Shock horror? Clothes to blame

March 20, 1998

Kam Patel reports on speakers at the Institute of Physics congress in Brighton this week

Fed up with getting a nasty electrical shock as you climb out of your car and touch the vehicle body? Blame your clothes and the car seat materials, John Chubb of John Chubb Instrumentation told IOP delegates.

Research by Dr Chubb has found that receiving a shock has virtually nothing to do with the car body being charged up through faulty electrical wiring as many people tend to assume.

"The problem, simply, is you. You become charged up because of the twisting and rubbing of clothes against the car seat when you are getting out. You can pick up 20,000 volts quite easily," he said.

Touching the car body, which is effectively earthed through the tyres, results in the shock, a discharge of static electricity built up because of the friction between the clothes and seat material.

The experience occurs with any ordinary car at ordinary humidity levels, he says.

At his Cheltenham-based firm, Dr Chubb has been carrying out a series of studies aimed at finding out how the potential of the charge picked up by the body varies with the type of clothing worn.

His approach has been to measure body voltage continuously as a person gets out of a car and record it in real time using an electrostatic voltmeter linked to a computer. The use of the voltmeter ensures there is no drain of charge from the body during the measurement period.

"What is clear from these readings is that the timescale for charging up the body is pretty quick. Typically you can go from zero to a peak of 20,000 volts in one second."

Tests with drivers, including Dr Chubb himself, wearing wool, cotton and nylon showed that wool picked up the biggest charge and cotton the lowest.

Dr Chubb repeated the experiments using different seat materials. Static build up was much lower using materials used as garment fabric for clean room applications in the electronic component-making industry, where stray charges can be disastrous.

But the exact mechanism by which this is being achieved is not understood.

"It is clear that body voltage can be effectively limited to below shock level (about 3,500 volts) by the choice of seat materials but we do not know what kind of material construction for the seat would be the ideal. It is something we are working on."

Dr Chubb's work has attracted the interest of several car makers keen to eradicate the problem.

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