Design for Approval - SafetyFollow article
Exploding phone chargers have been in the news recently. Is this a systemic problem or just a statistical probability?
Suddenly product safety is in the news. It has always been there, but it takes something which impacts “joe public” to raise awareness of a safety issue. Cheap phone chargers and more recently e-cigarette chargers are newsworthy when they explode and in some cases may have resulted in death or injury as for example reported by the BBC. Is this due to the sheer number of products or are we seeing an increase in poorly designed products?
It is worth exploring the recent history of safety regulation. 40 years ago switched mode power supplies were in their infancy, limited reliability and low voltage semiconductor switches made off-line switchers a non-starter. Instead a large, heavy and expensive transformer was used to reduce the mains voltage to a useful level to be further transformed and regulated by a low voltage switcher (this is still the best solution for certain applications). The principal safety components were the mains transformer, fuses and the associated cables and connectors. The transformer used enamelled wire, varnish and paper to insulate the primary and secondary circuits. Reliable, big, heavy, expensive and inefficient (don’t forget those iron and copper losses you learned about at school!). Progress in component technology, miniaturisation, and volume production methods has resulted in small, efficient and inexpensive products, but at what cost? A simple mains-to-USB power supply can be constructed using an IC and a small number of components by relatively inexperienced persons, but do they understand the hazards they are dealing with?
Safety is all about protecting users, operators, domestic animals(!) and property from harm in normal operation. This is extended in some technical standards to include “foreseeable and unforeseeable conditions”. The main hazards which must be considered for most consumer products are electric shock, energy, overheating, fire, chemical emissions and radiation (this is not an exhaustive list).
So are the problems we are repeatedly seeing, such as the examples of e-cigarette and ‘phone chargers that cause numerous fires and/or electric shocks a systemic problem or a statistical probability? If just one in a million chargers had a safety defect, whether by design or manufacture, then there are going to be hundreds or even thousands of potentially lethal chargers out there in the marketplace across the globe.
Can any system prevent power supply fires and electric shock events – absolutely? Probably not, without over-engineering and costly 100% post production testing. Cost is a big factor as well in safety. The manufacturer has to arrive at a trade-off between product safety level and product cost.
A system such as that used in North America which involves certification of products by a third party and ongoing compliance controlled by routine manufacturing inspections would undoubtedly improve the situation, but rogue suppliers will always find a way past these controls!
The European system of self-certifying products to meet the CE marking regulations has merit in many ways, but is simply very open to abuse by unscrupulous manufacturers. There is an EU-wide system for reporting non-compliant products which at least can warn EU member states when one country has discovered a problem. This is the RAPEX database and there is certainly no shortage of examples of product being found to be dangerous – look at any RAPEX report.
More on this in a later article on Market Surveillance, Enforcement and ongoing compliance.
The principal hazards which should be considered are as follows:
Electric Shock – probably the most obvious hazard in electrical equipment and the most mis-understood. Voltages in double-insulated secondary circuits of less than 42.4V peak or 60V dc are considered to be Safe Extra Low Voltages (SELV), and as such are safe under normal operating conditions, and under a single fault condition. Equipment and users must be protected against shock hazards by two levels of protection so that if one fails there is still one level of protection. These can take various forms: insulation, earthing, spacing (‘creepage’ and ‘clearance’), enclosures, protective components e.g. opto-couplers & transformers. Some forms of insulation don't provide two physical insulating barriers (such as with solid insulation), but an equivalent level of protection is achieved.
Energy related hazards – energy stored in capacitors even at SELV levels and high current supplies can result in burns, arcing and even ejection of molten material. A particular incident many years ago involved large 50V capacitors in a power supply. The engineer shorted a capacitor across his steel watchstrap which melted into his wrist!
Fire – Man’s worst enemy and best friend! Fire can result from overheating, component failure, breakdown of insulation and loose connections. Measures to prevent overheating are generally down to good design and use of suitable components. Flammability rated materials which limit or preferably prevent propagation of fire, nor eject molten materials should be used.
Heat related hazards – not fire but too hot to handle. Overheating can cause burns, failure of insulation and degradation of components. Once again, good design and use of suitably rated components will reduce this hazard.
Mechanical Hazards – not obvious in electrical equipment, but simple things like smooth edges on metal cabinets and preventing fingers from touching moving parts such as fans and motors. Even less obvious are hazards such as flying objects from broken glass envelopes e.g. CRTs (Cathode Ray Tubes), valves (still used in some applications such as high end hi-fi and guitar amplifiers) and lamps. These are somewhat old fashioned in our LED, LCD and digital electronics environment but still need to be taken into consideration.
Radiation – this covers the whole of the spectrum acoustic, radio frequency, light and ionising radiation. A high intensity noise, LED light or laser can be just as damaging as alpha radiation. Prevention is better than cure, intentional sources must be screened or user interlocks provided to prevent exposure. Warning marks are a must!
Chemical hazards – some materials and components used in the construction of a product may emit vapours or fumes which are hazardous to humans (and animals) under fault conditions. Avoid wherever possible. The regulations on use of some materials are aimed at reducing hazardous materials in the recycling supply chain. However, the same measures have resulted in less lead, cadmium, bromium, mercury, chromium IV in components (Ref. EU RoHS Directive)
The two levels of isolation can take a number of forms. The simplest is solid insulation which can be the insulation on a cable.
• Double Insulation (Basic + supplementary) • Reinforced Insulation • Solid Insulation • Protective separation (basic plus safety earth)
All of the above achieve an adequate level of safe separation from hazardous live circuits to humans. A single solid layer on say a transformer is permissible, but additional requirements about the quality of the material under different environmental conditions will apply.
The other important insulation system is the much misunderstood ‘creepage’ and ‘clearance’. Creepage is the distance along a surface between two conductors and clearance is the shortest distance between two conductors in air. See Fig 2. If you inspect a small power transformer you may note that the partition between the primary and secondary windings is more than a simple layer of plastic. It probably incorporates an air gap (>1mm) to increase the creepage distance.
Components incorporating isolation barriers are a good solution: relays, opto-couplers and transformers are available off the shelf. A word of advice: easiest solution is normally to use certified barrier components. They will have a current test house certification, e.g. VDE, UL, etc. which should ensure that you are using a component which will do the job! Buyer beware - don’t be caught out by the “complies with…” or ‘designed to meet…” statement it which is not the same as a certification. Test houses will not accept a safety component without performing further tests unless it has a valid certification.
A word about selecting “safety critical components”. Ensure that the component is suitable for the job. Is it operating within the specified voltage, current and frequency limits? If not, it is not fit for purpose and should be replaced. Some components have several further safety critical parameters which must be checked for the application (examples include flammability rating of polymeric materials, tracking index of PCB material and so on).
Probably the most useful form of protection, when all else fails the earthing, if available in the design/application,will protect the system and the user from harm, but only if it is connected to a reliable external earth. Connect all “dead” metalwork, user accessible parts such as handles and enclosures to earth. It will also help with EMC regulatory compliance Fuses and breakers These are important safety components when used in conjunction with other safety methods since they are usually there for the hopefully rare occurrence of a fault or a more likely potentially hazardous condition such as an overload. A standard fuse will take a finite time to break under overcurrent conditions, the higher the overcurrent, the faster the fuse blows. Fuses are often used in conjunction with varistors and clamp diodes. Don’t expect these to protect against a high power fault, they may catch fire or melt if not used with a fuse. Circuit breakers are usually more sophisticated electro-mechanical or even fully electronic devices with better defined performance parameters, and much higher prices! They are popular in industrial applications as they can simply be reset when the cause of a fault or abnormal condition has been removed.
User protection barriers and enclosures
The user barrier is there to prevent the user coming into contact with hazardous voltages, moving parts and hot components. This presents another set of problems, namely cooling and airflow. A vent large enough to allow adequate airflow may allow the user to touch a hazardous part of the product. The air vents may be shaped to prevent access or incorporate mesh, louvres or baffles. Vent openings can be problematical under a fault condition – a failing component near to a ventilation opening may emit molten particles and could set fire to flammable materials located near to the product’s vent opening.
The enclosure is more complex than it may seem at first sight. It may be constructed of a plastic material for cost, weight or design complexity. The flammability of the material becomes an issue. Only use materials which have a flammability rating e.g. UL 94 V0 supported by a competent test house report. Alternatively, the material can be tested as part of the safety assessment, but it may be too late to change at that stage if it fails!
When assessing the compliance of a product the first thing a product safety engineer will look is at the construction. The status, (but not the effectiveness!) of many of the safety systems discussed above will be evident by a simple inspection. Secure anchoring of cables, covers properly installed (and not hand removable if covering hazardous voltages), no sharp edges, no entry points for fingers and the all important finger test, does it get hot! Safety standards and manufacturer’s guidelines will give information on maximum allowable temperatures.Temperature dataloggers used with appropriate types of thermocouples are useful tools for the designer, especially on more complex products.
Back to our USB power supplies!
So why are there so many unsafe USB and similar chargers / mains adaptors on the market? Inspection of these products reveals that the safety barriers are inadequate or non-existent, cheap unsuitable components have been used, plastics catch fire and the enclosures not only catch fire, but fail to contain molten material when the inevitable happens and they explode! Basically one or more crucial safety principals is being broken!
Safety requirements are there for a good reason, learn and understand how to design a safe product and save the world from dangerous products!
If you have any questions or would like to find out how this applies to your product, please contact:
Product Approvals Ltd - www.productapprovals.co.uk
Look out for the next article in the series:
Design for Approval - Compliance
Design for Approval - Safety
Design for Approval - EMC Can't Hurt You Can It?