Are your 3-prong receptacles really grounding?




Grounding-type receptacles feature a solidly-bonded fault current path in the form of wire all the way to the service disconnect equipment. In old homes, this path can be incomplete due to innocent wiring errors; unfortunately, it is at least as common for grounding-type receptacles to be installed as a way to fake the appearance of newer wiring methods, or at best because of total ignorance of proper installation methods. Hard to say which of these two bad options you might prefer to have in your home.

A tamper-resistant 15A duplex grounding-type receptacle being installed in baseboard.

Equipment grounding connections at receptacles (the ‘ground pin’) provide two basic functions: a) to drain harmful voltages induced at-a-distance by nearby high-voltage lightning discharges, due to electromotive force around the lightning’s discharge current, and b) to cause breakers to trip if a short occurs from circuit conductors to grounded equipment; the breaker ‘opens’ the circuit under this fault condition, preventing the grounded external surfaces of equipment from becoming energized and posing a shock hazard to people or a fire hazard to the building. Both functions depend on a solid connection, intentionally bonded where splices and junctions are made in wire, between the metal mounting yoke of the receptacle and the metal ground electrodes (ground rods and water pipes) of the building grounding system.

Simply twisting grounding wires together does not reliably bond them, although this half-way splicing method appears to be the ‘standard’ of about 50% of the existing installations I encounter.

The lazy twist will likely test ok immediately after installation, and perhaps even for decades. Ground testers operate at 120V and insignificant current. However, it is the high-energy short circuit or lightning-induced surge that the bond must carry efficiently and withstand.

Detail_Lazy ground twist no wire nutq

Detail showing not only no wire nut, but also no good mechanical contact between lazy-twisted equipment grounding wires

Lazy-grounded receptacle hanging from box

This is not a properly bonded fault current path!

The more resistance across the splice, the less likely a breaker will trip to clear a short circuit. A lightning surge will also be more likely to damage equipment connected on a resistive ground fault circuit. And in the rare event of a direct lightning strike that typically finds multiple parallel paths to ground, energy in the resistive ground fault path will more likely arc out of the lazy-twisted, resistive wired fault path to find other paths to ground, like metal pipes. This will increase damage to pipes and hazards for people in the building at the time of the strike.

 

 

False-grounded outlet (receptacles)

Ignorant or dishonest installers may install grounding-type receptacles on old wiring that provides no actual ground path to the breaker panel and grounding electrodes. They may do this simply, by not connecting anything to the ground terminal behind the faceplate, or deviously, by bonding the neutral and ground terminals.

The simple fake ground  is a negligent professional version of the common (and improper) consumer practice of bypassing the ground pin on cord-ends of equipment that needs a ground connection for safety. Either bypass tactic improperly connects equipment furnished with grounding-type cord ends, where such equipment has been designed and underwriter-approved based on the requirement that the equipment be connected to a true ground by way of a dedicated equipment grounding conductor. Bypassing ground requirements is particularly dangerous, considering that the grounding requirement was often determined in the first place because the appliance is high-power.

Simple fake-grounded receptacles also endanger sensitive or expensive electronic equipment, since the surge-suppressors end users commonly connect to them rely on an equipment grounding path to dissipate surge energy and will not function without one. A fuse, if present on the suppressor device, will still disconnect equipment in the event of a very powerful surge of sufficient duration, with or without equipment grounding.

Trick-bonded grounding type receptacles are the worst type of false-grounding and the most intentionally devious, since the method is designed to fool simple plug-in type receptacle grounding/polarity testers.

back side of NEMA 5 receptacle hanging from Wiremold box, with white wire bonding neutral and grounding terminals.

Trick-bonded neutral and ground at the receptacle introduces dangerous stray current on everything metal through which the wiring passes.

The technique involves bonding the neutral and ground at the receptacle (this should only ever be done at the service equipment of the building, where the most robust grounding connection runs direct to earth). Trick-bonded ground receptacles are the worst type of false ground because a) you can test for ground and still not detect the problem, and, b) it goes beyond the passive action of not connecting a safety feature in the circuit, and actively creates a hazardous condition — whether any equipment is connected or not. Bonding neutral and ground anywhere except the service equipment allows voltage to be present, and  current to flow in the bare equipment grounding/safety wire, which was not designed to carry current except for the split-second necessary to trip a breaker in case of a direct short-circuit. This fire hazard due to uninsulated conductor is bad enough, but it also creates an electrocution hazard, since voltage and current on the bare conductor transfers to all the exposed metal the conductor is attached to. For example: the metal cover on the receptacle (or metal screws holding a plastic cover); metal surface conduit such as EMT or Wiremold, that people can and will touch (or children may lick); metal pipes in the house, including those running to a bathtub, etc.

Even if no fire starts and no one is shocked or electrocuted from voltage and current through the bare metal grounding/bonding system, this system will carry 1/2 of the grounded current path, splitting current that otherwise would be all on the insulated neutral conductor. Since the bare metal grounding/bonding system does not run close to, and parallel to, the phase conductor, the electromagnetic field generated around current flowing on this system will not be canceled by the phase conductor’s counter-EMF field. I have not run into data on the human health effects of stray current, but there is data showing that dairy cow milk production may be reduced by 20-40% in facilities affected by stray current conditions. I suspect that fetuses and infants must be impacted by significant stray current conditions.

Interestingly, I located the specimen in the photo while troubleshooting tripping of an AFCI breaker newly-installed on old wiring. AFCI and GFCI breakers both are highly sensitive to faults between neutral and ground. They will trip upon contact between ground and neutral, even when the phase conductor (‘hot’) has been interrupted by a switch (presumably because the switch is an imperfect insulator/interrupter and minute voltage remains across the circuit conductors?).

 

PROPER USE OF A GROUNDING CORD-AND-PLUG ADAPTER AT NON-GROUNDED OUTLETS

Ever wonder what the metal pin on your adapter is there for? It allows you to provide your own equipment ground connection where the building wiring does not include it. Proper use of the adapter requires that you connect the tab to a grounding terminal using a wire. This remote grounding terminal could be a listed grounding clamp around a copper water pipe, or the cover-plate screw of another, properly-grounded receptacle.

Without this wire and remote grounding termination, consumer type, cord-atached grounding adapters create the same hazard as false-grounded receptacles: they allow you to connect equipment that was designed to have a dedicated grounding path back to the circuit breaker, to be connected without such a path.

 



 

See also:

Loading

Articles Categories

Recent Projects

Electrical (off site) ->

LOVEphilly