testing tt systems

[paul copeman]

hi

i have been doing testing for a few years but always in town on TNCS or TNS systems

receently i have started working for a company that tests pubs and i have beeen going out into the sticks and had a few tt systems to test

what i wanna know is how to get figures for r1+r2 as when i test in the normal way i.e extremety of circuit i gat a unexpectedly large reading

for example 3ohms and above fo a small ring when i am expecting to find more like 0.5ohms

my firm has told me to just get the Zs for each circuit and LIM the r1+r2 but i wanna do it properly

someone mentioned linking the neutral and earth together but not sure if thats how to overcome this problem

any help appreciated

paul

 

[Electroglow]

How do you test r1+r2??

What does this test and shod the earth system effect the reading??

 

[Manator]

I take it you do not have your 2400, sorry just remembered its now 2391 with variations to suit!

You are right in wanting to do it properly, there are no short cuts to testing, its experiance that allows you to know what is going on, as with all testing. Interpretations of results is an art, and gained by repetative testing. All you have to remember is to follow the guidlines, BS7671, and if you are never sure read up on the allowed maximum results for any installation. Whatever you do, do not let those who are not qualified to test tell you what to do.

 

[Andy™]

R1R2 will always be the same regardless of earthing type. if you have a higher than normal R1R2 then you have a fault, or a badly designed circuit

Zs will usually be higher though

 

[paul copeman]

im not saying 3 ohms overall im saying 3 ohms on top of the 100 or so i get for the Ze

maybe its because the overall reading is high the instument is not that accurate when reading high or maybe because every time you test the Ze on TT systems you get a slightly different reading so therefore the R1+R2 are going to be not very acurate

 

[Electroglow]

U answerd my q to the op lol.

R1 + r2 has nothing to do with the earthing system !

The Zs will usually be higher than normal and so will the Ze but not the continuity

---------- Post Auto-Merged at 21:40 ---------- Previous post was made at 21:37 ----------

Yes but do u know the max Zs reading for TT system? An would those reading me ok?

 

[paul copeman]

sorry everyone i have not wrote the questioin right

what i mean is i get a reading at the main switch and earth bar

i.e Zs at main switch

Then i get a reading at Extremety of circuit I.E Zs for cicuit and then i usually take away Zs main switch to give me R!

R2 for cicuit but on TT system this doesnt work out

 

[Andy™]

im not saying 3 ohms overall im saying 3 ohms on top of the 100 or so i get for the Zemaybe its because the overall reading is high the instument is not that accurate when reading high or maybe because every time you test the Ze on TT systems you get a slightly different reading so therefore the R1+R2 are going to be not very acurate

read post 4...

---------- Post Auto-Merged at 21:42 ---------- Previous post was made at 21:41 ----------

sorry everyone i have not wrote the questioin rightwhat i mean is i get a reading at the main switch and earth bar

i.e Zs at main switch

Then i get a reading at Extremety of circuit I.E Zs for cicuit and then i usually take away Zs main switch to give me R!

R2 for cicuit but on TT system this doesnt work out

R1R2 is a dead test, and must be done before the circuit is energised, therefore you cannot calculate is by Zs-Ze....

 

[Electroglow]

I dot know what ur on about mate? Zs at main switch and take what away from what to get R ??? Explain

 

[Robin Spark]

Well as TT installs will have an RCD protecting the circuits then your maximum Z's will be governed by the rated tripping current of the rcd. eg for a 30ma rcd then max Zs theoretically could be as high as 1667ohms for the rcd to safely disconnect before 50volts appears.

Remember calculcation Idelta n * Ze < or equal to 50volts

You need to know your Ze which really is carried out the sameway as on TNS and TNCS and this will give you an indication that if you are getting Zs of 3ohms on a ring final circuit then you know it will be parallel paths (maybe metal water or gase pipes that are bonded).

However dont forget that a recommended max of Ze is 200 ohms for TT installs, otherwise the earth rod is possible unstable and in very dry or freezing weather then the value cna go considerably higher.

Abit slow at typing and now a few posts have appeared... are you doing Ze correctly ie main switch open with earthing conductor disconnected from main earthing bar so that you dont get inaccurate reading due to parallel paths?

 

[batty]

:C

 

[paul copeman]

for a tncs for example

taking ZS Zs from main switch at say 0.05ohm

found 0.4ohm at supply

so r1+r2 between supply and main switch i.e tails and main earth cable is 0.01ohm

then i take a Zs at extremety of final circuit say 0.68 ohm

so i take away 0.05 and get r1+r2 0.63ohm

???????

 

[Robin Spark]

:C

:C :C :C

 

[paul copeman]

the thing is the pubs we are testing wont let us turn off curcuits to do testing as we are in there when they are open sothats how we do it im afraid

 

[Robin Spark]

Paul, with all due respect do you know how to do the tests properly as per GN3? Because you are confusing me, unless its the Penfolds :Y :Y :Y cufuddling my brain?

 

[paul copeman]

very helpful battyman

---------- Post Auto-Merged at 22:08 ---------- Previous post was made at 22:05 ----------

u should be joinig r1 and r2 together at board and dead testing at extremety but thats not how we do it at this firm and i was told that is ok as pubs dont want us shutting down systems

 

[Andy™]

the thing is the pubs we are testing wont let us turn off curcuits to do testing as we are in there when they are open sothats how we do it im afraid

so its a PIR then

in which case, use limitations for R1R2 if necessary

if you put a value in there thats only obtainable by measurement, then that implies you have measured R1R2 and not calculated it

and how do you test Ze? or the resistance of the rod? or IR?

 

[Electroglow]

I test pubs and to carry out a test properly u need to turn the power off to each cct. U can calculate Zs by adding r1+r2 and adding it to the Ze reading! I'm confused with the Zs at mail switch?

 

[Manator]

If we can help you we will, but you will also have to understand that we are getting snippits of information, there is no way that anyone can give precise detailed information over the internet, especially if the information supplied is not detailed.

 

[Robin Spark]

the thing is the pubs we are testing wont let us turn off curcuits to do testing as we are in there when they are open sothats how we do it im afraid

Well, either put "Lim" in the Ze box and a note to say you werent allowed to isolate supply or go to pub before they are getting ready to open so you can do the Ze test. But really if you cant isolate the supply then as you say its more of a quick visual and a few Zs tests, however I dont think its really going to prove that much.