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One time at my job we had some electrical work that needed done. The electrician had a small homemade switch box with a 120v mains plug running out of it and a switch on the front. The device basically threw whatever breaker that plug was attached to so he new which one needed work on and made the circuit safe. Where can I get one of those boxes, or a way I can trip the breaker from the outlet?

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  • Just as an aside, while it's not really possible or practical to test circuit breakers in situ it is possible to test RCDs using specialist equipment. This isn't done for identification purposes, however, but rather for testing and verification.
    – Dan
    Dec 30 '16 at 16:25
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    Is there a safe way you can trip a breaker from the outlet? No. What are you actually trying to do?
    – Mazura
    Dec 30 '16 at 22:30
  • Thanks everyone for your insite. It looks I'm just going to have to turn off the main switch and swap out these outlets. The guy that had this device wasn't the one that wired my house, but they are cut from the same cloth. I don't have much knowledge in house wiring so I wanted to ask before doing something stupid. I'm glad everyone pointed out its dangerous. I do have a Fluke tone generator/probe for cat5 so I suppose I could use that to trace the wire.
    – Brent1409
    Jan 5 '17 at 5:17
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The answer which states that this is a bad idea is correct.

To answer your actual question: you could build such a device in about five minutes. Take an extension cord, cut it in half, attach the white and black wires to an ordinary light switch, and wrap the whole thing in a handy box.

When plugged in and the switch is closed, a short circuit is created that will either trip the breaker if it is working, or start a fire inside the walls if it is not.

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    While you answered the question, I downvoted because you didn't but in big bold text, "THIS IS A TERRIBLE IDEA AND YOU RUN A HIGH RISK OF DAMAGING BREAKERS AND POTENTIALLY BURNING DOWN YOUR HOUSE" Dec 30 '16 at 17:08
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    The answer clearly states that this is a bad idea and what the consequences are. I invite both of you to write your own answers that you like better. Dec 30 '16 at 17:14
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    I'm with Eric on this one. Stack exchange is polluted with answers that just say "don't." Sometimes I am looking for a bad idea. Not because I want to do it but because I want to understand how it works and why it is bad. By all means say "this is wrong," but at least answer the question. This answer didn't deserve a down vote. I don't think it is the best answer but I'm gonna upvote it so we don't punish answerers who answer the actual question asked. Also, @EricLippert, funny to run into you outside of stack overflow.
    – D. Patrick
    Jan 1 '17 at 0:21
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    @D.Patrick - "Sometimes I am looking for a bad idea." +1. I, myself, of course, have never made an outlet-popper... I mean um, a thingamajig whatchamacallit thingy. If you are going to use one; use your foot. You don't want your face anywhere near it. (This is if you have to ask territory. DVs should be directed at the question.)
    – Mazura
    Jan 4 '17 at 4:50
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    @EricLippert Thanks for the answer. Even when something is wrong there is still value in knowing why its wrong. You learn something besides being told what a dolt your electrician was and to check the label. I know something's wrong, but I'm not going to go sticking a rigged contraption up.
    – Brent1409
    Jan 5 '17 at 5:25
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That box should have been a warning that the guy was incompetent; it's a very bad idea. If the breaker malfunctions, it can start a house fire.

There are cheap devices available in most home centers and hardware stores which can put a signal onto the wire and pick that up at the breaker to identify which breaker controls that outlet. There's a better version that a pro should have which can do a few other things.

Or there is the traditional homeowner solution: plug a radio into the outlet, turn up the volume so you can hear it from the basement, and try breakers until you find the one which silences the radio.

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    Also, once you have found the correct breaker, write it down somewhere, preferably close to the breakers.
    – SQB
    Dec 30 '16 at 14:18
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    You could make a device with a suitable resistor and fuse greater than the breaker which would be reasonably safe. But I agree with your conclusion.
    – Chris H
    Dec 30 '16 at 18:56
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    I think this answer would be better if it included a guess as to how the box worked and details on why it is unsafe. It mentions starting a fire but doesn't explain how or why that would happen. I don't know the OP's intent but it seems like that would make this answer complete.
    – D. Patrick
    Jan 1 '17 at 0:25
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    @D.Patrick: It is not something I want to encourage people to try. I cited the right answers. Use them.
    – keshlam
    Jan 1 '17 at 6:58
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    @kategregory, we will have to agree to strongly disagree on that one. Yes, working alone can be dangerous, but that's true of almost any task from gardening to cooking to you-name-it. The better solution is to work properly so things don't go wrong in the first place. House current can be dangerous, but following fairly basic rules does manage the risk for simple tasks to an acceptable level. And part of what we're here for is to answer questions about how to do so.
    – keshlam
    Jan 1 '17 at 14:41
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That box works by connecting the live and neutral wires of the circuit to each other, i.e. it makes a short-circuit.
If all goes well, the fuse trips/blows. But as @keshlam said, it's a dangerous way to make a breaker trip: you're overloading the circuit. I did this once by accident, and had a 10 cm long flame blast out of the breaker box as the fuse tripped. Not the kind of thing you want to do on a regular basis.

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    Not to mention the extension cord could melt in your hands.
    – Dave
    Dec 30 '16 at 16:00
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    Exactly - it's kind of like testing your fire proofing by starting a fire. While things shouldn't go wrong if all is well, if all is not well there are multitude of possibilities - you could take out the main incoming fuse, the main circuit breaker (if applicable), an unknown fuse between (For example, some plugs in kitchens might be on a fused spur) among other things or maybe just start a good old fashioned fire. For the love of god, do NOT do this stuff deliberately.
    – Dan
    Dec 30 '16 at 16:29
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    Guys doing electrical work in our highschool did this. Put multimeter probes in each outlet, waited until the breaker tripped, and moved on to the next outlet (there was single breaker per outlet).
    – Cysioland
    Dec 30 '16 at 19:35
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    @Dan: In some cases there could be value in applying a moderate overload which should trip any properly-functioning breaker within a second, and ensuring that whether or not the breaker trips the overload won't last more than a few seconds. An overload which is small enough that a UL-approved breaker could take a second to trip shouldn't cause dangerous overheating in less than five. If things are wired correctly, stress-testing shouldn't be necessary, but if e.g. there's a connection between the hots coming out of two breakers, such a fault might allow part of a circuit to draw...
    – supercat
    Dec 30 '16 at 21:57
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    ...twice as much current as it should without tripping a breaker or worse--it could allow part of a circuit to draw four times the rated current while a breaker took as long to trip as it would for a 2x overload. Since a 4x overload would heat the wire four times as much as a 2x overload, that could easily get into dangerous territory.
    – supercat
    Dec 30 '16 at 22:00
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The reason this isn't completely deranged at that industrial location is that certain industrial sites follow slightly more permissive rules owing to having on-site electricians with scheduled, proactive maintenance. You see this phraseology in certain NEC rules. Their work is done to a higher caliber than home wiring; THHN in metal conduit, no backstabs on the premises, frequent inspections, most circuits meggered, etc.

Doing this in a residential location with lax residential rules and work of unknown provenance is suicide.

Obviously the switch is basically dead-shorting the hot and neutral to induce a trip, but if you do that in a residence, you have about a 50/50 chance of inducing future arc faults at a backstab, or at the least, fusing it open. We get lots and lots of "plugged in a (well within legal range) large load and a backstab failed" questions. You also have a chance of other stupidity, like discovering why FPE and Zinsco panels are panela nongrata.

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The safe answer to this would be to have an electrician install remote controlled circuit breakers, with a suitable trip circuit installed at the outlet where you want to remotely trip the breaker. (Eaton says they come in a CLR form factor for residential panels.)

Of course, properly labeling the panel, perhaps with a map showing where outlets/lights for each circuit would be a much cheaper and easier solution, and arguably more useful.

enter image description here

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Let's assume the guy with the box was not incompetent or insane and that he had actually devised a reasonably safe means to do this. What would it take? Below, I present a design that should work. I want to emphasize that I have not built this or tested it - this is purely theoretical. If you want to try it, do so at your own risk. I understand the fact that commercially available devices are out there that can identify circuits. Nonetheless, I have often wanted something similar to what was requested here, and the interest in this post tells me I am not alone. So here are my thoughts.

First of all, a couple of electronic principles. Ohms law states that Voltage (V) is the product of Current (I) and Resistance (R), V = I x R. A typical home circuit is supposed to be 120V. The standard circuit breaker is rated for 15 Amps. Using Ohms law, we can rearrange the equation as follows: R = V / I. So, 8 = 120 / 15. This means that an 8 Ohm load would draw 15 amps at 120 Volts. Resistance less that 8 Ohms would draw more than 15 Amps, and should trip the breaker.

Now, don't go out and buy an 8 Ohm resistor from radio shack and hook it up just yet. The other factor we need to consider is power dissipation. Power (Watts) is the product of voltage (Volts) and current (Amperes), Watts = Volts x Amperes. So, at full load, a 120 V, 15 Amp circuit provides up to 1800 Watts of power. This means that, with 8 Ohms of resistance and 120 Volts, the box would need to be capable is dissipating 1800 Watts. The resistors you see on circuit boards are typically something like 0.25 Watts. That won't work.

What we need now is a resistive element capable of dissipating the power. A simple method would be to create a box that has multiple screw-based light bulbs, and to get some high wattage bulbs, and have enough of them connected to exceed 1800 Watts. You could add toggle switches for each bulb, and then switch on one at a time until the current draw was too high and the breaker tripped. The advantage here is that you would be dealing with components normally designed to be powered by 120V, and all the wiring and switching would be standard stuff. The problem is that such a box would be bulky and it is getting harder to find high wattage bulbs. Plus, light bulbs break.

An alternative would be to use wire-wound power resistors. For about $18 each, you can buy power resistors capable of dissipating 300 Watts each. Power is additive, so if we have at least 6 resistors, we could handle 1800 Watts. That being said, it is not a good idea to run things at their limit. So I would design a circuit with at least 8 such resistors, so it could handle 2400 Watts, a reasonable safety factor. Home voltage is often as high as 125 Volts, and the breaker might allow more than 15 Amps to flow before tripping; 2400 Watts gives a safety factor of 33%, so we should be OK.

These resistors come in various values. A 16 Ohm, 300 Watt power resistor would be my choice. Back to basic electronics. In a series circuit, Resistance of the Total circuit (RT) is the sum of each individual resistance: RT = R1 + R2 + ... + Rn. In a parallel circuit, Resistance of the Total circuit (RT) with multiple resistors in parallel (R1, R2, etc.) is calculated by the following equation: 1 / RT = (1 / R1) + (1 / R2) + ... + (1 / Rn). Putting two 16 Ohm resistors in series makes a 32 Ohm resistor. Placing four 32 Ohm resistances in parallel makes a total resistance of 8 Ohms. That should draw 15 Amps and may or may not trip the circuit. Adding a fifth 32 Ohm pathway in parallel makes a total resistance of 6.4 Ohms, which would draw over 18 Amps and should trip the circuit.

So the idea would be to make a box, with a switch for each pathway, that has at least 5 32-Ohm pathways, each capable of dissipating 600 Watts. Switching on the first 4 would draw about 15 Amps from 120 V, switching on the fifth would go over 18 Amps and should trip the breaker. If you wanted to add a sixth, you could also potentially trip a 20 Amp circuit.

Here is a diagram with five pathways.

The resistors would cost you something like $120. The light switches could be standard stuff. You would need to make sure to wire it with wire capable of carrying enough current. Each 32 Ohm pathway would carry 3.75 Amps. Heavy gauge lamp cord would do the trick.

enter image description here

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    Breakers don't immediately break above their rating. 18A will trip the breaker thermally after 1000s or 17min. That's impractical. To trip within 1s magnetically you need to draw 300A, and that's dangerous. For trip charts see diy.stackexchange.com/questions/200353/…
    – P2000
    Apr 6 at 14:59
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    OK - thank you for explaining. Seems like there really is no good way to do this. I should have checked the performance characteristics of the breaker. Very interesting - thank you for helping me understand. Apr 6 at 19:56

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