What happens when a ground fault occurs inside the main electrical panel before circuit breakers?

Question

I am aware of what happens if a ground fault occurs outside of the electrical panel. For example, if a live wire were to touch the metal enclosure of some appliance, provided that the enclosure is connected to the ground conductor back to the panel, there should now be a path for current to flow back to the power source, creating a short circuit that trips the breaker the appliance is associated with (provided neutral and ground conductor are bonded in the panel). If someone were to touch the appliance, they would not be shocked.

However, if one of the live wires coming from the meter base somehow comes loose in the main panel (SEP) and touches the metal enclosure of the panel, there will now be a shorted circuit since the enclosure of the main panel is bonded to the neutral bar.

However, since this live wire is no longer connected to the main breaker in the panel, what happens next to clear the ground fault? Would the short circuit remain until someone realizes the fault? What if someone touches the cover of the panel?

Similarly, what if the same things happen but at the meter base? E.g., a live wire from the utility touches the metal enclosure of the meter base, how is the fault cleared? While I understand how ground faults are cleared after the main panel, it is unclear to me what happens from the main panel going back to the meter base.

Answer 1

BAM!

That's the sound of the meter exploding.

But seriously, that is why the hot wires coming from the meter into the main panel are connected to special lugs and torqued really tight. And covered with non-conductive material after installation so that even with the main panel cover off, those hot wires are inaccessible without extra, deliberate, effort.

That is also why (if it is not a combination meter/main) the main panel is supposed to be really close to the meter, such as immediately on the other side of the wall, not the other side of the house. Keep the area where possible damage could bypass the main breaker to an absolute minimum.

In other words, yes, that would cause a serious problem. But there are multiple additional safeguards to prevent this from happening. Far more likely is swinging wires from the weatherhead from a really bad storm, which would be dangerous but would manifest as a power outage for your house (or a partial outage, depending on how many wires swing free) and not present any direct danger in your panel. For this to happen in your panel is extremely unlikely short of the sort of damage that would result in numerous other problems, like in a tornado.

Answer 2

It will burn (an electrical arc, typically around 5,000-35,000 °F) until:

1. It's burned itself out of contact and the arc self-extinguishes. (Other things may continue to burn, having been lit, though.)
2. A power company upstream breaker blows (this takes a lot.)
3. Some part of the wire/cable feeding it fails, acting as a fuse.
4. Or, the power company, having been called to report the fault, shows up and shuts it off from the supply side.

It's an extremely unlikely event, by design. Service equipment, and who can work on service equipment are both designed to reduce the odds of such a failure. Short of illegal modifications, it's down in the "winning the lottery" level of liklihood.

The grounded case, if properly grounded, will remain grounded. Hopefully you have the sense not to try touching it if it's obviously arcing and on fire, though. It will burn you, at the least.

Answer 3

In one building deconstruction instance the mains was not correctly isolated. When the main panel was crushed all those pretty lights and magic smoke escaped.

The result was a short across multiple high voltage circuits, which only stopped being a short when the rather solid wires finally melted... all the way down the road in the underground ducts. Lots of emergency vehicles and a couple of days to get everything back to normal service.

Answer 4

If a high capacity circuit encounters a very low resistance current path, something somewhere will ignite as noted in other answers. Here is an example of what happens when a service wire is shorted to wet ground. The road ignited. Unfortunately (or fortunately?) I wasn't there to get the picture while it was happening.

If this sort of thing happened inside your main panel, hopefully the arc would self-correct, IE it would destroy the parts of the wires that are shorting and thus end quickly. But if that doesn't happen, the meter might ignite, or the service wires, or the pole transformer ... everything will heat up until something is sufficiently destroyed to break the circuit. Usually that will happen VERY rapidly.

Answer 5

Ground faults don't always cause overcurrents. Sometimes they simply provide an alternate path for current to flow to ground and said current never returns to the breaker panel. The breaker doesn't trip because the current remains lower than the breaker's limit. These situations are less of a threat for fire than for human electrocution because the "alternate path" might be through someone's body. For this reason Ground Fault Circuit Interrupters (GFCI) were invented. They detect not overcurrent but a difference between current supply and current return. If such a difference exists, they open the circuit.

I think you were asking about overcurrent situations specifically, but it's important to know that the term "ground fault" doesn't always describe an overcurrent situation.

https://en.wikipedia.org/wiki/Residual-current_device

Answer 6

1. Answer: If you mean the main electric panel, everything before it is the property of the electric company, everything after the main circuit breaker panel is your property. The circuit breaker panel is owned by you or the electric company, its different in different nations.

If there is a fault before the circuit breaker, the electric company sends 2 technicians to replace the cable as thats all they can do or the quality the current decreases until someone finds the problem.

2. Additional explanations: Both the ground and 0(neutral) cable are working all the time, the 0 can not be "boned onto the panel" or enclousure as you write. The idea of the ground cable is that when you touch the phase or 0(if you are not dead of course), the current flows through the ground cable to ground, which is the minimum resistance, your body has up to 1000Ohms resistance, depending on your condition(wet, drunk and so on), while the ground resistance is "the NFPA and IEEE have recommended a ground resistance value of 5.0 ohms or less". If you are not wearing your boots, it becomes interesting.

As for the 0 and the circuit breaker, it normally falls because it works by comparing the current that passes through the phase and the 0(it should be the same amount), if you touch, some current will pass through you, then 1 of the phase and 0 cables will have less, so the circuit breaker falls.

Answer 7

Depending on area, there usually will be multiple levels of breakers to catch the fault.
Starting from the wall socket, the first thing that goes pop is a regular breaker in the panel.
If short happens to be within panel, there are usually "main" breakers before that panel. These are still normally accessible to user.
On next level, there will be often a set of fuses before the meter, in sealed compartment, so nobody can bypass the meter. If you manage to burn those, expect a lengthy call with electric services and explaining how did you manage to do that.
Beyond that level, the wires go straight to transformer station - the station may have some kind of heavy duty breakers, or it may not, because shorting the overhead wires is something that just happens sometimes.
In general, places customer can access while doing own electric works will have some kind of breaker or fuse.