Behind the Label

People responsible for the safety of workers, or the public at large, have learned to "look for the label." National Electrical Code (NEC) sections 90-7 Examination of Equipment for Safety and 110-3 Examination, Identification, Installation and Use of Equipment encourage code-enforcement authorities to accept safety listing and labeling of electrical products, by qualified inspection and testing agencies, as evidence that the products are suitable for installation according to the Code, and as evidence that those products meet certain agreed-on minimum standards for safety. Many Code articles require that equipment be agency-listed for safety. For example, section 600-3 states that "electric signs and outline lighting ... shall be listed and installed in conformance with that listing, unless otherwise permitted by special permission."

Product safety standards are developed through a consensus process, similar to the process by which the NEC and other building codes are developed. Compliance with these product standards, and the manufacturer's submission of these products to an independent agency to verify that the products meet requirements of the standards, is usually voluntary; however, market forces usually necessitate listing. If several manufacturers offer listed products, competitors who do not are at a serious disadvantage. In some localities, product safety listings are required by law. Frequently, the NEC is incorporated by reference into the local or state law.

Code-enforcement officials, specifying engineers, and installers should be familiar with the ways in which an agency's label helps them to assure compliance with the Code. This article will attempt to show how the activities of independent safety evaluation agencies support these efforts.

This article will also describe how manufacturers of electrical equipment work with testing laboratories to become authorized to apply a listing mark to their equipment. We shall discuss some of the potential hazards that are addressed by product safety standards. By applying the standards, product safety engineers seek to minimize the risk of harm to people and property.

Product safety evaluation, listing, and labeling activities are conducted by a relatively small number of independent laboratories. The general approach described here is one that is followed, with minor detail differences, by all nationally recognized agencies.

Upon receiving a request to perform a safety evaluation of a product or family of products, the representative in charge of the investigation will request that certain specimens be provided for inspection and testing. It is necessary that the laboratory has a large enough number and variety of specimens that it is confident that the sample is representative of the entire product line to be listed.

Insufficient sampling could result in a product safety evaluation that is not thorough, compromising the integrity of the certification mark. Oversampling—thus overtesting—is likewise to be avoided because it causes delays in getting product to the marketplace, and it raises testing fees without a corresponding enhancement to product safety.

First, the laboratory will inspect the specimens for adherence to the standard's construction requirements. Among the construction requirements common to most standards are: integrity of frames and enclosures, accessibility and protection against shock, electrical insulation, spacings between live parts, separation of circuits, internal wiring, connections to the power supply, grounding and bonding, and components. The underlying principles of these examinations are discussed below. Frames and enclosures are examined to determine that they are strong enough to withstand the abuse the product is likely to encounter during installation and use (including effects of weather if used outdoors), and to determine that they are able to keep untrained persons from contacting live parts inside, to contain an internal electrical disturbance or fire, and to resist damage by fire.

Evaluating the electrical insulation material, checking the clearance (through-air spacing) and creepage (oversurface spacing), and examining separation of circuits whose voltage levels differ all have a common purpose: to reduce the chance of arcing between energized parts or from energized parts to ground.

Engineers examine the specimen's internal wiring to determine whether the conductors are adequate to carry the load currents without overheating, whether there is protection against physical damage—use of raceways or extrathickness insulation, bushings in sharp-edged openings, routing away from hot spots—as well as insulation voltage rating compared with the potential on the conductors, and proper splices and terminations.

Products to be connected to the electrical supply system generally must be installed in accordance with NEC, so in accordance with product standards, the specimen's supply connections are evaluated in order to determine the feasibility of installing the equipment per code. Cord-connected appliances are examined for the size and type of supply cord; correct attachment cap for the voltage, phase, and current; and strain relief on the cord. Permanently connected equipment is checked for the size of lugs compared to required supply wire gauge, ability of the enclosure to support conduits, integrity of knockouts, and accessibility of the connection point for inspection after the wiring is installed.

Standards usually—not always—state that products shall be fitted to facilitate connecting them to the source of supply in a way that is consistent with NEC (or National Fuel Gas Code, etc.).

Cord-connected appliances are examined for the proper type of cord, as specified in the standard, and correct attachment cap for the voltage, current, and number of phases. Cords usually need some method of holding them in place so they don't create fire or shock hazards when they are pulled on or similarly abused. Some standards have specific requirements for attachment caps, such as hospital grade, hazardous location, and horsepower rated.

With permanently connected equipment, standards require that the lugs are suitable to accept the code-dictated wire size and that the equipment enclosure can support conduits. The integrity of knockouts and the accessibility of connections for inspection once the field wiring has been connected must also be checked.

Standards covering gas-fired or oil-fired apparatus tend to be less specific in Code references than those covering electric equipment; nonetheless, certifiers need to make sure that the connection means provided would not cause the installer to violate the National Fuel Gas Code or other relevant installation guides.

Nearly all mains-operated electrical equipment—and even some battery-powered equipment—is Code required to be connected to safety ground (protective earth, in IEC terminology). Exactly what things are required to be grounded (earthed) is covered by Article 250 of NEC, and grounding requirements in the standards are intended to coordinate with Article 250.

There are several reasons equipment grounding is required. One is that if there is a fault (short-circuit or low-impedance path) within the equipment, the low-resistance ground return will cause the current drawn by the apparatus to rise quickly, and that will open the fuses or circuit breakers protecting the equipment and interrupt the flow of electricity in the circuit, thus reducing risk of a fire.

Another reason for having the equipment connected to ground is so the intentional path to ground will be of much lower resistance than the accidental path through a person's body. This is the way that grounding reduces the risk of electric shock. It is also the reason that standards for electrical equipment generally require that conductive parts of equipment that can be reached by people—including children, who have small-diameter fingers—are electrically connected to the equipment grounding connection point. This connection to ground is referred to as bonding.

Some standards require an installation/use manual, others require a manual that is "suitable" for the purpose (the certification body does not need to determine how suitable), and some standards dictate exact language to be used in a manual. Accompanying literature is evaluated to the extent the standard dictates; no more, no less.

Standards contain requirements that reflect how different devices are assembled, as well as how, where, and by whom they are used, maintained, and serviced. Because of that, product standards are unique. Many tests are common to a broad range of standards; this section describes some of these.

Testing the temperatures of components, wiring, and surfaces of products when continuously operated under normal conditions is fundamental to product safety evaluation. This test locates possible fire hazards by measuring the temperatures of surfaces that could contact combustible materials. The temperatures of components and electrical insulation within the product are also measured in order to minimize risks of fire or electric shock from thermally induced insulation failure. For example, electrical insulation can break down at high temperatures. Therefore, if a lighting fixture were wired using Type SSF fixture wire, rated 150° C, the laboratory would apply temperature sensors to a number of locations on the product in order to determine whether or not the wires exceeded their rated operating temperature.

An appliance or switching device that may become overloaded in operation is subjected to overload tests. A wide variety of overload tests are described in safety standards, simply because different types of equipment are subject to different conditions of overloading. Ac motor starters, for example, are tested by making and breaking six times rated current, 50 times in succession. The starters must complete the cycle without severe contact damage or mechanical failure. General purpose transformers are made to deliver twice their rated current for 20–30 minutes, without exceeding their windings' rated temperature.

Commonly, standards specify a minimum fire resistance of nonmetallic materials in particular uses. The tests most commonly used for investigating flammability include small-scale Bunsen burner tests of plastic materials, vertical burning tests of insulated wires and cables, hot-wire ignition tests, and flame-spread tests conducted in a standard Steiner tunnel. The Steiner tunnel test is used to establish comparative surface burning rates of different materials, when exposed to a standard test flame. The tunnel is used to establish comparative flame-spread ratings of self-supporting materials, wires, and cables.

Apparatus intended for use outdoors is commonly subjected to a simulated rainstorm of at least one hour's duration (longer, for some products) to determine that the apparatus will prevent rain or snow from creating hazards by reaching live electrical parts, pooling in enclosures, or by extinguishing the pilot or main flames of fuel-burning equipment. Interior parts of equipment are visually examined to see if they have become wet as a result of the test. Electrical insulation resistance is measured as a part of this test as well.

The fault currents that flow in short-circuit situations may be destructive. The heating effect of high fault currents can start fires. In addition, fault currents create strong magnetic fields. Large magnetic forces are sometimes capable of moving cables and even bus bars. For these reasons, many products are required to undergo short-circuit current tests. The nature of these tests can vary widely, depending on the design and probable uses of the product.

Switching devices and circuit protective devices such as fuses and circuit breakers must interrupt specified fault currents.

Materials used only to conduct electricity, such as cables and busways, must withstand fault currents. Appliances, which are generally applied on branch circuits, and which have substantial internal impedance, must undergo limited short-circuit tests in accordance with the comparatively low fault currents the equipment is likely to encounter. Generally speaking, damage to apparatus during fault-current testing is allowable, as long as the apparatus under test contains any fire that results from the fault current and maintains its electrical insulation system largely intact, so as to preclude risk of shock.

Among the most fundamental tests of electrical apparatus are insulation resistance and dielectric strength (commonly known as hi pot) tests, which test the dc resistance of insulation, and the insulating system's ability to resist electrical breakdown, respectively. Insulation resistance is measured on a megohmmeter. Dielectric strength of insulating systems is tested by applying a high test voltage—generally 1000 V or more between live parts and nonenergized metal parts of the apparatus. Virtually all electrical equipment must be tested against dielectric breakdown in order to qualify for listing.

As grounding (and bonding of internal parts to ground) is so important for safe use of electrical apparatus, there are tests for impedance of grounding paths. These generally involve passing a current of at least 25 A through the grounding circuit and measuring the resultant voltage drop. For example, it may be necessary for the laboratory to test a control panel having a removable cover, to determine that the cover cannot become "hot" in case of an internal fault. In some cases, it is required only that the laboratory establish grounding continuity by test, without measuring the actual impedance.

Electrical construction materials, such as conduit and heavy-duty cord, which may be exposed to crushing action where used in their intended or expected applications, are tested for their ability to withstand standardized crushing forces without damage. As an example, elevator traveling cables are tested by placing a section of cable between two steel plates, then applying an increasing force, up to 2000 lb, to the plates. If the cable's conductors short to each other or to either steel plate, the cable is not acceptable.

Cord-connected appliances are subject to accidental pulling on their supply cords, which can result in damaged cord insulation or the bare end of the cord coming into contact with metal that is not intended to be energized. A strain relief test establishes that the cord will not be damaged or displaced under a certain amount of pulling force. Typically, the test technician hangs a 35-lb weight from the cord for one minute, then examines the section of cord inside the appliance for evidence of damaged insulation or separation of the cord from its connection to the appliance.

Cord-connected appliances, except for double-insulated ones, are often required to be tested for leakage current. Leakage current is current that passes from accessible parts of the appliance to ground when the equipment is operating. Leakage currents are direct indications of electric shock risks. Most standards require that leakage currents not exceed 0.5 or 0.75 mA through a circuit designed to approximate the impedance of a human body. In most of the portable-appliance standards, this circuit consists of a 1500 resistor in parallel with a 0.15 µF capacitor.

To deal with special circumstances, some standards have lower permitted leakage currents. Patient-care medical equipment, for example, permits a leakage current of only 0.1 mA. This lower allowable leakage current is due to patients being less able to withstand electric shock than the general population. Also, the resistance of the test circuit is 1000 for these tests, rather than 1500 .

Motor-operated appliances are tested to determine that they will start, and run, where protected by the specified overcurrent protective devices, without tripping those protective devices.

Nearly all equipment that consumes electric energy has some reasonably foreseeable abnormal modes of operation. The safety standards require listing agencies to conduct abnormal operation tests based on these conditions.

As one example, an electric space heating apparatus is tested with the warm air outlet blocked by an inch-thick felt pad covered with cheesecloth to simulate upholstered furniture pushed against the heater. The heater under test is unacceptable if the pad or cheesecloth glows or flames in the test. Similarly, electronic equipment containing ventilation fans is operated with the ventilation means disabled, while component temperatures are measured, as described earlier in this article, to ascertain that increased risks of fire or shock do not result from loss of the cooling system.

After a thorough examination of the sample, the manufacturer is advised whether or not the product meets requirements. If it does, the product is deemed suitable for listing and to bear the listing mark. In the event there are deviations, these are communicated to the manufacturer, inviting a response. Usually, after some corrective changes to the product, it is determined that the product does meet requirements, and a listing report is issued. Occasionally, a manufacturer is unable or unwilling to make the changes needed to bring the product into compliance. The result is that the product will not be eligible for listing or labeling.