From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: Blowing Fuse (12 Volt) in MH Date: Sun, 18 Mar 2007 19:02:42 -0400 Message-ID: <mkgrv21vrv5ki24j9k6ai00561vvv7pdna@4ax.com> I had one of those in my rig that almost drove me batty. Turned out to be a sheet metal screw penetrating a wall over the wiring that just touched the wire. Over the years vibration wore through the insulation and the screw would occasionally touch, blowing the fuse. I eliminated everything else and was starting to think the wire had worn and was touching the frame somewhere. I was yanking on the wire trying to determine its exact routing when the short became continuous. I was removing the screws that held the portion of cabinet on when the troubleshooting lamp I had hooked across the fuse holder suddenly went out. Bingo! After I got the hardware off I found the worn spot in the wire and the little marks from arcing on the point of the screw. Start by physically tracing out the wire's path. Remove the fuse and connect a suitable lamp across the fuse socket. When the short is present the bulb will burn brightly. My first place to look would be under the light fixtures. I've seen a lot of RVs where they use a hole saw to cut a hole for the wires and leave the sharp edges unprotected. Likely that vibration has worn the insulation on a wire where it comes through the wall. Don't be surprised if your test light goes out as you remove the fixture. If it does then you've found the problem. If removing fixtures doesn't find the problem then grab the wire at every point you can and yank. Fairly hard but not hard enough to break it. The movement should cause the short to break. If you haven't found the short at this point, my advice would be to snip the wires loose on both ends and run another. The fault is likely behind something that will be time-consuming to remove. Just not worth the effort. John From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: Thanks Re: Blowing Fuse Date: Sun, 18 Mar 2007 23:27:41 -0400 Message-ID: <il0sv2d7c11iutkoavg7lcsofc4erdi4bb@4ax.com> On Sun, 18 Mar 2007 20:30:43 -0400, Lee <lbray5032@bellsouth.net> wrote: >Hi Ben, Hunter and Neon John > >Thanks for your suggestions, will print it out and will give them >a try tomorrow. As Neon Joh said it can be frustrating and I have >used quite a few of those 20 amp fuses just trying to find the >problem. Everything else was working fine so even if I don't find >it before we leave we can run everything else. Save your fuses by using the test light. Just hook any old 12 volt bulb you have handy across the fuse clips. A short will make the bulb glow full brilliance. A load such as another lamp will cause it to burn more dimly because the two are in series with each other. Place the lamp where you can see it while you work so that you can notice even a brief flash as the short comes and goes. John From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: Blowing Fuse (12 Volt) in MH Date: Mon, 19 Mar 2007 15:30:23 -0400 Message-ID: <6votv2t37upr5nfaac901l52bob8b99e2e@4ax.com> On Mon, 19 Mar 2007 09:07:58 -0400, Lee <lbray5032@bellsouth.net> wrote: >Bob AZ wrote: > > > Unless you remove the power don't use an ohm meter. Instead use a >> voltmeter. When the short is removed the meter will go to zero. >> Connect the meter to the fuse holder. Positive to the feed and the >> negative to the load. > >Thanks Bob > >If I remember correctly my meter does both, so will use it as directed >will refresh my memory by looking at the instructions! (grin) Lucky >me there are only 3 lights in that circuit but as far as the routing of >the wire it would be damn near impossible to follow it. So am hoping it >will be near the lights! If not, time for a new RV? (right) Don't try to use the meter. It will only confuse you. Even a tramp leakage to ground will cause the meter to give a voltage indication because it has such a high input impedance. You can demonstrate this easily enough. Hook one side of your meter to the battery positive. Hold the negative lead. Touch something grounded. Unless your skin is parched dry, your meter will indicate almost full battery voltage. The few tens of thousands of ohms of resistance your skin presents is practically a short compared to the standard 10 megohm input impedance of a DVM. Use the lamp like I described. It draws enough current that leakage won't affect it. John From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: Blowing Fuse (12 Volt) in MH Date: Mon, 19 Mar 2007 16:15:09 -0400 Message-ID: <s7ptv2lo9gkvrcpoh9kqgt927ad82i7saf@4ax.com> On Mon, 19 Mar 2007 08:46:53 -0400, HD Matt <nospammbode@multiprintinc.comnospam> wrote: >John, >Is there some sort of reasonably priced device that would allow me to >trace a 12 volt line through the ceiling? I have three lights in the >bath area of my MH that work some of the time. I am sure it's a loose >connection somewhere. I have pulled all the lights down starting at the >one that still works on the circuit and soldered all the connections >followed by a crimp on cable nut to insulate. Worked for a while but >now the three light don't work at all. I suspect that there is a >junction somewhere up inside the ceiling that is loose. Ceiling in not >easily removable without major work and would be fairly hard to take >down and reinstall without it looking like crap. I know there are >"tone" injectors that allow the tracing on 110v line in a home and >finding the correct breaker in the panel but, have never heard of such >for 12v circuits. Thanks Yes, there are devices for tracing telecom wires that work like the 120 volt ones except that they're battery powered. I believe that Home Depot sells one. It may or may not work, depending on whether your inside skin is metallic (metal or foil coated) or not. You can rig something up that is much more likely to work in the presence of metal because it works on the magnetic field instead of the potential field. Go to Rat Shack and get a crystal earphone. Less than $5. This is the cheapo earphone that came with transistor radios in the 60s. A low impedance dynamic headphone, the more common type, won't work. If your Rat Shack doesn't stock the crystal earphone then for a little more money (<$20), you can get a monaural amplified speaker that has a high input impedance. PC and iPod speakers won't work, as the input impedance is low. The one I have is stock number 32-2040. I've had it a long time so that may be an obsolete number. It's labeled "amplified speaker system" and has a "MIC" input jack on the back. Next you'll need a solenoid with lots of turns. The more the better. An old relay is a good place to get one. Buy a relay at Rat Shack if you don't have any laying around and rip out the coil. If the core doesn't come out with the coil, find some nails or something similar to stick through it. Connect the solenoid terminals to the earphone or speaker. You've just made a magnetic field detector. Make up a Jesus cord consisting of a 120vac line cord, a socket for a 100 watt bulb and some gator clips. The bulb should be in series with the black lead. Disconnect the circuit in question. Connect the bulb lead to the circuit at the fuse panel. Connect the other end to the neutral (white) lead or simply ground it. The bulb should light as current flows through the circuit. The current flow through the circuit sets up a weak magnetic field. You're going to detect it with your homemade detector. Put the earphone in or turn on the speaker. As you pass the coil near the energized wire you'll hear a hum. It'll get louder the closer you get and the closer you align the axis of the solenoid with that of the wire. If you move across the wire horizontally you'll hear the hum get louder, be loudest as you're directly over the wire and fade away. It becomes obvious that you can locate a wire by passing the coil back and forth and orienting the solenoid to produce the loudest hum. Because the low frequency magnetic field easily passes through non-ferrous metals like aluminum, you can find the wire even behind metal. This same trick will work to find wires inside the walls of your house. Simply plug a load into the outlet of the circuit you want to trace, the larger the load the better, and start moving the coil across the wall. In this case, the sound will increase as you approach, then decrease. When you're over the wire the sound will usually null out as the hot and neutral fields cancel. I have a very old antique Western Electric telephone wire tracer that works similarly. It is built into one of those gorgeous furniture-grade wooden boxes that instruments used to be built in. It contains a vibrator induction coil similar to a Model T ignition coil except that there is no secondary and it vibrates much slower. When the points on the vibrator open, there is an inductive kick generated of several hundred volts over a broad frequency band. The coil is tapped at several places so that various voltages can be generated. The wire under test (WUT) is attached to these terminals. The high frequency impulse goes out over the wire and is radiated and capacitively coupled away. This results in current flow in the wire even though it is open circuited. The detector is a long coil that contains tens of thousands of turns of fine wire hooked to an old-fashioned telephone operator's headset. This coil is passed over the wire whereupon loud clicks are heard. It is amazingly sensitive, audible from several feet away from the wire. The field penetrates aluminum like it wasn't there and even penetrates thin steel such as what control cabinets are made of. This is something that modern single-frequency tracers can't do. I got the idea for the technique I described above from using the detector wand to trace 60 hz wires. I noticed that it was VERY efficient at detecting the 60 hz field. Thus I could detect energized wires without having to de-energize them and connect the induction coil. I'll mention in passing one other technique for finding hidden faults, one that power companies use on buried cables - high voltage arcs. The idea is to cause an arc at the fault strong enough to be detected at a distance. The gadget is a trailer-mounted rig called a "Thumper". The trailer contains a generator and a bank of high voltage capacitors and the necessary controls. It dumps a high joule high voltage pulse into the cable at about one second intervals. One then walks the right of way. The arc is intense enough that it thumps the ground. One can feel it as he stands over the buried fault. The shots are strong enough to blow out most shorts and cause the arcs that can be felt. Sometimes when the cable is shallow buried the arc actually blows away the dirt. Sometimes spectacularly. If I were going to try this technique on your problem I'd get one of those disposable cameras that contains a flash. The strobe capacitor stores about 360 volts and considerable energy. After unhooking everything from the circuit, I'd dump that cap into the circuit using simple jumpers and using the flash tube as the high voltage switch. Just hook the circuit in series with the tube and take a picture :-) If the short is against aluminum there will be a little aluminum vapor/air explosion (well, really just a pop) that is easy to locate. I know that this is probably more than you wanted to know but I thought I'd do a brain dump while I'm at it. Please let me know what you find. I'm curious. Also how you end up finding it. >By the way haven't seen any "travelogs" from you lately. Boring places >or just tired of writing them. I am assuming you are still on the road. Funny you'd ask. I'm at home now for a little vacation. I turned in my truck last Thursday. The company has apparently gotten itself in financial trouble and has lost a lot of customers. It got a little tiring getting my butt chewed by customers because of the bad service when I walked in to pick up my load. I've spent much of my time since Christmas sitting and what running I did was repetitive short haul stuff. Nothing much to write about other than being in the mini-blizzard of about a month ago. I don't know if I'll go back out driving a semi or not. Probably not, as several other opportunities have presented themselves. One discussion I'm having involves setting me up in a motorhome as a traveling tech support engineer. Basically on the road full time supporting the company's industrial controls customers. If I were to write my own job description this would be it so I'm hoping it comes to pass. I have some friends in Texas who've been pestering me to come there and re-open my BBQ joint using their money. That's also an enticing possibility, though I could do without the heat of Texas. Right now I'm taking a month off and maybe the whole summer to relax. I don't much like working in hot weather so I may just hole up in the mountains for the summer. Between RV excursions, of course. John From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: Blowing Fuse (12 Volt) in MH Date: Mon, 19 Mar 2007 18:21:21 -0400 Message-ID: <e03uv2dcmn334utje7lrmjt7lq98pdlsq7@4ax.com> On Mon, 19 Mar 2007 16:25:31 -0400, "Steve Wolf" <news@w8iz.com> wrote: >A friend of mine obtained a dynamite deal on drywall labor. After they were >done he realized they covered all his outlets. I lent him my tracer and he >was able to find each of the lines and boxes without a problem. Ask around >to see if anyone has one of these: > >http://www.mcminone.com/product.asp?catalog_name=MCMProducts&product_id=74-335 Heh. I'm reasonable certain that this is the one I saw at Home Depot, only under the Greenlee brand. About the same price. > >You'd be surprised how many have them as they are so useful. Disconnect the >power, hook the transmitter up to the circuit and you're likely to be good >to go. Yes, the signal is weak. Even so, I have also found wire routings >quite deep in a wall. Yep, very handy thing to have around. I still use the old inductive one I described when I know the circuit doesn't have anything connected to it because the signal is so much stronger. The inductive kick will probably damage anything solid state connected to the circuit so I have to be careful. John From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: Blowing Fuse (12 Volt) in MH Date: Mon, 19 Mar 2007 18:22:37 -0400 Message-ID: <a73uv251c7nracoikij30utdonp1e7n6n3@4ax.com> On Mon, 19 Mar 2007 16:15:09 -0400, Neon John <no@never.com> wrote: >The current flow through the circuit sets up a weak magnetic field. >You're going to detect it with your homemade detector. Put the >earphone in or turn on the speaker. As you pass the coil near the >energized wire you'll hear a hum. It'll get louder the closer you get >and the closer you align the axis of the solenoid with that of the >wire. Oops, meant to say "perpendicular to the wire axis." Think one thing, type another. John |
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