Multimeter Continuity Reading Doesn't Always Mean 0 Ohms

[StephenO]

Some users out on this forum may know this, but I never gave it much thought until now, so hopefully this information is useful to somebody.

I was gathering information for a couple of projects this spring for our trailer, which mainly means I was watching YouTube , and I was viewing the clip below where he is troubleshooting a short circuit on the 12v system. BTW, and I have no affiliation with him, but I really like the videos posted by that user.

As he is checking for any shorts between the +12v and return line, he is using the continuity mode on his multimeter where it will give the operator an audible tone when it detects a connection. As he's doing that at the power distribution panel at the fuse posts, and is getting a tone between a fuse post and ground, I think to myself, "If there is a short between positive and negative, why is the fuse not popping?". That question was based on me thinking that when the multimeter indicates continuity, there is essentially zero ohms on the line.

Well, I quickly disproved that after reading some of the specs on multimeters. For example, Fluke indicates that their multimeters will say continuity upto approx 50ohms. At 12v, 50ohms will only give you 1/4amp, easily not blowing a fuse.

So long story short, if you're testing your electrical system, be aware that even if your multimeter is telling you the connection is good in the continuity mode, check it also using the resistance mode...

[howson]

StephenO -- not only what you wrote in the post above, but some individuals will define a "short" as any deviation from the as-manufactured configuration of a circuit. For example, at school the instructor would use the term "short circuit" even if only one small part of a circuit was bypassed. It's a classic case of "Who's on First"...using a term (short) that two people interpret (or use) differently.

In class there was a representative circuit where the fuse wasn't blowing but the appliance wasn't working correctly. There was a "short" in the circuitry. Technically, the instructor had it right using the term "short"...but it took me a bit to get out of my paradigm that a "short" could mean the supplied voltage is not going right back to the source. To me, a "short" lets all the magic blue smoke out of wires and blows fuses (or pops circuit breakers).

In addition to learning a new way to view a "short", it blew my classmates minds when they tried to wrap their heads around the fact that their multimeter wouldn't always measure the resistance value calculated from V/I. In some of the circuits, especially AC circuits, some of the R value calculated is due to reactive components (thus there was impedance in the circuit in addition to the resistive elements).

Ghost voltage is another fun topic...can drive you crazy if it's not understood what's happening (there's voltage according to the multimeter but component "x" isn't working!).

Bottom line (and a good reminder)--the tool we call a "multimeter" is only as good as the experience level of the individual using it.