How to measure the resistance of ESD chairs

1. Purpose

Define the sequence for measuring the resistance of seating used in an ESD protected area and assess the chair before installation and use.

2. Scope

Measure resistance from different chair components to a groundable point, such as conductive castors or a drag chain.

3. Application

Assess ESD seating using suitable resistance-measuring equipment. This procedure does not cover electrically initiated explosive devices, flammable liquids or dust.

Related article: ESD S20.20 electrostatic-discharge control training at Goertek Vina

4. Reference documents

• ESD ADV1.0: EOS/ESD Association terminology
• ANSI/ESD S4.1: Work surfaces - resistance measurements
• ANSI/ESD S7.1: Floor-material properties
• ANSI/ESD S8.1: ESD symbols
• ANSI/ESD S6.1: Grounding
• ASTM D2240: Rubber-property test - durometer hardness

EOS/ESD Association - STM12.1, seating and related documents

5. Personnel safety

Test equipment can present electrical hazards. Competent personnel must assess the risks, follow the manufacturer’s instructions and comply with applicable safety requirements.

Use suitable protection against earth faults, such as an RCD where appropriate. Do not disconnect protective earthing as a safety measure.

Control exposure to electrical energy and verify correct protective-earthing connections before testing.

6. Test method

Reference procedure. Before use, confirm the applicable ANSI/ESD STM12.1 edition, ESD programme, equipment and acceptance criteria. The values below do not replace the complete controlled test method.

Equipment and samples

• Chair-testing equipment: a suitable point-to-point resistance meter and electrodes.
• ESD chair samples: three chairs.
• Insulating support surface: R > 1.0 × 10¹² Ω, measured using STM11.11.
• Conductive metal plate: R < 1.0 × 10⁴ Ω.

• The reference procedure specifies a meter capable of measuring from 10³ to 10¹¹ Ω.
• The reference procedure uses 10 V below 1.0 × 10⁶ Ω and 100 V at or above 1.0 × 10⁶ Ω. Confirm the test method and instrument instructions.
• Clean the chair, castors, supporting surface, metal plate and test equipment according to the applicable method before testing.
• Conditioning under the reference procedure: 12 ± 3% relative humidity and 23 ± 3°C for at least 48 hours. Do not test until the prescribed conditioning has been achieved.
• Measure resistance from the relevant chair components to the groundable point: front and rear of the backrest, seat, both armrests, castors, drag chain, base and foot ring.
• Record each measurement from the chair component to earth or the designated groundable point.

Note: Test every castor.

Results and possible causes of failure

The reference procedure specifies resistance below 1.0 × 10⁹ Ω from a chair component to earth or the groundable point. Confirm the acceptance criteria against the applicable ESD programme and seating test method.

Incoming inspection can produce a failing result, or a result close to the acceptance limit, for reasons such as:

• Failure to meet the required environmental conditioning.
• Inadequate cleaning of the chair, test equipment or supporting materials.
• Continuing to read a resistance above 1.0 × 10⁶ Ω on the 10 V range instead of changing to 100 V and repeating the measurement where the procedure requires it.
• An incorrect chair-test arrangement.

Review conditioning, preparation, connections, test voltage and recording before concluding that the chair is nonconforming.

Maintaining the chair’s ground path

The reference procedure specifies ESD-compatible legs or castors with resistance below 1.0 × 10⁹ Ω. Verify both the required components and the complete chair-to-ground path using the applicable test method.

A grounding ball alone does not demonstrate compliance. Check whether the arrangement maintains a reliable ground path in actual use. Potential problems include:

• A small contact area may make the grounding connection unreliable; resistance can increase during use even if an initial test passes.
• Dirt can impair floor contact and charge dissipation. Regular inspection and cleaning help maintain the intended grounding path and protect ESD-sensitive items.