How to Choose Insulating Rubber Mat Thickness: A Voltage-Based Selection Guide

Thickness does not set a mat's voltage rating — the class does. A guide to the IEC 61111 Class 0–4 table, the selection rule, and how to write a specification that survives an audit.

Specifying an insulating mat by thickness alone is one of the most common problems we see in incoming RFQs. A mat is not selected because it is 5 mm or 10 mm thick. It is selected because the installation requires a particular electrical class, and the thickness only has to fall inside the range that class allows. This guide sets out how thickness, class and test voltage relate to one another under IEC 61111 and ASTM D178, and how to write the specification so that what arrives on site passes your audit.

1. Thickness follows the class, not the other way round

Under IEC 61111 the product is classified by the maximum voltage of the installation it protects. The class is the primary field on the marking, the datasheet and the type test report. Thickness is a consequence of that choice: the standard sets a recommended thickness and a maximum permitted thickness for each class, and any value inside that window can be supplied, provided the mat passes the electrical tests for the class.

This is why two compliant Class 3 mats can be supplied at different thicknesses. One manufacturer may publish 3 mm for Class 3; another may publish 4 mm. Both can meet the same class test requirement. When a purchase order fixes the thickness first and the class second, the outcome is usually one of two failures: a mat of the requested thickness in the wrong class, or a compliant mat rejected because it was not the thickness the buyer had in mind.

2. Reading IEC 61111 classes 0 to 4

IEC 61111:2009 (second edition) is the edition still referenced by most published mat datasheets and compliance files in circulation. A third edition, IEC 61111:2026, cancels and replaces the 2009 edition. A current specification should therefore name the edition it is written against instead of assuming that "IEC 61111" always means one fixed set of numbers.

DC ratings are not the same numbers. Manufacturers commonly publish a higher DC maximum service voltage for the same class: a Class 0 mat at 2 mm listed at 1 kV AC and 1.5 kV DC, and a Class 4 mat at 5 mm listed at 36 kV AC and 54 kV DC. Where the installation is DC, ask for the DC column specifically rather than assuming the AC figure covers it.

Colour is an identification aid, not a selection method. Where the colour code is applied to the live-working double-triangle symbol, Class 0 is red, Class 1 white, Class 2 yellow, Class 3 green and Class 4 orange. The class designation and the product marking are what an auditor checks.

3. The selection rule: match the maximum service voltage of the installation

The underlying rule is straightforward: the mat must be adapted to the maximum service voltage of the installation. On multiphase circuits this is the phase-to-phase nominal voltage. Where the exposure in the working area is limited to phase-to-ground potential, the phase-to-ground value is the figure to use.

1. 1Establish the maximum service voltage of the installation — not the room name. "Switchgear room" is not a voltage.
2. 2Decide whether the evaluation should be phase-to-phase or phase-to-ground for the way work is actually carried out.
3. 3Read off the class whose maximum use voltage is equal to or above that figure.
4. 4Confirm the intended thickness sits inside the class window, and choose within it for mechanical reasons.
5. 5Verify the marking and the type test report for the class being specified.

Class 0 is where most low-voltage work starts, covering 600 V and 1000 V boards. Classes 3 and 4 cover the medium and higher voltage switchgear most often quoted around 3.3, 6.6, 11, 15, 26.5 and 33/36 kV. Those groupings are market shorthand used for planning; they are not a substitute for step 3.

4. Where thickness earns its place: environment and mechanical duty

Once the class is fixed, thickness becomes a mechanical decision. The conditions below are the ones that justify moving to the upper end of a class window or to a heavier construction.

• Oil, grease and hydraulic fluid. Select a compound rated for the exposure. In the ASTM D178 Type II series, oil resistance is the IIC property, and it is normally expressed as a limit on volume swell after immersion rather than as a marketing word.
• Abrasion and foot traffic. Walkways, trolley routes and door thresholds wear faster than a fixed work position. Thicker material lasts longer, and a fine-ribbed top surface keeps grip after the surface has been polished by use.
• Impact and dropped tools. Puncture resistance is a published figure; values around 70 N minimum are typical for insulating mat products.
• Heat and cold. Published operating ranges commonly sit near −25 °C to +55 °C, with low-temperature grades extending the cold limit to −40 °C. An unheated outdoor enclosure in a cold climate needs that low-temperature capability confirmed in writing.
• Slip resistance. IEC 61111 requires both faces of the mat to be slip resistant and names corrugated and diamond surfaces. Anti-slip is a functional requirement, not a cosmetic option.
• Storage and ageing. One published recommendation is storage between +10 °C and +21 °C, away from heat sources, direct sunlight, artificial light and ozone, without compression or bending. Rubber has a shelf life, and its storage conditions are part of the specification.

5. Roll, sheet or custom cut: sizing to the work position

Coverage is a purchasing decision with a maintenance consequence. A 1 m × 10 m roll is the normal trade form. Width is commonly offered up to about 1220 mm, with roll lengths up to roughly 15 m for heavier thicknesses and longer runs available in lighter material. Sheets and cut-to-size panels suit a defined operator position.

The operator is expected to stand at the centre of the mat, so the mat dimensions should follow the standing and access zone rather than the dimensions of the room. Custom cutting is standard, which means a mat can follow a cable trench cover, a panel frontage or a switchboard footprint exactly.

6. Common selection mistakes that fail an audit

• Choosing thickness first. The enquiry reads "10 mm mat" with no class, so the supplier has to guess, and the guess may not match the installation.
• Treating the colour as the class. Colour identifies the symbol; it does not carry the rating.
• Confusing proof test voltage with maximum use voltage. A mat proof tested at 20 kV is a Class 2 product with a 17 kV maximum use voltage. The lower figure is the one that binds.
• Using one class across a site. LV boards, MV switchgear, transformer bays and battery rooms rarely share a class.
• Ignoring the marking. A compliant mat carries the manufacturer identification, the double-triangle live-working symbol, the IEC 61111 reference, the month and year of manufacture, the category where applicable and the class designation. In roll form these are repeated along the length — commonly every 400 mm, and not less than once per metre — so that cut sections still carry identification.
• Skipping the retest plan. Mats that have not been used for 12 months must be visually inspected and electrically tested before use. The type test voltage is 10 kV for Class 1, 20 kV for Class 2, 30 kV for Class 3 and 40 kV for Class 4, each applied for one minute; Class 0 requires visual inspection only.

7. How to write the specification on a purchase order

A specification that survives a factory acceptance test names the standard and its edition, the class, the dimensions and the tolerances. A workable line reads as follows.

Add the environment — oil exposure, low temperature or outdoor use — and the acceptance requirement: the type test report for the class and, where site assurance calls for it, the pre-shipment test record for the batch.

Where the tender names ASTM D178 instead of IEC, the structure of the specification is the same but the thickness ladder changes: 3.2 mm for Class 0 through 12.7 mm for Class 4, with a typical thickness tolerance of ±0.8–1.2 mm and Type I as the default grade unless Type II properties (ozone, flame or oil resistance) are specified. On projects where an IEC-based specification sits alongside an ASTM-referenced inspection requirement, state both on the order and ask for a single test report covering the class test voltage.

8. Frequently asked questions

Can I use a thicker mat for a lower voltage?

Yes, provided the class is correct. Thickness above the recommended value is permitted up to the maximum for that class, and it buys durability rather than rating. Extra thickness is not a substitute for the correct class.

Can I use a thinner mat for a higher voltage?

No. The class carries the voltage rating, and thickness cannot be reduced below the range that class requires.

Is a thicker mat always better?

Not for the electrical function. Above the maximum thickness permitted for the class, the product sits outside the standard's own window, which is a specification problem even when the extra rubber looks like additional protection.

If you are specifying mats for a switchgear room, a substation bay or a distribution board, we can confirm the class and thickness against your network voltage. See the IEC 61111 high voltage insulating rubber mat and the 10 kV / 35 kV standard insulating mat for the thickness and roll options held in stock.