# Short Circuit but Breaker does not trip

Discussion in 'Electrical Forum discussion & Blog' started by vino82, Jan 28, 2010.

1. ### vino82New Member

Joined:
Jan 28, 2010
Location:
plainville, MA
My wife plugged in her vacuum as soon as she turned it on the power went out to the circuit. My first intuition is to check the breakers to see if it tripped. I take a look and no breakers are tripped (I have the old push button breakers) but the entire circuit went out (all the outlets in the room and some random lights). I take out my meter and check the plug that my wife plugged into, no power but I have an infinite short between the hot and neutral. the same goes for all of the other plugs on the circuit. I take out the outlet to see if the short occured there, nope it looks fine. I did find a junction box in the cellar that is feeding that circuit, I pull the wires apart in the box and narrow it down to one of the wires that has a short between the hot and neutral. I check the plugs upstairs again, no infinite short at any of them, however I am reading 53 ohms between the hot an neutral. How is it that I have a short but the breaker does not trip and should I be reading 53 ohms between the hot and neutral?

Ryan

2. ### jimboPlumber

Joined:
Aug 31, 2004
Location:
San Diego
You need to get your terminology straight. INFINITE resistance means OPEN circuit, not short. 53 Ohms is far from a short. You could be reading the resistance of a light bulb or other load that is still on the circuit.

4. ### jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
Some vacuum cleaners draw quite a bit of current, so depending on what else was plugged in and operating when the vac was turned on, you probably just overloaded the circuit, not a short. You'd have to unplug or disconnect everything on that circuit to verify there was no load or fault in the wiring. 53 ohms is about the equivalent of three 100w lightbulbs, but the resistance changes when they heat up, so it could represent more, or throw in a clock or tv or anything. Look at the labels on the things that were plugged in and add up the wattage to see what load there really is, then see where you stand.

5. ### cacher_chickTest, Don't Guess!

Joined:
Jan 5, 2008
Occupation:
Test, Don't Guess!
Location:
Land of Cheese
Could have tripped a GFI between the panel and the load or you could have a bad connection anywhere in the circuit.

6. ### vino82New Member

Joined:
Jan 28, 2010
Location:
plainville, MA

Last edited: Jan 29, 2010
7. ### jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
Depending on the meter and the scale, zero is not always zero and the buzzer may not mean a short, but a low resistance. If there was a loose wire, or one sticking out too far, and the outlet was not tight in the box, pushing in the plug could have cause it to short against something. Usually, when a receptacle fails, you end up with an open, not a short, but anything's possible. If the plug is loose, the receptacle is worn out, replace it.

8. ### nickdelNew Member

Joined:
May 8, 2009
Ok, I'm a little confused on a couple of things. First, what is your proof that you have a short circuit? You tested resistance(?) between one hot wire and a neutral wire, got 0, and your meter buzzed, so that's a short circuit? If I read it wrong, please correct me. And you can't have a dead short with one wire.

You say you have a short but the breaker didn't trip. If you had a short on the circuit but the breaker didn't trip, you would have burned something up - a wire, a terminal, a piece of plugged-in-equipment, a receptacle. Something would have gone bad, because when there is a direct short, the amperage will continually increase towards infinity until something fails. And this happens very quickly (think fractions of a second). This is the purpose of the breaker: to open the circuit before everything burns up, and if a component of the circuit did burn up, it will create an open. If you're breaker hasn't tripped, there is no short. Because of this I don't think you had/have a dead short. When your meter buzzes when you're reading resistance, that means there is voltage, not that there's a short. No resistance between hot and neutral is also NOT an indicator of a short circuit. Also, just because a wire is white does NOT mean it is a neutral.

I'm not really sure why you're testing for resistance anyway. Do you have a voltage tester or a multimeter? If you feel comfortable with this, open the main panel and check to see if you have voltage coming from the circuit breaker. Test from the breaker (the screw that clamps down on the wire coming out of it) to the ground/neutral buss. See if you read a voltage. Turn the breaker off and take another reading. Then turn it back on and test it again. If you have voltage coming out of this breaker, you now have to inspect the circuit (if you don't read a voltage, change the breaker). Start opening affected boxes, starting with constantly hot (not switch-controlled) receptacles first. When you test for voltage in these boxes, test from the hot wire to the ground wire. If you have voltage, then test from hot to neutral. If you have voltage, move to the next box. If you have voltage from hot to ground, but not from hot to neutral, you've lost a neutral somewhere. When you come to switch boxes, you need to identify the cable that brings power into the box, which will be difficult if no power is coming in. If you have voltage at the breaker, but not coming into a switch box/junction box/receptacle box, you have an open on the hot somewhere. You'll need to check all your connections. If you can find the wire bringing power into the switch box, trace its path in the box, see what switches it feeds (if there are more than one), and if it also leaves the box before it goes to the switch(es). On the switches, test from the terminal where the power comes in to the switch to the terminal where the power goes to the light. For single pole switches: if the switch is on you should get 0 volts; if the switch is off you should get 120v. Any other reading than that and you have a bad switch (or you're testing improperly).

Of course there are exceptions: 3-way and 4-way switches, and switched loops. These are the reasons that white wires can be hot wires. If everything else above fails to find the problem - or if you know for sure a 3-way/four-way switch or a switched loop is involved - post so, and someone will tell you about those circuits.

And stop taking resistance readings and start testing for voltage

Last edited: Jan 29, 2010