Electrical Fault Circuit Interrupter (EFCI)

An EFCI can detect fault conditions in the premise wiring and on the power line and then disconnect electrical devices before they become damaged or ignite fires. The premise wiring faults include both high resistance points in electrical boxes and at outlets, as well as “open-neutral” wiring conditions. 

A EFCI in an electrical outlet measures the line voltage, compares the actual conditions to safety limits, and disconnects electricity to the load if actual conditions exceed the limit(s). ANSI C84.1 lists the normal service voltage levels and normal utilization levels at outlet receptacles. PFCI continuously monitors for violations of these limits.

High line voltages can overheat appliances and other electrical devices and cause fires, or simply destroy electronics and light bulbs without causing a fire. Low line voltages can stress switching power supplies and thereby damage electronics and PCs.

Recovery from blackout conditions can cause large grid loads and power on-off cycling. The cycling can stress and damage electronics from closely spaced surges. An EFCI can reduce the total inrush current associated with blackout recovery by delaying the turn on of loads until after good power returns and becomes stable.

How does EFCI protect from high resistance connections in premise wiring?

Loose screws, loose wire nuts, bad crimps, damaged wires, insulation, and corrosion can cause high resistance connections. High current flowing through a high resistance connection can create a large localized heat source. For example, 10A through a 2 Ohm junction results in 200 Watts of heat. When the resistance is in a small area such as a junction, it can cause the area to heat up to 2000 degrees F (1100 degrees C), cause brass and copper to glow red hot and ignite flammables. The same junction with just a 60 Watt lamp operating would only draw 0.5 Amp current and the 2 Ohm junction would only dissipate ½ Watt of heat. So a bad wire junction could exist for years and never cause a problem until a high-current load is plugged in.

The EFCI circuit measures the line voltage at the outlet. High current through a high resistance junction will cause a voltage drop. For example, 10A through 2 Ohms will cause a 20 V drop. PFCI detects the low voltage and disconnects the load to prevent the high resistance junction from overheating.

How does EFCI protect from “open neutral” conditions?

An open neutral condition means that the neutral circuit return wire has become disconnected in a single phase, split leg, 240 VAC residential electrical system. When this occurs, current flows across the entire 240V; no direct return path exists for each leg. Current can only flow if a connection exists through loads on both legs.

The voltage drop across devices on each leg is normally 120VAC. However, in an open neutral situation, the actual leg voltage depends upon the size of the active load in that leg relative to the size of the active load in the other leg. If the loads are identical, then the voltage will continue to be 120 VAC. However, if the loads on the two legs are imbalanced, then one leg will experience more than 120 VAC and the other leg will be equally below 120 VAC. For example, with a space heater on one leg and a cell phone charger on the other leg, the space heater could see only 10 VAC while the cell phone charger has 230 VAC across it. The cell phone charger becomes an immediate fire risk, since the device may not have been designed to handle double its voltage rating and four times its power rating.

An EFCI measures the leg voltage. It can detect either the lower voltage across the larger load leg or the higher voltage condition on the smaller load size. The EFCI electronic circuit measures the single-leg line voltage. If the line voltage is outside acceptable limits (typically 106V to 130V) the load is disconnected from the supply. EFCI will keep the loads disconnected until the voltages balance. Preventing the high voltage condition is especially important, since the associated smaller loads usually have thinner wires (e.g. 20, 22, 24 gauge, etc) that can quickly overheat from the higher voltages and cause a fire.

How does EFCI help during blackouts?

As the line voltage dips, EFCI will immediately isolate the load device from the line to prevent damage to the device. When the blackout has ended, the power company brings the grid back on line with its generators. The large inrush current, mostly from inductive loads like motors, can place an abnormal load on the generators. Often part of the grid must be taken back off-line. These on-off cycles can damage devices such as appliances, motors and electronics.

To protect these devices and reduce the inrush load on the grid, an EFCI delays the turn-on of each receptacle load differently. First it waits 15 seconds after the voltage level reaches acceptable levels to ensure that it will not surge off again. Then it pseudo-randomly turns on each outlet from 0 to 15 seconds later. Thus loads controlled by EFCI turn on 15 to 30 seconds after the power comes back on line, thereby distributing the demand on the grid more evenly over time. Additionally, on each two-receptacle EFCI outlet, the bottom receptacle turns on 2 seconds after the top one.

An EFCI will detect and disconnect multiple fault conditions but cannot correct them. EFCI assists in troubleshooting the fault by identifying the fault type using flashing front panel indicators on the outlet. SafePlug brand receptacles containing EFCI indicate status conditions such as the following:

• Outlet powered, line conditions normal
• Outlet tripped due to a power or premise-wiring fault
• Outlet end-of-life reached.

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Standard circuit breakers located in distribution panels are designed to prevent fires caused by overloaded in-wall wiring. Circuit breakers are matched to the wire gauge and type. The smallest allowed in-wall wire gauge is 14 AWG and is designed to carry up to 17.5A of current for extended time periods. Thus panel circuit breakers cannot adequately protect the smaller wires found in many appliances, electronics, motors, light fixtures, extension cords, and strip outlets. These smaller wires may be 16 AWG, 18 AWG, 20 AWG, 22 AWG or smaller and could overheat at much lower current levels.

Therefore, EFCI protection must be placed at the junction between the circuit wiring and the smaller gauge appliance wiring, usually in the electrical outlet. An EFCI circuit measures current draw from an electrical load device, compares the actual draw to the safe limit, and disconnects electricity to the load if actual draw exceeds the limit. The trip limit is higher than the maximum operating current of a device (called the hold level).