The Circuit Detective    -  Gadgets and Stories

HOME   Sitemap

Testers and Tips for Electricians


I am starting this page with ideas of some homemade electrical testers for special troubleshooting purposes. I'll let one of you guys patent and market these.

1. Power-restored buzzer. To know if power has returned to dead outlets in one room when you are testing outlets or purposely disturbing connections in another, I have found a 120-volt buzzer out of an older smoke alarm can be heard across much of a house. Rig it to a short cord-and-plug and plug it in.
2. Short-gone buzzer. As you disturb or disconnect one thing after another to narrow down where a short is happening on a tripped circuit, it is a nuisance to have to keep doing a resistance check or to keep retrying the breaker. So I rigged a 9-volt-powered buzzer that alarms when its two leads lose continuity to each other (through the shorted wires you attach them to). It uses a little relay. Of course, if you are after a line-to-neutral short, it won't alarm when it should if there are loads (below a certain resistance) still connected on that part of the circuit.
3. Power-lost buzzer. This is for an intermittent open that is not now open. As you disturb connections here and there, it will let you know immediately (from the next room) that power is gone again. It uses a cord-and-plug and a different relay.
4. Tripped-GFI indicator. This is for an open-hot standard receptacle when you wonder if it is dead from a tripped GFI receptacle upstream. People do sometimes protect strings of outlets that aren't required to be protected. For this, I rigged a very short cord-and-plug out the back of a single-AA flashlight, with the ground and neutral wires making continuity
through the flashlight bulb and battery (I cut off the prong for the hot). It's just a special continuity tester to see if there is continuity between ground and neutral (back at the panel), which would normally be the case. Tripped GFI receptacles (though not most GFI breakers) disconnect the neutral as well as the hot. Of course the test won't be conclusive, because it depends on a good ground and because there are other conditions that can make both hot and neutral discontinuous from their sources.
5. Fancy outlet tester. I still use 3-prong testers for outlets, but I have encountered enough strange readings that I have designed a more sophisticated model. It uses power brought by an extension cord from a good outlet to positively tell me whether each of the three receivers of the outlet I am testing is hot, grounded, or just plain dead. It uses seven neon lights.

Even though I give a link below to some good stories, I have more recent ones, in case you are interested. Nov. 10, 2008. I was called to trace down a short at a home I had been to before. This one was making the breaker hum before finally tripping it -- typical of underground. And the circuit did mostly serve the front landscape lights -- two bricked post lights and five pagoda lights, including two locations I had made underground splices to repair previous problems. Was MY work going bad? Unthinkable! Early in the process I was surprised to find that the short measured from hot to neutral, not hot to ground. You don't tend to find that with underground problems. I unscrewed all the light bulbs to be sure this ohm reading wasn't just from the path through the seven filaments or ballasts. No difference. The UF cable was quite shallow, which let me inspect my old splices and some portions of cable. Nothing there. I finally took enough of the black wire splices apart to see which one cable had the problem -- a piece leading to a receptacle bellbox (T-box) that supported one of the lights. I had looked in there earlier with my flashlight and seen nothing. This time I removed the receptacle. The two cables running through this box were both sleeved in a piece of half-inch nonmetalic flex coming up from the ground and entering the back of the box. The connector that had been used was for metal flex -- just a twist-in type. I didn't want to undo all that unless there was a good chance the problem was there. So I turned the breaker on again, thought I heard a noise immediately, and went out to see smoke coming from the area of that connector! I put the breaker back off (it hadn't tripped this time). Once I pulled the cables out, I saw that the installer had stripped sheath off them where they passed through the connector area (just to get them through), but for some reason he had also twisted those blacks and whites around each other quite a bit (to finally screw the connector into the box threads?). Apparently that had skinned some blacks and whites and time and moisture had cooperated with the metal of the connector to set up this short. Not technically underground after all! I spliced these two cables in the ground with a third piece now feeding the box all by itself. Fun, huh?

OK, here's a couple more from the spring of 2009. Nursing home tiled shower area. People occasionally feeling a shock when touching the metal shutoff handle. The maintenance people could measure up to 60 volts at times, and mostly, they began to think, starting with the weekend nurses shift. The first time I came (as a consultant only, since I'm not licensed for commercial), I helped find which breaker, in which of a dozen panels, was the one responsible. Using an independent ground, I also showed them that it was not the piping that was live. It was the floor! Indeed, holding a non-contact tester, it would light up even for the walls of the hallway for 30 feet (until the hall floor transitioned to another section of floor). On my return trip, maintenance had identified which outlets and lights were part of the circuit. My current tracer was inconclusive about which item to look into, but while the head maintenance man was holding his tester by the hall wall, I was plugging my outlet tester into a floor outlet in the nurses' station. He said that's when the tester's light when out. Of course, a floor plug! I let him take it apart. A black wire had been run to the ground screw of this receptacle! And when he went in the crawl space, he did find that that wire was connected to other blacks. Looks like the live metal box was in better contact with the floor at some times than others. The flooring was wood, and probably a bit of water leakage in the shower area over time had made the floor in general conductive enough to be dangerous. Under the circumstances.

The power company approached me to be the fifth party in an investigation. For a year, the owner of a new house had complained to the developer that loads like the dryer and disposal would flicker his lights excessively when they came on. He even saw this effect at times when he could not ascribe it to anything in the house. That's why he and the developer, brought a utility engineer in, as well as the electrical contractor who had wired the place. They both ended up with no answer. The utility's transformer and wires were checked and should result in only about 2% voltage drop for a sudden 20-amp load. The electrician even replaced the panel and all its breakers! It was my turn. I wasn't too sure I could find anything. I watched the effect that turning the disposal on had on its own circuit and on other circuits. It should have been more pronounced on its own circuit, but it wasn't much more. In fact I found that on some of the other circuits the voltage spiked up instead of down. That told me that something was hazy about the neutral at the panel, meter, or power company's things. On some kind of hunch, I wanted to see how the main neutral was behaving with loads running on it. To subject it to a heavier load, I turned off all the breakers that used phase B and clamped my meter around each of the three main wires in turn. Phase B had 0 amps, phase A had 6.5 amps. And the neutral had 5 amps. What?! Where were 1.5 amps going? Clamping on ground-rod and main bond wires, they had nothing. The 1.5 must be going back to the utility neutral by the screw that bonds the neutral bar to the panel's box and from it to the conduit to the meter's box, which the utility's neutral is bolted to. Once I backed the bonding screw off, all 6.5 amps of phase A now showed up on the main neutral! Why had current prefered the bond-screw path to the degree it had? I saw a theory: the main neutral lug of this panel is bolted to a horizontal bar that passes behind the hot lugs to feed the vertical bars on right and left, where the circuits' neutral wires attach. I could see a notch in that horizontal bar, on the right side of where the neutral lug is mounted. The notch is to hold a ridge on the back side of the lug, so the lug doesn't rotate when the wire is being tightened down in it. Could this bar have cracked right there from being over-torqued? The area itself was hidden by plastic that insulates a nearby phase lug. Well, I think my theory was born out by the 20% improvement against voltage-drop (for 15 amps) that I measured after jumping a bond wire from the right-hand neutral bar to a small lug built in right next to the main neutral lug. The story is not over. Even though the effect of loads on voltage appeared to have been lessened, the homeowner still reported some blinking going on in some rooms. Had he been oversensitied by now? (Or was he always too sensitive? -- a thought I think occured to all of us trying to please him.) I have another theory, which I wish some lighting engineer could confirm. It is this. The effect of sudden voltage change on the lumen output of bulbs (whether incandescent or fluorescent) will be greater (proportionally) than the voltage change itself. This is from Ohm's law. But the way filaments' resistance and their light output change under a drop in voltage increases the effect. Regarding a house like this one, even the 2% "voltage flicker" right at the panel will make a bulb right at the panel have a "lumen flicker" of up to 8%. Then beyond that, out in a far room in the house, bulb filaments will have dropped their lumens even more than 8%. This is because they were operating at a little lower voltage to begin with, closer down to the voltage level at which no light will be emitted, even though significant current still flows and makes heat in the filament. Neither the public nor Code have insisted on measures to keep this from happening as much as it does. Yet.

Early 2010. Two blocks from my house. A house "flipper" had had this house remodeled and was approaching the real estate "closing," when some troubles showed up. First, Some bedroom heaters stopped working. Then the dishwasher stopped working. Then he found all the lights and outlets in the place dead. Were these related? If they happened at different times, probably not. Could it be one main wire from the power company having trouble? That wouldn't tend to kill ALL the lights and outlets. Testing in his split-bus panel, I found the submain breaker tripped. Since signs of past water intrusion were visible in the panel, I replaced this 60-amp breaker instead of simply resetting it. Now the lights and outlets worked. The other problems remained. Based on labeling in the panel and the owner's recollections, I determined that the dishwasher having trouble was part of a new circuit serving a GFI receptacle and another kitchen outlet nearby. The GFI was not tripped or having any connection problems. However, only one load cable left the GFI's box, whereas there was only one cable at each of the items controlled by the GFI -- that is, the other outlet and the diswhwasher. A splice in the wall made sense, for someone's remodeling purposes. I was prepared to fish a new cable straight down from the GFI to the dishwasher, but when I removed the GFI box (remodel type), there was the splice -- with the dishwasher's black wire easily pulling out from its wirenut! Since luckily the cables could all reach into the box, I put them into it. Actually, I put them into a new deeper box; I think the in-wall splice had been just to avoid having to cram the GFI into a small box that would have had these three 12-2 cables in it... Now, what about the baseboard heaters in the two bedrooms and (it turned out) in the bathroom. These three constituted one heat circuit. And for me, bedroom B's heater actually worked, but not bedroom A and not the bathroom. The connections at all three thermostats were good. Oddly, the stats for the non-working heaters would not make their click noise when turned up, and after I reinstalled the stat for the working heater, it wouldn't either (and the heat wouldn't run there now either). Three bad stats?! I was starting to notice that screwing these stats into their boxes was difficult. The part of the stat that intrudes back into the box was hanging up on the plastic surrounding one of the holes that the stat gets screwed into. Should I blame this model of box? Partly. But I should also blame the stat's designers, because the intruding part (which is the sensing and switching mechanism) was put too close to the area of the screw. Apparently whenever we screw these stats into this kind of box, pressure on that mechanism changes its calibration. Should I shave the plastic down? Get three new stats of a different brand? No, I decided to reinstall all the stats and then adjust the calibration of each, using the calibration set-screw that is on the face of the (coverless) stat. That insured that the click was there and that the stats and heaters worked!

Sept. 29, 2010. My neighbor was having her roof redone, and the roofers found something they weren't comfortable with -- several places where the thin, rigid sheets of plastic (used as a vapor barrier, I guess) under the sheating were melted, right where they rested on cables. I was asked to see what needed to be done. This roof was torch-down with a 3 in 12 pitch and was providing a cathedral ceiling inside the house where this problem was. So below the roofing itself were (in this order:) the plywood sheating, the plastic sheets, the cables (run through holes in the joists), the batts of insulation, and sheetrock. The cables were discolored where they went through rotten joists, about the same color as a scorched cable would be. The plastic sheets were indeed melted in several places where they rested on cables. This was odd to me. Unless a breaker was not the right amperage and the homeowner was thereby able to overload that cable, there should not have been excessive heat generated there. So I took a sample of the plastic sheet material and taped my meat thermometer to it. Then I ran my heatgun at it until the temperature read 160 degrees Fahrenheit (about 70 Celsius). At that point, the plastic was melting. Apparently this roof, not having good ventilation there (as evidenced by the rot), would get pretty hot in the summer sun. Not enough by itself to melt the plastic, but enough with a little heat added from current in the cables. The cables themselves were Type NM-B (newer than the house), and so the insulation of their wires will be OK for up to 90 degrees Celsius. So I doubt the wires were hurt. Since the roofers are providing better ventilation, even the 60 degree Celsius rating of the cables themselves (whose jackets still felt pliable) would now be honored. So this was largely a false alarm, but it was good that everyone had me check the situation.

"Wanted to let you know it's the best website I have ever had the pleasure to read. I do a lot of troubleshooting for various friend's homes and rentals and found your site very interesting and informative." -John WA

More troubleshooting stories

© 2006-2010 Larry Dimock

HOME   Sitemap