Keithley 2000 digital multimeter has earthed input "LO" measurement terminal?

[treez]

Keithley 2000 digital multimeter has earthed input "LO" measurement terminal?

Hello,

Is the inut "LO" terminal of the Keithley 2000 Digital meter connected to earth ground?

...It certainly seems to have the earth ground symbol drawn near it.
......the user manual and datasheet dont say.........


Keithley 2000 Digital meter datasheet:
https://www.farnell.com/datasheets/1484284.pdf


Keithley 2000 Digital meter User manual
https://courses.engr.illinois.edu/ece445/documents/manuals/k2000_userguide.pdf

 

[sky_123]

Re: Keithley 2000 digital multimeter has earthed input "LO" measurement terminal?

Hi,

It can't possibly be connected to earth, it would make no sense. It is saying that that connection can be x hundred volts (I can't see it from the photo clearly) above
ground potential.
If it was connected to earth, then you'd be stuck using the multimeter for any circuit that is connected to earth if you wanted to measure a voltage with reference
to some other point.

 

[treez]

Re: Keithley 2000 digital multimeter has earthed input "LO" measurement terminal?

Yes i see your point.

But dont they say you should use some kind of differential probe to measure non earth referenced voltages?

.............but i agree that earthing it seems to make no sense............but then again earthing the output of the GW Instek GFG-8015G sig gen seems to make no sense either....................but it is earthed.

The earthing is necessary ( i think) to stop the output common from floating to a high voltage.(?)

 

[FvM]

Re: Keithley 2000 digital multimeter has earthed input "LO" measurement terminal?

Is the inut "LO" terminal of the Keithley 2000 Digital meter connected to earth ground?

If the front/back panel description isn't clear enough (I wonder if it could actually be), there's a text explanation in the user manual for you:

The maximum common-mode voltage (voltage between INPUT LO and the chassis ground) is 500V peak. Exceeding this value may cause a breakdown in insulation, creating a shock hazard.

But dont they say you should use some kind of differential probe to measure non earth referenced voltages?

Where do "they" say? I guess you are referring to suggested methods for instruments with grounded input like oscilloscopes.

The 500V peak voltage maximum specification for "LO" to ground however suggests a differential probe for measurement problems with higher common mode voltage, or using a different instrument.

 

[treez]

Re: Keithley 2000 digital multimeter has earthed input "LO" measurement terminal?

OK thanks, so i undertsand now that "LO" is not connected to earth ground.

That makes one ask the question, why are scope probe grounds connected to earth ground?

i.e. ,Why are scopes needing earth ground connection, and meters dont need it?


Engtips forums people always say diff probes are needed for non earth ground referneced measurments.
...also, vendors of diff probes say it.

How do they stop the "LO" connection from floating up to high voltages?........the basis of connecting the output common of instruments to earth ground is to stop them being induced up to high voltages................i know this first hand by having been electrocuted by touching the probe ground of a scope with its earth snipped out of its mains plug.

 

[FvM]

Re: Keithley 2000 digital multimeter has earthed input "LO" measurement terminal?

Some dedicated industrial electronics oscilloscopes have in fact floating inputs, e.g. Tektronix THS3000. It's primarly the nature of wide band inputs with single ended BNC connector that suggests grounding. A floating single ended input still involves a considerable ground capacitance and can't be used for true differential measurements. It might be used however for measurements with "live" common ground, e.g. in mains connected SMPS or inverters.

Multimeter measurements are relative small band and usually tolerate a floating single ended input without affecting the results. There may be problems with mV or very high impedance measurements though.

"Stop from floating" is a matter of suitable instrument design. High performance mains powered multimeters have usually the case connected to protective earth, also involving a screen between primary and secondary in the power transformer.

 

[treez]

Re: Keithley 2000 digital multimeter has earthed input "LO" measurement terminal?

Yes, my ex boss at Baldor used to have a THS3000 type scope................(i think his was a "TDS...." type and is now obselete, but it was the same in having isolated channels.)

............Anyway, 2 years later i was working at an electric drives place (electric cars) and i had to buy some scopes.................i offered to buy this Chinese PhD engineer an isolated scope.............and he said No!................he didnt want one with isolated channels.....................................i asked him why and he said that the degree of isolation between the channels is always only ever very small (say 40V) and that its easy to forget this and connect it to higher difference voltages and blow it up.

So isolated scopes are no use for mains voltage type work, so i was led to believe.

...................so i supposed that they must only be good for small signal (5V) digital work?