Elixir Part 9 : Avoid Car Sparks

The cause of car-door sparking is well known: contact-electrification between insulating surfaces, followed by separation of those surfaces. But what does this mean? Well, *YOU* are one surface, and THE CAR SEAT is the other. When you sit on a plastic car seat in dry weather, the contact between your clothes and the seat's surface causes the electrical charges within atoms of the material to transfer between the surfaces. This is our old friend "frictional" or "contact" charging. One surface ends up with more negative charges than positive, and has a negative charge-imbalance. The other surface has fewer negatives than positives, so it has a positive imbalance. This is nearly same thing as rubbing a balloon upon your hair: both surfaces become electrically charged. But rather than rubbing just your hair, instead you're rubbing your entire back, but, and legs upon the car seat surface.

However, nothing happens as long as you remain seated. Just keep yourself in one place and you won't get zapped.. As long as the surfaces remain near each other, the positives and negatives cancel out, and no overall "electricity" appears and no sparks are possible. But when you open the car door and step outside, you take just one polarity of charge along with you, while the car seat has the opposite polarity. At the same time, the charged-up car seat causes the whole car to become charged (by a process called "Faraday's Icepail Effect.") As you step out of the car, the voltage between your body and the car becomes huge, up to 10,000 or even 20,000 volts. Your shoes are probably insulating, so the charge has no opportunity to leak into the earth. You reach out to close the car door and ZAP!, the opposite polarities rejoin by leaping through the air while giving you a tiny, deep burn on your fingertip!

How to prevent this? One possibility: change the surface materials. Identify and avoid the specific clothing which makes the problem worse. These materials are usually wool sweaters and pants, certain manmade fabrics, plastic raincoats, etc. Or, replace your cheap plastic car seatcovers with cloth (stains easily!) or with leather (expensive dead animals.) Another method: mix up some anti-static solution and spray your car seats. This solution remains slightly damp for weeks, which halts the contact-charging process. The formula: a teaspoon of fabric softener mixed in one quart of water. This tends to work well at first, but after days it wears off and needs a re-coating. Another sillier method: always drive barefooted, so the charge will leak away when you step outside the car. Not good in winter! You could cover your car seats with a conductor such as aluminum foil, which screws up the contact-charging effect. Have a tailor make some custom clothing out of black conductive carbon cloth? Or you could eliminate the problem by eliminating your clothes. Skin is fairly conductive, so it doesn't create charge-separation when held against plastic. Driving while nude might cure the sparking problem (unless you are a very hairy person!)

A less frivilous method: the car-keys trick I mentioned earlier. Develop the habit of holding your car keys as you leave the car, then grip the keys firmly and touch the metal car door with the tip of the key. The spark will still jump, but it will not be painful, since it blasts a little hole in the tip of the key instead of in your finger. Or simply grasp the car door as you climb out, and this will drain off the charge-imbalance faster than it can build up on your body.

DANGER: GASOLINE FIRES. What happens when you climb out of your car at the gas station? Usually you'll zap yourself on the car door, or on the gas pump handle, or on the metal door that covers your gas cap, and usually nothing bad happens. However, suppose your passengers climb out of the car just as you're unscrewing the cap, or just as you jam the gas pump nozzle into your tank? The whole car becomes momentarily charged. ZAP! Or, suppose you turn on the gas pump and then climb back into your car. When you climb out again, you body is highly charged from the car seat. The very first thing you do is to reach into the cloud of gasoline vapors to grab the metal handle of the gas pump. ZAP! FOOSH! This obviously is a very rare event. However, it does occur sometimes, especially in the winter.


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THE CONTROL OF BODY VOLTAGE GETTING OUT OF A CAR

1. INTRODUCTION

Many people experience shocks from 'static electricity' when they get out of their car. Usually the car is blamed. In fact, it is not that the car is that is electrostatically charged - but your own body. Electrostatic charge arises on your body from the rubbing and twisting action between your clothing and the car seat as you get out from the car. If you are at more than about 3500 volts then you are likely to feel a shock when you discharge yourself on contact with the car bodywork or earth. As it is not plausible to specify or control the clothing people wear the action to limit shock risk needs to be part of car system design.

The investigative approach pursued at John Chubb Instrumentation has started with standard car with standard seats and normal clothing. This has provided background experience with normal situations and has enabled an appropriate test procedure to be developed. Studies have then examined body voltages generated with normal clothing and seats covered by test fabrics and laminates.

2. MEASUREMENTS

Measurements have been made voltage generated on the test person when getting out of a car with various clothing and various seat coverings. Body voltages were measured continuously from before preparation to get out of car, through exit actions and while standing still on a sheet of good insulation on the ground beside the car.

Body voltages were measured with an electrostatic voltmeter connected by a length of flexible high voltage lead to a metal wrist strap on the test person initially sitting in the driver's seat. Voltages were measured up to 20kV with resolution of 10V and were recorded directly into a Notebook microcomputer with a time resolution of 1/4 second. The electrostatic voltmeter and Notebook microcomputer were positioned on the front passenger seat. This gave easy movement on exiting from the car without tangling the high voltage connection and easy opportunity to see the variation of body voltage after exiting from the car.

3. MATERIALS

The three types of clothing used during testing were:

wool suit (100%)

cotton suit (100%)

nylon overall

The two cars used for testing were:

Vauxhall Cavalier (JNC)

Ford Mondeo estate (PH)

The seating surfaces used during testing were:

a) normal car seat

b) normal car seat covered by polyester fabrics that included core conductive threads (Negastat) as 5x5mm grid, as 5mm stripe and as 20mm stripe.

4. ENVIRONMENTAL CONDITIONS

The ambient environmental conditions during the tests were: temperature: 10-20C, relative humidity: 40-60%. The temperature and humidity conditions at the interface between the cloting and the seating materials may, of course, have been appreciably different from these values.

5. EXPERIMENTAL RESULTS

The main features of the studies were:

• Observations were fairly reproducible
• If stepping from the car on to good quality insulation (with care not to slide or scuff feet on insulation) the body voltages were observed to show a fast rise to a peak and then settle back to a steady plateau. The rise time to peak voltage was about 1 second. If stepping out on to the ground then the body voltage decayed away after the peak.
• Even with natural fibre clothing (wool and cotton) quite high body voltages can arise.

Table 1 shows the results of body voltage measurements for a variety of car seat surfaces and clothing. It will be noted that individual test runs started and finished with the 'normal' seating. This was to allow checking of any progressive changes in clothing conditions during the conduct of tests. Table 2 shows results of studies with different testers with two different cars and a number of garment and car seat fabrics.

Table 1: Peak body voltages for different test conditions

Carseat studies 09/02/97, 09/03/97 & 11/03/97
Body voltages (kV) generated getting out of car (JNC)
	Date:	Test time	Temp (C)	RH%	Nylon	Wool	Cotton
Seat surface
Normal seat	09/02/97	12:40-13:11	9.5	5 7		10.3-13.7	3-10
Atlantis regular	09/02/97	13:24-13:42	9.5	57	18.3 - 21.2	6-9	6-7
Atlantis -no A/S Negastat (2)	09/02/97	15:48-16:07	9.5	57	8 - 9.5	7 - 9.5	5.3 - 5.5
Atlantis No A/S Negastat in scrim	09/02/97	16:27-16:53	9.5	57	12.3 - 16.1	11 - 13.8	7 - 7.5
Normal seat	09/02/97	17:00-17:07	9.5	57		13.6 - 17.8
	< /p>
Normal	09/03/97		14	65	9.5-10.5	11	4.1-5.1
SATI high Negastat	09/03/97		14	65	4.9
Belltron	09/03/97		14	65	1.4-2.25	1.75-3.1	0.15
Megana	11/03/97		18	48	3.7-4.0	2.5-3.5	0.34-0.4
SATI 15	11/03/97		18	48	2.3-2.5	3.3-3.8
Normal	11/03/97		18	48	15.0-16.0	11.2-12.0	6.7-7.2

Table 2: Peak body voltages for different testers with different test conditions

Car: JNC ? General Motors, Vauxhall Cavalier

PH ? Ford, Mondeo

Car seat studies at BTTG July 2, 1997
Time	Car	Seat	Tester	Clothing	Peak V	Plateau V
					(kV)	(kV)
12:05-12:08	JNC	Normal	JNC	Nylon overall	12.0-12.5	8-9.8
12:15-12:20	JNC	Normal	PH	Nylon overall	9.2-10.5	7.0-9.0
12:38-12:40	JNC	Normal	CE	Nylon overall	9.5-13.0	6.5-9.5
14:46-14:48	PH	Normal	JNC	Nylon overall	8.7	6.5
14:57-14:58	PH	Normal	PH	Nylon overall	7.7-8.3	5.5-6.3
15:05-15:06	PH	Normal	CE	Nylon overall	9.1-9.3	7.0-7.3
15:16-15:17	PH	Al foil	CE	Nylon overall	1.3-1.7	1.3-1.7
15:23	PH	Al foil	JNC	Nylon overall	1.3	1.2
15:51-15:52	PH	SAT15 on Al	JNC	Nylon overall	2.4	1.7-1.9
16:00-16:02	PH	Normal	JNC	Polycotton	1.4-1.9	0.9-1.3
16:06-16:07	PH	Normal	CE	Polycotton	1.3-1.4	0.6-0.9
16:16-16:18	PH	Polycotton	JNC	Nylon overall	1.3-2.2	0.9-1.4
16:21-16:24	PH	Normal	PH	Nylon labcoat	3.1-5.4	2.3-3.7
16:30-16:31	PH	Normal	JNC	Nylon overall	4.6-7.4	3.7-5.3
16:53-16:54	PH	Normal	PH	Cotton lab coat	4.2-4.6	3.2-3.7
16:53-16:55	PH	Atlantis	JNC	Nylon overall	2.2-3.8	2.0-3.1
17:01-17:02	PH	Atlantis	JNC	Nylon overall	4.8-4.9	3.4-3.5
17:15-17:16	JNC	Atlantis	CE	Nylon overall	3.8-4.6	2.9-3.8
17:20-17:21	JNC	Atlantis	JNC	Nylon overall	4.6-4.8	3.2-3.7

6. CONCLUSIONS:

• Both with normal clothing and standard seating body voltages are often well above the threshold level of about 3,500V for shock
• Body voltages can be held below the shock threshold with alternative seat surfaces using fabrics that include a pattern of conductive threads

• The design features required for a seat fabric to give reliably low body voltages are not yet clear - or the relative importance of ?surface finish? of the seat fabric versus conducting thread characteristics.
• It is not yet clear what the mechanisms are by which the body voltage is limited. It seems likely it is corona discharging from the conductive threads in the seat surface as the contacting clothing separates.