How do you test your cables?

[MegaVolt]

I read the post in the R6581 thread and realized that I don't know how to check the cable easily and easily.

Because he didn't bring with him his own 4-wire cable, he carefully tested my 4 wire cable (after some testing, he considered that it was ok).

Can you share your testing methodology? That is, if there is an algorithm with a few steps and additional equipment to understand if the cable is good? Can it be used to measure voltage, resistance, etc.? What numerical criteria do you use for this?

[ramon]

Actually is was just a simple test to check how the wires and clips were connected.

Some Kelvin wires have wires to a single side of the alligator clips, while other has independent wire connections.

Which one is correct the upper row ones (where the cables connect to a single clip)?
or the lower side ones where each clip has its own wire?
 
or are both correct, but each type have their own different issues or applications?

 

[MegaVolt]

Actually is was just a simple test to check how the wires and clips were connected.

It's really simple )

But does anyone do any other tests? Allowing to clearly distinguish between a cheap Chinese cable from ali and a branded cable from Fluke?

[e61_phil]

Measure the offset voltage, isolation, resistance and so on

[mendip_discovery]

Depending on what you would like to test.

You could do an insulation test at 1000V to check they don't leak to the nearest finger when being used.

Mostly what I do is some real world tests using the leads and compare them. Record the differences and make some choices depending on performance. PTFE wire, twisted and with gold plated connectors is a good start. If looking for 4 wire test leads then test how the different placements on connection affect the readings. I have it on my list as I would like to build a set of wires for doing better resistance measurements.

[e61_phil]

Comparing differences can be quite hard if the effect is in the low ppm range. I prefer direct measurements of the cable properties.

And as already said it depends. If you use a cable with only 1G insulation between the poles it doesn't really matter for voltage measurements with low output impedance, but it really matters on higher resistance measurements.

[bdunham7]

But does anyone do any other tests? Allowing to clearly distinguish between a cheap Chinese cable from ali and a branded cable from Fluke?

Interestingly my 2X4 wire test probe set from Fluke was defective out of the box in a way that resulted in a not-immediately-obvious inaccurate reading.  If we're talking about 4-wire setups in general, the first step is making sure they are connected where they should be connected and not connected where they shouldn't be.  This would seem obvious, but experience has shown that it is not.  For example, a Kelvin connector that has both wires connected to both sides of the clip will appear to work OK on basic tests, but won't be as accurate as it should be.

[bdunham7]

Some Kelvin wires have wires to a single side of the alligator clips, while other has independent wire connections.

Which one is correct the upper row ones (where the cables connect to a single clip)?
or the lower side ones where each clip has its own wire?

The two sides of a kelvin clip should always be isolated.  Just because the wire only goes in one side doesn't mean they are connected that way and if they were it would be just plain wrong.  One wire typically goes in and around to the other side.  My preferred way of making these is with flexible coax, attaching the shield to the side that it goes in and then taking the center conductor around to the other side.  The shield should be the source, the core the sense connection.  This makes it easy to handle compared to running two wires to opposite sides of the kelvin clip.  It also eliminates a potential source of interference from the open loop created between the kelvin clip and the point where the two wires go back together.

[ramon]

The two sides of a kelvin clip should always be isolated.  Just because the wire only goes in one side doesn't mean they are connected that way and if they were it would be just plain wrong.  One wire typically goes in and around to the other side.

Yes, they are independent. But then one clip has around one inch (or few cm) extra wire on it (a.k.a. resistance).

Does this introduce any measurement problem? Or this is not an issue because the other pair of wires have also the same design: one clip with extra (miliohms? or microohms?) resistance than the opposite jaw?

[e61_phil]

The two sides of a kelvin clip should always be isolated.  Just because the wire only goes in one side doesn't mean they are connected that way and if they were it would be just plain wrong.  One wire typically goes in and around to the other side.

Yes, they are independent. But then one clip has around one inch (or few cm) extra wire on it (a.k.a. resistance).

Does this introduce any measurement problem? Or this is not an issue because the other pair of wires have also the same design: one clip with extra (miliohms? or microohms?) resistance than the opposite jaw?

You should think about the idea of a 4W measurement

[bdunham7]

Does this introduce any measurement problem?

The whole point of 4W measurment is to eliminate any effect from the resistance of the test leads by ensuring that no current (or almost no current) flows in the SENSE leads and, as long as the compliance voltages aren't exceeded, the SOURCE current is always the same.  There's no need to balance anything.  If you were just using four separate leads, they could be 5, 11, 14 and 37 ohms and it wouldn't matter which one you used where--at least on most normal 4W capable DMMs.

[ramon]

Thanks a lot for reminding me the theory. What a Yoke!    How could I have forgot about that! 
In any case, it is good to know the theory to exactly know where (and why) are the limits.



I think that the theory is one thing, but the practice (or should we call it "implementation") another totally different.

The SOURCE leads are carrying the current. So for 1mA, we probably don't care about lead resistance, but certainly we want as less as possible when the test current is 10A or 100A. MegaVolt asked for numbers and methodology. Megger provides a nice document related to their DLROs: "For the more rigorous tests, separate test leads are used and the current connections are positioned away from the potential connections by a distance that is 1.5 times the circumference of the sample being measured." This is the zone with uniform current density. (they refer to ASTM B193-65).

The SENSE leads should not carry much current, with typical DVMs the input impedance is at least 10M (usually much higher, like 1 or 10Gohms). Other measurement systems don't have that much input impedance, but this should not create any issue on modern DVM if the system calculates both the SOURCE currents and SENSE parameters and cancels out any imbalance.

But this is not always the case (on some systems) and IEEE Standard Test Code for Resistance Measurement (Std 118-1978) provides a few paragraphs about this:

"Some four-terminal resistance measuring instruments and techniques measure R(x) almost independently of the values of terminal resistances Ra, Rb, Rc, and Rd, the resistance of the leads connected to the terminals, and the contact resistance between the leads and the terminals. Other measuring instruments and techniques only reduce the effect of these resistances by putting them in series with resistances in the measuring circuit whose values are higher than R(x) or by connecting them in other less sensitive parts of the measuring circuit." (page 3)

Later on the Kelvin bridge section (4.5.3) it provides additional info:

"This arrangement permits four-terminal measurement of resistance elements, essentially eliminating the effects of lead and contact resistance errors in the measurement of low resistance (see 2.1)

...

equation 21 is useful, because it shows the necessity to keep the resistance of the yoke (Ry) as small as possible in order to minimize the error caused by lead and contact resistances to the unknown and standard in case of discrepancies between the ratios RA/RB and Ra/Rb. For the highest accuracy, care must be taken to ensure that the connection resistances are balanced, because Ry is not negligible."


And with all the above in mind then just you can now realize why the ESI 242D has a test leads balance system with two knobs that are called "Lead ADJ" and "YOKE ADJ".



Related thread here: https://www.eevblog.com/forum/metrology/4-wire(kelvin-measurement)-with-very-thin-lead-wires/

This was gold (as usual from Mr. Edwin G. Pettis):

"DVMs do have limits to compensating 4-wire connections, there is usually a specification called out in the manual for this and whatever effects it may have on the accuracy, DVMs measure and mathematically subtract out lead errors but it is not entirely accurate.  It still depends on the DVM's accuracy of measurement which is limited at such low values.  The Kelvin bridge was developed to MINIMIZE errors due to interconnecting lead resistance, it does not eliminate the effects.  In more modern bridges such as the ESI 242D, there are two adjustments which compensate for lead and yoke resistance up to 0.1 ohms by modifying resistance in the bridge arms to compensate for the extra resistance, these adjustments come very close to eliminating errors within the limits.  There is also a 6-wire modification of the Kelvin bridge which further compensates for other errors at very low resistances, the NIST website has information on the various methods of resistance measurement.

No measurement technique is entirely free of error, each one has its own sources of error and knowing just what those errors are can help in making more accurate measurements."

[bdunham7]

"DVMs do have limits to compensating 4-wire connections, there is usually a specification called out in the manual for this and whatever effects it may have on the accuracy, DVMs measure and mathematically subtract out lead errors but it is not entirely accurate.  It still depends on the DVM's accuracy of measurement which is limited at such low values. 

No measurement technique is entirely free of error, each one has its own sources of error and knowing just what those errors are can help in making more accurate measurements."[/i]

Although I agree with the second half and the result of the statement as a whole, the reasoning is not correct at least as applied to the many DMMs that I have dissected and repaired.  No DMM I know of (and of course that doesn't mean that there aren't exceptions that I don't know about) explicitly measures or does any calculations with the lead resistance in 4W mode.  The reasons that they would specify maximum lead resistances include SOURCE current compliance limits, voltage divider errors in the SENSE circuit--especially if they don't have or use high-impedance inputs--and the fact that the isolation between SOURCE and SENSE is not infinite and often not even all that high.