This piece was originally published in the June 2016 issue of ei, the magazine of the electroindustry.
Steve Montgomery, Chief Operating Officer, 2D2C, Inc.
Electrical residential safety is getting better every year, as progressively fewer incidences of electrical shock occur. Nevertheless, safety gaps still exist.
Electrical shocks, or injurious exposures to electricity, can occur wherever electrified metal contacts exist. Based on the age and health of the person touching the metal contacts, skin resistance, and path through the body, an electrical shock can be deadly. Seniors and young children are especially susceptible to electrical shock in homes.
The human body uses electro-chemical reactions to send messages through nerve networks to control muscles. If electricity flows through a person’s body, it can disrupt nerve signals and damage tissue. Most people can detect just one milliampere (mA) of alternating current (AC) passing through the body. About a 10-mA current can override normal signals to muscles, causing them to fibrillate or “lock up,” and disrupting blood flow to the heart. If a current fibrillates 30 mA AC across the body—or as little as one mA directly across the heart—the heart can stop pumping. Therefore, shock prevention can save lives.
In a home electrical distribution system, most live electrical contacts are covered by plastic cases, plastic wire nuts, electrical boxes, and cover plates. Electrical plugs and sockets, however, provide access points for people of all ages. Two technologies exist to prevent shocks in residential branch circuits: ground-fault circuit interrupters (GFCIs) and tamper-resistant receptacles (TRRs). GFCIs disrupt the source if leakage is detected. TRRs prevent access to live metal contacts. Both are required by the National Electric Code® (NEC) in the U.S. and the Canadian Electrical Code in Canada.
If properly installed, a GFCI protects against current leakage anywhere in a branch circuit. To protect a whole circuit, the GFCI must be located either in the circuit breaker or at the first outlet on the circuit. If the leakage exceeds a human safety threshold, the GFCI quickly disconnects power to the circuit.
Since 1972, the NEC has required GFCI shock protection at outdoor receptacles. It now requires GFCIs in basements, garages, kitchens, baths, pool areas, and outdoors—places where accidental grounding points exist. The NEC has mandated the use of TRRs in hospital pediatric wards since 1981. Today, they are required in almost all locations in new homes.
Plugging the Gaps
Unfortunately, the isolation provided by TRRs can be overridden with a non-TRR strip outlet, current tap, extension cord, or remotely controlled receptacle. Safety codes should require TRR protection in these devices, and consumers should replace non-TRRs in use.
TRRs use spring-loaded plastic socket shutters to prevent access to live contacts. These shutters are susceptible to jamming open or closed. An electronic approach to TRRs could eliminate these weaknesses. Electronic TRRs keep electricity off at an empty socket until an appliance plug is inserted. If a child sticks a metal object into the socket, the child will not be shocked because electricity would be off.
Leakage to ground exists through heating ducts, metal appliance shells, or other grounding paths where the absence of GFCI protection creates an electrical shock hazard (e.g., a second-story room). Infants have died from sitting on a heat duct while touching the line blade on a partially inserted plug. GFCI protection on all branch circuits or isolating electricity from partially inserted plugs bridges this gap.
Exposed plug blades create a live shock hazard, even on TRR-protected outlets. In Europe, most receptacle designs prevent this risk by recessing the receptacle face so that a plastic shroud inhibits access to exposed plug pins. Many European plugs also use round pins with ceramic insulating sleeves that cover all but the tips of the power pins.
NEMA WD6-2012 Wiring Devices—Dimensional Specifications defines plug blades but does not require blade body insulating material or shrouding to prevent shocks from partially inserted plugs. It is published by the NEMA Wiring Devices Section, which promotes competitiveness, technological progress, and public safety.
Mr. Montgomery is active with several NEMA sections and councils, including High Performance Buildings, Smart Grid, and Wiring Devices.