# Would ground fault cause breaker to trip if neutral not bonded to ground in main panel?

Suppose normal residential main breaker panel (not subpanel) is wired correctly except the neutral bar is not bonded to ground. Assume a 15A breaker in the panel powers an exterior light with a metal casing. The metal casing is connected to the ground wire. Suppose the hot wire to the light comes in contact with the metal casing (remember the ground and neutral are not bonded in the breaker panel). Will current flow through the breaker? How much current will flow? Would there be enough current to trip the breaker? Suppose a person in bare feet on moist ground touches the metal casing. Is the person going to be electrocuted or shocked?

Thanks for all the time and effort by each of you to answer my question. It is not an exam question. The breaker would not be GFCI. In the US, a main panel always has ground bonded to neutral; in a subpanel the ground must never be bonded with neutral. Perhaps a simpler scenario would answer my question. Suppose the single hot wire from my 15A breaker (not GFCI breaker) is connected via a switch to a copper ground rod in the soil. (Again, the breaker panel is wired improperly since ground and neutral not bonded together.) I have no idea what the resistance would be in the soil to complete the circuit back to the transformer. If you close the switch would there be enough current to trip the breaker?

• I don't see it making any difference whether neutral is bonded to ground in the panel or not. That's not done in the UK anyway. Jul 26, 2021 at 15:50
• As an addition to the answers, I would like to say, that grounding is very important. It has to be measured by a licensed electrician, approved instrument and of course the way it is done is probably illegal, but it is all written in the contract of the distribution company - TT, IT, TNS,TNC, TNCS,.. Jul 26, 2021 at 16:34
• Which country is this to know the electric system better? Is this an exam question? Is there a GFCI/RCD? Jul 26, 2021 at 18:02
• You forgot to mention the type of breaker. That makes a differelce. Jul 26, 2021 at 20:50
• Regarding your revised question, if the transformer neutral is also connected to a rod in the ground at the transformer, then PROBABLY enough current would flow through the earth to trip the breaker. However, if the transformer neutral is not connected to a ground rod, then probably the breaker would not trip. Jul 28, 2021 at 5:08

simulate this circuit – Schematic created using CircuitLab

Figure 1. The OP's setup.

Will current flow through the breaker?

Yes. A current will flow through ground back to the utility supply transformer.

How much current will flow?

That will be determined by Ohm's Law, $$\ I = \frac V R \$$. The value of $$\R\$$ will depend on how good a contact is made at the fault and the resistance of the wiring in the whole circuit from the transformer, house wiring and ground return.

Would there be enough current to trip the breaker?

See above. Generally yes.

Suppose a person in bare feet on moist ground touches the metal casing. Is the person going to be electrocuted or shocked?

Due to the resistance of the house's earth wire there will be a voltage drop across it while the fault current is flowing. It should be easy enough to imagine that if the live and earth wires are the same cross-section that they will act like equal resistors and the case of the lamp will be at half the supply voltage until the circuit breaker trips.

Electric shock is a possibility.

• This is a good answer. However, I don't think the transformer at my house has a ground electrode other than the ones right at the main panel (which is where the ground/neutral bond is). So if the bond was omitted at my house, I think there would be no well-defined path from metal case back to the transformer, not even traveling through soil. Jul 26, 2021 at 17:49
• I'd be surprised if the neutralising is not done at the transformer too. If not it risks your house earth bond and return wire carrying fault currents for everyone else that shares the same transformer. Jul 26, 2021 at 17:56
• It is a rural area. My house is the only one on the transformer. The transformer is up on a pole and the wires come right down aerially to my meter and main panel. I will check again next time I am able and see if there is a wire coming straight down from the transformer to the base of the pole that might be a ground wire. Maybe it is there and I haven't noticed it before. Jul 26, 2021 at 18:12

If the ground wire and neutral wire are not bonded together, then they will not have good continuity, and the hot wire shorting to the metal frame will not necessarily trip the breaker or GFCI (or RCD). The other answers to your question seem to be assuming that neutral and ground are bonded together somewhere somehow. And if they are bonded, then those answers are correct. But I am afraid they may have misunderstood the question.

The electrical code in the US requires that there be only one bond wire at any time. Normally this bond wire is in the main panel. So if the installation is normal except for the ONE bond wire connecting neutral and ground, then there will be no continuity between neutral and ground, and the ground wire will not be a reliable return path for fault currents.

• Are you sure? Wikipedia's Electrical_wiring_in_North_America says, "United States electrical codes require that the neutral be connected to earth at the "service panel" only and at no other point within the building wiring system." This doesn't mean the transformer cannot be grounded. I'm having difficulty finding any authoritative articles on actual transformer grounding practice in North America but nearly all high-leg transformer diagrams show neutral grounded. (In Ireland we don't do high-leg - just nice L + N.) Jul 26, 2021 at 19:39
• @Transistor I am not sure. I could be wrong. Jul 28, 2021 at 5:02

If the service neutral is grounded at the meter or at he service transformer, a hot wire that is grounded anywhere will draw a ground fault level of current that could potentially be in the low thousands of amps. Whatever the conditions under which the fault occurs the fault current should easily be enough to trip a breaker.

The answer by @mkeith covers an important point. There will only be one connection point between neutral and ground. That is often at the main panel for the service, but it is also often at the point near the connection to earth. For single-family structures the main panel is near the service drop and meter where the earth connection is commonly located. They are more likely to have neutral and ground bonded together at the main panel. Structures that have meters that are not near the service drop are more likely to have a ground wire from the main panel to a grounding point near the meter.

If the metal casing of any distribution panel or junction box does not have a ground wire that is connected to the earth ground for the premises, there is an electric shock danger for anyone who comes into contact with the metal structure.

• Hi, Charles. "There will only be one connection point between neutral and ground." I can see that the US code specifies this within the building but that would still allow it at the transformer and prevent my neutral-ground connection carrying your fault current should your bonding fail. Are you sure mkeith is correct? Jul 26, 2021 at 19:41
• Hi,Transistor. Just as many people say that NEC (National Electric Code) also stands for "Not Exactly Clear," I would have to say "not exactly sure." NEC has lots of related paragraphs, notes and exceptions. I live in a townhouse. My box has no neutral-ground bond. It has an uninsulated ground wire leaving the box. My meter is one of four on the side of the end unit. There is one large and one small conduit going into the ground below the meter box. The transformer is in a fiberglas box on a pad near the street. That may be the location of the only ground.
– user80875
Jul 26, 2021 at 20:22