3
\$\begingroup\$

Am using Klein tools ET600 to check if there is any hidden short to ground (like nail touching any conductor) in any of the 8-conductors in a 60 feet HVAC cable running from indoor air-handler to outdoor heat-pump. I completely disconnect all 8 conductors of HVAC cable from both AirHandler and HeatPump.

Then, I go to Heat Pump outside and connect MegOhmMeter’s Black probe to exposed copper-end of one of the 8 conductors (say the Green conductor). Then connect the MegOhmMeter‘s Red probe to the bare metal copper of the liquid line (as substitute for Ground). Set MegOhMeter dial to 250V. Press “Continuous Lock” followed by pressing “Test”. Display shows 4000 MOhms. Then, if I manually touch the copper end of any of the other conductors (say Orange conductor), display immediately dips down – to say 21 – and rises back up to 4000 MOhms within 2 to 5 seconds. The second time I touch the same conductor (after a minute or so), the dip is not so low – say it might dip to 250 before rising to 4000 MOhms. The third time (again after a minute or so), the dip is a little higher – say to 1000 before rising to 4000 MOhms. By the time I touch it 4th or 5th time, there is NO dip in reading – it stays at 4000 Mohms.

If the Green conductor and Orange conductor (in the above example) do not touch each other electrically, why does resistance value dip and raise? Is there any capacitance build-up in the insulation sheathing? Or should I worry about a short to ground? Is there any current leakage?

\$\endgroup\$
6
  • \$\begingroup\$ If the meter is set to 250 V, you are measuring voltage, not resistance. \$\endgroup\$ Commented Nov 16 at 21:31
  • \$\begingroup\$ @PeterBennett Not on a megger. For this type of insulation tester, the test is done by applying a specific voltage and measuring the leakage current, and on most of them you can select from a handful of test voltages. \$\endgroup\$ Commented Nov 16 at 21:33
  • 1
    \$\begingroup\$ wrt your comment on DAvide's aswer. Try touching and disconnecting several times and note dips. Then disconnect and leave for say 10's of seconds and touch again. Is the dip again larger? The capacitances are small AND the leakage is small and the time constant of the two is quite possibly in the order of a second. That would explain what you are seeing. \$\endgroup\$ Commented Nov 17 at 7:11
  • \$\begingroup\$ @RussellMcMahon Since my original post, I have reconnected the wires and put back the system. So, I am unable to do more experiments right now. One other thing I noticed: Instead of connecting MegOhmMeter's red probe to the bare metal copper of the liquid line, if I connected the red probe to any of the other remaining 6 conductors (say blue conductor), there would be no dip in resistance display when touching the Orange conductor in the above example. So, why does the dip occur only when red probe is connected to equipment ground? \$\endgroup\$ Commented Nov 17 at 15:20
  • \$\begingroup\$ @Sam Likely but not certain: The conductors in a 60 foot run of cable that is near ground or near grounded things will have a significant capacitance to ground. Making a few assumptions, you can expect about 10-20 pF/metre - so say 200- 400 pF over 60 feet. This capacitance is charged by the meters test current - when it is uncharged the apparent resistance is low and it increases as the capacitor charges. || The wire to wire capacitances will be similar or higher - highest for adjacent wires UNLESS they are individually screened or screened in groups. The common capacitance to ground will ... \$\endgroup\$ Commented Nov 19 at 10:23

1 Answer 1

Reset to default
3
\$\begingroup\$

Because two long wires next to each other form a capacitor. A very-low-capacity capacitor, sure. But a capacitor nonetheless.

When you touch one of the wires, you change their relative voltage. The resistance in the meter discharges that capacitance and is slowly settles to the final value.

\$\endgroup\$
3
  • 2
    \$\begingroup\$ dielectric absorption may be a factor too. \$\endgroup\$ Commented Nov 16 at 21:15
  • \$\begingroup\$ You write: "The resistance in the meter discharges that capacitance ..." If each touch discharges capacitance, how does that explain the subsequent touches resulting in increasingly higher dips and by the 5th touch, there is no dip (i.e., the display stays at 4000 MOhms on the 5th touch)? \$\endgroup\$ Commented Nov 17 at 3:54
  • 1
    \$\begingroup\$ It is rather "the output voltage of the meter charges that capacitance" :) \$\endgroup\$ Commented Nov 17 at 11:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.