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The shutter mechanism, slot by slot
Behind the face of a TR receptacle sits a pair of small plastic shutters, one covering each vertical slot, each held closed by its own light spring. The geometry is the whole trick: a shutter only slides open when something pushes on it and its partner at the same instant with roughly equal force. A two-prong plug does exactly that, both blades contacting both shutters together, so the pair retracts and the prongs seat normally.
Push on one slot alone (a paperclip, a nail, a hairpin) and that single shutter cannot open, because it is mechanically linked to require simultaneous pressure on both. The object simply stops at the closed shutter, never reaching the live brass contact behind it. This is a purely mechanical interlock with no electronics, no battery, and nothing to wear out on a normal service life.
Because the shutters add a little resistance, a TR outlet feels slightly stiffer on first insertion than the worn-in non-TR outlets people are used to. That stiffness is the mechanism doing its job, not a defect. Roughly seven children a day were treated for outlet-related shock and burn injuries before TR receptacles became standard, which is the statistic that drove the code change.
Why code requires them nearly everywhere
The 2008 National Electrical Code (NEC 406.12, originally 406.11) made tamper-resistant receptacles mandatory in dwelling units: living rooms, bedrooms, kitchens, bathrooms, hallways, garages, basements, and outdoors. Later cycles widened the reach to more locations and to certain commercial spaces such as childcare facilities and waiting rooms. When you replace a worn outlet in a home built to a recent code, the replacement must be TR.
A few narrow exceptions exist, mostly receptacles that are out of a child's reach or dedicated to a single fixed appliance: outlets more than 5.5 feet above the floor, those behind permanently installed appliances like a refrigerator, and some receptacles that are part of a luminaire or in an inaccessible location. Local amendments vary, so the exact list for your jurisdiction is something a licensed electrician confirms against the code edition your town has adopted.
The practical takeaway for homeowners: if you are buying replacement receptacles at a store, the standard stock is already TR, and choosing a non-TR part for a covered location would put the work out of code. There is no meaningful price penalty, since TR receptacles cost about the same as the old style, and the cost to install or replace an outlet is the same whether or not it is tamper-resistant.
The correct insertion technique
The number one complaint about TR outlets is "my plug won't go in." Almost always the cause is uneven pressure: the plug enters at a slight angle, one blade hits its shutter a fraction before the other, and the interlock refuses to open. The fix is to align the plug square to the face and push straight in with firm, even pressure on both blades together.
If a plug still resists, pull it back, make sure it is right-side up (a polarized plug has one wider blade that only fits one way), and try again perpendicular to the wall rather than rocking it side to side. A gentle, deliberate straight push beats a forceful wiggle, which tends to load one shutter and not the other. New shutters loosen slightly with the first few uses.
Genuinely defective shutters do happen, especially on bargain receptacles where the spring or plastic is out of spec. If a brand-new outlet will not accept a known-good plug after a careful straight insertion, the receptacle itself is suspect and worth swapping rather than forcing, since a cracked shutter can leave the slot permanently jammed. A stiff plug is different from an outlet that has stopped working, which points to wiring rather than the shutters.
TR receptacles vs plastic safety caps
The old approach (the little plastic plug-in caps) is now considered the weaker of the two by a wide margin, and safety groups recommend TR receptacles over caps. Caps are a separate object that has to be present in every slot to do anything, and the moment one is removed to use the outlet, that outlet is wide open until someone remembers to reinsert it. TR protection, by contrast, is permanent and automatic: the shutters re-close the instant a plug is pulled.
Caps also create their own hazard. Many are sized so a small child can pull them out, and once loose they become a choking object. Studies of childproofing have repeatedly found that toddlers can remove common caps in seconds. A TR receptacle has nothing to remove, nothing to lose, and nothing for a child to put in their mouth.
There is no conflict between the two in practice: a home with TR outlets does not need caps, and a home with caps still has unprotected outlets every time a cap is out. Where caps still have a niche is as a stopgap in an older home with non-TR receptacles, until those outlets are replaced with TR parts.
How to identify a TR outlet
Every tamper-resistant receptacle carries the letters TR stamped or molded into the face, usually between or beside the slots near the center screw. The marking is small and easy to miss, so look closely in good light. If you see TR, the shutters are present, whether or not you can feel them.
A second tell is visual: peer into the slots and you will see the small plastic shutters sitting just inside, rather than the open path to bare brass that an old receptacle shows. Many TR outlets also combine other functions, so you may see TR alongside GFCI or WR (weather-resistant) markings on a single device, common for outdoor and bathroom locations. If you are choosing protection for one of those spots, our guide to a GFCI breaker versus a GFCI outlet covers where each belongs.
If an outlet in a recently built or recently renovated home has no TR marking and an open slot path, it may be a non-compliant replacement someone installed using old stock. That is a low-cost thing to correct, and a good item to flag during any other electrical work.
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