You are an idiot if you can not see the potential for error. If you have multiple conductors carrying current nearby you will read voltage on an isolated circuit with a DMM. That voltage will not be read on an analoge meter or with a resistive voltage indicating device like test lamps.
I can see that none of you ever worked inside high energy panels.
I will try and explain it another way. voltmeters on DMMs are good for measuring the difference in potential, but they are not suitable for checking for the presence of a difference in potential inside a changing magnetic field.
Now if you can not see why I say it is not good practice to test for live circuits with a DMM you DO NOT KNOW ANYTHING ABOUT THE GENERATION OF ELECTRICITY.
Uhmm, it seems that someone is "writing aloud". Why do that? Here we are different people with different experience. I think that no one should think to have the "absolute knowledge", don't you?. I'm Italian and maybe I have missed some offensive statement in your debate that started this situation, but I don't believe that the owner of this thread wrote it to read some people offend each other. Paul started this thread just to let us know his problem. Some message ago, I guessed that the problem could be raised because of the capacitive DAC based analog to digital technology. The older ADC types just have an Hi-Z comparator at their inputs, while the cap-DAC-based ones switches their inner capacitance directly at their input via c- mos devices. It is my opinion that this could inject some (few) charges backward to the DUT, raising the problem on very Hi-Z capacitive DUT. I don't know whether I'm right or not, but I believe that (as technicians) we should be aware of this possibility in future. Because (if I'm right), it means that these new DMMs having that technology inside, could have a so Hi-Z that it could become "negative". Of course, it also means that we should never use them to determine the parassitic rasistance of a capacitor. Have a great day. Massimo
I'm sorry, I sent the previous message one more time because (for a strange reason that I can't figure out) I was unable to see it in the thread until I sent this new one. Anyways, It's not my intention to bore you. Have a nive day.
I really don't understand the resistance (if you will excuse the expression) to Arloe.
Of course both analog and digital meters (excluding, for practical purposes, the original topic) have resistive input. For the purposes Arloe uses a meter, the intentionally high impedance of most digital meters is a liability. The lower impedance of an analog meter, and some digital meters, is an advantage.
alt.home.repair gets more electric power questions than this newsgroup, with the OP usually less technical savvy (some at a.e.e that aren't either). It is rather common for someone to ask why there is 83 volts on a wire that should be dead. The common response is it is a "phantom voltage" picked up by using a high impedance digital meter and to use an analog meter (or light bulb). As Arloe said "An analog meter 'IS A LOAD' a DMM is NOT."
Arloe is just saying to use the appropriate tool. In addition, he can't kludge a lower impedance onto a high impedance meter. He may meter power circuits that have an available fault current of tens of thousands of amps. In the US you may need to wear a "flash suit" to merely meter the voltage. The meter must be designed to safely fail on those circuits.
Same applies to responses to Stewart, who is speaking from professional experience.
Posted through googlegroups (yech) because my news source apparently doesn't like cross posting.
The issue is his (and others') insistence that a high impedance meter (and indeed even a digital meter) cannot be used in these circumstances. The fact is that it can, though one has to understand that what one is reading may not be correct. These errors are easily prevented/corrected, something the sparkys simply cannot understand.
Of course, "analog" has nothing to do with it (as some have claimed). How does he know that even the analog meter isn't lying to him for similar reasons? The fact is that one has to understand the tools and work with them. A "load" is easy. Preventing a low impedance meter from loading a circuit is difficult.
No, he's saying that that is the only tool possible. The fact is that a high impedance meter *can* be used, though one has to be careful. Something evidently beyond the average sparky.
That isn't the issue, and you know it.
Perhaps "professional" experience, but limited knowledge.
High impedance DMMs "safely fail" on ALL equipment as well.
And no, it matters not that there is a high current capacity available. If the operator does not know how to take measurements in such environments, he should not even be in such an environment, much less be there attempting to take readings from power equipment. With ANY meter.
Arloe understands =93that it can=94 and =93understands what one is reading may not be correct=94. Arloe understands that if he reads 83 volts with a high impedance meter he doesn=92t know if the voltage is real or =93phantom=94. So how is the measurement useful? Maybe electricians are too fussy about knowing whether the circuits the work on are live?
Arloe does understand. That is why he uses an appropriate tool. And I am a licensed master electrician. I guess I must not understand either.
A high impedance meter reads 83 volts. What do you know? How do you prevent? How do you correct?
The term =93analog=94 is used because (without an amplifier) they are inherently low impedance in the context of the measurements Arloe is making. (With a good 50 microamp movement you might still see some phantom voltage. AC ranges are likely way under 20k ohms/volt.)
Arloe does understand the tools. That is why he wants to use a lower impedance meter on power circuits.
Not explained - how do you =93work with=94 a high impedance meter? What is the =93work with=94 procedure if you measure 83 volts?
How do you =93easily=94 provide a load if you are measuring power circuits with a high impedance meter?
Of course. If you have a high impedance circuit you use a high impedance meter. Use the appropriate tool.
Power circuits are not high impedance.
He is saying that a low impedance meter is the appropriate tool for the measurements he is making.
Arloe measures 83 volts with a high impedance meter. What does that tell him?
May or may not be. You haven=92t explained how you =93work with=94 a high impedance meter. What is the =93work with=94 procedure if you measure 83 volts?
Your Ouija board is very accurate and, of course, is the appropriate tool. Is it high impedance?
Only if they are designed for high energy circuits.
It matters greatly that there is high current capacity available.
There is a meter classification based on available current. (Category
1, 2, 3; probably from the IEC.)
If you are in the workplace and covered by (US) OSHA using the wrong type of meter could be real expensive.
And if you don't use the appropriate tool you could wind up wearing the meter. You could also wind up wearing the equipment you are metering.
Of course. But a competent person working high energy circuits will use a meter that has been designed and rated for high energy circuits. If you are poking around in a PC it doesn't much matter.
It *is* a phantom voltage. It's not so much that measurement is useful but the fact that it's not. How to take the measurement to
*not* get this reading is the point. That takes some brains he ain't got, apparently.
No, apparently they're too dumb to use the instruments correctly and to know when to *not* take what they're reading as fact.
No, he doesn't understand. His "low" impedance meter isn't all that low and can still have these measurement artifacts. He's too stupid to understand what he's seeing.
That it most likely ain't 83V.
A number of things. Mainly, no matter what the meter, not to trust it.
Load the circuit. Turn on a light bulb. Use a resistor. Any number of things, depending on the circuit and the wire in question.
They can still be thousands of ohms per volt.
No, he's been told what tools to use. He understands nothing. A high impedance meter can certainly be used, though one has to know what he's doing. Arloe is clueless.
I told you, several times. I can't help it of you're in Arloe's league.
Maybe a light will come on.
No, *understand* your tool *AND* your business.
They certainly are, if not connected.
No, that's not what he's saying. You're coming in in the middle here.
How many times are you going to ask this silly question?
I have many times. I can't help it if you, like Arloe, are too dumb to read.
They, like all the *sensible* engineers I have come across, have a desire to reach the age where they are able to draw and enjoy their pensions.
The Avometer Model 8, which I still have, has an impedance of 1000 ohms/volt on AC. It has switched ranges to 1000V f.s.d and a further range, via separate terminals, to allow measurement to 2500V f.s.d
The AVO model 40, which was favourite when working more on the "power engineering" side, was lower. I cannot quote an exact figure but I believe it was more like 200 ohms/volt. The most sensitive AC current range was
12mA f.s.d.
And he is absolutely right.
Should anyone care:
I spent the best part of 40 years as a transmitter engineer maintaining, repairing and caring for, transmitting and associated plant for the BBC.
I have worked on everything from DC to 7GHz (They wouldn't buy me the test gear to repair anything higher in frequency than that but since we only had a few links at higher frequencies it probably made economic sense to return to manufacturer to repair).
I have covered everything from milliwatts to megawatts and battery power to 11kV distribution systems. (Perhaps KRW would like to take his favourite high-Z DMM and try testing the spouts on an 11kV switch-gear panel) I was qualified as an "authorised" person for the purposes of the HV rules.
I have repaired everything from the simplest DC power supply to the latest digital TV and radio transmission equipment. I have worked on cooling plant, rotary converters, standby power plant and done design and prototype work. I doubt there is a single piece of electrical/electronic test equipment which I have not handled and known the *proper* usage of, at some point in that time. (not forgetting general workshop equipment such as lathes, drills, milling machines....)
Most meter leads for standard voltage measurements are very small gauge. Not much will pass across those.
When I read current, I make my own 10Ga. Silver Plated Copper short jumpers that allow a current reading without changing the circuit much.
The meter, however is limited internally on current readings by one or even two fuses, and the voltage is limited by the breakdown voltage of the inputs, and yes, it is isolated to 400 or 600 or more volts.
You're an idiot. Even for an HV reading, one would be required to use an HV probe, so no threat is posed to the meter itself under proper operation yet again.
Dead shorting a modern meter amounts to the few joules it takes to blow the internal circuits or explicit fuse. There is no energy greater than that that you can pump into one.
Take typical meter probes and a big copper shorting bar. Hook up to power. The leads flash. Likely before the friggin fuse even blows. The meter will flash open far sooner than the leads will.
Almost all analog meters on the market are in the oder of 10's of Kohms / Volt. My Triplett 630, a very typically rated industry standard analog meter for example is 10-20K/V on DC and 5-10K/V on AC volts. The most sensitive analog meter I've ever owned (and from memory, saw) was rated at 100Kohms/V.
Those that are higher are FET input meters and they are NOT rated in ohms/volts, as the impedance is no longer based on the meter movement. My Tandy FET analog meter is 10Mohms fixed on DC volts, and non-FET input on AC, rated once again in 10's of K per volt.
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