Branch circuit wiring problems lead to more residential electrical fires than any other single cause. Hidden trouble inside a branch circuit can also lead to electrocutions and the failure of sensitive electrical equipment.
How serious is America's fire problem? Based on data from the National Fire Protection Association (NFPA) and the U.S. Consumer Product Safety Commission data, an estimated 406,000 residential structural fires occurred in 1997, resulting in 3,390 civilian deaths and 17,775 injuries.
About 14,600 of these structural fires and 110 deaths were determined to be the consequence of problems in the electrical distribution system. High-resistance connections within a branch circuit, such as a loose or corroded connection, a bad splice or an improper installation, were overwhelmingly the culprit in these fires.
Fire hazards arise when current flows through these high resistance connections, resulting in hot spots. As more current flows through the connection, heat continues to build up. Depending on the ventilation, the heat might dissipate safely. If not, it could lead to a fire.
IDENTIFYING HIGH-RESISTANCE CONNECTIONS
Most fixed wiring and receptacle hazards are hidden from inspection. A visual inspection in the rough-in stage of residential construction may identify obvious problems, such as a staple cutting through the conductors, but they may not identify a loose wiring connection or a bad splice. Normal instrument testing of a static circuit reveals little about the quality of wiring or the integrity of the circuit. However, testing under load and calculating the voltage drop can identify 90% of these hidden defects.
Voltage drop is a measure of how much a circuit's voltage fluctuates (or drops) once a load is applied. Voltage drop can be calculated by comparing a voltage measurement with no load on the circuit to a voltage measurement under full load.
VOLTAGE DROP=V no-load - Vload/Vno-load
The voltage drop calculation will be most accurate when no-load conditions are compared to full- load conditions. When using a digital multimeter to calculate voltage drop, remove all loads from the circuit to take the no-load measurement. For the full load measurement, use a space heater or some other appliance that will draw close to 15A.
Voltage drop can also be measured with some circuit analyzers now on the market that are capable of placing a full 15A load onto the circuit without tripping a breaker or causing any interruption to equipment on the line. These convenient, time-saving testers will compare the voltage measurement at a full 15A load, with a measurement at no load and calculate the voltage drop.
HOW MUCH VOLTAGE DROP IS ACCEPTABLE?
The National Electrical Code (NEC) recommends that the combined voltage drop of the electrical system (branch circuit and feeders) not exceed 5% for optimum efficiency. It is important to note that this is a recommendation and that local inspectors, or other governing bodies, may use their own judgment on an acceptable level of voltage drop for the electrical system.
For example, the Philadelphia Housing Development Corporation (PHDC) requires contractors to calculate the voltage drop prior to installing blown insulation in existing homes. If the voltage drop is 10% or higher, the contractor must replace or repair the circuit prior to proceeding with the insulation. Prior to instituting this requirement, half a dozen smoldering fires resulted from the blown insulation installations. In the 2,500 homes insulated during a two-year period after this electrical integrity test was instituted, no fires were reported. At least 15 other municipalities have followed the PHDC's lead in requiring the load test as part of their winterization programs.
TROUBLESHOOTING A CIRCUIT
Troubleshooting to identify the cause of the high impedance within the electrical system is actually quite simple. First measure the voltage drop at the furthest receptacle from the panel on the branch circuit under test. If the voltage drop is high, then further investigation is necessary.
Testing the remaining receptacles in sequence, from next furthest from the panel to the closest to the panel, will identify the problem. If the voltage-drop reading changes significantly from one receptacle to the next, then the problem is a high impedance point at or between the two receptacles. It is usually located at a termination point, such as a bad splice or loose wire connection, but it might also be a bad receptacle.
If the reading steadily decreases as you get closer to the panel, with no significant decreases between receptacles, then the wire may be undersized for the length of run, or for the load on the line. Check at the panel to see if the wire is sized per Code, and measure the current on the branch circuit.
The reading may not decrease at all from the last receptacle to the first. This would indicate that the problem could be at the first splice, or at the panel itself. Most poor panel connections show up as hot spots on the panel. These can be checked quickly with an infrared temperature meter.
Arc Fault Circuit Interrupters (AFCI), a new protection device, became an NEC requirement in 2002. AFCIs will be required to protect all bedroom outlets, including lighting outlets. AFCI circuit breakers monitor the branch circuit, looking for the waveform patterns given off by an arc fault. These arc faults, which can be caused by a breakdown of insulation in wiring or small appliances, are a large contributor to residential fires.
Chad Reynolds is a product manager with Ideal Industries, Sycamore, Ill.