When testing R1+R2 in a ring I understand that r1 + r2 the loop impedance is 4x the value of R1+R2 due to one being a serial measurement and the other parallel. Obviously you need to get a reference value from the table in the regs, OSG or GN3, but it does not explain how the value is arrived at. I presume that the tables are for single length of cable and fine to read directly off for lighting or radial circuit, but I think I have to halve the value tabulated for a ring circuit. Is this correct?? Please help! I have my first pre written exam for G&G 2330 L3 tommorrow and have to get this straight in my head.
I think you are confused. Measure R1 Measure R2 add them together, then divide by 4. Record this figure. You have to do this because you have a incoming and a outgoing cables. Join RI Incoming to R2 outgoing at CU then measure the R1 and R2 at the sockets this figure should be within 0.05ohmn of the figure above. If it's not, then either the wrong cables are joined at CU You are measuring a spur, bad connections etc.
Hi aelect, When measuring the ring continuity we're taught to connect the outgoing live to the incoming CPC then connect incoming Live to outgoing CPC and then measure at the sockets and take the highest reading. Basically that gives you a parallel net with a one leg of the ring + one cpc in one branch and similar in the other, so r1 would be 4x the parallel measurement. The equation we're given is (r1 + r2)/4 = R1 + R2. My problem is not with the measurement, indeed I feel comfortable with the theory aswell, but not sure what the R1 + R2 values in the tables are giving me, parallel or series values?
I might add we know r1+r2 as the loop series measurement and R1+R2 as the parallel. Only applied to ring circuits.
Sorry there's a mistake in my first reply. As r1 is the two legs of the ring measured in series. Then r1 is 1/2 of R1+R2. The main equation as I mentioned earlier is (r1 + r2)/4 = R1 + R2. This presumes the ring and cps sizes are the same.
There are two reasons to measure R1 + R2 earth loop impedance which is unlikely to be a limiting factor especially after July 1st and the volt drop the latter needs R1 only so we need to divide by 47 and times by 18 to get ohms of R1 assuming 2.5mm and 1.5mm earth. Seems a lot of work when both ends of R1 are available at the consumer unit? On 32 amp this must be less than 0.29 ohms on 16 Edition or 0.36 on 17 Edition unless it also supplies lights then 0.22 ohms. i.e. 4% 5% and 3% I know we must measure R1 + R2 in case the earth has become disconnected but with RCD's on all sockets working out if it passes should not really need calculating as the pass value is so high. If you read book three it give 1.67 instead of my 47 and 18 but I though you would see relationship with resistance per meter.
Unfortunately the 17th has come half way though my last year at college, and the colleges take is to continue course using the 16th. I much appreciate your advice, but with all my garbled text I think I have confused the point. If I go to page 159 appx 9 of the OSG and read off the R1+R2 fig say 2.5/1.5 at 19.51ohm/m I believe this is the value for a series measurement of cable typically T/E. What I need to know is: if I want to compare the parallel measured value of R1 + R2 from the test I explained earlier do I halve the value?
Assuming the ring is perfect. After you measured your R1 + R2 /4 = ? + Ze = max Resistance, a voltage will see, if it develops a pure earth fault. Because it's a ring. A radial will be R1 + R2 = ? + Ze = max Resistance a voltage will see, if it develops a pure earth fault.
Sorry I don't get you. I will refer to pg 71 OSG that shows the method of measuring R1 + R2 in a ring. Taking a reading at a socket between the L and E pin will give you a direct read off of R1 + R2 as a it is in use, a parallel circuit between the L and CPC cables. I would add this reading directly to Ze to get Zs. The very same as if this was a lighting circuit or radial. I would not divide R1 + R2 x4. I would multiply x4 to derive the end to end values of Live and CPC added together, that is first step in the test sequence. I would only do this to verify my R1+R2 measurements. However this end to end test is useful to prove continuity of each ring.
Step 3: The above step is then repeated but with the phase and cpc cross-connected (see Fig 2c). The resistance between phase and earth is measured at each socket-outlet. The readings obtained at each of the sockets wired into the ring will be substantially the same and the value will be approximately one quarter of the resistance of the phase plus cpc loop resistances, i.e. (r1 + r2)/4. As before, a higher resistance value will be recorded at any sockets wired as spurs. The highest value recorded represents the maximum (R1 + R2) of the circuit and is recorded on the Schedule of Test Results. The value can be used to determine the earth loop impedance (Zs) of the circuit to verify compliance with the loop impedance requirements of the Regulations (See Section 2.7.14). This sequence of tests also verifies the polarity of each socket, except that if the testing has been carried out at the terminals on the reverse of the accessories a visual inspection is required to confirm correct polarity connections. Page 34 Book 3 so yes divide by 4
Poolking, you are getting confused. The (R1 + R2) values in table 9a, p158, OSG are only relevant for radial circuits. Are you asking how to work out (R1 + R2) for a ring final circuit given circuit lengths? If so, use the values for <u>single</u> conductors (7.41 for 2.5 sq mm and 12.10 for 1.5 sq mm) to work out r1, rn and r2 (end-to-end values). Then apply the formula (r1 + r2)/4 to get a calculated value for (R1 + R2). (In real life this value is, of course, not recorded; it is used to verify your stage 3 measurements.)
MGW Thanks for an intellagable reply. I've only done a few tests at college and find it hard to remember the expected values for the different tests. Therefore I have to fall back on the reg's to find a value to compare my readings to. I happy to calculate R1 + R2 for lighting and radials directly from the ohms/m given. The rub comes when I do the calculation for a ring. Surely a ring circuit has a L out and a L in and also a CPC out and CPC in. Therefore I believe a R1 + R2 value of half the resistance of a radial of the same size.
Thanks dingbat That comes now, to two sensible replies, except for the glib comments that keep the forum alive. I understand exactly what your saying. Just I'd be comparing one reading with another, with no yard stick. Is it just not done or bad practice to manipulate the table to get a value, as it does make some arithmetic sense to me, bet never mentioned in any literature.
PK, the table is there for design purposes. For instance, if you'd installed a 20m-long radial circuit in 2.5/1.5, you'd expect the (R1 + R2) reading to be in the region of 0.4 Ohms (approx 20m x 19.51 milli-ohms-per-metre) For a ring, you <u>could</u> take the (R1 + R2) value from the table, multiply it by the end-to-end length of the ring and then divide that by 4... you'd just be doing the same calculation via a different route. In any case, it's one thing to use known values to calculate expected test outcomes in an exam paper and quite another thing to use known values to verify measured values in real life.
Hi Dingbat Thanks for your succinct replies, all making good sense and putting my mind at rest. It means now when I test I'll know fairly well what to expect an even pick up a fault thrown in to catch me out. Like you say it probably best if you know how much wire you've put in a ring. Take that length to multiply by the ohm/m value from the table then divide by 4 and you have a yard stick. Otherwise the distance to the last socket in the ring ohm/m value divided by 2. And I think that's all for today, so I can get back to my revision. Thanks again for everyone's help even the good intentioned but well off the mark replies.
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