Radial Zs Testing Question

[kerching]

I am sure there was a pic of him in PE doing a test at a light whilst up a pair of steps....may not have been EFLI though.

It was within the last 6 months or so

Just vaguely remembering

 

[Manator]

Manator ...... Insulation tests should be carried out with all earthing able is sound aNDnd cpcs connected

If you are conducting tests on a single cable all three or five cores need to be tested individually.

Insulation testing is one of the most neglected tests carried out,I carry out insulation tests prior to second fix, this is carried out with no cables connected to supply.

I fail to see how a connected cable to earth, neutral or otherwise would give a satisfactory reading, when doing a simple insulation test.

The insulation test is to prove the integrity of the cable installed. Nothing more and nothing less. It proves that the cable is suitable for use, free from damage and capable of performing to its use and installation method.

I do not think that my tester can throw an insulation test down a connected cable, Its always been dead testing.

 

[steptoe]

a global IR test is/was permitted within a PIR during 2391 and if results were dubious then individual testing was required,

dunno what the watered down versions allow,

initial verification did NOT allow global testing IIRC

I may be wrong..

But I think you will find one of the points of consideration when doing a Zs that they are referring to is not just your health & safety..

But the safety of others at the same installation where you are working...

You are putting a test voltage down the CPC to all associated earthy parts connected to it...

There is a comment in guidance note 3 about risks of shock hazard to The testing personnel,

Other persons Or Livestock on site.. 

when doing direct live measurement of Zs.

i.e.

An elderly person trying to switch on the light in a room on the same circuit, touching the nice decorative metal light switch while you stick your test voltage down while plugged into the light fitting next door...

May get a bit of a stray voltage as they become a parallel earth path...

(especially if real earth path is a tad faulty and not as low resistance as it should be)

Obviously the Loop Tester should cut itself off if the test voltage is getting to high..

And you should have verified your earth continuity first on your dead tests...

But I think the video comments (I haven't seen it) may be relating to the dangers of just going to an existing live installation & doing a Zs thinking it posses no danger to anyone.

Hence the comments in GN3 & OSG in the Zs testing bit!

It is not only you that has to work safely...  you must also be aware of hazards, risks and dangers you can impose on others that you may not know what they are doing..

opcorn

many moons ago as an apprentice I carried out a Zs/Zdb on a submain that was on a TT installation,

and got a lot of colourful language from the plumber on site,

I soon learned all about earthing systems after that.........

 

[apprentice87]

Hi Manator,

So far as i know it is in the regs that IR is tested with the cable CPC connected to the earth bar [and earthing and bonding in place].

There is a reason for this;

I installed a cable once. I did all the IR tests, firstly on the cable itself [as I always do], and all was fine.

Once i had connected it up to the CPC though, and retested [as i always do!], the original reading between one of the conductors [cannot remember which one] and the CPC fell to about half. It was still about 400 odd Mohms though [how you do the symbol for Ohms??] but i was not happy, there HAD to be something wrong, something MUST have changed purely by sticking the CPC in the earth bar.... and i was determined to track it down..

Turns out that a screw in a damp brick wall had just nicked the cable, so my tester was, instead of measuring the insulation between the cable cores, was measuring between the damaged conductor, through the brick wall, the ground, and up the gas pipes or whatever back to the earth bar. If you like, the wall and the ground it stood on, was forming a conductive path between the MET and the cable conductor.

If i had not tested with the CPC in the earth bar, i would have NEVER have found this fault. Now, imagine if the screw that nicked the cable was touching something conductive that was not bonded, a metal window frame or something, could have been a problem then...

john....

 

[Sidewinder]

Now I do this 2 ways.

I check the disconnected ends of the cable to each other to check for cable damage, then connect the cpc @ supply end and test again.

That way I believe that I can be sure that the cable is clear internally before connecting, then be sure it is clear to earth after connecting.

Just me being a fussy so and so.

 

[Sharpend]

Have to agree Sidey. My preferred method.

 

[apprentice87]

Yep, as i explained, i do this too!

john..

 

[Evans Electric]

If theres a regulation  , H&S I assume, that says you cannot test when ....how would you test to see if a circuit was live or dead ?   

If someone says this machine doesn't work anymore , the first thing I do is stick the testers across the supply at the machine to see if theres a feed . 

If I follow this Regulation  that is spoken of , I'd have to switch OFF the supply  ...test if its LIVE or DEAD .............  I'm betting its going to be DEAD ...Hmmm  !  But I need to see if all the phases are there , say,  but I can't do that while its LIVE , so I'd have to carry out dead continuity checks    , so I'd have to pull the fuses in the distribution board .......but wait ! ...Its LIVE in there ...so  I'd best pull the fuses at the Main Distribution Panel ...Oooops Its LIVE in there too .....  so ..ermm.... I guess I'd have to phone the DNO and ask them to shut down the Midlands while I pull three fuses .  

 

[Sidewinder]

I was in an IET presentation given by our local Chairman, at the time, who was also the Head of Electrical for HSE in Wales.

He made a definite distinction, between Live, WORKING, and Live TESTING.

Live testing was, in their opinion, fine, with the usual caveats, suitable equipment, competent people, safe methods etc. etc.

However, he was dead against any form of live working.

Except by DNO's and the like, it was felt that it could be justified in their case.

The DNO's have a lot of competence, and tried and tested proven procedures for live working, which work!

Hence the distinction.

 

[Sharpend]

Now that is sense!! maybe that's why he's the head of electrical at HSE?

Or is that not a case of stating the obvious?

 

[Andy™]

I was in an IET presentation given by our local Chairman, at the time, who was also the Head of Electrical for HSE in Wales.

He made a definite distinction, between Live, WORKING, and Live TESTING.

Live testing was, in their opinion, fine, with the usual caveats, suitable equipment, competent people, safe methods etc. etc.

However, he was dead against any form of live working.

Except by DNO's and the like, it was felt that it could be justified in their case.

The DNO's have a lot of competence, and tried and tested proven procedures for live working, which work!

Hence the distinction.

exactly. massive difference between live testing and live working

 

[Max6979]

I asked same question to my tutor at college as I have my practical inspection and testing exam  (C&G 2365 level 3) on 9.02.15 and was told that I need to use the R1 + R2 value from the dead test and then use the OSG having determined the supply type ( TN-S / TNCS) to get the Zs value as R1 + R2 + Ze=Zs.   This is what I have been taught but I would appreciate any guidance or professional opinions.

 

[Andy™]

and what if the Ze is high or non-existant? or so low that your PFC is too high?

 

[pewter]

I asked same question to my tutor at college as I have my practical inspection and testing exam  (C&G 2365 level 3) on 9.02.15 and was told that I need to use the R1 + R2 value from the dead test and then use the OSG having determined the supply type ( TN-S / TNCS) to get the Zs value as R1 + R2 + Ze=Zs.   This is what I have been taught but I would appreciate any guidance or professional opinions.

could be because some of the test boards at collages have a high Zdb, so by doing calculation your result can be checked against maximum Zs.