Short in Circuit or Circuit breaker?

I have a problem with an electrical circuit in my house. I have a circuit where the breaker trips repeatedly, but not immediately when I flip it back on. What I mean is this; I see that the breaker has tripped, so I flip it back on. After 4 or 5 minutes, the breaker is invariably trip again. It never trips immediately, but there is that gap of time between when I turn it on and it trips.
The circuit is barely loaded. Straight from the breaker is an outlet in the garage (where I usually plug in my water softener but I don't have one at the moment). From there, the wire goes outside and is buried all the way to a shed. I have enclosed the buried wired in that grey conduit all the way from the outside of the house into the shed. In the shed, I have four outlets (with rarely anything plugged in unless I'm weed-whacking or something) and four switches going to four lights. That's it.
I've wondered (and dread) that there's a short somewhere in the outside part of the wire but I don't know why the breaker wouldn't trip immediately after I flip it back on. Could it be the breaker is no good. Any thoughts on the best way I could begin testing where the problem is?
Thanks in advance for any advice. I greatly appreciate anyone's help!
Greg
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I'm no electrician, but it would take about 2 minutes to disconnect the underground circuit and see if the breaker still trips. Follow this process of elimination back to the breaker.
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Greg wrote:

Hi, If it's not dead short breaker may trip like that. Some breakers have time delay built-in by design. If you can disconnect the wire going out to shed, that will make testing easier. Tony
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Put a new circuit breaker in and see if that makes any difference-- or if you didn't want to buy one, swap one from another circuit in your panel being sure to use the same amperage.
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You could remove the wire from the breaker and put the breaker back in and flip it on to see if it still trips after 4-5 minutes. If not, then I think the breaker would be good but the best time is as stated earlier, exchange it with another breaker of the same Amperage.
Greg wrote:

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What is happening is common when the line is being shorted through water. My guess would be a cut in the underground line, or if there are any outside outlets connected to it, check them

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Greg wrote:

Tripping a short time after flipping it back on indicates either a bad breaker but or, more likely, a load that is very close to the maximum allowed by the breaker. As others suggest, with the breaker off, go to that last outlet and disconnect the wire that goes underground, then flip the breaker on and see how long it takes, if ever, for the breaker to flip off. If it flips off, reconnect in the garage, and disconnect the wire in the shed at the first connections. That way you will be able to figure out if the problem is the underground wire or something in the shed. You would hate to dig up the wire and find the problem to be something in the shed. Probably gophers, ground squirrels, etc. munching on the wire.
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measure zero amps from the breaker load screw to the disconnected load wire with all devices off. or replace the breaker with the same amperage gfi breaker. or maybe unplug the electric heater the kids have in the shed. general faq: http://www.faqs.org/faqs/electrical-wiring/part1 /
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when you get this figured out, please let us know the resolution.
cheers Bob
I would have added something is all the bases had not been already covered :)
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Try switching off all the breakers except the main breaker and the one that trips, then take a look at you electric meter to see if it's really drawing much current.
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Why not just put an amp meter on the wire?

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Greg wrote:

Watch your meter while resetting the breaker (or have someone else watch the meter) to see if it starts spinning faster when you turn the circuit on. If there's a mystery load large enough to trip the breaker, you should see a difference in consumption.
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Hi Greg
Any circuit protection device that works on an overload state is spec'd at a certain time delay at a certain load. (that includes fuses) ie if its a 15A breaker it might trip after 30 seconds at 15A or 3 seconds at 18A. Dead short (lots of amps) situations will give an immediate trip. (Note these times are guesses!)
Keep in mind that many breakers nowadays are core balance as well as overload. ie they look at the current difference between inbound and outbound (eg a phase and neutral) Any disparity usually means an earth flow (like through a human body) hence it trips. Pretty well all medical equipment running on mains has this protection.
Assuming the breaker isnt faulty I would assume you have a load just over its rating. It is conceivable you have attached it also to a device that doesnt try to power up immediately (eg some high power heating)
What have you got attached to the circuit the breaker is protecting?
Cheers Bob
Bob wrote:

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First, thanks for all the help and advice from all of you.
Second, I was finally able to spend some time on this problem yesterday. I measured and disconnected this and that wire just like the advice above. I also switched the circuit to other breakers, etc. I'm certain that the problem lies in the wire that is buried (the very result I dreaded). I even ran a parallel wire from the breaker box to the shed above ground to see what would happen (~50') and the breaker never tripped.
What I didn't try, yet, was the advice about measuring the amperage on the circuit with a meter. I'm going to do that, still, because now I'm interested. I'll let you all know.
Again, thanks for the help!
Greg
PS - now that I'm facing the prospect of digging (again), has anyone ever found a less torturous method of removing sod WITHOUT a sod kicker. I think I'd rather pull out my fingernails next time than use one.
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