Mercury relays have been used for decades as industrial controls. Is it time to consider switching to solid-state devices?

The internal workings of a solid-state relay consist of electronic components such as triacs and photocouplers, which means there are no moving parts that could contribute to product failure.

Why use solid-state relays rather than mercury relays? The answer is quite simple. First and foremost, solid-state relays do not contain mercury, which means they are environmentally friendly. Secondly, they do not have any moving parts, which greatly reduces the risk of downtime. Lastly, they can be used in conjunction with PLCs and other industrial controls components.

Mercury is considered a toxic substance and is banned in Europe and many other countries. According to Mark Hartwig of Safety-Kleen, Columbia, S.C., an environmental services company that collects, processes, recycles and disposes of hazardous and nonhazardous waste: "Mercury has been used in our society without us even knowing." Some examples would be the preparation of felt hats where mercury was used in the brim, hence the phrase "mad as a hatter." It also was used in controlling mildew in paints that were applied to our houses and office environments and in weed killers.

Mercury is so dangerous that when a person comes in contact with it, it can cause nervous system disorders and kidney damage. It also can impair vision, speech, hearing and coordination. Mercury can make its way into daily life if it is not properly disposed. A prime example is if mercury is incinerated improperly, it will make its way through the air and through rain or snow. Then, it descends into the lakes and settles in the lake bottom in the mud or sand. After it settles in the mud, it gets into the algae, which is eaten by small fish. Those are eaten by large fish, which are ingested by humans. This is known as mercury food chain dynamics.

The subject of reducing mercury and mercury products is a serious issue in North America. It is so serious that the last U.S. mercury mine was closed in 1990. Today, mercury is imported into the United States to be used in batteries, mercury and fluorescent lamps, dental amalgams, measuring instruments, thermostats and electrical control equipment.

Industrial use of mercury is getting a second look by some manufacturers. According to a white paper presented to Congress by the Environmental Protection Agency (EPA), "Data from the Mercury Study indicate that industrial manufacturers are shifting away from mercury use. As a result, domestic demand for mercury decreased more than 75 percent between 1988 and 1996. The EPA believes this shift is largely a result of federal bans on mercury additives in paint and pesticides; industry efforts to reduce mercury in batteries; increasing state regulation of mercury emissions and mercury in products; and state-mandated recycling programs."

In addition, according to a U.S. Geological Survey performed in 1997, "Industrial manufacturers who use mercury are shifting away from its use except where mercury is considered essential." The survey also states "mercury consumption in the categories of electrical and electronic uses and instruments and related products is still growing and is expected to grow due to the increase in the manufacture of computers and other electrical equipment."

Given all these concerns, it would seem prudent to have a two-pronged approach toward mercury-containing equipment in industrial applications. Where possible, consider replacing mercury-containing devices, including mercury relays, with suitable products that do not contain mercury. If mercury-containing devices are replaced -- whether with mercury-free or mercury-containing devices -- the processor must find a proper way to dispose of them.

Solid-state relays do not violate any EPA guidelines or regulations and they are compatible with other industrial control components. They also consume less power than mercury relays and can be mounted on a DIN rail.

Solid-State Devices

A solid-state relay's internal components consist of electronic circuitry rather than mercury. This feature provides two benefits: a mercury-free device and the risk for failure is greatly reduced compared to mercury relays. There also are no disposal concerns if solid-state relays fail.

The internal workings of a solid-state relay consist of electronic components such as triacs and photocouplers, which means there are no moving parts that could contribute to product failure. Mercury relays contain moving parts that can lead to mechanical failure and can possibly fail in normally closed position, possibly causing harm to the equipment. Both products are silent in operation and can withstand high cycle operations. But, the solid-state relay consumes less power than the mercury relay, is smaller than the mercury relay and can be mounted on a DIN rail.

Along with concerns of its contents, one needs to be aware of which countries -- including Japan and many European countries -- will not accept products that contain mercury. In those countries that do not allow mercury, the onus is on the equipment manufacturer to find a suitable replacement that will possess the same or better operating characteristics than the mercury device. Solid-state relays do not violate any EPA guidelines or regulations and may outperform mercury relays. In addition, solid-state relays are compatible with other industrial control components.

Many applications today require programmable logic controllers (PLCs). The growth of integrated process control is reducing the need for mechanical relays and driving the use of solid-state. In PLC applications, solid-state relays provide switching speed, frequency, DC input and lower power consumption than electromechanical and mercury relays. In many applications, 24 DC voltage is becoming more common in industrial applications for circuit control voltage because OEMs do not wish to provide a separate power source to run indicator lights and other control components. The switching speed and DC input features provided by solid-state relays allows OEMs to avoid needing a separate power source.

SIDEBAR: Disposal Concerns Start with One Relay

When working with mercury relays, one needs to be concerned with disposing of the relay because of its mercury contents. Disposal can be a drawn out process if the proper channels are not followed. If a mercury relay is not disposed of properly, fines can be issued from local governmental officials as well as the EPA.

On an average, each pole of a 35 A mercury relay contains about 251.6 g. According to the EPA, this amount would be considered Low Mercury Subcategory. But if two- and three-pole units are being used, they would be classified in the High Mercury Inorganic subcategory. Both subcategories need to be disposed of properly no matter what their status.