Download Solid State Relays vs. Mercury Relays

Solid State Relays vs. Mercury Relays:
Solid State Relays Pull Ahead
By Dan Nigro, Solid State Relays Product Marketing Specialist
Omron Electronics LLC
www.omron.com/oei
Why use solid state relays over mercury relays? The answer is quite simple. First
and foremost, solid state relays contain no mercury, which means they are
environmentally friendly. Secondly, they do not have any moving parts, which
greatly reduces possible downtime. Lastly they can be used in conjunction with
high-speed PLCs and other industrial controls components.
Mercury is considered a toxic substance and is banned in Europe and many
other countries as well. According to Mark Hartwig of Safety-Kleen: “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 was also used in controlling mildew in paints that
were applied to our houses and office environments, and also in weed killers and
the silver fillings that are in most of our mouths.
How dangerous is mercury?
Mercury is so dangerous that when we come in contact with it, it can cause
nervous system disorders and kidney damage. It can also impair our vision,
speech, hearing and coordination. Mercury can make its way into our daily life if it
is not properly disposed. Another question that one asks is, “If mercury is so
dangerous how does it make its way into our daily life?” A prime example is if
mercury is improperly incinerated, it will make its way through the air and
precipitate as rain or snow. Then it descends into our lakes and settles in the
lake bottom in the mud or sand. After it settles in the mud, mercury is taken up by
algae that is eaten by small fish that are eaten by large fish that are ingested by
humans. This is known as the “Mercury Food Chain Dynamics”.
The subject of reducing mercury and mercury products is a serious issue in North
America and is so serious that the last mercury mine was closed in 1990. Today
mercury is being imported into the U.S. from countries that banned it, to be used
in batteries, mercury and fluorescent lamps, dental amalgams, measuring
instruments, thermostats and electrical control equipment. As stated in a white
paper to Congress from the 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.” According to a U.S.
Geological Survey study performed in 1997 “industrial manufacturers who use
mercury are shifting away from its use except where mercury is considered
essential.” The study 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.” With the growth of mercury products
in our industry one needs to find proper way to dispose of it and find suitable
replacement products that do not pose a threat to our environment as well as
ourselves. One such product to replace mercury relays is solid state relays.
Since a solid state relay’s internal components consist of electronic circuitry (that
does not contain mercury) the risk for failure is greatly reduced as compared to
mercury relays. There is also no need to worry about disposal when and if they
do 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, on the other hand,
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 DIN rail whereas mercury relays cannot.
Proper Disposal Issues
When working with mercury relays, one needs to be concerned with disposing of
the relay because of its mercury content. Disposal can be a drawn out process if
the proper channels are not followed. If a mercury relay is not properly disposed,
fines can be issued from local governmental officials as well as the EPA. On an
average, each pole of a 35-amp mercury relay contains about 251.6 grams.
According to the EPA this amount would be considered Low Mercury
Subcategory. But if two and three pole relays are being used, they would be
classified in the High Mercury Inorganic subcategory. Both subcategories need to
be disposed of properly no matter of their status. Along with concerns of its
contents, one needs to be aware of which countries will not accept products that
contain mercury because it is banned in many countries (like Japan and many
European countries). Mercury relays are therefore not a globally accepted
product and the onus is then on the equipment manufacturer to find a suitable
replacement that will possess the same if not better operating characteristics
than their current solution. That suitable replacement is solid state relays. Solid
state relays do not violate of any EPA guidelines or regulations and in most
cases will outperform mercury relays. Another advantage that solid state relays
have over mercury relays is that they are compatible with other industrial control
components.
Signal Reliability Justifies the Change
Many applications today require PLCs, therefore, the need for mechanical relays
and mercury relays is being reduced and the need for solid state relays is
increasing. The advantages solid state relays have over electromechanical and
mercury relays in PLC applications is switching speed, frequency, DC input and
lower power consumption. In most applications the switching speed and DC input
is very important because 24 DC voltage is becoming more and more common in
industrial applications for circuit control voltage. The reason behind this is that
more OEMs do not wish to provide a separate power source to run indicator
lights and other control components.
In closing, solid state relays make a better choice of control products over
mercury relays because there is no need to worry about disposal. They can be
used in multiple applications where mercury relays cannot be used and in
addition, they are environmentally friendly. For information regarding proper
disposal of mercury relays or other dangerous chemicals in your plant, please
contact your local Safety-Kleen office or contact Safety-Kleen at 888/298-8096.
Omron continuously strives to be both good corporate citizen and advocate for
stewardship of environmental resources. These activities are based on Omron’s
corporate motto: “At work for a better life, a better world for all."