You can understand why manufacturers would want to test EMC performance first, as it is a regulatory issue; by submitting a pristine product for testing, the best possible results can be expected. But this an approach could, ultimately, see the product fail and, in turn, create an association with unreliable products that do not perform as promised.
Electromagnetic fields are as much a part of the environment which interacts with a product as are temperature, shock, vibration, dust and water ingress. Over a product’s lifetime, environmental factors can have a significant influence on EMC performance, potentially jeopardising the product’s integrity later in its life. It therefore makes sense for manufacturers to look at integrating EMC and environmental compliance throughout the product development lifecycle.
Formal test requirements?
While there are no regulatory requirements for a product to be aged artificially prior to EMC testing, it is necessary to demonstrate that EMC related failures will not degrade a product’s functional safety. Indeed, the EMC standard general methodology for the achievement of functional safety – IEC/TS 61000-1-2 – stipulates the need to consider ageing.
It notes that a product’s immunity may be affected by environmental parameters and proposes that an aspect of the testing is to quantify the impact of factors such as stress, ageing and foreseeable misuse on the electromagnetic characteristics of the equipment or system.
After the equipment has passed initial EMC testing, ageing testing should then be performed, if it can be reasonably and foreseeably expected that the electromagnetic characteristics will change during the equipment’s life time. As an example, this testing should include the evaluation of the reduction in effectiveness of electromagnetic mitigation measures associated with the equipment or product due to corrosion or mechanical movement during the anticipated lifetime of the system. As appropriate, during or after these stress/ageing tests, the electromagnetic characteristics should be measured to determine whether they have been degraded excessively.
For example, ageing needs to be taken into consideration when it comes to medical devices. Ageing can be caused by condensation, liquid spillages and sprays (including human body fluids), mould growth, particulate matter, dust, cleaning and maintenance – as well as the usual wear and tear caused by multiple operations of controls, opening and closing of doors and access panels and temperature cycling.
The Medical Devices EMC standard – IEC 60601-1-2 – proposes tests that simulate the reasonably foreseeable operational life of a medical device. Accelerated life tests, as mentioned above, are recommended to help verify that the design is adequate to maintain safety over the expected service life. Where such tests are performed, it is also suggested that the EMC characteristics of the medical device are assessed before and after the tests, to verify they have not become degraded to the point where risks have risen to unacceptable levels.
Testing guidance
The ideal approach to determine whether a product can maintain electromagnetic compliance throughout its life would be that suggested in the functional safety specifications. This is to apply environmental conditions which simulate the proposed life, then perform EMC testing. However, the validity of using accelerated environmental tests to simulate the full life of a product remains the subject of debate, since unexpected failure modes can occur. Even if accelerated tests could be relied upon to produce only ‘real’ failure mechanisms, there would be difficulty in determining exactly how much acceleration had been applied and whether the end of life point had been exceeded accidently.
|
Environmental testing covers a wide range of issues; even the packaging in which products are despatched.An alternative approach
The electromagnetic performance of a product is characterised by the design and layout of the product’s circuits, along with any protective measures put in place to shield, contain or reduce electromagnetic effects. Measures taken to improve performance – such as bonding, shielding and the addition of filters – can all be degraded by environmental conditions.
Due to the difficulties of simultaneous application of EMC and environmental tests, combined testing tends not to be performed, however simultaneous application of environmental and EMC conditions can be analysed, as can the effect of through life degradation on EMC.
Whilst the validity of accelerated environmental tests is uncertain, the range of effects caused by environmental exposure is generally well understood. An analysis of a product’s predicted environmental lifetime exposure would help to identify the environmental threats – their duration and severity. The suitability of measures taken to provide EMC can then be assessed against the environmental threats, with appropriate environmental testing performed to support the assessment.
To ensure continued compliance, however, the conditions taken into consideration should not only be limited to the climatic and mechanical environments, but also to the wear that may result from operational or maintenance conditions. For example, even if the product is mechanically static, the repeated activity of opening for inspection could result in wear or fatigue of measures put in place for bonding or shielding, especially if the materials used also degrade with age.
Manufacturers that want to ensure their products are of a high quality could learn from those that supply the defence and aerospace markets, where there is often a contractual requirement for EMC integrity over a defined lifespan.
In these markets, products are tested against a defined build standard and that standard is maintained through the quality management process. A well-defined maintenance schedule, including visual and insulation resistance checks, ensures there is regular surveillance of components that will affect the EMC performance of a product.
Don’t isolate
It is clear that environmental and EMC compliance should not be thought of as isolated events. In many cases, lifetime compliance for quality products will be best achieved if environmental and EMC requirements are considered together from the outset.
However, it seems that many manufacturers still prefer to rely on the warranties they provide, rather than addressing this issue at the design stage. By eliminating the inconvenience of breakdowns, they can use extended warranty periods to gain a competitive advantage with their products, whilst adding value to their brand by assuring consumers of continued EMC compliance.
By taking this converged EMC and environmental compliance approach, designers and manufacturers could measurably increase the perceived brand value of their products by ensuring that EMC integrity lasts a lifetime.
While it may cost a little more to instigate from the start of the development lifecycle, the benefits of enhancing a brand’s reputation for reliability significantly outweighs this and may also minimise the risk of legal action by customers.
Author profile:
Jean-Louis Evans is managing director of TÜV SÜD Product Service.