Unfused Digital Multimeters

[GranTotem]

Hi,

I have a question to which I couldnt find a clear answer and that is, why does a DMM have a 20a UNFUSED socket?

Why would you fuse the 200ma Max. and leave the 20a one unfused!

The DMM I have is a low end, an Unitrend UT58C, looks nice and works well but I feel kinda insecure knowing that it has this "issue".

Should I bother to add a 20a 1000v fuse? There is space inside to add a cylindrical fuse holder (looks like a tube that you unscrew and put the fuse in). As a hobbyist I havent had to measure more than a few milliamps so, should I bother?

Thanks in advance,
GranTotem

 

[thylacine1975]

The high current socket of a DMM is generally not fused to avoid the voltage drop across the fuse at the (intended) high measurement currents.

Also, a fuse would be quite useless in protecting the meter against *slight* overloads. For example, most meters impose a time limit on their high current measurement - mine says "20A for 30s" - this is to limit the heat dissipated within the internal current shunt. A fuse capable of passing 20A won't blow with any sort of rapidity until the fault current is substantially larger - say, 60A+. Since the meter can be readily destroyed through overheating at currents far below this value, the manufacturer's don't bother installing a fuse (and imparting the user with a false sense of security). Plus, they save (=pocket) a few $s in the process!

The one time where a fuse would be handy though is in the case of an extreme overload - where a fuse would be expected to blow within milliseconds.
Interestingly, I have found that the leads themselves can perform this role I had forgotten to swap the connections back from "Amps" to "Volts" when I placed the probes across the bussbars of a (very large) lead-acid backup battery bank.... *KAPOW!!* ...when the smoke cleared (and my pulse rate returned to normal) I found I was holding only the rubber handgrips of the probes and the tips and wires no longer existed!

Oh, to answer your question then - no, don't bother installing a fuse. A new meter might be cheaper on the one day that it mattered!

 

[pjmelect]

I agree with the above post, the meter may not have a fuse, but often the manufacturers put in a weak spot say a small section of PCB track that blows first, saving the rest of the meter. You don't need to ask me how I know this

 

[GranTotem]

Now I understand, thanks!

Your story reminded me of when I was building a power supply and the mains where shorted by its metallic case...
Heart attack + big spark + all 3 bedrooms out of electricity (safety switch, thermal switch(whatever you call it in english) switched off)