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THE CONTROL OF BODY VOLTAGE GETTING OUT OF A CAR

Based on a paper presented at the Institute of Physics, Static Electrification Group meeting ?Static and Textiles? meeting 18 March, 1998 during the Institute of Physics Annual Congress in Brighton.

Jump to: Introduction | Measurements | Materials | Enviromental Conditions | Experimental Results | Conclusions

John Chubb

John Chubb Instrumentation,

Unit 30, Landown Industrial Estate, Gloucester Road, Cheltenham, GL51 8PL, UK.

(Tel: +44 (0)1242 573347 Fax: +44 (0)1242 251388 email: jchubb@jci.co.uk)

 

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 v oltage 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

57

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

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

  < td width="13%" valign="TOP" height=16>

9.1-9.3

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

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 vo ltages 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.

Annex 1:

DRAFT TEST METHOD

Assessment of body voltage getting out of a car

John Chubb Instrumentation, Unit 30, Landown Industrial Estate,

Gloucester Road, Cheltenham, GL51 8PL, UK.

(Tel: +44 (0)1242 573347 Fax: +44 (0)1242 251388 email: jchubb@jci.co.uk)

1. PURPOSE:

To measure the electrostatic voltages generated on a person getting out of a car

2. TEST EQUIPMENT:

1) Electrostatic voltmeter with analogue and/or serial data output, with ability to measure voltages to at least 20kV with a resolution of 50V or better, a leakage resistance 1014 ohms or greater at voltages over 10kV, an internal capacitance less than 10pF and a response time below 0.1s.

2) Signal reco rding equipment capable of storing data with time resolution of 0.1s or better

3) Springy or coiled HV lead with high quality insulation cover to connect the electrostatic voltmeter to the person. This needs to provide no noticeable impediment to personal movement in getting out of the car and no dragging over surfaces during such actions and providing a leakage resistance 1014 ohms or better at 10kV.

4) Sheet of good quality insulation to place on the ground at the point of contact of the feet with the ground and for standing after getting out of the car. This needs to provide 1014 ohms isolation at voltages up to 20kV.

5) For assessing the characteristics of the materials of the seating and the clothing:

a) Charge Decay Test Unit (e.g. JCI 155 with sample support JCI 166 and associated microcomputer running DECAY18 for recording, analysing and storing charge decay curve data. Note that FTS 101C type test equipment is NOT suitable).

b) Resistivity Meter able to measure to 1014 ohms per square

3. ENVIRONMENTAL CONDITIONS

The temperature and humidity in the car at the time of testing are to be measured - for example using a Whirling Hygrometer. Because the characteristics of materials can be strongly influenced by absorbed moisture the car, any alternative seat ing materials and all clothing worn shall be exposed to the test conditions for several hours before the start of testing.

For standardised studies measurements are to be made at 23C 50%RH and at 23C 12%RH. The test garments and the car shall be under these conditions for 24 hours before starting tests.

 

4. TEST PROCEDURE

Place the electrostatic voltmeter on the other seat than that to be used for testing and link it to the signal recording equipment. Bond the earth connection of the voltmeter to the metal bodywork of the car. Link the HV connection from the electrostatic voltmeter to the tester - for example to a wristband or metal watch strap.

Place the sheet of good quality insulation on the ground at the point of contact and covering the position of standing after getting out of the car.

Perform a test action of getting out of the car to check that the HV connection lead moves freely and does not impede activity.

Switch on the electrostatic voltmeter and signal recording equipment and check operating satisfactorily. Record relevant test parameters and the date and time.

Perform at least 3 activities of getting out of the car in a normal manner from the selected seat with each set of test conditions. Leave the car without the body touching the metal bodywork and with the feet impacting squarely on the layer of insulation on the ground. Avoid any sliding or scuffing action of the feet on the insulation. After leaving the car stand still on the insulation for at least 5s to allow recording of the natural rate of voltage decay.

Where studies are made on a number of different seat fabrics and/or wearing various clothing the sequence of tests shall preferably start and finish with standard or reference combinations of materials. If significant changes are observed then additional tests shall be performed to establish if the changes are due to changes in ennvironmental conditions, transfer of fabric finish or other factors.

5. TEST REPORT:

The following information shall be reported:

- peak and plateau values of body voltages observed shall be listed

- description of the make and model of the car and the seat used for testing

- the seat material

- the type and fabric of clothing worn (including component fibers and weave if possible)

- the charge decay times of seat area of clothing worn with open and earthed backing and of the car seat surface in situ (front and rear areas)

- the surface resistivity of the seat area of clothing worn and of the car seat surface in situ (front and rear areas)

- the temperature and humidity in the vehicle.

The performance with a particular set of test conditions shall be in terms of the highest value of the peak voltage value observed - not the average. If a larger number of tests are performed under particular test conditions, say at least 5, then it will also be appropriate to calculate the mean of the values observed and the standard deviation.

The description and type numbers of the instrumentation and the data recording equipment used shall be recorded together with the serial numbers and date of most recent calibration.



John Chubb Instrumentation,
Unit 30, Lansdown Industrial Estate, Gloucester Road, Cheltenham, GL51 8PL, UK
Tel:+44 (0)1242 573347 Fax: +44 (0)1242 251388
email: jchubb@jci.co.uk

© John Chubb Instrumentation.