December 2016

Enclosure Sealing to Prevent EMI Leakage

The silicone gasket around the door or access panel of an electrical enclosure has two jobs. Not only does it keep dust out, but it also keeps electrical noise in. Or it does if made from the right material.

The Noise Problem

Electrical noise, sometimes called electromagnetic interference (EMI) or radio frequency interference (RFI) is a problem in some environments. By interfering with wireless transmissions EMI makes phone calls difficult and disrupts wireless data transmission. It can also induce currents in other conductive materials. That can lead to spurious data signals, possibly giving rise to false alarms or affecting process control equipment.

Conversely, sometimes sensitive electrical equipment needs shielding from environmental EMI. Placing it in a metal enclosure keeps the EMI outside, avoiding problems of signal interference.

EMI Sources

EMI travels through air, spreading out from a source much like ripples on a pond. Circuit breakers, relays, transformers and switches can all produce EMI. This is one reason they’re often placed in an enclosure. Motors, power cables and welding equipment also produce EMI, but aren’t so easily shielded. However, sensitive electronics near these items may need to go in an enclosure.

Conductive Shielding

When waves of EMI meet a conductive material the energy spreads out over the surface rather than continuing on. Gaskets and flanges however provide an opportunity for leakage.

With most enclosures, closing the door leaves an uneven gap. That could allow dust inside, which is the main reason a gasket is fitted around the opening. Electrical enclosures are usually sealed with silicone gaskets, which offer good compression and stand up to elevated temperatures. However, silicone is not naturally electrically conductive. That results in a leak path where EMI can escape.

Ask for Electrically-Conductive Gasket Material

When replacing gaskets around an electrical enclosure, or specifying new, always consider the need for EMI shielding. Electrically-conductive gasket materials are available, but it’s important this issue is raised when speaking with the material supplier. In many cases conventional silicone gasket material will be sufficient, but if EMI could be a concern, let your vendor know.

Fiber or Rubber Gasket Material

One of the most important properties in a gasket material is compressibility, and this leads many gasket buyers to think they need rubber. In many cases though there is an alternative: fiber gasket material. “Fiber gaskets” is a broad heading as there are many different types. Here we’ll explain what “fiber” means, how it differs from rubber and other rubber-like elastomeric materials, and when you might want to use it.

What is a Fiber Gasket?

Fiber gasket material is made through a process similar to papermaking. Strands of fiber are spread out and impregnated with a resin material. This dries to form sheets that are easily cut to shape.

Many different type fibers are used to produce gaskets with differing strength, compressibility and temperature ratings. These range from vegetable fiber and cellulose to more exotic materials like aramid, (a strong, heat resistant synthetic fiber.)

When additional compressibility is needed cork or a rubber binder (often NBR) is added. Alternatively, cellulose fiber material can be vulcanized to make a paper-like material that’s both hard and lightweight. When this has electrical insulating properties it’s known as “fish paper.”

Possibly the oldest type of gasket still in use is the vegetable fiber or “Detroiter” gasket. This is made from vegetable fibers impregnated with a glue-glycerine compound. It remains a popular choice in some applications.

Fiber Gasket Properties and Applications

Fiber material makes gaskets with high tensile strength, (so they’ll resist internal pressure,) and an upper temperature limit of 250 – 350°F. They have excellent resistance to chemicals, particularly oils, so are used in many industrial situations, especially chemical and petroleum product manufacturing.

The Contrast with Rubber

While true rubber is a natural product, the rubber used in gaskets is almost always a synthetic version. Synthetic rubbers function over a wider temperature range and are less vulnerable to damage by UV light. Commonly used materials are nitrile-butadiene rubber (NBR), styrene-butadiene rubber (SBR), neoprene and EPDM. Available in a range of thicknesses and grades of hardness, these have generally good compressibility but are vulnerable to the effects of petroleum oils.

Hennig Gasket & Seals manufactures custom gasket from a large variety of fiber gasket material.  Contact us today for a fast quote.