High Ze on a TNCS system

[loopy]

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

 

[Spudnik]

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

 

[Guest123]

Agreed.

External DNO problem there somewhere. Replacing the 16 for 25 wont make an ounce of difference over that distance.

TN-S and TN-C-S are two different beasts with a different set of parameters altogether.

Have a look here.

http://www.electriciansforums.net/r..._matters_earthing_your_questions_answered.pdf

Cheers.

 

[Scott F]

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 .

 

[WB Scott]

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

 

[trebor]

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

 

[james king]

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

 

[sparkswillfly]

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

 

[james king]

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

 

[trebor]

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

 

[james king]

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?

 

[trebor]

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

 

[james king]

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

 

[Guest123]

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.

 

[james king]

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?