Does an ESD lining have to be grounded?

Answer: Yes. Surface resistance only shows charge can move; ungrounded, a dissipative liner still holds above 800 V after rubbing versus 60 V once the 1 MΩ path is fitted.

Is black foam automatically dissipative?

Answer: No. Black EPE with 3% colour masterbatch still reads 10¹⁴ Ω. Only an ohmmeter decides, with 10⁶ to 10⁹ Ω marking the dissipative band.

How long does a coated liner last?

Answer: Count the strokes. After 500 passes with steel wool under 500 g the surface climbs two decades, so at 20 insertions a week retest after about two years.

How should low humidity change acceptance?

Answer: Test at 12% RH rather than 50% RH, where the same liner reads two to three decades higher, and sign off on the dry figure.

ESD Lining: Surface Resistance and Grounding

Quick answer: An ESD lining works only when surface resistance and a full ground path agree: 10⁶ to 10⁹ Ω bleeds charge slowly and 1 MΩ limits the return.

Three Resistance Bands Inside a Case
Carbon-Loaded Foam or Coated Surface
Volume, Surface and Point-to-Point
The Ground Path: 1 MΩ and 0.5 mm²
Proving Triboelectric Voltage Under 100 V
Splices and Adhesive: Three Breaks in Continuity
FAQ

1. Three Resistance Bands Inside a Case

Surface resistance decides which band a liner belongs to. Conductive sits at 10² to 10⁵ Ω, dissipative at 10⁶ to 10⁹ Ω, insulative above 10¹² Ω. Cross one decade and the behaviour changes.

Band	Surface resistance	Volume resistivity	Typical material	Where it fits
Conductive	10²–10⁵ Ω	10²–10⁴ Ω·cm	carbon-loaded XPE	earthed benches
Dissipative	10⁶–10⁹ Ω	10⁴–10⁸ Ω·cm	loaded EPE, fabric	ESD lining, gauge boxes
Insulative	above 10¹² Ω	above 10¹⁴ Ω·cm	plain EPE, EVA	packing with no ESD need

An ESD lining usually takes the dissipative band, and the reason is how fast charge leaves rather than how completely. A conductive liner dumps 500 V in under 10 ms, spiking current into the amp range that a thin gate oxide will not survive. The dissipative band takes 0.2 s to 2 s for the same 500 V, holding current in the milliamp range.

The insulative band fails by accumulation. Plain EPE at 10¹⁴ Ω traps 3 kV to 8 kV after rubbing and releases it through the next hand that touches it. Black foam proves nothing: colour may come from carbon or from pigment, and only an ohmmeter separates the two.

2. Carbon-Loaded Foam or Coated Surface

Two ESD lining routes reach 10⁶ Ω on paper and behave nothing alike once cut.

ESD lining cut edge in close-up, coarse dark grey foam with a stepped cavity wall, a knurled metal instrument against it, light from upper left

Carbon loading makes the whole section conductive. XPE or EPE takes 15% to 25% carbon black before foaming, pulling volume resistivity to 10³–10⁵ Ω·cm, so a cut face still conducts. Three costs follow: colour is limited to black, rebound drops 10% at 20% loading, and density climbs from 30 kg/m³ to 60–90 kg/m³, adding about 200 g per case.

Coating stays surface-only. Plain EPE is sprayed with 20 μm to 50 μm of conductive coating or laminated with conductive fabric, reaching 10⁴–10⁶ Ω on the skin while the bulk keeps 10¹⁴ Ω·cm. Charge stops at the cut, because the exposed section is an insulator.

Coatings age. After 500 strokes of 0000 steel wool under 500 g, surface resistance climbs from 10⁵ Ω past 10⁷ Ω, and any breach exposes 10¹⁴ Ω. A laminate splits the difference: fabric on top, 30 kg/m³ EPE below, spot-bonded with conductive adhesive, 15% heavier than plain foam and 20% cheaper than a fully loaded sheet.

3. Volume, Surface and Point-to-Point

Three numbers get quoted interchangeably and come from different fixtures.

ANSI/ESD S4.1 covers work surfaces, with two 2.3 kg electrodes 254 mm apart at 10 V or 100 V, read after 15 s. ANSI/ESD S11.11 covers planar surface resistance with rings or bars, again at 10 V or 100 V. Volume resistivity follows ASTM D257 with a 63.5 mm electrode and comes out in Ω·cm.

Voltage choice shifts the answer by one or two decades. Use 10 V below 10⁶ Ω and 100 V above it, and a dissipative liner straddles that line, so both readings belong in the report rather than whichever passes.

Point-to-point is the number that describes an ESD lining honestly. It asks whether the far cavity corner connects to the grounding stud through foam, splice, adhesive and wall. Four or five interfaces sit in series and one insulator voids the chain, so a liner reading 10⁶ Ω as a sheet often reads 10¹¹ Ω once fitted.

4. The Ground Path: 1 MΩ and 0.5 mm²

An ESD lining with no ground is a floating capacitor.

ESD lining in a black rotomoulded case on a bench, egg-crate foam in the lid, empty cut cavities below, a catch on the side

The 1 MΩ resistor at the stud protects people, not components: at 250 V it holds current to 250 μA, below the perception threshold. Without it the liner ties straight to earth and current depends on skin resistance alone.

A 0.5 mm² conductor is enough, kept under 1 m and measured below 1 Ω. Thicker cable buys nothing, since the bottleneck is the termination, where one badly crimped lug contributes 5 Ω to 20 Ω.

Bonding is where most builds fail. Foam pressed against plastic touches over a few newtons and lands above 10⁹ Ω, an open circuit in practice. Fit a conductive fabric tail bonded with conductive adhesive to a metal insert, holding the joint under 10 Ω, or add a stud through the 3.0 mm wall.

5. Proving Triboelectric Voltage Under 100 V

An ESD lining that passes as a sheet may still fail once installed, so charge generation needs its own test.

Wrap a 500 g slider in nylon, stroke the liner 10 times over a 200 mm path, and read peak voltage with a field meter at 25 mm. The target is under 100 V, because above it a part is at risk the moment it is lifted.

Run the harshest condition. At 23 °C and 12% RH the reading peaks, since the water film is thinnest and charge cannot escape; the same liner at 50% RH often reads a third of that. Quoting only the 50% figure is quoting good weather.

Decay time tells more. Charge the liner to 1000 V and time the fall to 100 V; the dissipative band allows 2 s, and that figure folds the stud, splices and adhesive into one result.

6. Splices and Adhesive: Three Breaks in Continuity

Large cases rarely take one sheet, and every joint is a candidate break.

The first is the butt splice. Two sheets each read 10⁶ Ω, yet across the seam the meter shows above 10⁹ Ω, because cut faces carry no path. Overlap 10 mm and press conductive tape across the joint, extending 20 mm onto each sheet.

The second is hot melt. At 10¹⁴ Ω·cm a few dots insert an insulator between conductors, so use conductive adhesive or keep bonding off the discharge path.

The third is the folded corner, where 20 μm of coating crazes at R5 and surface resistance jumps from 10⁵ Ω past 10⁸ Ω. Carbon-loaded foam has no such failure, which is why it holds up in irregular cavities.

Accept by sampling three points per metre and reading the two furthest apart; under 10⁹ Ω passes, three decades above the material figure to allow for interfaces.

Conclusion

An ESD lining rests on three numbers: 10⁶ to 10⁹ Ω at the surface, a 1 MΩ resistor at the stud, and a 0.5 mm² conductor holding the path under 1 Ω. Material choice is only the first step, since failures cluster at the bond to the shell, the splice between sheets and the crazed coating at a folded corner. Hold triboelectric voltage under 100 V and decay under 2 s, and the certificate stops being the only evidence.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.