FEATURE TEST & MEASUREMENT
POWER SUPPLY SAFETY TEST A
By Matthew Emery, electrical safety supervisor at TÜV SÜD product service, a global product testing and certification organisation
key step in the test regime for power supplies is to review a product’s
intended environment, as this will affect the power components selected. This should include considerations relating to temperature and humidity; whether it will be used indoors or outdoors; and the industry and country in which it will operate. For example, military electrical supplies typically operate at 50, 60 or 400 Hz, while domestic supplies operate at 50 Hz. You must also review how the product
will be used and what the end-user will have access to. For example, is the product a mobile phone charger that must be safe for the user to touch, or is it a piece of laboratory equipment where the power output is not accessible? If it is an open-frame design, you must consider the appropriate fire, mechanical and electrical enclosures, to prevent access to the hazards within the product. For example, who will provide a rigid mechanical enclosure and prevent foreign objects from entering and starting a fire, as well as prevent flaming/molten parts escaping? Will this be provided within your design, or will it be expected that the end-user will provide this as part of the products integration?
SAFETY CRITICAL COMPONENTS Early on in the design phase, safety critical components must be identified. These are typically all components containing hazardous voltages, or ones that are used to protect the product. This includes fuses, mains switches and isolating transformers. It is also vital that each component has the appropriate certification, such as NRTL recognition for North America and IEC component approval for Europe and the rest of the world. Of course, it makes sense to ‘source smart’ by using dual certified components. Approval certificates and current
listings on certification body websites are the only acceptable evidence that components have the correct approvals. Do not just rely on manufacturer data sheets, and ensure that the most up to date standard(s) has been applied. Also, part number changes could mean that the certificate applies to another/old component. It is also very important to follow any conditions of use stipulated within the components’ pre-approval. It is becoming more commonplace to
18 MARCH 2019 | ELECTRICAL ENGINEERING
them. Typically you will find the following circuits: Hazardous - operate above the safe to touch limits; Earth: 1. Functional – provided for reasons of functionality or performance (e.g. for EMC), 2. Protective – are proven as performing a protective (safety) earth function; 3. Output - usually required to be isolated from the hazardous circuit and are typically safe to touch. You must also ensure that the main
use software within the power supply to improve performance and reliability. If software is required to prevent a hazardous situation then the software will become safety critical and you must ensure that you are meeting the requirements of the safety test standard. For example, some will have requirements within the test standard for the assessment of the software, while others will reference other standards such as IEC 61508-1.
IDENTIFICATION OF CIRCUITS It is very important to identify different circuit types and the appropriate levels of safety isolation required between
circuits are adequately separated, and your overall design should adhere to the following: Basic insulation - required between hazardous circuits and the protective earth. If the basic insulation fails, the fault current would flow to protective earth. Reinforced insulation - required between hazardous circuit and functional earth. This is required to provide ‘two degrees’ of safety insulation as the functional earth does not provide any degree of safety isolation.
FAULT CONDITIONS No matter the intended use of the power supply you are designing, all electrical safety standards will require a safety strategy regarding fire. This is usually tackled in three ways. Use a fire enclosure, which will prevent metallic foreign objects entering a power supply and stop molten material leaching from it if there is a fire. Use components and materials with the appropriate flammability ratings, to minimise fire propagation. Test simulation of single fault conditions will verify the fault tolerance of the design. Single fault conditions are any failure of an unapproved component, either open circuit or short circuit. For example, capacitors, resistors and diodes. Function insulation barriers are also typically short circuited. It is also important to consider abnormal conditions or misuse. Such as loss of forced air cooling or blocking of air vents. Use in unintended orientations must also not present a hazard to the user. The test regime for power supplies is complex and can seem daunting. However, the overriding approach should be to design for compliance and verify by test, and do not test for compliance and then redesign as this will delay time to market.
TÜV SÜD
tuvsud.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
