Lynn Demartini
New member
Can someone please explain this phenomenon in laymens terms and hoew to rectify it?
Net current
- Thread starter Lynn Demartini
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- Lockport, IL
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- Semi-Retired Electrical Engineer
A commonly held, and absolutely wrong belief is that current will take the shortest, or least resistance, path to ground. That is wrong on two levels. First, current is not seeking a path to ground, but rather is seeking a path back to its source. Second, current will take every path it can find, including paths of low resistance and paths of high resistance. Any path will do.
Let’s talk about a simple, two-wire circuit (three, if you include the ground wire), like one that will feed a receptacle outlet. Current travels to the outlet on the black wire, and returns on the white wire. If you measure the current on each wire, you should get the same value. If you get different values, it means that some current is finding its way back to the source via some other path. The difference between the current leaving the source on the black wire, and the current returning to the source on the white wire, is called “net current.” If the value of net current is not zero, it means that there is a leakage current flowing somewhere.
The two most likely causes of net current are equipment failures and improper wiring. If in some appliance or machine in the facility, the insulation on some internal wire has worn thin, it is possible for current to leak from the black wire to the case of that equipment, and find its way back to the source via conduits, pipes, or other metal objects, or even via the dirt below the facility. The most likely type of improper wiring that can lead to net currents is to have neutral (white) wires and ground (green or bare copper) wires connected to each other in a panel other than the main service panel. That is explicitly forbidden by the NEC.
Net current can cause a particular kind of problem at facilities that use equipment that is sensitive to stray magnetic fields. Current flowing in a wire creates around itself a magnetic field. Normally, the magnetic field created by the current flowing outwards on the black wire is cancelled out by the magnetic field created by the current flowing back to the source on the white wire. If, however, there are net currents, if the currents in these two wires are not identical, then the magnetic fields will not cancel each other out. That will mean there is a net magnetic field created by the circuit, and this can interfere with some types of equipment.
Let’s talk about a simple, two-wire circuit (three, if you include the ground wire), like one that will feed a receptacle outlet. Current travels to the outlet on the black wire, and returns on the white wire. If you measure the current on each wire, you should get the same value. If you get different values, it means that some current is finding its way back to the source via some other path. The difference between the current leaving the source on the black wire, and the current returning to the source on the white wire, is called “net current.” If the value of net current is not zero, it means that there is a leakage current flowing somewhere.
The two most likely causes of net current are equipment failures and improper wiring. If in some appliance or machine in the facility, the insulation on some internal wire has worn thin, it is possible for current to leak from the black wire to the case of that equipment, and find its way back to the source via conduits, pipes, or other metal objects, or even via the dirt below the facility. The most likely type of improper wiring that can lead to net currents is to have neutral (white) wires and ground (green or bare copper) wires connected to each other in a panel other than the main service panel. That is explicitly forbidden by the NEC.
Net current can cause a particular kind of problem at facilities that use equipment that is sensitive to stray magnetic fields. Current flowing in a wire creates around itself a magnetic field. Normally, the magnetic field created by the current flowing outwards on the black wire is cancelled out by the magnetic field created by the current flowing back to the source on the white wire. If, however, there are net currents, if the currents in these two wires are not identical, then the magnetic fields will not cancel each other out. That will mean there is a net magnetic field created by the circuit, and this can interfere with some types of equipment.
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