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?
No, because there will be two extra resistances between the points of the probes and the circuit.
As ever, what are you trying to measure, and why?
Continuity between PE and different points of the enclosure.
Continuity between PE and different points of the enclosure.
It's not so useful to know with high accuracy. If the chassis is mains connected (as you have PE), PE is meant to be able to conduct mains voltage and short circuit current. Appropriate wiring for high current should be used. PE wire must be secured to chassis with a strong screw, preferably using a star washer that bites into the metal.
But if you have any kind of conductivity that your DMM can show (such as under 1 ohm), it will probably be fine. Of course, depending on application, you might want all parts of the chassis to have good connection between them (for shielding, EMI, ESD etc.). But again, you just want good conductivity, but the exact value isn't a big deal.
As ever, what are you trying to measure, and why?
Continuity between PE and different points of the enclosure.
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?
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.
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.
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..
As ever, what are you trying to measure, and why?
Continuity between PE and different points of the enclosure.AFAIK you need a special tester for that which uses high AC currents (up to tens of Amps). At least if you want to adhere to standards.
The meter I'm using is a Brymen BM869s but as it has mentioned even a 4W meter isn't suitable for this job.
Probably a hi-pot tester is the appropriate instrument for such measurements but unfortunately I can't afford one at the moment (they seem to go for over 1k) and secondly I don't feel very confident to use one as I've never done it before. AFAIK, you have to somehow discharge the DUT after the test and I have no idea how to do this. Also, although I appreciate the DIY recommendation I don't feel competent to to do it myself.
So, I was wondering whether a PAT tester would be sufficient. For some reason it's almost impossible to buy one in EU. It seems to be a UK thing. Searching around I found this one (which will set me back a bit over 500 euro) but I don't know anything about its quality and whether it makes sense to buy one. For example in a youtube video there's the following comment:
"I would love to know how this even qualifies as a PAT tester, it has a FAIL threshold of two Meg on a class one device according to the manual, yet the official threshold is one Meg. On a class two device it has a fail threshold of four Meg, when the the official threshold is two Meg, this all seems so very wrong when other testers are correctly calibrated, and this one seems to pass items other machines would fail."
https://www.firststopsafety.co.uk/product-page/memorypat-tester-with-display-and-memory
The meter I'm using is a Brymen BM869s but as it has mentioned even a 4W meter isn't suitable for this job.
Probably a hi-pot tester is the appropriate instrument for such measurements but unfortunately I can't afford one at the moment (they seem to go for over 1k) and secondly I don't feel very confident to use one as I've never done it before. AFAIK, you have to somehow discharge the DUT after the test and I have no idea how to do this. Also, although I appreciate the DIY recommendation I don't feel competent to to do it myself.
So, I was wondering whether a PAT tester would be sufficient. For some reason it's almost impossible to buy one in EU. It seems to be a UK thing. Searching around I found this one (which will set me back a bit over 500 euro) but I don't know anything about its quality and whether it makes sense to buy one. For example in a youtube video there's the following comment:
"I would love to know how this even qualifies as a PAT tester, it has a FAIL threshold of two Meg on a class one device according to the manual, yet the official threshold is one Meg. On a class two device it has a fail threshold of four Meg, when the the official threshold is two Meg, this all seems so very wrong when other testers are correctly calibrated, and this one seems to pass items other machines would fail."
https://www.firststopsafety.co.uk/product-page/memorypat-tester-with-display-and-memory
The meter I'm using is a Brymen BM869s but as it has mentioned even a 4W meter isn't suitable for this job.
Probably a hi-pot tester is the appropriate instrument for such measurements but unfortunately I can't afford one at the moment (they seem to go for over 1k) and secondly I don't feel very confident to use one as I've never done it before. AFAIK, you have to somehow discharge the DUT after the test and I have no idea how to do this. Also, although I appreciate the DIY recommendation I don't feel competent to to do it myself.
So, I was wondering whether a PAT tester would be sufficient. For some reason it's almost impossible to buy one in EU. It seems to be a UK thing. Searching around I found this one (which will set me back a bit over 500 euro) but I don't know anything about its quality and whether it makes sense to buy one. For example in a youtube video there's the following comment:
"I would love to know how this even qualifies as a PAT tester, it has a FAIL threshold of two Meg on a class one device according to the manual, yet the official threshold is one Meg. On a class two device it has a fail threshold of four Meg, when the the official threshold is two Meg, this all seems so very wrong when other testers are correctly calibrated, and this one seems to pass items other machines would fail."
https://www.firststopsafety.co.uk/product-page/memorypat-tester-with-display-and-memory
Are you confusing earth continuity tester (that tests that grounding wiring is low enough resistance) with an insulation meter (HI Pot that tests that insulation is proper i.e. very high resistance)?
That tester has both of these functions. It can test insulation leakage from live to ground with 500V applied, and separately with 150mA it can test that housing is connected to earthing on plug with low enough resistance..
As for insulation test, more is better .. few MΩ is already very low resistance... If instrument threshold is 4MΩ and standard is 2MΩ it means tester is more rigorous than standard, which means it errors on the safe side... Which means it will show failed on some marginal devices that would PASS on strictly calibrated devices.
Problem is that even 4 MΩ is actually very bad insulation on most devices outside industrial environments.. 100W toroidal transformers i tested recently had resistance between primary/secondary in GΩ ranges. One that was outside in damp, salty, environment for 5+years was more than 50 MΩ still...
Please can you explain what you are trying to do?
If low resistance PE to case and do not need to test to specification and have a power supply, Set current limit on power supply to 1A, connect the points you wish to test. Then measure the voltage drop with the Brymen. Thus use Ohms law to calculate resistance. Wire it 4 wire, which means that you connect to the measuring points, not to the power supply leads or clips. You can measure the power supply current as well to improve accuracy.
It's not often you see Class I audio gear, normally it's Class II..? At least in domestic settings, obviously pro grade stuff that goes on stage and tour will have a PE.
It does sort of sound like a PAT/ISITEE machine is what you need, if you're making Class I devices. An installation tester will do an insulation resistance test, but it won't be able to test the PE with any significant current. If you're making Class II devices, any insulation tester would be fine, I should think.
It's not often you see Class I audio gear, normally it's Class II..? At least in domestic settings, obviously pro grade stuff that goes on stage and tour will have a PE.
It does sort of sound like a PAT/ISITEE machine is what you need, if you're making Class I devices. An installation tester will do an insulation resistance test, but it won't be able to test the PE with any significant current. If you're making Class II devices, any insulation tester would be fine, I should think.
As far as I'm concerned, almost all pro audio equipment has PE and in most cases it's class I. Definitely all the gear I make.
Don't quote me on this one, but I believe some PAT testers can be plugged on the mains and provide a significant higher current than the battery powered ones (?).
Tt least according to this video (3:40) but this specific model seems to be discontinued.
With this power cable it is impossible to pass any test and this does prove, that any sticker, which should guarantee safety is bullshit! The cable is marked 0.75mm², but this cannot be the case with 315mOhm: