Hi guys/gals,

I've been struggling to come up with a solution to my problem on my own and have decided to consult with the internet for help.

My name is Max and I am an electrician in Calgary, Alberta. I am currently working on a prototype tool for use in electrical construction. I dont have a very strong "electronics" background as most of my experience is with 120VAC and up with very large circuits. So i need help with the small stuff!

The tool is simply a device with a switch, when flipped, trips the circuit breaker that powers that particular circuit. For years I have just used a switch, and wire it up to cause a dead short when flipped, however, i have some interest in making a prototype that can be sold in wholesale stores and the current method isn't exactly safe. Because of that, I have to design a circuit that will limit the current in the circuit to 25A in order to not blow up the contacts in the switch, and not exceed the maximum ampacity of #14AWG.

What I need help with is this, if i go with the power resistor route, i run into two issues:
1 - a resistor big enough to handle the current is outrageously expensive.
2 - the resistor gets incredibly hot (obviously)

The solution needs to be relatively inexpensive, small in size, and not melt in your hand.

Any insight you can provide would be great. The nominal system voltage here is 120VAC and the current needs to be limited to no more than 25 Amps.


A product does exist that is branded as a 'circuit analyzer' so I know that a device that functions in this way can be made safe. If you google "EXTECH CT70" you can have a look at it. It does quite a few other things and because of that it is very expensive. which is why i want to build a simple version that just trips breakers.

Thanks so much!

Max

 

Not really sure what you exact intentions are, but if you just need to trip a breaker at some point in time without waiting for a OL to achieve it, there is both a relay, but also a breaker with a trip coil, that when energized trips the breaker that has to be reset manually.
What is the end purpose?
Max.

 

Yes, simply put, i want to plug the device into a receptacle, flip the switch, and and have the breaker that powers that receptacle trip. Either by means of an OL or some other way. Using a switch to cause a dead short isnt really the asfest way to do it because you force a large fault current through the breaker and conductor.

 

Plug the device in and get the breaker to trip? I must be missing something. Why do you want to force a breaker to trip?
As I mentioned, if it is a protection breaker you need to trip, you can get them magnetically operated by low current operation circuit.
The alternative is a disconnect relay, why is this no good?
Max.

 

I am a Multi family residential electrician, the point of the device is to make quick work of finding out what devices are powered by what breaker. when you finish 10+ apartments per day, alot of time is spent going to and from the panel flipping breakers on and off. this method will speed up the process and make it so that one person can do the job, instead of two. Most people just use a 20 amp switch wired to a cord end, the hot and neutral from the cord and are wired to the two terminals on the switch and presto, flipping the switch cause an OL which trips the breaker.

 

I read this online:

The instantaneous portion of the trip unit is tested by injecting short pulses of current (5 to 10 cycles long) below the instantaneous trip point and slowly increasing the amount of current until the CB trips.

Im not sure what electronics would be involved to provide such short bursts of current.

 

I am thinking more on the lines of something less 'drastic'.
Max.

 

as in simpler?

 

You don't say where your located, but this is what is used around here to do what your wanting. My grandson that is in his last year of electrician apprenticeship has one and likes it. http://www.homedepot.com/p/Klein-Tools-Digital-Circuit-Breaker-Finder-ET300/202330830

 

You don't say where your located, but this is what is used around here to do what your wanting. My grandson that is in his last year of electrician apprenticeship has one and likes it. http://www.homedepot.com/p/Klein-Tools-Digital-Circuit-Breaker-Finder-ET300/202330830

+1 on this. These have been around for quite a while and are far less dangerous than slamming a breaker.

 

You don't say where your located,

In the OP,
Calgary AB.

I was thinking of something along these lines, I did not realize they were already out there!
Max.

 

The crux of the problem seems to be an inexpensive way to limit current. Being an electrician, you will know better than I whether using a breaker rated at a higher current and a longer trip time than the breakers in the installation you are working on would be a suitable solution.

 

Using a switch to cause a dead short isnt really the asfest way to do it because you force a large fault current through the breaker and conductor.

Circuit breakers are designed to protect the wiring. You seem to be operating on the premise that circuit breakers don't do their job properly. I can't remember seeing any residential wiring that is labeled for how many times it will trip the breaker before insulation damage starts to occur. In other words I doubt your starting point.

Be that as it may, Watts Law says you want to inflict a load of 125V x 25A = 3125 watts in a small, convenient package.
As a first guess, a 5 ohm, 300 watt resistor costs $27.51 and is rated to survive 3000 watts for 5 seconds.
That should do the job.
Enough research and math might find a smaller, cheaper resistor that will do the job.
How long does it take to trip a 15 amp breaker at 25 amps (166% of rating)?
How many kinds of circuit breakers are possible in this residential position and which style is most forgiving of a 66% overload? (Time required to trip.)
Again, research is required if you're going to arrive at the optimum device.

There is also the idea of 13.2 millihenries or 530 uf which would create a lot less heat while limiting the current. The 530 uf would be a dry electrolyte "start capacitor". A 13 mH choke would need to be wound with 10 Ga wire.

Grainger has a capacitor at $17.07, but they are notorious for high prices. You can do better.
https://www.grainger.com/category/c...edirect=motor+start+capacitors&searchBar=true

An inductor to do this resembles plugging a 500 foot roll of 10Ga wire into the outlet.
I don't think this is going to work.
http://www.pronine.ca/multind.htm

So there's some numbers for you.

 

The crux of the problem seems to be an inexpensive way to limit current.

I believe what the OP is trying to achieve is a way to purposely trip a breaker in order to identify the circuit it is feeding where the breaker is remotely situated.
In order to save excessive time going back and forth when working alone.
Max.

 

The company my grandson does a lot of apartment building repair work and since he's an apprentice he gets stuck with a lot of it. The "slamming" of a breaker to find a circuit scared a lot of elderly folks so he looked into another way of doing it and found those. now all the service trucks have them.