The initial purchase price of an electromechanical relay or contactor is sometimes two to three times less (or more) than a comparable solid state relay solution. This often influences design engineers and procurement managers to lean towards EMRs when choosing a switching solution for their equipment. In some cases this may very well be the right decision, depending upon the demands of the application. If acoustical noise, shock & vibration, switching speed and current draw of the relay's coil are not a concern, then opting for an electromechanical solution is understandable. However, what is often misunderstood or underestimated is just how much that choice can cost a company over the life of the product in which the relay is used.
The contacts of a mechanical relay have a finite life, which is usually specified within the product's datasheet. A typical economical EMR will have a life expectancy between 100k and 300k operations, with more expensive contactors reaching a life expectancy of one million operations or more. This may seem like a life expectancy sufficient enough to meet the needs of most applications, but how quickly those cycles accumulate over time is commonly understimated, and often by a significant amount.
Figure 1 depicts the calculated cost of ownership of a solid state relay and electromechanical relay when used in a system with a six month warranty period. In this example, we used $20 for the cost of the EMR and $60 for a comparable solid state relay power controller. We also used $150 as an estimate for the cost of repairing the product under warranty. As can be seen from the chart, the EMR solution is less expensive than a solid state relay solution in applications where the relay switches power to the load fewer than 5x per minute. However, in applications where the relay is subjected to more than five cycles per minute, warranty services can be expected.
In figure 2, we calculated the TCO using the same criteria as above but with a twelve month warranty period more commonly seen in the industrial market. In this example, warranty repairs can be expected in applications with operating frequencies as low as two cycles per minute - approximately nine months after the product first goes into service. Even though the initial cost of the solid state relay was more than the EMR, the replacement and warranty costs incurred as a result of the abbreviated life of the EMR cost the company an additional $130 over a twelve month span. Therefore, the total cost of ownership of an EMR in this application was $190, compared to $60 for a solid state relay solution.
There are obviously a lot of variables involved in calculating the total cost of ownership of a solid state relay and an EMR. The examples above are meant to give a fundamental understanding of how that cost can drastically change as the number of operations in the system increases. Every application is different and must be evaluated according to its own unique operating conditions, type of load, warranty terms and system cost, along with consideration for any impact on brand image and other corporate objectives. The HBControls TCO Calculator is a tool meant to simply facilitate that evaluation.
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