Fluke LoZ limitations?

[FliesLikeABrick]

Hi all,

As most of you are aware, a number of Fluke DMMs offer LoZ mode, which offers a lower impedence for testing while eliminating ghost voltages.

If my understanding is correct, this means that usage on a 120vac circuit will lead to approximately 5w dissipation inside the meter.   At 480vac, this is 75 watts!  And of course the power grows exponentially with any higher voltages under test.

Reading the documentation for this:
http://www.fluke.com/fluke/uses/comunidad/fluke-news-plus/articlecategories/electrical/dualimpedance
http://www.techni-tool.com/site/ARTICLE_LIBRARY/Fluke%20-%20What%20you%20need%20to%20know%20about%20input%20impedance.pdf

And specifically (for example) the 289's documentation:
http://assets.fluke.com/manuals/287_289_umeng0100.pdf

I do not see any specifics about how this is managed, or what protection may be in place to avoid damage to the DMM.

My questions are:
- Can sustained usage on live circuits damage the DMM?
- If not, how is the protection accomplished?
- If it can damage the unit, where is there documentation about voltage or duration limitations for this mode?

While the input network may be able to dissipate 5W for a period of time, I am wondering how usage on 480 or higher voltages (even briefly to check for liveness or eliminate ghost voltages) can avoid blowing up the input side of the DMM.

Thank you, and apologies in advance for any fundamental misunderstandings or terminology abuses in the questions above.

[borjam]

It's intended to be used as a "voltage detector".

I tried it once on a mains outlet and it tripped the GFI. 220 volts through 3 kohms is more or less 73 mA.

So, imagine that you are wondering wether you can touch a live wire or not. You test with the "normal" high impedance mode and you
see a voltage that might not be real. Then you can try with the LoZ. I guess it's not intended to be left unattended, just for a quick check.
The resistance is low enough to make "ghost" voltages read as 0.

[FliesLikeABrick]

Thanks.

So I guess what I'm wondering is what would you expect to happen at higher voltages that are within the spec of the meter (480vac for example)?  If this blows up a resistor in the input network immediately, is that the user's fault?  I can't find documentation to know where the limits are.

[technogeeky]

Thanks.

So I guess what I'm wondering is what would you expect to happen at higher voltages that are within the spec of the meter (480vac for example)?  If this blows up a resistor in the input network immediately, is that the user's fault?  I can't find documentation to know where the limits are.

It seems kind of hand-wavey. Every single Fluke instrument manual I found (five or six of them) which had LoZ, simply said something like "do not use LoZ with circuits that could be damaged by this mode's low impedance".

Then again, they also explicitly state that the LoZ mode is single-range, and that the range goes up to ~ 600V RMS.

On the other hand, I looked at some Fluke 287/289 and I certainly can't see any major differences or big power handling devices. So I'd have to guess that one should limit onself to momentary usage and not leave it connected.

[Cerebus]

It's intended to be used as a "voltage detector".

No, it's intended to avoid 'ghost' voltage readings where a high impedance voltage source is present, such as an induced voltage from an adjacent live circuit making a significant voltage appear (but with no actual useful power) in a disconnected circuit that's being measured.

This can happen in electrical AC circuits where a tiny current capacitively couples from one conductor to an adjacent one. The low impedance mode sinks enough of this capacitively coupled current to stop the meter registering a significant voltage.

[Cerebus]

The resistors used to implement Lo-Z modes are typically PTC resistors. So, they start off at 3-4K, but put any significant power into them and they heat up, their resistance goes up thus reducing the power that they consume. Typically you'd design this kind of part to self limit its temperature rise to something reasonable.

[FliesLikeABrick]

It seems kind of hand-wavey. Every single Fluke instrument manual I found (five or six of them) which had LoZ, simply said something like "do not use LoZ with circuits that could be damaged by this mode's low impedance".

Indeed, that is literally the only cautionary statement anywhere - and it seems to be entirely about protecting the device under test by from drawing too much current if the device under test doesn't have protection against overcurrent situations

[borjam]

It's intended to be used as a "voltage detector".

No, it's intended to avoid 'ghost' voltage readings where a high impedance voltage source is present, such as an induced voltage from an adjacent live circuit making a significant voltage appear (but with no actual useful power) in a disconnected circuit that's being measured.

This can happen in electrical AC circuits where a tiny current capacitively couples from one conductor to an adjacent one. The low impedance mode sinks enough of this capacitively coupled current to stop the meter registering a significant voltage.

Yes, I meant that, but I phrased it really poorly.

[FliesLikeABrick]

Ok, so down to the point now -- My 289 was damaged after I used it on some ~400v circuitry under test a few weeks back.  I sent it to Fluke, paid to have it repaired.  I was using LoZ intermittently throughout my work, for testing whether components could provide some power instead of just no-load voltages, and draining some capacitors after disconnecting mains.

The damage seemed to be to a larger 1k resistor near the lower left of the back of the board, near the inputs.  It makes  sense that this may be for the LoZ operations.

The reason I am asking these questions, as some of you may have guessed -- is could this have been from using LoZ at ~400V?  If so, should I have known not to do this, and if so from what documentation? 

Here is a picture of a 289's board (not mine, I didn't think to take a picture before sending it for repair).  The resistor in question is the green 1k in the lower right corner of the picture.

I am trying to understand where I went wrong, and if so in retrospect how I could have known to acted differently.