What are all those markings on my power supply?
Did you ever wonder what those odd markings are on the “dataplate” of that power supply (or appliance or electronic gadget). They all have a specific meaning to those who know (or care…). Many confirm that the product meets various product safety requirements and others are often to confirm meeting electromagnetic compatibility requirements. The markings can be domestic and/or international in nature.
If you’ve purchased a computer you may have noticed that the information displayed on a label on it (or on its power supply, if that’s separate) includes not only the product’s electrical ratings (voltage and current) but also some markings and symbols that may not be immediately identifiable. I’m not going to try to identify each of them, but I thought it would be informative to discuss them in general. Some apply domestically and others may apply for products intended for sale or use in other countries.
The National electrical Code (NEC) specifies that “appliances” which get power from the mains (in our case, the electrical outlet in the wall) must be “listed” by a Nationally Recognized Testing Laboratory (NRTL). “Listing” means that the NRTL has tested the product and found it to comply with the appropriate American Nation Standards Institute (ANSI) standard for product safety. The product is added to the laboratory’s “list” of products found to comply.
Over a hundred years ago, the insurance companies were taking a beating as a result of products that were damaging property or injuring people in fires. The insurance industry started laboratories to test products to try to reduce the likelihood that their use would result in a claim. Eventually the idea caught on as being necessary to the safety of the public. The initial concentration on fires led to the development of product safety standards for many types of products. Most product safety standards are developed and maintained in the US by Underwriters Laboratories (UL), now a NRTL and one of the first organizations testing products for safety for the insurance companies. Their UL safety standards become established as ANSI standards to which all NTRL’s evaluate products. The standards are prepared and maintained by a combination of UL’s own staff, representatives of the industry for which the standard is created, and other experts in the field of product safety. The standards have to be effective, but not so onerous that they make products impossible to design or economically unfeasible to produce. The standards typically address a specific class of products and they include both design criteria to be met and tests to be performed.
The “Listing” process typically involves providing NRTL with samples of the product and various critical components and information about the components used. The information is reviewed to determine the “pedigree” of the components being used. For instance, if a product employs a dc power supply, if that power supply has already been evaluated as a component and has suitable “pedigree” (a NRTL component certification or “recognition” mark) then it only has to be evaluated for performance in the new application rather than also being evaluated for its own construction. Some components are relied upon just for safety (for instance, thermal overload limit devices). Those are given special attention.
The products are subjected to the required testing, some of which can be very destructive. Generally, the criterion for passing the test is that no safety hazard be created. The product doesn’t have to perform its intended function at the end of the test. The tests are intended to confirm the design factors that affect safety, such as strength of enclosure materials, their resistance to the spread of fire, stalling or overloading motors, overloading transformers – basically doing anything that could result from a “single fault” condition. But they also test for conditions that occur under normal operation, such as the dielectric strength of insulation (it must withstand the expected high peak transients to which the product may be exposed), or the “leakage current” to which operators may be exposed when touching an ungrounded product, or the temperature of insulation on components that heat during normal operation (windings of motors or transformers).
The origin of some tests is a little obscure. There is a test which requires certain types of motors to endure an 18 day “locked rotor” condition. The motor’s rotor is prevented from turning, thermocouples are attached to the winding insulation (to measure the temperature), and power is applied for 18 days. During that time, temperatures must not exceed certain values (based on the type of insulation) and no other hazards must arise. Where did the 18 day requirement come from?
Long ago, refrigerator compressor motors were literally burning up as a result of bearing failures. The standards writers somewhat arbitrarily considered that this might happen on the first day of a long weekend preceding a two week vacation, so the motor would have a long time to “cook” unattended, hence the 18 day duration requirement.
The NRTL, in cooperation with the product’s submitter, also prepares a document describing the product’s significant components and features which, if changed, would require further review and testing for continued conformance with the standard. That is in preparation for factory inspections by the NRTL to confirm that the product’s manufacturing does not change during the life of its “listing”.
The result of this whole process is a product that is demonstrably safe. But it’s a lengthy and expensive process. Not all manufacturers choose to do this. While it is in their interest to market a product that is safe, it’s not a requirement that they have the product “listed” for safety. It is the user’s duty to connect only “listed” products to the electrical mains. You can often find a less expensive product to perform the same function but it may not have a safety pedigree. That doesn’t mean it won’t perform safely for its useful lifetime, but if it does fail in an unsafe manner – such as burning your house down – insurers may balk at claims since it didn’t meet the NEC.
Another variation on this applies for products intended for use in Europe. There, manufacturer’s can “self-certify” that products meet the pertinent European safety standards. The “CE Marking” on products is a manufacturer’s declaration that the product meets all pertinent European standards. There is no requirement for an independent evaluation and confirmation. Sometimes in the US you may see a product that has only the CE Marking on it. It will likely perform safely, but that is not as well assured as those meeting domestic standards and the associated listing processes.
Most people don’t understand the time, effort, and expense manufacturer’s have to invest to make sure their products are “reasonably” safe. Nothing in life is absolute (well, death and taxes, maybe) and that’s the case with safety as well. But responsible manufacturers go to significant lengths to reduce the likelihood that their products will create safety hazards. Those markings are the evidence that they have done so.
P.S. You may already be asking yourself, “Well, what if I want to make my own power supply? I’m not going to get it ‘Listed’. How does that square with the NEC?” My answer is “I don’t know.” I’m not aware of an exemption in the NEC for such situations, but I’m not an expert on the NEC (although I used to work with a guy who helped write it). I have constructed my own power supply, but I applied everything I knew about making a safe one. But that has now been superseded by a Listed unit. I personally look for a NRTL mark on equipment I buy to plug into my home’s electrical system.
Did you ever wonder what those odd markings are on the “dataplate” of that power supply (or appliance or electronic gadget). They all have a specific meaning to those who know (or care…). Many confirm that the product meets various product safety requirements and others are often to confirm meeting electromagnetic compatibility requirements. The markings can be domestic and/or international in nature.
If you’ve purchased a computer you may have noticed that the information displayed on a label on it (or on its power supply, if that’s separate) includes not only the product’s electrical ratings (voltage and current) but also some markings and symbols that may not be immediately identifiable. I’m not going to try to identify each of them, but I thought it would be informative to discuss them in general. Some apply domestically and others may apply for products intended for sale or use in other countries.
The National electrical Code (NEC) specifies that “appliances” which get power from the mains (in our case, the electrical outlet in the wall) must be “listed” by a Nationally Recognized Testing Laboratory (NRTL). “Listing” means that the NRTL has tested the product and found it to comply with the appropriate American Nation Standards Institute (ANSI) standard for product safety. The product is added to the laboratory’s “list” of products found to comply.
Over a hundred years ago, the insurance companies were taking a beating as a result of products that were damaging property or injuring people in fires. The insurance industry started laboratories to test products to try to reduce the likelihood that their use would result in a claim. Eventually the idea caught on as being necessary to the safety of the public. The initial concentration on fires led to the development of product safety standards for many types of products. Most product safety standards are developed and maintained in the US by Underwriters Laboratories (UL), now a NRTL and one of the first organizations testing products for safety for the insurance companies. Their UL safety standards become established as ANSI standards to which all NTRL’s evaluate products. The standards are prepared and maintained by a combination of UL’s own staff, representatives of the industry for which the standard is created, and other experts in the field of product safety. The standards have to be effective, but not so onerous that they make products impossible to design or economically unfeasible to produce. The standards typically address a specific class of products and they include both design criteria to be met and tests to be performed.
The “Listing” process typically involves providing NRTL with samples of the product and various critical components and information about the components used. The information is reviewed to determine the “pedigree” of the components being used. For instance, if a product employs a dc power supply, if that power supply has already been evaluated as a component and has suitable “pedigree” (a NRTL component certification or “recognition” mark) then it only has to be evaluated for performance in the new application rather than also being evaluated for its own construction. Some components are relied upon just for safety (for instance, thermal overload limit devices). Those are given special attention.
The products are subjected to the required testing, some of which can be very destructive. Generally, the criterion for passing the test is that no safety hazard be created. The product doesn’t have to perform its intended function at the end of the test. The tests are intended to confirm the design factors that affect safety, such as strength of enclosure materials, their resistance to the spread of fire, stalling or overloading motors, overloading transformers – basically doing anything that could result from a “single fault” condition. But they also test for conditions that occur under normal operation, such as the dielectric strength of insulation (it must withstand the expected high peak transients to which the product may be exposed), or the “leakage current” to which operators may be exposed when touching an ungrounded product, or the temperature of insulation on components that heat during normal operation (windings of motors or transformers).
The origin of some tests is a little obscure. There is a test which requires certain types of motors to endure an 18 day “locked rotor” condition. The motor’s rotor is prevented from turning, thermocouples are attached to the winding insulation (to measure the temperature), and power is applied for 18 days. During that time, temperatures must not exceed certain values (based on the type of insulation) and no other hazards must arise. Where did the 18 day requirement come from?
Long ago, refrigerator compressor motors were literally burning up as a result of bearing failures. The standards writers somewhat arbitrarily considered that this might happen on the first day of a long weekend preceding a two week vacation, so the motor would have a long time to “cook” unattended, hence the 18 day duration requirement.
The NRTL, in cooperation with the product’s submitter, also prepares a document describing the product’s significant components and features which, if changed, would require further review and testing for continued conformance with the standard. That is in preparation for factory inspections by the NRTL to confirm that the product’s manufacturing does not change during the life of its “listing”.
The result of this whole process is a product that is demonstrably safe. But it’s a lengthy and expensive process. Not all manufacturers choose to do this. While it is in their interest to market a product that is safe, it’s not a requirement that they have the product “listed” for safety. It is the user’s duty to connect only “listed” products to the electrical mains. You can often find a less expensive product to perform the same function but it may not have a safety pedigree. That doesn’t mean it won’t perform safely for its useful lifetime, but if it does fail in an unsafe manner – such as burning your house down – insurers may balk at claims since it didn’t meet the NEC.
Another variation on this applies for products intended for use in Europe. There, manufacturer’s can “self-certify” that products meet the pertinent European safety standards. The “CE Marking” on products is a manufacturer’s declaration that the product meets all pertinent European standards. There is no requirement for an independent evaluation and confirmation. Sometimes in the US you may see a product that has only the CE Marking on it. It will likely perform safely, but that is not as well assured as those meeting domestic standards and the associated listing processes.
Most people don’t understand the time, effort, and expense manufacturer’s have to invest to make sure their products are “reasonably” safe. Nothing in life is absolute (well, death and taxes, maybe) and that’s the case with safety as well. But responsible manufacturers go to significant lengths to reduce the likelihood that their products will create safety hazards. Those markings are the evidence that they have done so.
P.S. You may already be asking yourself, “Well, what if I want to make my own power supply? I’m not going to get it ‘Listed’. How does that square with the NEC?” My answer is “I don’t know.” I’m not aware of an exemption in the NEC for such situations, but I’m not an expert on the NEC (although I used to work with a guy who helped write it). I have constructed my own power supply, but I applied everything I knew about making a safe one. But that has now been superseded by a Listed unit. I personally look for a NRTL mark on equipment I buy to plug into my home’s electrical system.