[panfilero]

how do you feel about measuring resistance or continuity in circuits that are hot, i see people doing it all the time, and it seems to work, but something about it doesn't seem right... is it an ok thing to do? or is it ok only in so and so situation, or you should never do that?

thoughts?

 

[Cydrome Leader]

how do you feel about measuring resistance or continuity in circuits that are hot, i see people doing it all the time, and it seems to work, but something about it doesn't seem right... is it an ok thing to do? or is it ok only in so and so situation, or you should never do that?

thoughts?

seems like a bad idea, unless you have a good meter that doesn't fail if
you abuse it. Even then, the readings you can get could be questionable.

I should probably test what it takes to trick a fluke meter into beeping
on the continuity test, it's probably not very much.

 

[mike]

seems like a bad idea, unless you have a good meter that doesn't fail if
you abuse it. Even then, the readings you can get could be questionable.

I should probably test what it takes to trick a fluke meter into beeping
on the continuity test, it's probably not very much.

 

[mike]

seems like a bad idea, unless you have a good meter that doesn't fail if
you abuse it. Even then, the readings you can get could be questionable.

I should probably test what it takes to trick a fluke meter into beeping
on the continuity test, it's probably not very much.

Here's a thought experiment for you.
What's the resistance of a battery?
Put your Fluke on it.
How many ohms does it measure?
Does that agree with your expectation?
What does that say about the wisdom of trusting ohms
measurements on a live system with a Fluke?

 

[Spehro Pefhany]

how do you feel about measuring resistance or continuity in circuits that are hot, i see people doing it all the time, and it seems to work, but something about it doesn't seem right... is it an ok thing to do? or is it ok only in so and so situation, or you should never do that?

thoughts?

If you need to ask, don't do it.

 

[panfilero]

If you need to ask, don't do it.

yeah i don't think the people doing it are asking, but good advice

 

[panfilero]

Are you sure you're not seeing people measure resistance in-circuit with

the power off, or measuring voltages? Both of those are valid things to

do.



Ohm meters work by injecting a current into the part then measuring the

voltage developed. Some circuits will be upset by having the current

injected into them, and most circuits will have some standing voltage

between most of the nodes -- so the readings will be off and the circuit

won't be working right.



I could see it being valid if you were making a DC measurement to a spot

in the circuit that was known to be AC-coupled -- but that rules out a

whole lot of opportunities.



--

My liberal friends think I'm a conservative kook.

My conservative friends think I'm a liberal kook.

Why am I not happy that they have found common ground?



Tim Wescott, Communications, Control, Circuits & Software

http://www.wescottdesign.com

you know, it's really more the continuity test I see people do with live circuits, not the resistance measurement... and it's the continuity test that seems to work most of the time.... scratch resistance out of it, just continuity

 

[Adrian Tuddenham]

Here's a thought experiment for you.
What's the resistance of a battery?
Put your Fluke on it.
How many ohms does it measure?
Does that agree with your expectation?
What does that say about the wisdom of trusting ohms
measurements on a live system with a Fluke?

It's even more spectacular to measure the resistance of the mains with
an AVO. (I'm told the flames are purple.)

 

[Spehro Pefhany]

Sure, it really comes down to the knowledge (or lack of) of the
operator. All methods are valid if used in the right context. To read
resistance directly in a live circuit using the meter set for resistance
would at least be wrong and as mentioned leading to a world of pain

I don't want to encourage dangerous behavior (so, don't do this!), but
consider a live* circuit with a series switch of some kind. When the
switch is open, you can measure the load R, perhaps even measure it
accurately depending on common-mode voltage and rejection. Close it,
deliberately or accidentally, and you may well experience a "Joerg
moment" or much worse. Google 'arc flash' and 'arc blast' for
examples.

* I'm using 'live' in the electrician sense- power has not been
switched off (and it should also be locked out) at the breaker panel.