[Jeffhs]

Originally Posted by Electronic M

House fuses and breakers don't always catch failures. When I was a kid I had a 60's GE TOASTER-oven (looked more like a toaster) with the original rubber cord. I had put it out of use because the house wiring would hum when it was on, but one morning I had to make my own food as we were in a rush to pick up Dad from the airport and I dragged it out to make my food. Well it hummed the whole time it was on, finished it's cooking cycle, and a moment later...POW the insulation between the leads at the plug failed, and it let out a bunch of smoke and a pinky red flame from the plug after a moment of surprise I grabbed the cord away from the plug and gave it a good yank which managed to unplug it. The aftermath: a good 1/4" of one of the cord wires starting at the plug moving towards the toaster was vaporized, and the outlet and plate that I had changed months earlier was charred and had to be replaced. The fuse never blew!

In my next house before I knew my way around a DMM I tried to find which two leads on a 4 pin connector for a portable set were 120V by looking for 120V with a DMM...Well I had the test leads in the 10A current measurement position which is a dead short between the leads...I put the test leads in the pin holes in the plugged in cord, and knew I had found 120V when smoke came billowing out of those holes...The breaker did not trip then either.

Because fuses and circuit breakers do not always open the circuit in the event of a short or overload is exactly why I do not, under any circumstances, leave any of my Zenith radios, including a very old one with a 35W4 rectifier tube, plugged into the line when I'm not using them. The line cord to my K731 Zenith AM-FM set, in fact, is presently coiled up and stored in a dresser drawer (I had to replace the cord several years ago). The C845 still has its original cord, even after 54 years (the radio was made in 1960). However, I did replace the cord on my Zenith H511 some years ago because the insulation of the original was cracked; the cord was showing bare wire at several points. I would not leave that radio plugged in (even when the radio was switched off and with a new cord) due to the sheer age of the set--63 years.

Your account of your experience with a DMM smoking from the range selector jacks reminds me of a similar problem I had, now forty-plus years ago, with a small, cheap Olson Electronics (made by the now long defunct Olson Company of Akron, Ohio) analog multimeter. This meter had several pin jacks into which the test leads were plugged to change ranges. Well, one day I wanted to check the line voltage at a multi-outlet AC power box (six AC outlets arranged in two rows, in a large box with a master switch and, of course, a fuse) in my workshop at the time. I forgot to change the test leads from the resistance to voltage jacks on the meter, and when I threw the master switch on, I saw a small plume of smoke coming from one of the range selector jacks.

Needless to say, I yanked the test probes out of the outlet immediately. The meter, amazingly, was not seriously damaged; in fact, the only damage I could see was that the plastic casing surrounding the zero adjust potentiometer had melted slightly. However, that experience taught me a lesson I knew I would never forget (and never have, even to this day): Always check the range switch on a DMM before measuring anything, be it current, resistance or voltage, and be absolutely certain said switch is set to the correct range for whatever you are measuring. Trying to measure voltage with the meter set on one of the resistance ranges, for example, can and often will ruin the instrument in an instant, even though most DMMs (including my current Velleman one) are fused to prevent accidents.

Even then, however, one should never put blind trust in that fuse, for the very reasons you mentioned. In fact, my DMM has a warning right on the front panel to the effect that one of the current ranges (the 10-amp one, if I remember correctly) is not fused. Connecting any level of voltage to this jack will almost certainly destroy the meter immediately.

Your statement that your house wiring hummed when a toaster oven was plugged in suggests a potentially dangerous situation. I don't know how old your wiring was, but if it hummed when a high-current appliance was plugged in and switched on, I would think the electrical system in that house was almost certainly pre-'60s, even World War II or earlier vintage, and the toaster oven was overloading said wiring. These wiring systems were not designed to handle very high current appliances (except electric stoves, which had their own dedicated circuits), and would often blow fuses or the wiring would hum if overloaded. My grandmother had a summer cottage, built just before WWII, that had a separate circuit for the electric range, but that was the only high-current appliance in the place at the time. The only other electric appliances in that cottage, in fact, were a couple of table lamps, an overhead light in the kitchen, a small radio, and, much later, a TV set I put out there for her. This was a 1950s Emerson table set with a defective power switch, so when it wasn't being used, it was unplugged from the AC socket so there was little chance of a fire hazard when she wasn't there. There was also a "crowfoot" outlet in the living room of the cottage, but it was connected to the standard 120-volt electric service; this, of course, was likely because the cottage had been constructed in 1939, two years before Pearl Harbor, and repair parts for just about anything were in short supply at that time. Building contractors had to use what was available; if that meant using a 220-volt AC outlet to replace a defective standard one, that is exactly what was done.