What tamper-resistant products are UL-listed and what test methods are used?

Tamper-resistant receptacles are the only devices that are specifically tested by UL for safety and reliability in respect to compliance with NEC Sec. 517-18(c), including the exception. Although the exception, added in the 1996 code cycle, states that "a receptacle cover shall be permitted to be used in place of a tamper-resistant receptacle provided such cover, by its construction, limits improper access to the energized parts of the receptacle," a receptacle cover is not UL-listed for safety/tamper resistance. Products such as plastic plug-in inserts and wall plates with contact shutters also are available for tamper resistance; however, none of these products is UL-listed for safety/tamper resistance. Plastic plug-in inserts are listed as insulating devices aimed at energy conservation only; wall plates with contact shutters are UL-listed strictly as wall plates.

The NEC requires that tamper-resistant receptacles be installed in pediatric areas; however there are many other settings where children may be at risk. These include day care centers, children's play areas, elementary and nursery schools, doctor's offices and lobbies, retail establishments featuring children's attire or toys, and your home.

Background information

The concept of tamper-resistant receptacles is not a recent development; these devices have been a standard product offering of wiring device manufacturers for the better part of 10 years. The devices are designed to protect individuals (primarily children) from accidental shock resulting from the insertion of foreign objects.

Several code cycles ago, the need for tamper-resistant receptacles emerged as an NEC issue. The NFPA, through specific code-making panels, was concerned about the well being of children in pediatric units of hospitals and the inclination of a child to unknowingly endanger him- or herself by inserting keys, pins, paper clips, or other items into unprotected receptacles. As a result, the use of these receptacles has been a requirement of the NEC since 1981, under the title "tamper proof," with a qualifying definition. In 1984, the NEC expanded the application of these devices to psychiatric wards as well. It wasn't until the 1990 code cycle that the NFPA changed the classification to "tamper resistant."

Once the Code requirement was adopted, it was difficult to determine feasible requirements needed to prevent adult access to receptacles. Keeping children out of receptacles was one thing; adults presented another problem. Not until the NEC dropped the psychiatric ward requirements in the 1993 Code [See 517-18(c)] were UL and wiring device manufacturers able to identify realistic requirements and develop the test procedures necessary to list tamper-resistant receptacles.

How tamper resistance works

There are several methods to achieve tamper-resistance operation, the most common being the use of a spring-loaded shutter mechanism. Inside the face of the receptacle is a spring-loaded thermoplastic safety shutter. Under normal, unused conditions, the shutters are closed, and both contact openings are covered.

Upon insertion of a grounded or ungrounded plug, the blades of the plug simultaneously compress the shutters against the spring. The simultaneous force allows the shutters to slide and open up access to the receptacle contacts. The plug becomes fully inserted and securely fits into the receptacle. When the plug is removed, the shutters instantly close, covering the contact openings.

When a foreign object, such as a paper clip or small bladed screwdriver, is inserted into only one of the openings, the safety shutter will not allow access to the live contact.

For these receptacles, the safety mechanism covers only the line and load contacts, and not the ground; this is to allow application versatility. Having a safety mechanism over the ground would require a ground pin on the inserted plug and limit the type of equipment that could be used with these devices. Because they accept a two-pronged plug, these receptacles can be used with standard household and office appliances like light fixtures, clocks, and radios.

New UL testing

In April 1994, specific UL testing requirements and product markings became a requirement for a tamper-resistant receptacle to receive a UL listing. The UL testing measures the performance, against minimum standards, of the receptacle's internal mechanism that prevents the insertion of inappropriate objects. Prior to this, there were no specific tests for tamper resistance; all that was available was the UL 498 requirements for a standard receptacle.

The new testing procedure requires a probe test, impact test, mechanical test, and dielectric test, details of which follow.

Probe test. This test tries to defeat the safety mechanism by inserting a single weighted (8 oz.), .031-in. die probe into the blade opening in the face of the receptacle. The probe is then manipulated in an effort to defeat the system and make contact with the receptacle's contacts.

Impact test. This physical abuse test uses a 1.18 -lb, 2-in. die steel ball, which is swung on a pendulum, to make direct contact with the face of a mounted tamper-resistant device, with an impact force 5 ft-lbs. After this test, the probe test is repeated.

Endurance test. This is a cycle test in which a plug is inserted and removed from the device 5000 times, after which the probe test is again repeated.

Dielectric test. This test is performed after the impact and endurance tests. It tests for internal shorting, with twice the device's voltage rating plus 1000V applied for 60 sec.

After passing all of the above performance tests, all tamper-resistant receptacles must have either the words "tamper resistant" or the letters "TR" (minimum 3/16 in. high) on the device as a clear indication that this is a tamper-resistant receptacle.