Multimeter safety voltages ?

[floobydust]

If the entire DMM was floating at say 300V, there is a shock hazard. I'm not talking about the voltage between the test leads.

A multimeter's common-mode voltage rating is dictated by the insulation around pushbuttons and rotary switch, and the battery compartment and housing screws, LCD window - any opening or seam.

It's relevant when measuring line-line voltages on a 3-phase power system, or those with a high-leg delta.
The DMM will then have a net potential wrt ground.


Also, some DMM's crash when there is a lot of common-mode voltage or noise present.
A metal foil shield inside a DMM is to help prevent this, as a DMM crash can indicate the wrong voltage or zero, which is a safety hazard.

I thought the old 2002 Fluke 17x recall was on this issue:
"...recalling 40,000 multimeters after discovering that some of them can take up to 18 seconds to display AC voltages above 500V, possibly leading users to believe that no voltage is present, with potentially hazardous consequences."

[IanB]

The instructions are clear and match what the graphics on the meter are telling us.

But ... it makes no sense. In both cases the CAT rating is higher than the "potential to ground" rating.

Can I only measure a 600V DC circuit with this meter if the 'neutral' wire is at -300V DC relative to ground?

Take an ordinary 9 V battery and measure the voltage between the (+) terminal and ground (use the ground pin of a nearby mains receptacle or a bonded water pipe).

Now measure the voltage between the (-) terminal and ground.

Lastly measure the voltage between the (+) and (-) terminals.

Explain what you see.

[wael]

Well I've to say that this is a very nice explanation from you. I don't know why this haven't crossed my mind. Many thanks !

[joeqsmith]

Just to be clear, I am having some fun with Mr Fungus and suspect that was mutual.  He brings up the case where the negative supply is attached to earth.  Attaching the meter's common to the negative gives a common mode of 0 volts and we can read the 600V according to the manual (assuming some sort of CAT II or less).   If the leads of the meter are swapped, we now have a 600V common mode, exceeding the manuals maximum 300V limit.  That was my take all along but I didn't want to let him off the hook.    

I think if you look at most handheld meters, you will find that the common mode actually follows the CAT rating which is what I suspect Fungus was wanting me to post all along, admitting that the meter was a POS and that they had no clue how to write a manual or mark a meter (which was already pointed out to be the presumed work of their marketing department).         So in this case, the 300V  meant the 300V CAT III and could have shown a max common mode of 600V for CAT II and less.   

For others actually trying to follow the mess, ignore my posts.

[Fungus]

... which is what I suspect Fungus was wanting me to post all along, admitting that the meter was a POS and that they had no clue how to write a manual or mark a meter (which was already pointed out to be the presumed work of their marketing department).

And my point was that I don't see how that voltage can be less than the CAT rated voltage (it goes bang if you swap the leads round??  )

It also can't be greater than the CAT rated voltage, so that means it must be equal to it.

I guess that's why Fluke mark their meters like this:


Conclusion: Either the markings are wrong/misleading or the meter is completely messed up internally (the voltage can leak through the case to your hand from the COM socket but not via the other one?)

[Fungus]

If the entire DMM was floating at say 300V, there is a shock hazard. I'm not talking about the voltage between the test leads.

But only via. the COM socket. The other sockets can be connected to 600V, no problem!   

[stj]

i read that marking as "relative to ground"

so if your metering a line with negative bonded to ground at some point, it can be upto 600v
BUT,
if your metering a 600v line that has no ground reference there could be several thousand volts difference between the lines and ground.
and bad shit could happen!!

[Neilm]

i read that marking as "relative to ground"

All CAT ratings are to ground - it has to do with the transient that can be applied to the mains.

One other point - IEC61010 requires testing of all combinations of inputs at the worst case (unless the leads are not compatable). Hence current inputs will have to be tested shorting the terminals with maximum voltage and a very low impedance source.

[floobydust]

If the entire DMM was floating at say 300V, there is a shock hazard. I'm not talking about the voltage between the test leads.

But only via. the COM socket. The other sockets can be connected to 600V, no problem!   

If you are measuring current on the high-side of a 300V rail, both sockets (COM, A) are hazardous live.

The 61010 test is basically wrapping metal foil around the DMM and doing the voltage tests:
"ACCESSIBLE  insulating  parts  of  the  ENCLOSURE  are  covered  with  metal  foil  everywhere  except  around TERMINALS."