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In a typical U.S. home, medium-voltage power in the electrical distribution system is stepped down to 240 volts at a utility transformer located near a residence. Electricity at this voltage, or electrical potential, is sent to the home via two “hot” conductors - or “legs” - that form a loop, or circuit, between the home and the transformer. The flow of power in this loop alternates its direction of flow 60 times a second, and at any instant, the measure of electrical potential between the two hot legs is approximately 240 volts. Some items in a home, like air conditioners and electric vehicle service equipment, utilize both legs to deliver a large amount of power at 240 volts; while lighting, outlets, and other devices like TVs and refrigerators are designed to utilize power at only 120 volts, half of the service voltage. One-hundred and twenty volts is created by the addition of a third wire at the transformer, halfway between the two hot wire terminals. One-hundred and twenty-volt circuits use one hot wire and this third, “neutral” wire to create a circuit through which power can flow.
The neutral wire gets its name due to the fact that it should be electrically bonded to ground, meaning that though there is power flowing through the wire, it has no voltage, or electrical potential, to the surrounding earth. The neutral wire is required to complete a 120-V circuit, and though the neutral doesn’t conduct power in a 240-V circuit, it still serves an important function. A correctly-installed neutral tap on a split-phase transformer splits the 240-V service in half, keeping each of the hot legs at approximately 120 volts relative to ground. This has the effect of “centering” the 240-V service around the ground. Since our electrical devices, the materials in our homes, and our bodies are all directly or indirectly in contact with the earth, this is crucial as it prevents larger, more dangerous electrical potentials from forming between our electrical systems and the ground. If the neutral wire is broken or lost at one of its source sides - that is, the distribution transformer or generator - the closed circuit’s neutral conductor will lose its reference point and “float”.
What is a floating neutral?
A floating, open, or loose neutral is a dangerous electrical issue that exists when the neutral wire is not connected or poorly connected, whether through improper installation or a faulty device, posing a fire hazard or shock risk. If a floating neutral is detected, it needs to be fixed by a licensed electrician or the electric utility (depending on the location of the problem) and the affected device or appliance should be removed from service until it is repaired and tested or replaced entirely.
On the residential customer’s side of the meter, branch circuits use the neutral wire to create a complete circuit. There is also a neutral running from the main service panel back to the electric utility. When the two legs of a split-phase service are perfectly balanced, this neutral does not conduct current, but in most cases one leg conducts somewhat more than the other, and the neutral wire is responsible for conducting the difference in current back to the utility transformer.
In a distribution system, the load is always unbalanced, or delivering an unequal amount of power. This is why we need a neutral wire. When the load is balanced, the electric current floating through the neutral wire is zero. The job of the neutral wire is to carry the unbalanced current and help keep both halves of the split phase wiring an equal 120 volts to ground. However, a break in the neutral wire will create unequal voltage across the loads, which can lead to some homes having higher than normal electricity flowing to their appliances, and some having lower than normal electricity flowing to their appliances. For example, one might notice the plug on their hair dryer causes a spark when plugged into an outlet, or a lamp in the living room flickering on and off for a brief moment. These fluctuating loads are the floating neutral, meaning the value of the loads keep changing or “floating” because there is no neutral wire to balance the now open circuit.
Causes of a Floating Neutral
If the neutral wire is broken at the source (i.e. distribution transformer), many customers will experience electricity variation, and likely the majority of the equipment connected to the distribution center will get damaged. Floating neutral can also happen if the overhead power line is damaged from natural occurring events such as extreme heat or severe storms. Finally, poor workmanship or a loose connection from rust or corrosion can cause a floating neutral wire.
Effects of a Floating neutral
If a floating neutral occurs, it can be very dangerous for a number of reasons. Without a neutral keeping the 240-volt service tied to the earth’s potential (“ground”), one leg of power in the home may measure significantly more than 120 volts, while the other leg may measure significantly less than 120 volts, even though the two voltages still add up to 240 volts. In addition to the increased potency of a shock from a circuit experiencing overvoltage, these voltage irregularities can damage computers, TVs, motors, compressors, and other sensitive electronics. In some cases non-standard voltage can even cause devices to catch fire.
The neutral conductor is designed to be the low-impedance path for current to return to the utility transformer. If the neutral is opened or damaged in such a way that resistance on the neutral path increases, electrical current may find a lower-resistance path through the earth back to the transformer, or through plumbing pipes, low-voltage coaxial wiring, building materials, and people. These unintended current paths can lead to electrical shocks and fires.
There are well-documented cases of loose neutrals leading to unintended current on the exterior of metallic electrical boxes. This current can then flow through the wooden framework of a home, using the moisture within the wood as a conductor. Over time this current heats the wood and dries it out, eventually turning it into a type of charcoal with a low ignition temperature. At that point the wood can easily ignite and cause a fire.
A further danger occurs with open neutrals because they simultaneously raise the risk of unintended current flowing through material that should not be conducting electricity and prevent a standard circuit breaker from clearing the fault as the path through the ground back to the transformer is high-inductance and doesn’t allow the kinds of large current flows that would trip a breaker.
In a home, a floating neutral can cause flickering lights. You may also notice metal surfaces in your home such as sinks or pipes giving you a tingle. If current is returning through cable TV or other wiring, you may see or smell smoke or notice melted frayed wire insulation on visible wiring running back to the utility pole. If you are experiencing any of these symptoms, call an electrician and/or the electric utility company immediately as your home is at high risk of causing electric shock or a house fire.
A floating neutral occurs when the neutral wire that balances electric loads becomes disconnected. This can occur because of mechanical issues, weather events, human error or other issues like corrosion. If a floating neutral does happen, this creates a dangerous situation that can lead to electric shock or fire. If you suspect a floating neutral event in your home, contact an electrician immediately.