Floating Neutrals

A floating neutral is a serious electrical issue that can cause damage to electrical devices in your home and potentially pose a fire or shock hazard. If Sense detects a floating neutral in your home, we will reach out to you to share the data that we see and help you diagnose. However, Sense might not detect faults perfectly, so if you think you might have a floating neutral then you should contact an electrician as soon as possible. 

What is a floating neutral?

In a typical U.S. home, 240V electricity flows into the electric panel via two phases from a utility transformer. The phases are inverses of each other, carrying +120V and -120V relative to a third neutral (or “ground”) wire. Residential electric panels have a neutral bar that connects individual circuit neutrals to ground. Since electricity flows through a closed circuit, electricity flows to an appliance and then returns to ground via the neutral wire.

A “floating” neutral occurs when the connection to the ground breaks or becomes loose, which causes the neutral bar to “float.” This can happen in your panel or between the utility and your electric panel. It can be caused by a mechanical issue or other issues like rust or corrosion.

It’s a serious issue because the neutral balances voltage across the two phases. In a floating neutral situation, the difference between the phases remains 240V, but the 0V reference (the neutral) no longer exists. This knocks voltage out of balance, so some appliances may draw more voltage than they should while others draw less. You might notice flickering lights. In a severe case, an outlet could malfunction and could be energized with the full 240V, which can damage devices and pose a shock or fire hazard.

How does Sense detect it?

Sense detects a floating neutral by spotting events where voltage on the two phases drifts away from each other. The larger the load imbalance, the more serious the issue. For example, the graph below shows wattage and voltage on each phase when a 120V device starts up. In the first example, the neutral is intact so we see a spike in wattage as the device turns on, and flat voltage on each phase. However, in the second example the neutral is floating, so when the device starts up, voltage rises on one phase while the other falls, which is a sign that the neutral isn’t well connected and is not balancing voltage correctly.

In a normal situation, voltage stays balanced across the 2 phases:

 
A floating neutral causes voltage to rise on one phase, while falling on the other.