XPost: sci.electronics.equipment
From: rich@example.invalid
In sci.electronics.equipment Commander Kinsey wrote:
> On Fri, 17 Jul 2020 15:02:16 +0100, Rich wrote:
>
>> In sci.electronics.equipment Commander Kinsey
wrote:
>>> On Fri, 03 Jul 2020 02:20:53 +0100, RheillyPhoull
wrote:
>>>
>>>> On 2/07/2020 10:41 pm, Ralph Mowery wrote:
>>>>> For a real scare you should see some of the safety movies that
>>>>> Fluke put out. They show under test conditions what can hapen to
>>>>> inexpensive meters and their meters under different conditions like
>>>>> having the meter set for amps and putting across a 480 volt circuit
>>>>> that has plenty of amps .
>>>>>
>>>> Ahh the old "Leaving it on amps" trick. How many of us can say they
>>>> never did it ?
>>>
>>> I find it crazy that you can select volts and have the wires in the
>>> amps holes. The switch should change the contacts. I've broken a
>>> meter doing that, just measuring the voltage on a car battery. £100
>>> meter, but UNFUSED FFS! At least the mA range was fused, but they
>>> couldn't be bothered putting in a 20A fuse for the big range....
>>
>> The separate amps jacks exist for at least two reasons:
>>
>> 1) If the range switch also switched in/out the current shunt, then the
>> user could accidentally create a short circult across the probe tips
>> simply by turning the range switch to or across the amps measurement
>> ranges. Even if they were quickly turning /across/ the amps settings
>> ranges, a brief short circuit would be created, which would cause
>> damage to the meter as well as the device under test depending upon
>> what was connected at the time.
>
> Then you put the amps ranges on one end of the dial. You would never
> go through them.
Some meters have range switches that rotate through a full 360 degrees
with no stops - there would be no "end of the dial" in those instances.
And even with an 'end of the dial' there is still the possibility of
landing 'off by one' while turning the dial.
> And what idiot turns the dial while it's connected?!
The same idiot who'd leave the leads plugged into the amps jacks and
then attempt to measure the voltage of a car battery (or some other low
impedance high current voltage source).
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.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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