XPost: sci.electronics.equipment   
   From: presence@MUNGEpanix.com   
      
   In sci.electronics.basics Ralph Mowery  wrote:   
   > In article , rich@example.invalid says...   
   >>   
   >> I suspect you would find changing the range switch while connected is   
   >> much more common than your response implies.  Esp.  for switching   
   >> up/down a range for meters that are not auto-ranging.   
   >>   
   >> >> 2) By having the amps jacks separate, the range switch itself does not   
   >> >> have to have contacts beefy enough to carry the current for the amps   
   >> >> settings.  Remember, when measuring amps, the current being measured   
   >> >> flows *through* the meter itself.  This would require very different   
   >> >> (and likely much more expensive) range switch contacts.   
   >> >   
   >> > Or a relay.   
   >>   
   >> Which also equates directly to added expense vs.  having the user   
   >> themselves be that "relay".  And for battery powered meters, shorter   
   >> battery life (due to the current consumed by the relay coil).   
   >>   
   >> It is also possible (this is a guess in my part) that the CAT ratings   
   >> specify separate amps jacks for an added safety factor of "user must   
   >> deliberately move lead to obtain short circuit through meter".   
   >>   
   >> As well, the user who'd leave the leads plugged into amps, and then try   
   >> to measure voltage on car battery or other high current source is also   
   >> just as likely to leave the range switch in the amps setting (after   
   >> having measured current somewhere) and subsequently try to measure   
   >> voltage.  It is not possible to fully protect users who don't pay   
   >> attention from doing stupid things.  Either they forget to unplug from   
   >> the amps jack, or they forget to switch away from the amps range,   
   >> either way they get a local fireworks display of their own doing.   
   >>   
   >>   
   >>   
   >   
   > I often leave the leads connected to a Simpson 260 and change voltage   
   > ranges.  Where I worked there were so many wires in a conduit carrring   
   > 120 VAC control voltage it was difficult to tell if the voltage was   
   > 'real' or induced by the coupling.  I found out that if I started on the   
   > 500 volt range and then switched to the 250 volt range if the meter   
   > pointer stayed in the same relative position I could then switch to the   
   > 50 volt range and the meter pointer would still be near the same   
   > relative position if the voltage was induced.  If the pointer changed   
   > positions to follow the 120 VAC when going to the 250 volt scale the   
   > voltage was real.   
      
   The simpson meters are interesting in that they have as many jacks as they   
   they positions on the switch. You really need to move the leads around all   
   the time to do anything. I guest it makes you think, a little. I did catch   
   myself reading the ohms range wrong though recently.   
      
   > I doubt that the CAT rating requies seperate Amps positions for the   
   > leads.  Fluke uses special fuses in their meters to get the CAT ratings.   
   > They do not have a seperate jack for the ohms settings either which is   
   > more likely   where the meter would be set.  Most of the time the   
   > circuit would have to be broken and the meter inserted to measuer amps.   
   >   
   > As you say it is impossiable to fully protect everyone from doing   
   > somethen forgetful or stupid, but having a $ 300 meter instead of a $ 5   
   > dollar one goes a long ways.   
   >   
   > I do have a Fluke test meter that is purty much fool proof up to 600   
   > volts and rated CAT 4. A T1000.  It only measuers AC amps by passing a   
   > wire through the prongs.  It does have a switch for voltage and ohms.  I   
   > have on purpose set it to ohms and put it across a fuse in a 480 volt AC   
   > circuit to see if the fuse is good or bad.  No problem to do this.   
      
   I just got one of those clamp current multimeters. I noticed some glaring   
   limitations vs the full out Fluke 87 V, but at the same time, it's way   
   more "idiot-proof".   
      
   The current ranges, even for DC at only available though the current   
   clamp. It's just not possible to short anything out with the test probes.   
   The downside (not really surprising) is no low current ranges. Ok fine.   
   The input impedance is pretty low at 1Meg as well, but for poking at line   
   voltage wiring, this is fine. Again, no matter what range you are set it,   
   it appears to be impossible to blow up the meter as it has no low   
   resistance across the leads modes. The ohms range seems to max out at 40k   
   or something surprisibly low like that, again, no big deal for prodding at   
   lighting circuits or an outlet, or some 24 volt circuit.   
      
   Getting all sorts of wonky reading with the banana jack type K   
   thermocouple though. It seems the meter has an internal thermometer and it   
   compares to the junction at the end of the probe. Not really sure how that   
   all works and how the temperatures of the banana jacks affect things as   
   they are not the special metals in the leads of the probe. I know with   
   normal process controllers, and deviation of the correct connector or   
   metals used will result in really strange readings.   
      
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