Preliminary Report On Electric Blanket

Press, Volume XCVIII, Issue 28800, 22 January 1959, Page 10

Preliminary Report On Electric Blanket

More than 150,000 electrically-heated blankets have been sold to the New Zealand public ; but many afe unsafe. About 800 fires, directly attributable to electric blankets—some with loss of life—had been reported Up to last March and of these 114 were caused by faulty elements, 85 by improper use, 53 by defective flex and joints, and 37 of unknown origin were probably distributed over all three categories, leaving a few more than 1 per cent. arising from faulty thermostats. Investigations made in the last six months by the Canterbury University Industrial Development. Department indicate that even the.-- voluntary Standards Institute specification for electric blankets (which is nbt met by about a dozen of the 57 types) is deficient and it is likely, that even stiffer requirements—possibly mandatory—will be -recommended.

When the 300 officially-reported electric blanket fires were discussed at a meeting convened by the Standards Institute, the situation was considered sufficiently serious to warrant a fullscale investigation by the Industrial Development Department, which had earlier been testing blankets for the present standard specification. “It was felt that manufacturers and importers of blankets would have considerable interest in the project and would contribute to the costs of the research,” says the director of the department (Mr T. R. Pollard) in an introduction to the interim report. “There are approximately 50 importers and 16 manufacturers of electric blankets in New Zealand; but only 11 industrial concerns have responded and contributed £257 2s. The estimated cost of the project is £l5OO. The estimated value of blankets already sold in New Zealand is £750,000, an estimated annual sale value being about £100,000.” _ The firms giving financial support are:— A. and T. 'Burtt Ltd., Falks (N.Z.) Ltd., Guscott Brothers, Independent Traders, J. Isbister, Latex Rubber Company, Neill Cropper and Company Ltd., Ralta Ltd., W. Shattky (Golda Products), Spedding Ltd., and Turnbull and Jones Ltd. ... Since the investigation began, there has been detailed examination of another 75 blankets which caught fire in recent months. The x remains of these blankets were made available through cooperation of insurance companies and local electricity authorities, and owners of the blankets were questioned not only to discover how particular fires were caused; but also to ascertain what they considered “normal” usage. Abnormal Usage

Apart from cases oi taulty construction, which predominated in the earlier 300 fires, these were some of the examples of “abnormal” use which caused fires:*— (1) One person used two electric blankets in one bed. (2) A woman switched on her blanket in April—for the winter. (3) Another “customer” used a * s blanket between two mattresses. (4) An elderly woman., folded her blanket carefully and put it . under the pillow—omitting to switch it off. (5) One blanket was used regularly to air damp sheets. (6) A boy aged 11 was expected to switch off his blanket before going to sleep. Discussions with both the users of electric blankets and with those concerned with their manufacture, repair, and sale have shown the following general requirements:— As most people prefer the heat below them, electric blankets should \ be capable of working safely in this position, supported by a mattress. An electric blanket so placed tends to resist chronic rumpling and creasing which causes the element to fracture and generate an arc hot spot. An electric blanket should attain working heat in 20 to 30 minutes—that is, heat itself and the sheets. But it must be capable of continuous, heating. The lead in must be strong. Though a single heat control satisfies many, there is a demand for multiple-heating control. Double and single blankets are required. Source of Fires Early in the investigation it was evident that fractured elements were a source of blanket fires and the survey shows one third in this group. English statistics. for 1950 and 1953 give a similar proportion. Most of these fractures arose from the blanket being creased into an S bend during sleeping movement. When the l element breaks, the hellices are pressed together, arcing com-

whicb often Carbonises, and the whblebfeaks down 'to”a high resistance joint Although less normal current flows,. considerable wattage is dissipated in a small area, blanket, temperature rises above ignition noint.and the blanket catches- Are.’ <■ ‘ Pursuing this source of Ares, the Industrial Development Department built a machine which will bend five element samples into an omega shape 00 times a minute while current flows through them.'Elements so tested varied from an average life ol 600 cycles in a poor element to 42,000 cycles with a good and entirely different type of element Testing is continuing to And the types of “onstruction prone to early failure and those which will better withstand creasing or flexing. The newly-developed testing machine is known to be more severe than a flexing test of the Electrical Development Association laboratories in England; but it is considered to better simulate normal use.

Interim Conclusions Conclusions- reportod so far are that multi-strand elements are more prone than single-stranded wires to initiate the arcing conditions which lead to fires; nonwatertight fractured wire ends in a damp blanket will produce similar conditions; tracking between adjacent elements can be minimised by moisture-proof barriers, compromise between circuit and element lay-out, and ensuring that adjacent runs do not carry full potential; overcurrent and undercurrent devices might prevent fires from fractures, pillows above the electric blanket area or excessive bedding may cause excessive rises in temperature and fires; the test conditions for the New Zealand standard specification do not fully represent “normal bed conditions.”

Improvement of this specification will be suggested. “It may be that all blankets sold should conform to the new standard and that a re-examination service, similar to that employed by the underwriters’ laboratories in the United States, should* be introduced to support and to maintain the standard,” the interim; report says.