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THE CONTROL OF BODY VOLTAGE GETTING OUT OF A CAR

The overhead slides of a paper presented at the ?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

Common experience of shock on getting out of a car.

It is not that the body of the car is electrostatically charged - but you are.

Charge arises on you from rubbing and twisting action between your clothing and the car seat on getting out of the car.

 

It is not plausible to try to limit the clothing people wear - so action to limit shock risk must be part of car system design.

 

Investigative approach:

- start with standard car with standard seats

- use normal clothing

- try seats covered by test fabrics and laminates

 

2. MEASUREMENTS

Instrumentation and method for measurements:

  • Recording electrostatic voltmeter with flexible connection to metal wrist strap. Body voltage measured continuously from preparation to get out of car, through exit actions and afterwards.
  • Body voltage measurement to 20kV with resolution of 10V and 1/4 second (JCI 148 with JCI 140C and JCILOG for data recording on laptop microcomputer)

 

 

3. MATERIALS

Clothing:

  • wool suit (100%)

  • cotton suit (100%)

  • nylon overall

Car:

  • Vauxhall Cavalier (JNC)
  • Ford Mondeo estate (PH)

Car seating:

  • normal car seat

fabric coverings for seat:

  • 5x5mm grid (Negastat)
  • 5mm stripe (Negastat)
  • 20mm stripe (Negastat)

 

 

4. ENVIRONMENTAL CONDITIONS

Temperature: 10-20C

Humidity: 40-60%RH

Note possible influence of microclimate

 

 

 

5. EXPERIMENTAL RESULTS

Features of observations:

  • fast rise to plateau if on to insulation or decay if on to ground
  • fairly reproducible results
  • high body voltages can arise with natural fibre 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< /td>

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 f oil

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< /td>

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

A tlantis

JNC

Nylon overall

4.6-4.8

3.2-3.7

6. CONCLUSIONS:

  • body voltage often well above threshold for shock with normal clothing and standard seating
  • body voltages can be held below shock threshold with alternative seat surfaces

  • design features for seat fabric not yet clear

(e.g. relative importance of ?surface finish? vs conducting threads)

Annex 1:

JCI 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 recording 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 ru nning DECAY18 for recording, analysing and storing charge decay curve data. 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 seating 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: jchub b@jci.co.uk

© John Chubb Instrumentation.