Total Cost of Ownership
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.
Additional information on the TCO Calculator can be found in the FAQ section below. Please contact our technical team at 800.879.7918 / , or via our Contact Us page, if you can't find the information you need or if you'd like to discuss your specific application in more detail.
Frequently asked questions
What is the purpose of the TCO calculator?
The intent of the TCO calculator is simple; we like providing engineers with tools to help them make an educated decision when selecting a power switching solution. While we’re obviously biased and would prefer that every solution involve an HBControls solid state relay or power controller, we genuinely care about our customers and their applications. Therefore, we want to help them select the best solution for their application needs, which sometimes just might be something other than a product we offer. With regard to how often a product is utilized within the application, the TCO calculator does just that.
How accurate are the calculations in the TCO calculator?
In the simplest of terms, they’re about as accurate as we can make them using Excel. The math itself is relatively straightforward and actually much easier to perform on a calculator than it was to create the formulas in a spreadsheet. However, we also understand that some people don’t like or have the time to do math, hence the calculator. We can say that it’s been tested multiple times over the years by multiple users, and we’ve yet to identify a mistake. This is probably due to the fact that many mistakes were found in the initial drafts, which were all corrected before release. However, that doesn’t mean that there isn’t something buried somewhere that we simply haven’t stumbled across. If you happen to find anything or doubt your results, please contact us at 800.879.7918 / firstname.lastname@example.org so that we can correct it accordingly.
Why is “Heat Sink Cost” not required for HBControls power controllers?
An HBControls solid state power controller is a solid state relay pre-assembled onto a thermally efficient heat sink. We included individual entry fields for the solid state relay and heat sink cost for customers that purchase those components separately. If you are using a solid state relay / heat sink assembly (a.k.a., an HBControls solid state power controller), then you only need to enter the cost of the full assembly in the “SSR Cost” field, leaving the “Heat Sink” field at $0.
Why can’t I change the life expectancy of the solid state relay in section B?
Unlike electromechanical relays, a solid state relay has no moving parts. Therefore, the life expectancy of a solid state relay is not primarily dependent upon the number of operations it’s subjected to in the application. A solid state relay mounted to an adequate heat sink and continuously carrying load current within its specification has an MTBF (mean time before failure) of >700+ years. Translating MTBF into a number-of-operations specification becomes difficult, since that would primarily depend upon thermal excursions to which the relay is subjected (how quickly the solid state relay’s internal temperature increases and decreases during normal operation). If a solid state relay is mounted to a large heat sink and frequently switching load current well below its specification, then its internal temperature will not fluctuate much during normal operation and, subsequently, it may have a life expectancy in the hundreds-of-millions of operations range. However, if a solid state relay is mounted to a small heat sink and frequently switching load current close to its maximum rating, then thermal excursions will be higher and life expectancy can decrease as a result.
Solid state relay manufacturers have looked at various methods over the years for estimating life expectancy in operations rather than an MTBF. Since there has yet to be a consensus for how to calculate such a specification, many manufacturers have instead settled upon looking at observed life expectancy. As stated above, this varies significantly from application-to-application but has been known to vary from roughly fifty million operations on to low-end, to over five hundred million operations on the high end. For the sake of simplicity, we populated the “SSR Life” field with fifty million operations.
Why does the type of load impact the total cost of ownership?
Resistive loads are fairly benign with regard to surge currents and, in the context of mechanical relays, arcing across the contacts when they switch power to / from the load. However, the stored energy in inductive loads will try to maintain current flow when the contacts begin to open, creating an arc that can damage the contacts of the relay. The more inductive the load, the more significant the arc. Therefore, the life expectancy of an EMR is negatively impacted by inductive loads, which we’ve accounted for within the TCO calculator.