PLEASE HELP CONFUSED APPRENTICE

[simon williams]

Hi guys been to work today and as i stated i've just started my apprenticeship. whilst in work i was helping someone test a ring main which we were putting in a school. the problem i have is when we took the r1+r2 the reading for example came out as 2.8 and the loop test came out as 1.7 (these are just just example figures b4 i get shouted at) but from what i gathered at first year college is that

zs(loop test )=ze+r1+r2

but in this case the r1+r2 are more than the zs to start with he did say something because the cables are above a form tile ceiling and there was metal all around it this occurs. if someones got the time to run me though this it would be much appreciated.

thanks

 

[fireman22]

something not right. Ze is the resistance external to the installation. R1 + R2 is the resistance of the phase and cpc from the furthest point therefore your Zs value must be higher than your Ze and must be higher than you R1 + R2, as your Zs is these two values added together.

 

[fireman22]

how did you take the R1 + R2 reading?

 

[ban-all-sheds]

I wonder if you are confusing R1+R2 and r1+r2?

Zs does indeed = Ze+R1+R2, but one of the tests done on a ring is to measure r1+r2 (the end-to-end resistance of phase + cpc) at the distribution board.

R1+R2 = (r1+r2)/4 so in your example figures, if Ze was 1.0 then the figures would make sense.

 

[simon williams]

now im really confused

he did the r1+r2 by link the phases and the earths at the db and testing at the socket.

and then just a normal loop test

i was told that when testing a ring like this you should devide the r1+r2 by 4 then that would sould about right.

 

[ban-all-sheds]

he did the r1+r2 by link the phases and the earths at the db and testing at the socket.

That's R1+R2 not r1+r2....

 

[industryspark]

here's a ring circuit example........

measure phase to phase and you may get 0.40 ohms.(this is r1)
neutral-neutral would be about same so =0.40 ohms (this is rn)
earth earth might be 0.67(this is r2, higher resistance because the earth has a smaller csa in twin and earth cable than the l and n)

next you would connect the incoming live to the outgoing neutral and the incoming neutral to the outgoing live, all at the db.

you may think that the resistance of this circle would be 0.4 +0.4 =0.8 but because of parralel resistance it is actually 0.20(hence where you get the divide by 4 from)

if you test between phase and neutral at any socket on the ring now you will get 0.2 ohms. any reading thats slightly higher, say 0.26 may indicate a spur on the ring. you can map out what you get on a piece of paper and it should give you an idea of how the circuit is wired or if it is wired correctly!

also connect the phase and earth together in the same manner as you did the phase-neutral and at each socket you should get 0.27 (0.4+0.67/4=0.27).

this 0.27 is also the R1+R2 value for the ring circuit.

note that only ring circuits are tested in this way, most others that you'l come across in domestic wiring are simpler to understand.

if ive missed anything off guys please feel free to add to it, my mind goes numb with these long posts!!

 

[The Jeep]

Simon, if I may put my oar in....I have recently finished testing a ring main with a Fluke 1653 tester.
I carried out my tests in the same way Industryspark describes, but still got some discrepancies when comparing my calculated value of Zs to that actually measured at the socket.
For example, my Ze (measured at the CU) is 0.08 Ohms; my r1+r2 for the ring is 0.20 Ohms, giving a calculated Zs of (0.08+0.20) = 0.28 Ohms. However, when I measure my Zs directly from a socket on the ring, I get something like 0.15 Ohms, which is even less than my r1+r2.
Concerned about this, I called up Fluke's technical help line and was told the following:
1. As the quantities being measured are extremely low, the way that you 'zero' the test leads is very important (i.e. don't just join the croc clips together as this gives a 'point-to-point' connection, but rather use a piece of copper bar). Likewise with zeroing the 3 -pin test plug lead: don't just use a couple of screwdrivers, use copper wire around the pins).
2. The accuracy of the measuring device is not 100% so this has to be taken into account, though this can be quite complicated to assess.
In my view, the Zs measurement taken directly from the socket is a single measurement with, therefore, a single error involved. With Ze plus r1=r2, there are two measurements being made here, so the error is compounded.
Sorry if this all sounds a bit long-winded, but it is an important issue and perhaps warrants further discussion. I feel I made all my measurements very carefully but still feel confused by the results. Also, with my meter, the EFLI (Loop) test has a greater error (or less accuracy) than the straight resistance measurement. However, it's always worth rechecking everything before putting discrepancies down to meter error...

 

[ban-all-sheds]

For example, my Ze (measured at the CU) is 0.08 Ohms; my r1+r2 for the ring is 0.20 Ohms, giving a calculated Zs of (0.08+0.20) = 0.28 Ohms. However, when I measure my Zs directly from a socket on the ring, I get something like 0.15 Ohms, which is even less than my r1+r2.

I still don't see what the problem is...

r1+r2 = 0.20 ohms

Therefore R1+R2 (calculated) = 0.05 ohms

Ze = 0.08 ohms

Therefore Zs (calculated) = 0.13 ohms

Zs (measured) = 0.15 ohms

 

[The Jeep]

Sorry, BAS, I meant the R1+R2 value is 0.20 Ohms, so my comments still apply. However, whichever way you measure it, the results are well within BS7671. It would be nice, though, if they tallied...

 

[securespark]

U sure Ze= 0.08??

 

[sparkybird]

Err, I know this might seem a bit obvious, but did you null the test leads before carrying out the continuity tests???

SB

Opps, just read long post above and seent his already mentioned - but did you??

 

[ebee]

two more points to consider:-
1/ Inaccuracies of tester gear and inaccuracies of your testing itself can conspire against you to make results not quite add up .

2/ Also the Zs reading might include parallel earths that were not in place when Ze was taken, this makes the actual Zs less than the calculated Zs anyway.

Case in point as per no 2/ above - yesterday I installed a short run of 10.0 t & e for a shower and the meassured Zs was significantly less than measured Ze.

Remember Zs = Ze + R1 + R2 is acceptable as a calculated Zs but Ze (calculated) does not equal Zs (Measured) - R1 - R2

Similarly Zs (Measured) - Ze (Measured) does not equal R1 & R2.

Another case in point here too -
I came accross an installation that had been PIR`d by an NICEIC firm and amongst all the other numerous mistakes Zs, Ze and R1 = R2 were noted, however there was a shower on a seperate shower protection unit (RCBO) and the installation earthing condutor was not present (ie only a cpc on the earthbar) , the bathroom had no deliberate supp bonding and it must have been incidental earthing of pipework in the house that was providing an earth reading to allow a Zs to be taken

 

[The Jeep]

U sure Ze= 0.08??

Yes, I know this seems very low. I originally measured it as 0.20 Ohms some months ago, but perhaps didn't zero the tester correctly. This time round, I went to great pains to zero correctly and then connected my earth lead to the MET, and the neutral and live leads to the neutral and live contacts at the top of the main switch (with the main switch off).

I then carried out the test repeatedly, getting the same result each time. Thinking about it now, perhaps just applying the tips of the leads to the exposed metal within the recess at the top of the main switch may not be the ideal way of doing things: perhaps I should pull the tails out and get the croc clips around them. Will try this later.

Mind you, the flat I am testing is in a tower block which may have a transformer very nearby (in the basement?).

Interesting comments, ebee...I will look into the possibility of parallel paths. How are you getting on with all of this, Simon? You started the topic after all!