Why Does an Energized Wire Trip a Breaker When Grounded?

[Crazyhorse2882]

I have a question. When an energized wire contacts a ground wire or goes to ground, I know there's an increase in current and the breaker will trip, but my question is why? Is the energized conductor pulling more electrons from the Earth causing it to exceed the breaker rating? Thanks

 

[davenn]

in most mains power systems, the ground and the neutral are tied together
so shorting the hot ( live) to either the neutral or ground IS a short circuit and causes a large current to flow ... no different to shorting out battery terminals and blowing the fuse ( circuit breaker)

ALL your electrical appliances, microwave oven, kettle, fridge freezer, hair drier, TV etc put a load between the hot and neutral. That load has xxxx Ohms of resistance ( different for different devices) This will result in a given fixed current through the device.

does that help ?

cheers
Dave

 

[Windadct]

IN agreement with Davenn - they are often tired together, and electrically the same, but their purpose, and often construction, is quite different.

There are 2 typical faults - if the Line contacts the neutral or ground you may get enough current to trip the CB. However - since you brought this up as a Ground fault - there is a different type of breaker or protection, for sensing ground faults, that will trip the breaker if a very small amount of current flows in the ground wire.

 

[Crazyhorse2882]

I read an article the other day that stated that what happens in a hot to ground is that the ground wire carries the current back to the source where it tied to the neutral (the transformer was the example they used) and then that current was cycled back through to the circuit breaker causing it to trip. That didn't make sense to me because that seems to me that it would trip the main breaker and/or every breaker in the panel and not just that pone specific breaker. That was one part of my confusion. The second part is that in my mind when I picture a hot going to ground, I would think that the circuit current would basically drain into the earth. I don't understand why their is a surge of current

 

[Crazyhorse2882]

I thought about it and I understand what you guys are saying but still not understanding why that particular breaker trips and not all the breakers or the main breaker of there is one

 

[Windadct]

It may be helpful to draw a diagram of what you are thinking and then trace a loop of where the current will be flowing. -- there is no fault curretn flowing through the other breakers.

 

[wirenut]

Usually the breaker for that particular circuit is a lower rating than upstream breakers (15 or 20 amp typical, but that can vary). If you have a 15A breaker for that circuit, you should have a larger breaker supplying power to that 15A breaker. That larger one could be anywhere from 60A to 400A or larger, depending on the installation. And so on up the line- feeder breakers usually get bigger as you go backward toward the source. At least until a voltage change via xfmr. Another note is as breakers get bigger, they can withstand higher short term overloads. If a 15 amp trips in 1/2 cycle @150% that is only 22.5 amps, whereas a 200 amp breaker feeding the panel that has the 15 would take longer to trip @ 22.5 amps more even if it were almost 100% loaded. In short the higher the overload the shorter time a breaker will stay "on" during overload.

 

[wirenut]

Sometimes if main breakers are close to 100% loaded (nec states should not be over 80%, but a lot of "electricians" will add circuits without checking loads.) you may see the breaker for the shorted circuit trip AND the main feeding it, or I have even seen the main trip before the individual breaker because the main is already @100% -and then some- load. BTW a 105% load probably won't trip a breaker .

 

[Crazyhorse2882]

Thank you wirenut that was very informative. One last question if you don't mind. Is the amount of fault current restricted by the size of the wire? I'm guessing yes but just confirming

 

[Windadct]

Yes the size of the wire will restrict the current, but you can still put enough current in the wire to start a fire! So this is not used as part of the protection, it is used to set or establish where the circuit breaker should trip.
There are different circuit breaker trip settings ( in larger systems), but standard Household breakers account for :
Overload ( relatively long time) - at 110% it may take a few minutes to trip.
Instantaneous ( this is often referred to a short circuit) often at 5 to 10 x of the breaker rating, and the breaker trips "with no intended delay" -
As mentioned Ground Fault protection is also different

 

[Crazyhorse2882]

Thanks everybody!