No maths needed - kelvin measurement is simply separating the paths taken by the test current and voltage measurement to eliminate the effect of resistance. The source current is actively regulated to be constant regardless of lead resistance ( or measured to compensate) , and the voltage measurement current is negligible so not affected.
The instrument is just applying ohms law to calculate the resistance. Dividing the voltage measured between the two 'P' terminals by the current measured flowing between the two 'C' terminals. That is why correct placement of the P and C probes onto the test object is required. At such low resistances, temperature also becomes a factor. I had the CA6250 for a while, but never really got on with it, it was forever overheating and getting errors whilst measuring winding resistances, also couldn't use the meter whist it is charging. One of the worst instruments I ever purchased. That model is discontinued now, but never used their replacement models, I guess they may have improved. I tested the old R1-R2 lead a while back so will be interesting to see what you find, especially if you have the newer ones from Klein. Probably not a good idea to spark the micrometer like that as you will be putting voltage spikes back into the instrument, not something you would want to do with such an expensive piece of test equipment if you have bought it. Dirt inside a high voltage joint doesn't cause an issue due to current, the current will be higher on the equivalent low voltage side. The dirt inside causes a voltage stress between the two surfaces in comparison to the rest of the joint and can lead to partial discharge within the joint at that particular point, that will eventually lead to its failure.
Great video Jamie. I have used the Megger "ductor" thousands of times on site whilst testing HV equipment, busbars, isolators, Etc. Not going to lie there is nothing more frustrating than going 40ft up in the mewp and having your ductor battery die on you mid way through testing. Essential piece of kit on the HV equipment as a high resistance joint (over 100µΩ) can easily get hot enough to melt something. I think in general we go with 10µΩ for a bolted joint and 20µΩ for a flexi joint when testing busbars. On new stuff thats easy to obtain but you try taking a 60 year old busbar clamp apart and try to get a good result. You normally end up taking the whole thing apart and clean each individual component. You would love some of the Omicron test gear we have onsite, expensive but amazing kit
@@jamieblatantsparky The ductor (DLRO) test often doesn't have specific number limits because it is used as a comparison test. Similar connections will be tested on each phase and per NETA (InterNational Electrical Testing Association) need to fall within 50% of the lowest value. I.E. if testing 3 similar bolted joints and A phase is 10, B phase is 12, and C phase is 14, then it passes because the limit would be 15 micro ohms. Often this test is used to confirm the connects were properly torqued. I have found a connection in a motor control center where the electrician reasonably thought he had torqued the connection, but it failed my test. Upon inspection I found that was actually little bits of sand between the two bus pieces. The bolted connection was torqued, but not making contact. When testing fuses the values need to fall within 15% of the lowest fuse. Breakers still fall under 50%, but often times manufacturers of high current (4000A +) or medium to high voltage breakers will also have specific values that the main contact should fall under. Typically this would be somewhere less than 50 micro ohms. A good rule of thumb is the larger the breaker, the lower the resistance should be. As you proved in the video when measuring across the bus with the hole in it versus the solid piece of bus, less conducting surface will give higher resistance. I recently tested a large number of 100A disconnect switches which were installed in bus ducting to and would be used to feed servers in a big name data center. ABB made the switches and had the requirement that the switch contact measure
Ductor testing of fishplate joints in busbar and risers. Need to put some sort of indication of testing points if you want to use the results for board maintenance inspection purposes. If you testing contacts in MCCBs need to do a volt drop test, not ductor test.
That was the original intent but the results were not as “Vidoe cool as you might think , you can see the rig in the background, I need the next tester up from the one I have to show the numbers
@@jamieblatantsparky yes it will, using the ammeter setting or the millivolt microvolts setting while shunting, it's exactly how a 4wire measurement is made even in a megger like the one in this video
Because an all in one testing solution is so inconvenient, let's roll up to site with a high voltage supply and shunt for your multimeter! 😂 Watch the site electrical engineers face when he asks to see the calibration cert and you tell him "aahhww ohms law is mathematics, mathematics requires no calibration " 😂 🤣 yawn! back to your workbench, oscilloscope!
hey jamie me and another spark rewired and changed all the old sodium lighting to the new led fittings with microwave pirs in 10 weeks in 2014 in that complex
@@jamieblatantsparky yes we were in the junior rates bar every night, we stayed in the old original accomadation at the end of the long corridor, shithole but warm
@@jamieblatantsparky you and dave just simply amaze me with you knowledge and skill sets, I’d give my left arm to be anywhere near to you, but allas I,m just a thick 16th edition spark from the 90,s, keep making amazing content as your an amazing teacher.
@@jamieblatantsparky I watched your video on doing the 18th regs exam, I’ve got the book and trying to do an hour or so a day with test exams but finding it really difficult, I will keep trying though mate.
Negative ohms would probably be better called “conductance” and in electrical engineering has the units of Mho, notice that is Ohm reversed. Typically it is symbolized with a Y instead of the R used for resistance.
