Test & Measurement - Production line testing: What price safety?

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Ben Croucher, applications and sales engineer, Clare instruments, looks at the  paramount importance of electrical safety testing in the manufacturing environment

With verification of the safe operation and functionality of electrical products being vital to ensure compliance with established industry standards and maintain customer confidence, the focus has switched to the extent of testing required.

This requirement to ensure conformance through manufacture is clear from both generic product safety standards and European Directives, but the common reaction still seems to be ‘does this mean I have to do 100% testing?' followed by a rapid retreat into discussions that aim to reduce an erroneously perceived time/cost burden, often quoting ISO9000 procedures and focusing on sample testing as a suitable solution.

Batch sampling and or product verification tests are essentially designed to determine type test and build instructions are being maintained via a set of ‘working standards' and rely upon there being a traceable scientific relationship between the ‘sample' and the rest of the batch.  The assumption being if the sample shows conformance, then the rest of the batch also complies. However when customer safety is paramount can anyone take this risk?
In order to maintain a proper scientific relationship, back to the ‘type approved product' testing of the batch sample should really involve a repeat of the ‘type test' which could involve the use of external test house or the transfer of the sample to a dedicated, in-house  test laboratory. In either case the test will require the use of skilled and expensive labour, specialised (and usually high cost) test equipment, complex, time consuming, test routines and/or possible destruction of test sample.

Taking a typical batch sampling routine as an example, the following scenario can be envisaged: Risk analysis determines a procedure for testing one sample product for every 100 that come off the assembly line.  The sample is sent to the laboratory where it undergoes rigorous testing and fails. Strictly speaking, production should now be halted until the cause and extent of the fault is identified. 

This should include recalling and testing not only the remaining 99 items of the particular batch, but any items produced / packed and shipped out since the sample was taken.

The cost of this exercise can be worked out in terms of re-call costs (time, labour, discard packaging etc) - even greater if products have left the factory - testing costs (which will now include skilled labour), rework costs (time, labour, parts if any), lost production (highly unlikely that all items are salvageable) and late delivery penalties.

To review the real on-cost to a business a useful investigation would be an ISO9000 ‘re-call' procedural review and cost them accordingly. It might be argued that this worst case scenario only applies if the sample fails - but would anyone feel comfortable knowing that the electric drill used in a workshop has only a one in 100 chance of NOT causing electrocution.

Similarly, it is clearly in the interests of manufacturers of finished products that the safety critical components used to assemble a product are satisfactory - preferably before being incorporated into the product. Many manufacturers now request ‘certificates of conformity' (CofC's) from their suppliers of safety critical components. However the question always has to be ‘how sure are you of their test regime?'

Against this background it is clear there are increasing numbers of manufacturers of electrical products who wish to check supplied components before or during their own product assembly.

Among such companies, there is recognition of the advantages which can result in the pro-active identification of problems and defects before assembly, increases confidence in finished products, reduces the likelihood of product re-work and permits the cost of failures to be recovered from the supplier more easily.

By completing the cycle with 100% product testing, significant information can be gathered and used to improve and refine manufacturing processing and techniques. Identifiable reasons for product failures can be highlighted and quickly acted upon. Even simple fault counters can indicate particular areas of the build phase that may require further investigation.

Another major plus for 100% testing is the development of a competitive advantage, in that a company's ability to offer full testing during their own production processes reduces the need for the customer to carry out their own testing, thus offering a level of added value that can be translated into increased profitability, plus customer confidence and loyalty. But what is meant by 100% testing?

Firstly it should be noted we are talking about electrical safety requirements. Manufacturers will review their own processes for Class I and Class II products and accordingly introduce the three main tests for ensuring product safety: High current earth bond measurement, insulation resistance measurement and high voltage flash (or dielectric strength) test. In addition many manufacturers will be driven by standards/customer requirements or even their own in-house guidelines to complete functional tests (also known as run or load leakage testing).

A number of criticisms have been made against 100% testing, again usually on the basis of time and cost.

On the time factor, concerns normally arise from misconceptions between type testing requirements and the established practices for 100% routine production line testing. A typical regime of electrical safety testing to meet these routine test requirements can be completed in less than five seconds. Referring back to the earlier example, all 100 products could have been tested in less than nine minutes.

In terms of cost, equipment can be expensive if the type test requirement is to be employed. However for routine production line testing, there are a number of  systems available that can cost from as little as £1500. With simple to use set-up and control features, they can be readily incorporated into the production environment without the need for highly skilled labour.

For type testing, a flash test can require high current levels (sometimes in excess of 100mA) and extended test times (several minutes for some standards) and consequently this type of test can require application under closely controlled conditions, involving the use of highly skilled and experienced test personnel. 

However for routine production line testing, electrical safety standards define not only a lower safer trip level, but also the setting up of the test area is well defined to keep safe the operator. Experience has shown that routine test parameters provide a realistic evaluation of electrical safety and does not harm equipment that is designed to comply with the relevant standards for creepage, clearance and insulation properties. 

Where delicate electronic components are involved, far from omitting the flash test, various techniques can be incorporated to soft-start (ramp) the test voltage, apply DC voltages with discharge circuits etc...thus removing any likelihood of damage occurring - a procedure recognised by EN60950 for information technology (IT) equipment. 

Far from costing time and money, 100% electrical safety testing on the production line makes sound economic and business sense creating a competitive advantage and peace of mind. After all only 100% testing can categorically show 100% conformance.

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