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Discuss High Ze on a TNCS system in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

L

loopy

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Whilst doing a Periodic Inspection on a premises that had a TNCS system i got a reading of
0.68 at the M.E.T that was four inches away.... the link between the head & M.E.T was a 16mm2 Main Earth

due to the limits being 0.35 on a TNCS system what actions should be taken... should there be a increase in the main earth to a 25mm2
plus as i always asked questions alot when i was a boy,,, why if in a TNS system 0.8 is the max then this situation would of being acceptable..

question 3 being then if all circuits still fall inside the max earth loops for the mcbs then is there a problem ... or am i from another planet ? thanku if anyone has any thoughts on this :)
 
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S

Spudnik

  • Thread Starter Thread Starter
  • #2
I would maybe contact the DNO and ask them, as they have supplied the earthing point and it should be .35 as you know.

I dont think increasing to 25mm2 would bring the reading down that much to be honest.

Even if the circuits are well within the max allowable ZS, you still need to record and measure Ze, even on a MWC, so i would not be comfortable writing that down.

This would also make your PFC higher as well:)
 
S

Scott F

  • Thread Starter Thread Starter
  • #4
Know this one from experience if you have high ze readings and zs .First thing i would do is check the main earth connection from dno main fuse to board (relavant for a tncs system )Had all the sockets off checking connections twice!turned out although looked connected was nor tightened up properly at brass connection at side of fuse .
 
W

WB Scott

  • Thread Starter Thread Starter
  • #5
Had this problem this week, Was getting a constant reading of 0.42 ohms on the incomer, after checking suppliers side(naughty). The installation was protected via RCD Main switch and all circuits fell within the allowed limits. I made note on my Observations sheet, Let the occupier know and sent them a covering letter explaining if full and that they should contact there Supplier and ask them to look into then matter
 
T

trebor

  • Thread Starter Thread Starter
  • #6
Did you check the PSCC ? it is extremely unusual for a TN-C-S to be this high, testing the PSCC will confirm whether there is perhaps a loose connection of the earthing conductor in the head, some of the earlier heads had internal links which had to closed by the the installation inspector on connection, if the PSCC is also high then the problem is a high impedance neutral on the network as obviously PSCC and PEFC should be the same on PME
 
J

james king

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  • #7
hi can some one explain the max ze result allowed for TNCS= 0.35?
TNS=?
TT=?
and also the max loop inpedance (zs) for each system?

also what reading is good reading for r1 plus r2 and the lowest readin for IR test ?? would realy help thanks
 
S

sparkswillfly

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  • #8
jason what mw certs do you use as i have never recorded ze on the ones i use only zs
 
J

james king

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  • #9
help i have a zs reading of 2100 ohms on tt system what should i do ?????? to make it lower alot lower!!!???????
 
T

trebor

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  • #10
Re: High Zs on a TT system

Hi james

if you have this high a reading on the electrode you will need to instal a much longer electrode to get into some decent soil or use a soil conditioner, what type of ground are you in and where are you measuring from, the max now for BS7671 is 200ohms

hi can some one explain the max ze result allowed for TNCS= 0.35?
TNS=?
TT=?
and also the max loop inpedance (zs) for each system?

also what reading is good reading for r1 plus r2 and the lowest readin for IR test ?? would realy help thanks

The DNO`s quote 0.35 for tn-c-s
0.8 for tns
21 ohms for tt which is the supply electrode, when you add your electrode and earth conductor resistanc to this it should not exceed 200ohms for the 17th requirements

max earth loop impedance is related to the protective device operation not the particular system

r1 + r2 ? how long is a bit of string, it depends on circuit length, csa,a nd resistance per metre of the conductor

IR 1 Mohm for low voltage with a reading under 2 Mohms to be investigated when measured at the tails or bus bar
 
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J

james king

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  • #11
i am measuring from sockets with a new megger mft 1553 its at my house but just want to get use to the tester if u coulp help with some question it would rele help i thought max zs for a 30ma tt is 1667ohms 17th Edition Maximum Ze & Zs Values at bottom of page

right this is my readys testing from a socket!

loop = 2110 ohms
pfc= <1 A

now i dont understand pfc what reading should i be getting? the circuti is on with al earthing in place! i can not seem to get a ze reading from my megger either?
 
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T

trebor

  • Thread Starter Thread Starter
  • #12
there are 2 different issues here james, THE 17TH quotes 200 ohms max is for the installation earth electrode of a TT system when measured between your earth conductor (disconnected and live, same as measuring Ze on tn systems) WARNING !if you are going to try this dont for get to turn off the ccu as you will be leaving the installation without an earth during testing, this the correct way to measure the electrode resistance when combined with rcd`s


1667 ohms is the max resistance (Zs) for a 30ma rcd/rcbo to operate within the required time to prevent a touch voltage exceeding 50V on exposed mwork etc ie 50/0.03A = 1666.66, under normal circumstances you would never expect a reading this high or indeed readily accept it

with your reading of 2100 ohms I would suspect it is the high electrode resistance otherwise you have serious circuit problems
 
J

james king

  • Thread Starter Thread Starter
  • #13
lol well my dad wired it all up, he done the whole house been sparly for 25 yrs lol. but has no clue of regs or testin as retired.

ok so i want a min readin of 200omhs for tt (zs) what min reading for tns tn cs? sorry to keep anoyin u by asking loads of question!

what is a good pfc result?? as i am geting <1A

Ze testing = turn off supply test between L and E and N with earting disonected????

sorry have not tested for over a yr and for got every thing got essesment in 2weeks.. need all knowlege i can get

thnaks
 
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G

Guest123

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  • #14
James,

If you have your assessment coming up in 2 weeks as you say then first thing Monday go out and buy yourself a copy of Guidance Note 3, seriously it'll help you no end and has the answers to all things testing.


Have you got jobs lined up for your assessor to see, these will need completed test certification for him to see aswell.

Good luck.:)
 
J

james king

  • Thread Starter Thread Starter
  • #15
ye i tryed to get that but no one had all whoe salers neva had one. even tryed water stones!!! im sure to get one ne way. yes i have got 2 jobd lined up one being a full wiring of a flat and other being as wiring of a garage all in pvc conduit with new CU. 2 radial sockets and 1 light. what cert do i need to show him is it the green one?

also can u please help with a pfc test ? what is good reading?
 
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G

Guest123

  • Thread Starter Thread Starter
  • #16
Hi James.

Sounds like you will need an Electrical Installation Certificate for each one of those jobs.

You can download certificates from the IET website or you should be able to buy them in the wholsalers i.e Robin own brand or Kewtech etc etc.

NIC will also sell you the green certificates I think you mean.


With regard to your PFC, this is in relation to your Ze as the impedance of the earth fault path will restrict the amount of fault current that will flow.

So with the Voltage of 230V and Ze of 2100 - 230/2100 = 0.11A, which isn't enough to cause your MCB/fuses to operate in the required time before the safe touch voltage of 50V is exceeded!!!! so that's why RCD protection is needed on TT system...I hope you have RCD there as you are in quite a bit of danger if not.

You really need to get the Ze (Ra) down to below 200ohms ASAP and RCD if not already present.


HTH
 
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J

james king

  • Thread Starter Thread Starter
  • #17
yes at the mo my metre reads <1A all circuits r rcd protected what is maz pfc reading on tt tns tncs?
 
G

Guest123

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  • #18
The max fault current is limited by the KA rating of your protective devices i.e, a Type B BS 60898 MCB is rated to a max of 6KA, BS 1361 Type 2 (suppliers fuses) are rated to a max of 33KA.
 
J

james king

  • Thread Starter Thread Starter
  • #19
thanks lenny ur a great help!1 other thing i have for got the R2 test for lighting and ridal sockets all that is is conect live and cpc at the CU and test from socket or lighting point eg only fil in r2 test slot leave r1 and r2 blank????

Ze test is from incoming live incoming nut but with a dissconneted earth and test between live and nutral and earth bar? i done it to day got a reading of 0.18ohms and a Zs of 0.08 i think off top of my head il have a lok at test sheet?
 
G

Guest123

  • Thread Starter Thread Starter
  • #20
thanks lenny ur a great help!1 other thing i have for got the R2 test for lighting and ridal sockets all that is is conect live and cpc at the CU and test from socket or lighting point eg only fil in r2 test slot leave r1 and r2 blank????

Right, the test you have described there IS the R1+R2 test i.e linking line and CPC at CU and testing at extremity of the circuit.

The R2 test is the "long lead" test whereby you connect one end of the lead to the CPC at the CU and test betwen the other end of the lead and CPC at the extremity of the circuit.


Ze is measured by turniing off all breakers RCD's and main switches, disconnecting the main earthing conductor from the earthing terminal/bar and testing between this disconnected conductor and L+N at the feed side of your main switch.


Zs is measured with all conductors re-connected, all breakers, RCD's and main switches re energised and taking a reading at the extremity of each individual circuit.


HTH.

Ps, make sure you get that copy of GN3.;)
 
J

james king

  • Thread Starter Thread Starter
  • #21
yes i will b going denmas mon morn to get one its all cuming bk to me now

r1 and r2 for final ring is crossing over cpc and live from oppisite cables.

got a diagram for u to c il put it on nw on ze

how do u do it lol
 
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G

Guest123

  • Thread Starter Thread Starter
  • #22
Dont be confused by r1, r2, and R1+R2.

They are different tests altogether.
 
J

james king

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  • #23
sorry can u explain?

r1=
r2=
r1+r2=

would rele help!!
 
G

Guest123

  • Thread Starter Thread Starter
  • #24
sorry can u explain?

r1=
r2=
r1+r2=

would rele help!!

r1 = end to end test of LINE conductor of a ring circuit at the CU
rn = end to end test of NEUTRAL conductor of ring circuit at the CU
r2 = end to end test of CPC of ring circuit at CU

R2 is the long lead test I mentioned earlier

R1+R2 is the linking out of LINE and CPC of a RADIAL circuit at the CU and measuring at the extremity of that radial circuit

OR

Cross connecting L1 + CPC2, and, L2 + CPC1 conductors of a RING CIRCUIT at the CU and measuring at each socket outlet on that ring circuit.
 
J

james king

  • Thread Starter Thread Starter
  • #25
yes got it now thank u!! funny how if u dont test for a long time u forget every thing isit it even tho i got 17th 2391 and 2330 L2 L3 LOL

ONE MORE TEST I WANT TO KNOW AND IF U COULD HELP IS SHORT CIRCIT TEST? I KNOW U HAVE TO TAKE THE HIGHEST READING EITHER FROM PFC OR SHORT CUIRCIT??
 
G

Guest123

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  • #26
PFC is the higher of the two readings of PEFC and PSCC.

PEFC is measured L-E, PSCC is measured L-N.

On a 3 phase system as a rule of thumb, the PFC is doubled as the highest fault current would be between phases.
 
J

james king

  • Thread Starter Thread Starter
  • #27
Ok thanks lenny very helpfull o ye if any one can help with what question the assessor will ask with the answers u would give with b good

need all info i can collect

cheers every 1
 
J

james king

  • Thread Starter Thread Starter
  • #29
Ok kl thanks voltman can u give sum advice on the assessment

i neeeeeeeeeed to pass no matter what will loose lots of cash if i dont
 
V

VOLTMAN

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  • #30
the thing is not to panic my assessor was great hes there not to pick holes but to help . he asked me questions on earthing and did tests on r1 r2 on a lighting circuit and some rcd tests make sure you got all paper work ready for him good and plenty of tea good luck
 
J

james king

  • Thread Starter Thread Starter
  • #31
paper work !

il give him bout 4 testing certs

invoices i hav gave to clients

and some quotation i have done thing is i have never done a quote all my clients just said they want me any way. have not done one in 2years. just give ruff estimate and thy say ok kl, shw them a few figuers and away i go what other paperwork will i need
 
V

VOLTMAN

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  • #32
he asked me for pubic liability ins and company health and safety policy .meter calibration certs etc took about half a day
 
J

james king

  • Thread Starter Thread Starter
  • #33
i got calibration certs ect the women said i can have insurance on the eassessment day? health a nd safty? what shel i do for that ? im going to fail i no it lo


how do i start a thread i cant seem to do it ?????????l
 
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V

VOLTMAN

  • Thread Starter Thread Starter
  • #34
youll be ok who are you goin with ? oh and remember always replace cu cover when ever you leave it to do tests
 
J

james king

  • Thread Starter Thread Starter
  • #35
ye isolate cu test incomer L and N for voltage make sure dead. re test ur voltage on a volatage detector carry out visual inspection , then dead teast r1 r2 and IR then i would carry out a ze test with dissconected earth then a zs test with all eartin back on and CU live. then rcd test. say i do a r1 r2 test and connect at cu then go off to test the last point shel i put cover back on even thought its dead? im going with napit! what r they like?
 
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V

VOLTMAN

  • Thread Starter Thread Starter
  • #36
yes i would prove dead then lock off make sure covers on when you leave it. heard good things about napitt electricians guide to building regs is a good book got loads of info in there
 
J

james king

  • Thread Starter Thread Starter
  • #37
well looks like i will be going shopping monday morn pop to wholesales then go water stones lol as got a massive cricket shed to rewire. thansk for the help voltman and lenny

o yea and i have not got a lock off kit or voltage proving unit ? shel i get one i no thy r exensive for what thy r!! if not how should i prove live amd dead properly ????
 
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G

Guest123

  • Thread Starter Thread Starter
  • #38
how do i start a thread i cant seem to do it ?????????l

Click on the forum you want to start a thread in i.e inspection & testing, commercial & industrial etc. At the top there is a button "NEW THREAD", click on it and away you go.:)
 
T

trebor

  • Thread Starter Thread Starter
  • #39
Whilst doing a Periodic Inspection on a premises that had a TNCS system i got a reading of
0.68 at the M.E.T that was four inches away.... the link between the head & M.E.T was a 16mm2 Main Earth

due to the limits being 0.35 on a TNCS system what actions should be taken... should there be a increase in the main earth to a 25mm2
plus as i always asked questions alot when i was a boy,,, why if in a TNS system 0.8 is the max then this situation would of being acceptable..

question 3 being then if all circuits still fall inside the max earth loops for the mcbs then is there a problem ... or am i from another planet ? thanku if anyone has any thoughts on this :)
Re high TNCS, you could try the Ze test on the neutral incomer (PEN conductor) to the main switch with the neutral disconnected and the installation islolated, this may useful for the dno as a good reading on the neutral will point to a connection problem within the service head, on the older heads there is a seperate link to be closed between neutral and earth terminals, its possible this could be loose
 

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