Even though safety is second nature to electrical product designers, testing laboratories still see their share of products failing safety testing. This is often due to circumstances that could have been prevented through simple yet effective safety measures. This article will provide a technical overview of areas of concern in regards to product design, testing and documentation.
Off to a Great Start… Or Is It?
After the product design is complete and the entire organization is in anticipation of a new, hot product hitting the market, there remains a question of product safety approval process. Naturally, the designers considered safety features so the laboratory can run the sample through and issue the certificate in time for the official product launch. In the ideal world, that is.
First, the laboratory might have other products in queue, so waiting till the product is complete before contacting a test lab is not a good idea. The equipment needed for testing
might not be available right away. Second, even if technicians begin testing right away, it is possible that they find non-conformances that could delay the product from getting to market on time.
The Devil Is in the Design Details
It is always a good idea to review basic safety requirements applicable to the product in the works. This reduces the chance of overlooking a minor technical detail that may turn into a costly mistake if the design team needs to make physical changes to the product during the safety approval process. While the safety standards will have many different features, the tricky ones are listed below.
Ground Is King
The laboratory will examine the ground path according to the applicable standard and look at such factors as the capacity of current-carrying parts in the ground path, reliability and prevention against accidental loosening. Remember to use the wire of the correct color. Ground is sacred in many standards as it will shunt the fault current away from a user in the event of a fault.
Watch Your Spacings
Spacings are the separations between circuits at different voltage levels and different circuits and user-accessible parts. The laboratory will check the creepage and clearance as required by the standard (refer to the Reference Guide to Terms and Basic Requirements at the end of the article).
Proper layout of the printed circuit board (PCB) is critical. Today, automated programs allow a PCB designer to input design rules. A good practice is to define all nodes on the schematic by the circuit type (primary, Safety Extra Low Voltage (SELV), ground, etc.) and then set design rules based on the standard used to evaluate the product. Designers must be careful on the tolerance. A well-designed PCB will often fail because the design allowed for under etching, which can cause a failure by as small a distance as one micron. A tight tolerance on the low dimension is recommended.
Regarding spacings, the other area to watch is next to the enclosure. Engineers need to ensure that component devices, such as a switch mode power supply, are mounted on standoffs tall enough to ensure proper clearance. They need to watch for sneak paths from the PCB in contact with a plastic enclosure through a seam. This is a valid creepage path and products often fail because many designers ignore the seam. The last thing an engineer wants to do is reduce a PCB size.
Enclosures Keep Fires In and Fingers Out
The enclosure prevents users from coming into contact with hazardous electrical or mechanical parts. It also prevents an internal product fire from spreading to the surrounding environment. That is why enclosures are evaluated for proper materials, openings and strength and suitability for the purpose. The openings in an enclosure must be examined for both accessibility and their ability to contain fire, and polymeric materials of construction must be of the type with a suitable flame retardancy rating.
There are a few major traps to watch out for. Plastic has flame ratings according to its thickness. If the enclosure for the product is thinner than the approved thickness for a flame rating, this presents a problem.
Also, plastics are approved in various colors. Make sure the color of the enclosure, as selected typically by the marketing department, is covered under the plastics’ approvals.
Additionally, the lab will put the enclosure through a series of abuse tests to make sure it can withstand long-term usage. Engineers are well advised to review the standard against which the product will be evaluated for details on these mechanical tests.
The Fine Art of Specmanship
Specmanship is the practice of assigning ratings, not tolerances, to a product based on the worst-case tolerances of parts inside the end product. Following are a few examples.
The power supply is rated 100-240VAC but its specs say 86 to 264V. Often, a manufacturer will rate the product 86-264V. For the heating test, this means the laboratory will test at 90% of 86V (77.4V) and 106% of 264 (280V) There will most likely be failures. Additionally, many components in the device are rated only 250V and, strictly speaking, cannot be used in a product rated as high a 264V.
A component inside is good to a 5,000-meter altitude so the manufacturer rates the product as suitable for use at 5,000 meters. In the laboratory, the assumption is that the product is good to 2,000 meters. If the product is rated higher than 2,000 meters, the spacings values change dramatically, a consideration sometimes missed by designers.
Shopping for the Right Components
Designers must pay attention to safety-critical components. It is always better to choose pre-approved components. They will still need to be tested in the end product but the safety mark on them goes a long way. Custom made parts without approval could add weeks and extra cost to the safety approval process. The laboratory will have to evaluate the component and will need information that a designer may not
have and a vendor may not want to provide. While a custom part allows engineers to add some great features, they need to check early in the design stage if its use will have an impact on the safety process.
Avoid the Test Traps
Below is a set of traps that manufacturers can fall into and end up with test failures.
Hipot: Engineers need to check the trimming of through hole components on the power supply. They also must make sure the standoffs for the power supply are tall enough. It is a good idea to check any possible arc paths and be prepared to add insulators.
Leakage current caused by EMI fixes (see hipot as well): Designers must be careful about adding too many capacitors to pass EMC tests. They are the reason they have a leakage current. A proper balance is always required.
Ground continuity: There are two main traps. The first happens when carrying product ground through a PCB. If this is done, a 1000A test is conducted and most traces are not designed for this test. The other trap is painted metal surfaces. Designers need to either employ masking techniques or utilize paint biting washers for any screws.
Heating: A lack of airflow is always the culprit in heating test failures. Ensuring there is enough airflow will keep the components from exceeding the allowable temperature limits.
Batteries: Lithium batteries will need approval to IEC 62133. Even user-replaceable AA batteries will need this approval, so it pays off to select approved batteries.
No Requirement Is Too Minor
One of the most common issues that delays any laboratory from completing a product safety review is the lack of labels and a manual. Documentation and labeling are an integral part of the safety standards but they are often overlooked, with the design getting all the attention. Typical labeling and manual requirements for generic electronic equipment are listed below.
Safety-related documentation accompanying an electric product must contain the following items:
- Technical specifications, instructions for use, name and address of the manufacturer or supplier for technical assistance and an explanation of warning symbols;
- Equipment ratings such as supply voltage, frequency, power, current and environmental conditions under which the equipment can operate;
- Equipment installation instructions, including those required for assembly, mounting, protective earthing, ventilation and similar actions;
- Equipment operation instructions, such as use of operating controls, interconnection to accessories, replacement of consumables and cleaning;
- Equipment maintenance instructions, including identification of a specific battery type, fuse types and parts that need to be supplied by the manufacturer or his agent.
The equipment must feature the following markings:
- Manufacturer’s name, trademark and model number
- Equipment ratings (supply voltage, frequency, power/current and IP)
- Fuse marking (current rating and type) according to IEC 60127 (e.g., 250 V F 2.0 A)
- Equipment protected throughout by double or reinforced insulation must be marked as such.
- Warning markings
- Safety instructions must be available in the language of the country of installation.
Other markings, which may include:
- Short duty cycles and mains voltage adjustment
- Power outlets in the operator accessible area must be marked with the maximum load allowed, voltage and current
- Fusing, if operator replaceable, must be marked with the rated current, voltage and characteristic. If it is in the service area, then a cross reference is acceptable: F1, F2, etc., with a replacement information in the service instructions; e.g., = 250V 3A. The following fuse characteristic markings should be used:
- FF = very fast acting
- F = fast acting (fast blow)
- M = medium acting
- TT = time lag
- T = time lag (slow blow)
Reference Guide to Terms and Basic Requirements
This section contains the most commonly used terms and basic requirements for product safety as well as guidance to help designers implement them.
When it comes to hazardous voltages, follow these ranges: >30 V r.m.s. or >42.2 V peak or >60 V d.c., according to IEC 60950-1, and >33 V r.m.s. or >46.7 V peak or 70 V d.c. respectively, per IEC 61010-1.
Enclosure flame ratings
When selecting materials for enclosures, remember the following requirements:
- For movable equipment having a mass of < 18 kg, use 94V-1 or the test of clause A2;
- For movable equipment having a mass of > 18 kg and all stationary equipment, use 94-5V or the test of clause A1;
- For decorative parts outside the fire enclosure, 94-HB is acceptable.
Electric Shock Protection
Protection against electric shock relies on three measures: a connection to protective earth; double insulation between hazardous parts and the operator; and supply by SELV. However, this last measure is not defined in EN 61010-x.
An electric device can incorporate one or more of the following five insulation types:
- While insufficient for safe electrical separation, operational insulation is nevertheless needed for the correct operation of equipment and is applied between line and neutral and in SELV circuits. There is no thickness specified for operational insulation. Dielectric is dependent on the working voltage and spacings are the same as for basic insulation. Abnormal short circuits or dielectric testing is allowed to show compliance.
- Applied between primary circuits and earthed parts, basic insulation supplies a basic level of insulation against shock. There is no thickness specified for basic insulation. Dielectric between primary and earth is 1500Vrms or 2121dc for compliance with EN 60950. Dielectric between primary and earth is 1350Vrms or 1900Vdc for compliance with EN 61010.
- When combined with basic insulation, supplementary insulation creates a double insulation for protection against electric shock. Independent insulation is applied to basic insulation to ensure protection against electric shock if basic insulation fails. The specified thickness is 0.4 mm when it is combined with basic insulation. Transformers must have two thin layers where one layer passes dielectric for supplemental insulation, or three thin layers where any two pass the required dielectric.
- Supplementary insulation is applied between primary circuits and SELV. Dielectric is 1500Vrms or 2121Vdc for a working voltage of 250Vrms for compliance with EN60950. Dielectric is 1350Vrms or 1900Vdc for a working voltage of 300Vrms or dc for compliance with EN61010.
- Double insulation is comprised of basic and supplementary insulation. Its main application is between primary hazardous voltage and SELV circuits. Dielectric for 250Vrms working voltage between primary and SELV is 1500Vrms (basic) + 1500Vrms (supp.) = 3000Vrms or 4242Vdc for compliance with EN60950. Dielectric for 300Vrms or dc working voltage between primary and SELV is 2300Vrms or 3250V dc for compliance with EN 61010.
- Reinforced insulation is a single insulation that provides protection against electric shock equal to that of double insulation. It is usually a thin sheet material used in transformers and comprised of at least two layers, where either layer passes the dielectric for reinforced insulation. Its minimum thickness must be 0.4 mm and its main application is between hazardous voltage circuits and SELV circuits. Dielectric between primary hazardous voltages and SELV for a working voltage of 250Vrms is 3000Vrms or 4242Vdc. Dielectric for 300Vrms or dc working voltage between primary and SELV is 2300V rms or 3250 dc for compliance with EN61010.
Understanding the Insulation System
Keeping in mind that, for various types of insulation, designers need to build an insulation system in an electric device. Any insulation system must include the elements described below:
- Creepage distance over solid insulation. It is the shortest distance between two conductive parts, measured through air.
- Clearance through air. It is the shortest path between two conductive parts measured along the surface of the equipment.
- Solid insulation material. There are no requirements for the thickness of material but it has to undergo a dielectric strength test.
Varying Electrical Protection Based on Equipment Class
The type of insulation used to protect a device will depend on its classification. Protection against electric shock in Class I equipment is achieved with both the basic insulation and a reliable earth connection to the metal parts that may assume hazardous voltage if the basic insulation fails.
To render Class II equipment safe, designers do not need to have a connection to the earth, but the unearthed metal parts are isolated by reinforced insulation from hazardous voltages. Class II equipment must be marked with symbol 5172 from IEC Publication 417, and the mark must be visible on the outside of the product in the operator accessible area.
Class III equipment is the type of equipment where protection against electric shock relies upon a supply from SELV circuits and in which hazardous voltages are not generated.
Playing It Safe
When it comes to safety of electric devices, it pays to spend extra time on shock and burn protection. Consideration of the technical factors discussed above will ensure a great degree of confidence in the outcome of the regulatory compliance process, and significantly increase the odds of the product passing the tests and getting to market on time and on budget.
Steve Williams is Technical Manager at TUV Rheinland of North America. He has 25 years of experience in testing and managing regulatory compliance for electrical products, and can be reached at firstname.lastname@example.org.
Uwe Meyer is a Technical Manager for the Business Field, Electrical and Product Safety, at TUV Rheinland of North America. He has over 19 years of experience in regulatory product testing and certification, and can be reached at email@example.com.