Just relays?

In today’s changing process and control environment, we constantly see advances in instrumentation, control and processing equipment. Our young engineers and technologists have a veritable ‘toyshop’ full of technological wizardry from which to select for tomorrow’s projects. What’s more, the general quality of process and control equipment manufactured today is also considerably high. Yet, among all the technical know-how and sophistication there are some devices that remain misunderstood.

Relays are used in almost every installation, yet the level of understanding of these devices is surprisingly low. They are much ignored, sometimes maligned and often an unknown necessity in nearly all process and control systems. Poor knowledge of relays can lead to inappropriate product selection with negative consequences. Capacitive or inductive loads that were not taken into account during factory acceptance tests can result in burnt contacts or even contacts welded shut.

The selection of the most appropriate electromechanical relays is further hampered by the variety and diversity of relays available today. Many engineers and technicians are overwhelmed by choice and perplexed by the data available from manufacturers, which cannot be easily compared and assessed. Questions such as “Would gold relay contacts be better than silver in this application?” or “Does a high resistance coil result in noise immunity issues?” often go unanswered. However, to improve a system’s reliability and safety, these are the very questions that require answers.

When selecting relays, the maximum current and voltage a relay can switch needs to be considered. Equally important are the relay’s minimum switching values. It is at this stage that the selection of contact material is of utmost importance. Users should closely examine the requirements of their systems before deciding whether to choose gold plated, silver oxide, gold flash or another contact material as an error here could lead to switching faults, or worse, system failure.

The rapid growth in the availability of solid state relays in recent years has been made possible by advances in MOSFET technology. Equally impressive, but not surprising, is the uptake of solid state relays by engineers and technicians. They are increasingly specifying solid state relays because of their exceptional reliability. As they do not have contacts, solid state relays open up a whole new world of possibilities that can be achieved. High speed, long life and ultra-reliability all become possible once the limits imposed by mechanical switching are removed.

When investigating relays, engineers and technicians should also consider that relays not only switch an input signal or an output to a load, they also provide electrical isolation. This fact is often overlooked but is intrinsic to the design of both electromechanical and solid state relays. It is often the reason why a relay is used, especially when signals are sent to, or received from, the field. The isolation level should always be closely assessed to ensure the safety of a system is not compromised.

Relays are used in nearly every industrial application. However, users should not consider them as simply ‘just relays’ because of their ubiquitous nature. They are a critical component of the system and, therefore, an important part of any process and control application.

*David Head has worked for Weidmüller Australia for the past 30 years. He is currently responsible for the company’s Electrical Connectivity and Application Specific Products.