AC and DC bonding

[slick 50]

Hello,
I been thinking about this but maybe someone has dealt with similar situation? Lets say you got a food truck that is powered by a 7,500 watt portable generator through a power inlet box. All the AC wiring in the food trailer have EGC's and all metal parts are grounded and bonded together back to the generator through the 4-wire cord. The generator does not have a grounding electrode established and is floating..?

Now, you leave the truck connected to the trailer while in-use and the 12VDC trailer plug is connected between truck and trailer as nobody would disconnect the plug unless they were disconnecting the trailer. All the metal components in the trailer will be factory bonded to the frame of the trailer therefore all the metal components are bonded to the 12VDC negative battery terminal as well as the ground from the 120/240VAC.

If there was a 120VAC short to ground in the food trailer, would that fault current travel to the truck battery and damage the truck's electronics or would it not take that path? I'm thinking it would flow to the truck's battery because there is no grounding electrode system at the generator?

Thank you!

 

[LarryFine]

There should be no damaging current because there is onlyone common conductor, the collective grounds.

It's really no different from the utility primary and secondary systems sharing a single grounded conductor.

 

[Jpflex]

Hello,
I been thinking about this but maybe someone has dealt with similar situation? Lets say you got a food truck that is powered by a 7,500 watt portable generator through a power inlet box. All the AC wiring in the food trailer have EGC's and all metal parts are grounded and bonded together back to the generator through the 4-wire cord. The generator does not have a grounding electrode established and is floating..?

Now, you leave the truck connected to the trailer while in-use and the 12VDC trailer plug is connected between truck and trailer as nobody would disconnect the plug unless they were disconnecting the trailer. All the metal components in the trailer will be factory bonded to the frame of the trailer therefore all the metal components are bonded to the 12VDC negative battery terminal as well as the ground from the 120/240VAC.

If there was a 120VAC short to ground in the food trailer, would that fault current travel to the truck battery and damage the truck's electronics or would it not take that path? I'm thinking it would flow to the truck's battery because there is no grounding electrode system at the generator?

Thank you!

Good point. This is something that had not came to my mind but I can se how 120 v ac can return to the trucks 12 volt battery negative cells.

This gets me thinking what would happen to the dc battery receiving such as high ac voltage and or current and electronics,

As a former mechanic I know that a battery can withstand low levels of ac current as it happened to me when my alternator’s diodes (bridge rectifier) failed. The analogue gauges such as speedometer used to intermittently fluctuate up and down

 

[don_resqcapt19]

Good point. This is something that had not came to my mind but I can se how 120 v ac can return to the trucks 12 volt battery negative cells.

This gets me thinking what would happen to the dc battery receiving such as high ac voltage and or current and electronics,

As a former mechanic I know that a battery can withstand low levels of ac current as it happened to me when my alternator’s diodes (bridge rectifier) failed. The analogue gauges such as speedometer used to intermittently fluctuate up and down

Can you describe that path?

 

[Jpflex]

Normally current is trying to return to the source so I think short circuit voltage would return to high voltage source as normal through EGC but may also return through trailer chassis to cord to truck chassis to negative battery post but will continue until trailer over current protection breakers open circuit

 

[jaggedben]

If there was a 120VAC short to ground in the food trailer, would that fault current travel to the truck battery and damage the truck's electronics or would it not take that path? I'm thinking it would flow to the truck's battery because there is no grounding electrode system at the generator?

In this paragraph it sounds like you think a 'fault' tries to go to 'ground' and that because it has no 'ground' it will go somewhere else. This is not true. Get the 'electricity wants to go to ground' thinking out of your head. Electricity always wants to return to the source.

In your post you described a 4-wire feed from the generator and implied that the truck frame would be bonded to the green wire from that feed. A 'short' from the generator hots would try to get back to the generator neutral. Therefore if the EGC is bonded to the generator neutral then that will complete the circuit back to the generator and hopefully trip the generator circuit breaker.

Whether neutral and EGC are bonded together at or near the generator is a thousand times more important here than whether either of them are connected to a grounding electrode. If they are not bonded together, then there will be no imediate 'short' if the truck frame. Instead the truck frame and and metal parts will become energized at 120V to the generator neutral until something or someone completes the circuit. Current will not try to go to the truck battery because that doesn't complete a circuit back the generator.

 

[don_resqcapt19]

Normally current is trying to return to the source so I think short circuit voltage would return to high voltage source as normal through EGC but may also return through trailer chassis to cord to truck chassis to negative battery post but will continue until trailer over current protection breakers open circuit

While it will flow on the bonded metal parts that provide a path back to the source, there is no path for it to flow through the battery.

 

[wwhitney]

If there was a 120VAC short to ground in the food trailer, would that fault current travel to the truck battery and damage the truck's electronics or would it not take that path?

It would not take that path, as doing so does not complete a circuit from a voltage source back to (the other side of) itself, as others have described.

But one thing worth noting is that if both the battery (DC-) and generator (neutral) are bonded to the frame, there are other undesired interactions possible between the two systems. If DC+ shorts to generator neutral, that completes a circuit through the frame and is a dead short on the battery. Conversely, if generator non-neutral shorts to DC-, that is a dead short on the generator. [This one is a bit more obvious, as you don't have many DC- wires, since the automotive components are typically directly mounted on the frame, so that would be a direct generator non-neutral to frame short.]

You'd also have an issue if generator non-neutral shorts to DC+. Then the battery would be trying to push 12 VDC through the generator's alternator; I wouldn't think that would harm the alternator.. But the generator would also be trying to push 120VAC through the battery, which would presumably damage the battery.

Cheers, Wayne

 

[slick 50]

Thanks for all the replies!