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Low R measurement on a DMM
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bdunham7:

--- Quote from: pope on March 27, 2023, 12:08:11 pm ---Considering a DMM, which reads a value of 0.1Ω when the probes are sorted. If we do a continuity test on a device and we measure 0.3Ω, can we assume that the actual resistance between the two points is 0.2Ω? i.e 0.3Ω minus the resistance of the probes (0.1Ω)? 

And if so, what's the benefit of a 4W multimeter?

--- End quote ---

If your meter uses a 1mA test current (typical but not universal) then a resistance of 0.3R will result in a voltage of 300µV.  So how well your meter can measure the difference between 0.1R and 0.3R depends on both the ability of the meter to resolve the 200µV difference and on any small contact resistance you introduce in the circuit when you change the position of the leads from shorted together to measuring your resistance.  You would want to do this a few times each way to see how repeatable your measurements are.  If they seem reliably repeatable, it wouldn't be all that unusual for  decent 4.5-digit meter (or better) to be able to resolve those measurements to at least 1 digit. However it is often a tricky and error-prone process.  Using a better meter and 4W kelvin-clip leads you can usually get 2-3 digits accuracy because you eliminate the effects of varying contact resistance and voltage drop in the leads.
TimFox:
Also, with many DMM test leads, you can see a noticeable shift in resistance depending on the pressure you exert on the probes, which limits your repeatability for 2W ohms.
DX1:
If you are trying to measure for safety then you need to use high current and measure voltage drop.
A simple way to generate high current is with a toroidal transformer and a single turn (OR small number of turns) adequately rated conductor wire through the core and connected to the measurement points. Ideally with a variac to adapt the primary voltage from 0 to 230V and a clamp meter to measure the current. Measure AC voltage across the measurement points directly (Not using the current injection connections.) AC voltage/AC current = resistance.

Use a hefty transformer to avoid burning it out.
You can get better accuracy using a current shunt adequately rated in the current loop and measure the voltage drop across the shunt.
You need to check the standard you are using to see what current you need and also the allowable resistance.
Be aware that the current can be high enough to melt cable, so ramp up the variac slowly from 0V. Also keep testing time short.

I did suggest using a Robin digital loop tester, but on checking there seems to be too much variation for it to be useful.
I am assuming you know how to do this safely. If not sure don't attempt it.
2N3055:

--- Quote from: alm on March 27, 2023, 02:32:55 pm ---For the normal case of measuring say a low value current shunt, you'd do a four wire measurement by feeding a current, for example from a bench supply, through the resistor and measure the voltage across the resistor at its terminals (separate connections for voltage and current). Then divide the voltage by the current, and you get the resistor. For very low value resistors it helps to use a larger current, like 1A, to increase resolution.

For earthing resistance measurements there will likely be specific standards in your local electrical code. I wouldn't assume a random DIY solution would be sufficient there.

--- End quote ---

DIY absolutely won't. For it to be done right you need to use proper certified earth continuity tester... I believe it is called PAT tester..
Currents used would be up to 25 A..
DX1:
The OP will be doing precompliance testing, or production testing or simply checking his equipment is OK.
DIY can solve this, see my earlier post. Again this does involve mains wiring and high current. If you do not know how to do this safely don't.

--- Quote from: 2N3055 on March 27, 2023, 04:38:32 pm ---
--- Quote from: alm on March 27, 2023, 02:32:55 pm ---For the normal case of measuring say a low value current shunt, you'd do a four wire measurement by feeding a current, for example from a bench supply, through the resistor and measure the voltage across the resistor at its terminals (separate connections for voltage and current). Then divide the voltage by the current, and you get the resistor. For very low value resistors it helps to use a larger current, like 1A, to increase resolution.

For earthing resistance measurements there will likely be specific standards in your local electrical code. I wouldn't assume a random DIY solution would be sufficient there.

--- End quote ---

DIY absolutely won't. For it to be done right you need to use proper certified earth continuity tester... I believe it is called PAT tester..
Currents used would be up to 25 A..

--- End quote ---
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