the electrode connection as per 3017 firgure 3.1? Correct?\"

An MEC should be measured from one end to the other. To get a valid reading you have to eliminate any parallel paths; so need to disconnect one end - generally the MSB end, leaving the connection to electrode intact.
This is the method illustrated in \"3017 Fig 3.1.


Q1B
\"What about with buried Earth\'s where from what I\'ve seen they normally bring part up into a wall, what if there was a break downstream or am I just over thinking it?\"

A buried horizontal earth electrode can be a bare copper conductor, in which case either the same conductor can be extended to act as the MEC; or a different, often smaller, conductor can be the MEC. If extended, there is no \"connection\" between the two, but one part (the MEC) is insulated and the other (the electrode) is bare. That point must be accessible for testing exactly the same as for other electrodes.

If there were a break downstream then chances are the electrode would not comply. But since it\'s a 25 mm2 conductor what\'s going to cause such a break?

Q2
\"I understand that in 3017 table 3.2 may be used as a guide to the PEC value for 0.4s disconnection time, and I understand these values are probably just another cause of making things super simple using them for all conductors but just for my understanding on things where the disconnection time is say the 5s time could value of continuity be higher than in table 3.2 and still be deemed as compliant provided they would still cause \"protective device to operate within a specified disconnection time\" which for said item is 5s?\"
There\'s a glitch in that Table (which is also Table 8.2 of \"3000\". The column headed \"fuses\" is for 5 sec disconnection time. And yes, with a longer permitted time, the EFLI can be higher. so of course the part of the EFLI that is the PEC of a submain or subcircuit can be higher. If you want to know just how high eg for an mcb instead of for a fuse; then you\'ll need to swot up App B and do some calculations. But generally, if the Re value is higher than allowed for 0.4 sec, you have a problem that should be found & fixed.

Q3
\"Do we need to check all protective earthing conductors or just those used for the equipment that needs them such as exposed metal or earth pin on sockets? Because in figure 3.2 at the top it has a picture or a lamp holder earth terminal being tested?\"

Fig 3.2 of 3017 is just an illustration of the test method. The rule is in \"3000, in both 8.3.3.1(a) and 8.3.5.1. Neither of these clauses allows any earthing or bonding conductor to be missed. ALL have to be tested. Including the ones that are required to be provided at every lighting point.

   

OctaneOutlaw
Oct 20 2018 14:21

Right so from your response if I understand you\'d simply disconnect the MEC at the switchboard to eliminate parallel paths and then test from here to the other end of this conductor and not the actual electrode, so for the buried type (with a smaller sized MEC in between let\'s say 6mm from the board to the 25mm buried electrode) are you just testing the 6mm earth as that is the insulated MEC? In the same way that with a stake it would just me the 6mm earth conductor that is being checked for the 0.5 value? And making sure that the end of this MEC where the connection is made is always accessable? Is my understanding correct now or am I still missing something?

Second part makes sense, basically can be higher but most likely if it\'s higher than that table for the 0.4s value there is normally something wrong causing it

But I just don\'t really get the last bit, why do we need to test these conductors if they aren\'t connected to anything? Wouldn\'t the technically make them not a PEC because they aren\'t connected or protecting anything? Why must we establish that they would trip protection in the allowed time in a fault if they aren\'t actually protecting anything? Does that make sense? Just curious as to the reasoning that\'s all. Is this not any different from pulling off every double insulated item with an unused earth conductor behind? Or are you suggesting we do the same there? Not saying you\'re wrong or anything just trying to better understand why and the reasoning

Also how do you suggest we test this? Pull down every down light for example and then use a trailing lead to the terminal or conductor connection behind?
   

AlecK
Oct 20 2018 15:08

You have to test them because you installed them. If you don\'t test them, how do you know you\'ve done the job correctly? If you have a string of lights, and there\'s a break anywhere from the switchboard to the end of the string, then there isn\'t a PEC at the last point. The ONLY way you can tell that you actually have a PEC at that last point (or any point) is to test it.

Later on someone else WILL change the luminaires, and the PECs WILL be needed - which is why they are required to be installed. And tested.

   

OctaneOutlaw
Oct 20 2018 15:34

Interesting, thank you for the responses, I\'d be interested in seeing what other peoples views on the matter is

I\'ve learnt a lot reading your responses on other things and has definitely improved my overall knowledge so keep doing what you do, it\'s much appreciated for us trying to learn and understand things!
   

OctaneOutlaw
Oct 21 2018 09:08

I\'ve just been thinking and have a further question for anyone

When there is a main board and a distribution board(s) how does testing Earth\'s work here?

Do I test the PEC between boards in the same way and use the value from the table?

When testing PECs that are supplied from the distribution board do I test from each point in the subcircuit back to the distribution board? Or must I test from them all the way back to the MEN location in the MSB?

What about with bonding earth conductors? Do they need to be 0.5 back to the distribution board or back to the MSB?

If there were distribution boards run off one another would this be any different also?

Thanks
   

pluto
Oct 22 2018 10:55

Octaneoutlaw

I note that you are able to use some constructive thinking in understanding of the fundmental outcomes for electrical installations, may I suggest that you get yourself a free textbook available on the web link.

It is a large file and best viewed on large laptop computer, and is 580 pages long but contains a lot of material over a wide range of electrical equipment. I have a soft copy on my computer (PDF format) and only copy the pages needed from time to time.

It contains the IEC recomendations for electrical installations based on the international wiring rules. Which are very simular to AS/NZS 3000 with 1 or 2 notable exceptions in which AU and NZ do their own thing!

The MEN system that we use in AU and NZ is an adaption of the international supply system TN-C-S and the publication also covers some other systems of supply which are likely to reach New Zealand in the medium term (3 to 5 years away) and number of other subjects which include disconnect time of protective devices.

I think you find it more instructive than AS/NZS 3000.
http://en.electrical-installation.org
   

OctaneOutlaw
Oct 22 2018 12:44

Thanks Pluto, I\'ll be sure to have a look, I am interested in furthering my knowledge best I can and understanding things past just the point of knowing what to do but why we do it

Appreciate it
   

AlecK
Oct 23 2018 09:28

\"When testing PECs that are supplied from the distribution board do I test from each point in the subcircuit back to the distribution board? Or must I test from them all the way back to the MEN location in the MSB?\"

As far as mandatory testing is concerned, we test the work we do. If what we install is only part of an installation, we test that part.

For PECs, the requirement [8.3.5.1] is to test each PEC. So where you install a new subcircuit from a DB, we test the new subcircuit. True there is a submain, but whoever installed it was required to test the submain PEC prior to \"placing it into service\". True the added impedance of the submain PEC increases the overall impedance of the fault current path, but the Re values in the Table are conservative; and (bad connections aside) when it comes to conductor impedance we\'ll be in trouble for volt drop before we have too much EFLI.

Think about the worst case. For impedance added to the EFL by a submain, the worst case is where the active and PE conductors of the submain have the highest impedance (ie smallest conductors). Say the submain is 2.5 mm2; the CCC of 2.5 mm2 is max 31 A and generally less; so worst case protective device will be no higher than 25A but more likely 20 A, for which Table 8.2 allows a Re value of 0.49 ohm max. The active is same size, so the absolute worst case for added impedance of a submain is 1 ohm. When it comes to disconnect times for fault protection; 1 ohm isn\'t enough to be a problem.


\"What about with bonding earth conductors? Do they need to be 0.5 back to the distribution board or back to the MSB?\"

Same as for PECs: test the conductor, to ensure it is continuous. If it originates at a DB, test to the DB.


\"If there were distribution boards run off one another would this be any different also?\"

Yes this scenario adds more impedance; but as above if each component part is OK the overall result will be OK.

The requirement to test is set by ESR 59.
Separately, we also have an obligation set by ESR 13 that the work done on part of an installation does not make any other part less safe. There are no specified tests for this; it\'s an area where we have to rely on an understanding of basic principles.

\"3000\" is not supposed to teach theory; it\'s job is to set minimum requirements and provide a little bit of guidance to people who have already been trained.

So it reminds us [8.3.5.1] that the basis for testing earth continuity is allowing a high current to flow under earth fault conditions in order for a protective device to operate; and that this is done in order to avoid exposed conductive parts being at dangerous voltage. And it sets a required result for the tests that\'s dependent on the protective device for that particular circuit. Woth a subcircuit fed from a DB, we don\'t want a fault on the subcircuit to operate the submain protection, we only want it to operate the prtoection for that subcircuit.

For bonding conductors, there\'s NO protective device, and they are not intended to carry high fault currents. All they are expected to carry are tiny currents in order to equalise the potential of various conductive parts. Hence the name \"equipotetntial bonding\"; and an fixed maximum value that is deemed to be OK.
Similar for MECs. The MEC isn\'t there primarily to carry fault currents, because under normal circumstances these will flow back to source via the main neutral and only a tiny part flows down the MEC, through the electrode to earth, and through earth back to source. An MEC is primarily an equipotential bonding conductor, keeping the neutral at or near earth potential. But under some fault conditions, eg a broken main neutral, the current from an earth fault will flow in the MEC (because there\'s no other path for it)- though unlikely to be enough to operate the overcurrent device (supply fuse) within required time (or at all). But enough to justify sizing the MEC according to the size of the active, rather than a purely nominal size as for equipotential bonding. We can control the impedance of the MEC, but we can\'t control the impedance of the mass of earth; so again we use a nominal value of impedance for MECs.


   

OctaneOutlaw
Oct 23 2018 13:25

Thank you AlecK, that has been very informative as always, helps a lot