Wall plug adapters:
Use of a three-prong adapter for two-slot electrical receptacles ("outlet" or "wall plug") when there are just two wires (hot and neutral) but no ground wire, or use of a gang adapter to expand the number of devices that can be plugged in at a single electrical receptacle is widespread but not always safe or proper. How to choose & use a power strip or surge protector to prevent electrical equipment damage from power spikes. Use of multiple plug adapters, gang adapters, & extension cord adapters.
This article series describes how to choose, locate, wire and use an electrical receptacle in a home. Electrical receptacles (also called electrical outlets or "plugs" or "sockets") are simple devices that are easy to install, but there are details to get right if you want to be safe.
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Power strips such as the simple unit shown just below are a popular method for plugging in more than two electrical devices at a single wall receptacle or "outlet".
In the rather sloppy installation above this reader-contributed photo of an electrical power strip illustrates use of a power strip as a short "extension cord" to connect a clothes washing machine. The power strip may, depending on its model, also offer limited protection against electric power surges that can damage stereo equipment, computers, internet routers and other sensitive electrical equipment.
Most of the electrical devices you may plug into a wall receptacle in your building operate at 120VAC (120 volts alternating current) or in the U.K. and some other countries at 240VAC (240 volts, alternating current). Depending on where you live, the line voltage being supplied to your building is nominally at either 120VAC or 240VAC but might vary slightly from moment to moment or during the day. In some communities line voltage may surge considerably above the nominal 120VAC / 240VAC rating to well over 300V or even higher in the event of a lightning strike or a power transformer failure.
Normal alternating current voltage is actually varying between 0 volts and about 170 volts 60 times a second. Your home appliances, computer, router, and TV are designed to operate at this power range.
During a power surge or spike, your electric lights may flicker in response: usually no harm done. But that same power surge can damage sensitive electrical equipment or even destroy it. A typical power surge is very brief, lasting just a fraction of a second, and varying in voltage level (above the nominal power of 120V or 240V depending on where you live) from a few volts to hundreds or even thousands of volts. That very high voltage, even if just for a millisecond, is enough to burn out electrical components.
Watch out: depending on model, power strips, also often referred to simply as surge protectors, may not offer much protection against electrical power surges. Cheaper (my dad said to say "inexpensive") power strips or surge protectors are more limited in their surge protection ability or may have a shorter service life. A better quality surge protector will typically "clamp down" a 600 volt surge in the electrical power supply down to under 200V while an inexpensive surge protector may clamp the 600V power surge to about 300V.
When buying a power strip you should be sure that yours includes surge protection. The product description will include the words "transient voltage surge suppressor", "surge protection", "surge protector", "fused power strip" or "power interruptor switch". If your product's description says only that it is a "power strip" it probably does not include this important protection.
How much surge protection you need depends on both the sensitivity of the particular electrical devices you are protecting and the frequency and sort of electrical power surges that occur where you live. A power surge protector with a higher joule rating (e.g. 3000 joule) will provide more protection than a smaller unit. In the U.S. surge protectors sold for home use will meet UL Standard 1449, "Standard for Surge Protective Devices". That standard covers several types of surge protection devices, but all of them are:
Surge Protective Devices (SPDs) designed for repeated limiting of transient voltage surges as specified in the standard on 50 or 60 Hz power circuits not exceeding 1000 V ... - UL Standard 1449 Standard for Surge Protective Devices, retrieved 1/2/2016, original source: http://ulstandards.ul.com/standard/?id=1449
The ability of a surge protector to protect your equipment is rated for its ability to absorb a power surge or energy surge, described as its "clamping voltage". The clamping voltage is defined as the level of voltage (over 120VAC or 240VAC) that will trigger the devices protection feature that in turn limits the voltage that can pass through to the devices plugged into it. Clamping voltage is expressed in joules (an energy rating number). If the clamping voltage of the device is over 400 Volts it's not going to offer good surge protection. You don't want to send 400 volts or more through your internet router that was rated for 120 volts.
Lower clamping voltage is better than higher clamping voltage.
But higher Joules rating for your device is better than Joules rating as Joules measures how much energy your device can safely absorb. A clamping voltage energy absorption rating of less than 600 or 700 Joules won't offer useful power surge protection, and a 3000 joule or higher clamping voltage is recommended for home use.
Watch out: the least-expensive power strips may offer no surge protection at all even if the power strip includes its own internal circuit breaker.
Watch out: some surge protectors will continue to deliver electrical power to the plugged-in devices even if its surge protection feature has worn out or failed. In that case you may not know that your electrical devices are no longer protected against power surges.
FYI: the relationship between volts and joules is volts = energy (in joules) x charge in coulombs or V = E(JO / Q (C) or Volts = Joules / Coulombs
and since most of us don't know a coulomb from a grapefruit, I'll add that coulombs combines energy and time as follows:
Coulombs = Amps x Seconds where 1 Coulomb = 1 Amp x 1 Second
We care about this time relationship because even though a power spike or surge is an event that lasts for some limited time, usually in seconds or even much less - milliseconds or nanoseconds, the ability of your power surge protector to handle this hazard has to consider both the energy level in the power surge and its duration.
An alternative to using a power strip (shown above) is the use of a six-plug gang adapter that plugs directly into a wall receptacle (shown below). Using the electrical power strip shown above offers some advantages over the gang adapter, including protection from electrical power surges and the ability to plug in more than six devices.
Using the multiple wall plug adapter above has its own advantage: less clutter and a neat installation.
Is the multiple wall plug adapter shown above safe to use? Well it depends. If you overload the electrical circuit by plugging in too many high-amp devices at one location, provided the electrical circuit is properly protected by a circuit breaker or fuse, the worst that may occur is you'll trip the breaker or blow the fuse - an inconvenience but not a fire hazard.
Notice the small round hole in the center of the wall plug adapter shown above? Best practice would be to remove the original center screw holding the electrical receptacle faceplate in place (red arrow in our photo below), but leaving the plastic wall receptacle cover in place.
Next plug in the gang adapter shown above, and then install a longer center screw that will hold the gang adapter firmly in place. This improvement will prevent the accidental disconnection of all of the pugged-in devices by keeping the adapter securely fastened to the wall as you un-plug a single device.
Watch out: gang plug adapters may be a convenient way to plug more than two electrical devices into an individual receptacle, but there may be electrical hazards in their use on multiwire branch circuits and there may be a safety hazard if the device wall plug is not inserted properly into the gang adapter - a mistake that is easy to make if the wall plug itself is an older non-polarized plug both of whose blades are identical in width
Below: a partly-disassembled gang adapter for an electrical receptacle, provided by reader D.N. [private email 2014/09/06 ] illustrates the actual electrical connections in a typical gang connector: the upper and lower receptacle trios on this device are electrically completely independent. This means that if the adapter is plugged into a split-receptacle wired electrical receptacle on a multi-wire branch circuit, devices plugged into the upper or lower sections of the adapter should not cause multiwire branch circuit operating problems.
Reader D.N. writes:
The construction is representative of what I've found among the three such adaptors I've examined from different manufacturers. All of them, like this one, have two completely separate halves.
See MULTI-WIRE CIRCUITS for an explanation of shared-neutral electrical circuits.
Also see ELECTRICAL CONNECTIONS for 2-WIRE RECEPTACLE CIRCUIT where we discuss safety requirements for electrical receptacles on circuits where no electrical ground is present.
5/14/2015 Chris said:
Hi, I just moved into a "new" home and all the receptacles are two-wire with no ground. Reading this, I don't believe it is safe to plug-in an adapter that ads the ground through the screw on the faceplate. However, our TV has the ground wire plug so I cannot use it.
Is here any option short of re-wiring to safely use devices that need the ground plug? Can I cut the cord on my TV and convert the plug into a two-wire style and forget the ground? (Also, we rent for now so I don't own the home). Thank you great article!
Do not cut or modify the cord to your TV. Doing so is unsafe and may also void the device warranty.
You are correct that it is not a good idea to connect a grounded appliance to an un-grounded electrical circuit. It is possible that there is a ground path available - you won't know until an electrician has examined your wiring.
And there are certainly three-prong adapters sold to plug into 2-prong receptacles such as the handful of them that I photographed atop this bureau in New Zealand. The 3-prong to 2-prong plug adapter typically has a tab that connects to the receptacle face plate screw - a feature that both secures the adapter in place and gives hope to the fantasy that there is an actual ground connection.
In fact there may be:
I gave this litany of at least a dozen connections in this makeshift electrical ground pathway to explain that if you use the adapter, your TV will probably work, and it might even have a "sort of" electrical ground. But in an unsafe situation such as a short circuit there are two hazards with this un-approved grounding approach:
1. the passage of the stray or shorted electrical current to earth has to go through multiple, potentially loose, unreliable screws, clamps, and connections that means that it may not work at all.
2. the grounded current - when it occurs - is passing not through a safe, properly-installed, code-approved grounding conductor wire but through the armored cable and other components not designed to carry current and that are exposed to touch. I watched a home owner, insisting his electrical system was safe, lick his knuckles and, standing on a damp basement floor, touch between a gas pipe and the outside of a "BX" or armored cable. He was shocked and knocked to the floor.
In sum, your TV will work, and it may be fine, in fact everything may be fine, until something isn't.
So the best approach is to install a grounded electrical circuit in the area(s) most needed, as soon as you can have that done. In the mean time if you decide to just buy and use a 3-prong to 2-prong adapter your TV will work but won't be properly protected.
On a polarized wall plug the wide blade is intended to connect to the neutral wire side of the electrical receptacle and the narrow blade of the wall plug connects to the hot wire. If you are connecting a polarized wall plug (having blades of two different sizes) you cannot plug it in incorrectly. But on a gang wall plug adapter such as the one shown below it is indeed possible to plug in a polarized wall plug in the wrong location or position on the adapter.
What happens if you plug in your wall plug into the wrong slots on the gang adapter at a receptacle?
Not much, other than wasting space at your wall plug gang adapter as long as you are using a modern wall plug whose blades are of different sizes. The wider wall plug blade will not fit into the more narrow wall plug slot and you'll be forced to insert the wall plug so that the hot and neutral wires to your device are connected properly.
However on some older devices that do not use polarized wall plugs, if you insert the wall plug as shown above you might cause reversed polarity that can damage some electrical devices such as some stereo equipment or even some coffee makers or other small appliances.
Watch out: Reversed polarity on an electrical outlet is dangerous. If you accidentally reverse these wires the device you plug in to the receptacle may "work" but it is unsafe and risks a short circuit, shock, or fire.
Details are at REVERSED POLARITY warning
Below we show the proper insertion position for a wall plug.
Continue reading at GROUND WIRE CONNECTIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see ELECTRICAL RECEPTACLE CONNECTION DETAILS - where to connect black, white, red, green, ground wires
Or see REVERSED POLARITY warning
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