this video is part three of the miniseries on finding shorts in electrical wiring circuits and this includes Earth faults specifically we're looking at radial circuits here Parts one and two covered ring circuits we should begin by getting the wording right especially if you are studying electrics and about to sit an assessment a short is a low resistance path between line and neutral or between two phases in a three-phase system then we have an earth fault this is a low resistance path between line and Earth or CPC or between phase and Earth in a three-phase system it's a short yes but it does have its own name and just a reminder that the CPC or circuit protective conductor is the earth conductor in a twin and Earth cable let's start with looking at low ohms resistance measurements in a good circuit a healthy circuit what sort of readings should we expect and how do we know if they are correct for this video we will assume a radial cooker circuit that's wired in 6 Square mm by 2.5 Square mm twin and Earth cable and that the circuit length is about 90 18 m in One Direction the first problem we have is that the test meter probes only have a lead length of around 1 M if we try to measure just one conductor the line for example at the consumer unit the second probe will not reach the cooker the same is true with ceiling roses and many other radial circuits there is an easy solution use a wo connector or similar at the cooker end the point of use so that we have the other conductors as a return path now we can low ohms measure between line and neutral all at the consumer unit in this example we have a measurement of 0.12 ohms and I've rounded up or down some of these measurements to make things easier to understand here we are using 6x 2. 5 Twi Earth cable and at 19 M the low ohms resistance reading between the line and CPC returns 0.2 ohms we would expect this as the CPC has a smaller cross-sectional area than the line neutral and will therefore have a higher resistance the neutral to CPC resistance check should be the same as the line to C PC and a reading of about 0.2 ohms putting these into a little table and it's good practice to write things down don't trust to memory we have a line to neutral at 0.12 ohms line to CPC at 0.2 ohms and neutral to CPC also at 0.2 ohms but can we check this how do we know that these numbers are correct we can begin with checking the line to CPC resistance in the tables these tables are very easy to use and I do recommend that you practice using them they will prove so useful in your career we can use either table B1 in guidance Note 3 or table i1 in the on-site guide it's the same table the same data we are using 6X 2.5 twin and Earth so begin by Looking Down the Line conductor column the leftmost column and find the block for 6 mm conductors Now find the protective conductor or CPC size in the second column which is 2.5 Square mm the third column will give you the combined resistance of 6X 2.5 copper cable and this is 10.49 9 milliohms per meter length measured just one way we can now use a very easy formula there are two variations of this formula and we give them both here we can find the total resistance of the cable if we know the milliohms per meter and the cable length by rearranging the same formula we can find the length one way if we know the measured resistance in ohms and the milliohms value for that particular combination of conductors pause the video and take a look at the two versions shown on the left the formula tells us that 19 M of 6 by 2.5 twin and Earth should return a low ohms reading of 0.2 ohms after rounding using the same data we can calculate the length and we have a rounded figure of 19 m one way next we can find the line to neutral resistance using the same tables B1 or i1 use the table to find the resistance of the line of neutral conduct is combined there is no column for the neutral but that doesn't matter find the line conductor size in the leftmost column and the neutral conductor size in the second column what was the protective conductor column both line and neutral are 6 Square mm in size so find 6 s mm by 6 squ mm the third column gives a milliohms value of 6.16 milliohms per meter length now we can use this information in the same calculation as before pause the video again and look at how the formula works the combined resistance of 19 M of 6 squ mm line and 6 squ mm neutral is 0.117 ohms remember that larger size cables will have a smaller resistance if we measure the resistance at 0.117 ohms we can put the data into the length formula and calculate a length of 19 M if you're never short of resistances or lengths always check against the tables it will save a lot of time so that is for a good fault-free circuit now we can look at what happens with a faulty circuit perhaps your customer has told you that the rcd or MCB keeps tripping always ask the customer what's happened when they notice the problem Etc they can give you some valuable Clues you will frequently find that if the rcd trips you may be looking at line to Earth faults and if the MCB trips this could be a line to neutral fault not always but often the circuit is showing a nail that has pierced the twin and Earth cable about halfway along its length the nail has broken through the insulation of the line conductor and now we have a low ohms path a nail connecting the copper line conductor to The Copper CPC or Earth because it involves the Earth conductor we call this an earth fault or at least we should do in assessments and exams what readings would a low ohms test on this circuit show perhaps something like this as you can see from the table below the readings are nothing like those for the good circuit and they don't appear to make sense but if you didn't know what the fault was or where it was how would we go about finding it if we low ohms resistance test the circuit then it's obvious that something is not right but the results can look confusing part of this is because we still have all three conductors linked together in the wo we need to take a step back and find some clarity instead test with just two conductors at a time as shown here just remove one conductor at a time from the wo and retest on low ohms record your results and compare them to what a good fault-free circuit should measure now we can compare what the test results are against what they should be for a healthy circuit now the results are clearer and it can be seen where the problem exists with only two conductors connected together at any one time we find that the line to neutral and neutral to CPC show the same results as a healthy circuit but line to CPC gives a completely different reading and we can be fairly confident that the problem is between line and CPC as the fault involves the Earth or CPC we call this an earth fault rather than a short or short circuit now we can move on to finding the approximate location of the fault one method to find the fault location is to use the line to CPC resistance and table B1 or i1 to calculate the distance from the point of measurement which for me is usually the consumer unit use the length formula as shown before and input the data into your calculator with this example the length is calculated at about 9.5 M from the consumer unit and should now be fairly easy to locate and this method of finding the distance to a fault is especially useful in industrial installations where the cable runs maybe many many meters in length it will get you into the right area close to The Fault a quick recap on what we've done there are two variations of the same formula a simple rearrangement to find either the expected resistance or the length of the circuit depending on what we already know use the tables from the on-site guide or guidance Note 3 and remember that the neutral conductor's resistance can be found by using the protective conductor colum the second column in the tables a radial circuit has no return path and it is unlikely that your test meter leads will be long enough instead use a wayo or similar connector to make a return path along the other conductors when a fault is identified it's often helps to only test between two conductors at a time and do record your test results don't trust all those numbers to memory take the time to understand the tables use them practice with them they help become familiar with table i1 in the onsite guide or table B1 in guidance Note 3 thank you for watching it really is appreciated and I hope that you found this video useful and informative please subscribe to our channel to get access to all of our videos and remember to click on notify to be sure of not missing our next video and don't forget you can type in learn electrics into the YouTube search bar to go directly to our Channel at any time from any computer and once again thank you for watching and we hope to see you again very soon