Ring r1r2 test

[steptoe]

its like a spiral,

then you join the middle bit to the outer bit, 

 

[Rob_the_rich]

No..... draw it out,, just a loop of the L & E

It ends up being just one great big circular conductor

That is true but we don't test end to end of a big circular conductor, we test between the two loops.

 

[NozSpark]

No you don't, you test between one side and the other

you really need to draw it out

EDIT... yes,, it's two loops at the points, but they're both connected together at the cross over which makes it one big loop

 

[Rob_the_rich]

If you're saying that the resistance increases from the inter-connection to the furthest point, you are correct (for r1<>r2).

But like you say, the increase is small, you'd just lose it in measurement error.

I've got a mathematical proof somewhere if you're really interested!!!

Looks as though you might have to dig that out after all Lusk.

 

[apprentice87]

Look, I am only a stupid welder, It is well known that welders are particularly stupid, and, are in fact, the 5WW version of a blacksmith, but nevertheless, for the benefit of the mathematically and electrically challenged, i shall explain how it works in easy to visualise "mechanical" terms...

Take your ring [comprised as it is of two conductors in parallel] and stretch it out straight. It makes no difference if the conductors are not of the same CSA, as this would just be the same as a "compound beam" in mechanics..

Say your "ring" is now 10 feet long....

Here comes the mechanical bit...

Take a horizontal beam exactly 10 feet long, and let it be suspended from the ends by means of two spring balances hung from the ceiling.

Zero the spring balances.

Now, place a weight [we do not care for any of that "mass" gibberish here] of exactly 10 pounds in the middle of the beam.

It will now be seen, that, as we might expect, each spring balance now reads 5 pounds.

Now, move the weight one foot to the left. It can now be seen that one spring balance reads 6 pounds, and the other, 4 pounds. BUT, 6 + 4 = 10.

Move the weight another foot. One scale now reads 7 pounds, the other, 3 pounds. BUT, 7 + 3 = 10

Move the weight all the way to the end. One scale now reads 10 pounds, the other 0 pounds. BUT 10 + 0 = 10.

No matter where we position the weight, the sum of the reactions [weights] at the end of the beam is always equal to the weight applied, that is 10 pounds..

Now, you can do it the other way about too. if you know the reactions at the ends, you can work out the weight applied and how far along the beam it is, BUT the important bit is; No matter where we position the weight, the sum of the reactions [weights] at the beam ends, will always be equal to the weight applied.

It is the same with the ring circuit, if you take two conductors, lay them parallel to each other on the ground, join the ends to form a ring, and then measure the resistance between the two conductors thus joined, it will not make any difference if you measure right at one end, or two feet along from one end, or anywhere you like. Nor does it make any difference if the conductors are of different CSA's or different materials.. This can be visuallised as follows..

Say our 10 foot long beam suspended from the ceiling was a compound beam composed, not all of the same stuff, but of a piece of wood glued to a length of steel, would this affect the reactions that we measured at the ends due to moving the weight?? of course not..

Likewise with our cables..

john...

 

[James1978.]

 Is COMPLETELY different from;

Which is exactly right..

john...

If you do a loop test at socket close to the cu the reading will be lower than a test at a socket half way around the ring. The fault path passes through the origin. 

 

[Andy™]

no it wont. it will be the same at any point. you may get some variation from the test kit

 

[steptoe]

If you do a loop test at socket close to the cu the reading will be lower than a test at a socket half way around the ring. The fault path passes through the origin. 

now you are talking about the impedance back to star,

continuity measures the RESISTANCE of the conductors, and has nothing to do with the impedance to star point

 

[apprentice87]

If you do a loop test at socket close to the cu the reading will be lower than a test at a socket half way around the ring. The fault path passes through the origin. 

No, it absolutely WILL NOT. Sorry, but you are wrong here. Go and read the regs section on testing, the OSG bit on testing, anything you like on testing, they will all tell you are wrong..

Sorry and all, but if you cannot grasp this, then you are NOT CAPABLE of any electrical installation work at all, as you have no understanding of verification and leaving the job in a safe condition.. I am not being nasty, but that is the facts of the matter....

john..

 

[Davethsparky]

I'm just planning a job for next weekend where a ring final circuit will be wired in MICC, I'd like to see someone do an R1+R2 on that, or even an end to end on the cpc  

(the only reason I'm doing it in MICC is that I'll be taking a scam inspector to it and I like winding them up if I can, plus I have a load spare from another job) 

 

[kerching]

So does this explain it any better and shed any light as tomwhy the readings may increase?

Step 2
The line and neutral conductors are then connected together at the distribution board so that the outgoing line conductor is connected to the returning neutral conductor and vice versa (see Figure 10.3.2(ii)). The resistance between line and neutral conductors is measured at each socket-outlet. The readings at each of the socket-outlets wired into the ring will be substantially the same and the value will be approximately one-quarter of the resistance of the line plus the neutral loop resistances, i.e. (r1 + rn)/4. Any socket- outlets wired as spurs will have a higher resistance value due to the resistance of the spur conductors.
NOTE:    Where single-core cables are used, care should be taken to verify that the line and neutral conductors of opposite ends of the ring circuit are connected together. An error in this respect will be apparent from the readings taken at the socket-outlets, progressively increasing in value as readings are taken towards the midpoint of the ring, then decreasing again towards the other end of the ring.
On-Site Guide    99

 

[Rob_the_rich]

Kerching, if the cross connection were connected wrongly, there would definitely be a measurable increase as you test further away from the connection.

I am talking of a barely measurable increase of 0.06 ohms when using 2.5/1.0 T&E and less when using 2.5/1.5. This would hardly register on our MFTs.

I have said before I thought there would be an increase, but did not realise how small this would be until I did the calcs in the spreadsheet.

Still waiting for Lusk to dig out the mathematical proof of this.

The OP is correct when saying that a spur taken from the CU would have no R1+R2 component of the ring. If the cross connection was done somewhere else other than at the CU, the R1+R2 of the ring would be included in the measured value, but would play no part in the actual fault path.

 

[Lusk]

@Rob_the_rich

OK Rob, here you go ...

View attachment R1+R2 Proof.pdf

 

[Rob_the_rich]

@Rob_the_rich

OK Rob, here you go ...

View attachment 6560

Thanks Lusk, but as I see it, it shows that the resistance measured at the mid point CAN be equal to that at the cross connection point. So doesn't really prove that the midpoint resistance IS greater than that at the cross connection. :facepalm:  oh well Guinness

Thanks for finding it though.

 

[Andy™]

so basically its showing what everyone else has been saying all along, in that the readings should be the same no matter where they are taken...

 

[Rob_the_rich]

so basically its showing what everyone else has been saying all along, in that the readings should be the same no matter where they are taken...

Andy, Apprentice, everyone else;

Ring length 100m

resistance of 2.5mm wire from OSG 7.41 mOhms/m

resistance of 1.5mm wire from OSG 12.1 mOhms/m

cross connected at CU (why not) so we have 1 big circle (or a figure of 8, depending how you look at it), half of which is 2.5mm and the other half 1.5mm. If we test at the CU we have 2 conductors in parallel one conductor is 2.5mm the other 1.5mm.

Using the formula Rtotal=R1xR2/(R1+R2)

Where R1=7.41 x 100=741 mOhms=0.741 Ohms, which in this case =r1

and R2=12.1 x 100=1210 mOhms= 1.21 Ohms, which in this case =r2

I get Rtotal=0.4595 Ohms

Can you explain why this figure is less than the figure (r1+r2)/4=0.4877 Ohms which is the midpoint reading for the ring, and is apparently according to everyone else here the figure we should get at every point in the ring, including that taken at the CU?

If the above is wrong, why?

 

[Rob_the_rich]

If I am wrong, I will eat my hat. Should go down nicely with my socks. :innocent

 

[steptoe]

OK,

small words

r1  &  r2  are readings taken on a single conductor

R1  &  R2  are taken with parallel conductors

 

[OnOff]

Ref R1+R2 for the ring: I like the single circle drawing in the Scaddan book. Never could get the figure of eight they kept on about at college! Had a massive ongoing argument with a theory lecturer who said all readings should be the SAME at each socket he even wrote it on his handouts. Scaddan uses the phrase "substantially the same" which I guess is the nod to MFTs not reading to that many decimal places.

I eventually got the lecturer to agree (understand?) about the whole parallel resistance bit - I'd even hang around after everyone else had gone and force him to look at paper examples I'd done to understand it myself. Cheeky S@d gave it a couple of weeks then stood up in front of the class to just remind everyone that readings at each socket should be "substantially the same" and suggested they get Scaddan's book to understand it better. Backtracking git! 

 

[Lusk]

Thanks Lusk, but as I see it, it shows that the resistance measured at the mid point CAN be equal to that at the cross connection point. So doesn't really prove that the midpoint resistance IS greater than that at the cross connection. :facepalm:  oh well Guinness

Thanks for finding it though.

Rob,

You need to interpret the result.

If r1=r2, then the last line equals 0 and RE does indeed equal RM, both conductors with same csa, so R1+R2 is the same wherever you measure it, this is undisputed.  Look at it as a special case.

However, if r1<>r2 (different csa cables), the last line of the proof is greater than zero, hence proving RE<RM

It might be clearer if you say ... for r1<>r2, I claim that RE<RM, then the last line would be (r1-r2)2>0, proving they are not equal. (Just remove the equals sign out of the proof)

It's all academic anyway, as we both know, the difference is almost negligible!