Relays: Out With the Old?

Is Replacing Old Mechanical Relays via PLCs a Good Idea?

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We do a lot of controls upgrades including panels with a lot of old mechanical relays. Our first response is to simply replace them all via PLCs. Some of the non-variable functions, though, are ideal for relays, and many of the installations are in high-EMI areas where relays do better than PLCs. Customer reaction is mixed. Some want every possible relay eliminated. Others who have had them a long time don't see the need to ban them entirely. Anyone dealt with something similar?

—From June '11 Control Design


A Balanced Response

A control system design is a balance between costs and reliability. Application considerations include: How many I/O points are there? What are the voltage and current requirements for the devices being controlled or supplying input (field I/O)? How reactive are the field devices (inductive/capacitive properties)? How often are the field I/Os changing state? Is maintenance a concern?

For a cost-effective PLC design, high-density digital I/O is desirable to achieve the smallest rack size and the best price per point. Digital I/O is normally low-voltage dc, for example 24 V, and current outputs are typically 500 mA. For those I/Os that do not fit the requirement of the voltage/current specifications for the high-density I/O PLC card, then you can add separate PLC cards to fit those specifications, or a relay can be used by the high-density I/O to either convert the voltage or to amplify the current.

For a reliable system, the designer must understand the properties of the field I/O. Both PLCs and electromechanical relays are concerned with reactive loads in regards to the reliability of the system. A highly inductive field device such as a 24 Vdc solenoid can produce a high-voltage spike that can damage the PLC output or weld the contacts of an electromechanical relay. A common design choice is to use relays in between the PLC and the reactive field device for isolation (interposing relays). This is a way to protect the more expensive PLC card and sacrifice the low-cost plug-in relay over time. Another example of a reliability concern would be field devices that have to change state very quickly and very often. An electromechanical relay has moving internal components, so it can cycle only so fast and will cycle only so many times. A solid-state device such as a PLC digital output has no moving parts and can theoretically cycle an infinite number of times at a very high frequency.

Maintaining the system is always a concern. A PLC has better diagnostics, and when the problem is found, the failed I/O card can be replaced. Relays have no intelligence, so troubleshooting can become an issue depending on the complexity of the relay scheme. If troubleshooting the wired logic is not an issue, then there is no debate on the cost of a PLC card vs. the cost of a plug-in relay.

The bottom line is that there could be a good reason to use electromechanical relays in an old design. It's also quite possible that a PLC upgrade makes perfect sense. Quite often, the combination of a PLC and electromechanical relay system results in the most reliable and cost-effective design possible.
Mike Garrick, product marketing manager—power solutions,

Phoenix Contact,

Not Too Shabby

This question brings up refreshing ideas. We're probably the world's largest manufacturer of solid-state relays, so it's sometimes easy to forget that there might be some applications out there for which a mechanical relay might be better. In the scenario described, leaving banks of relays in place and just changing out the control hardware might not be such a bad idea.

A mechanical relay doesn't care whether it switches ac or dc, whereas with SSRs, it might be necessary to change the relay if the nature of the signal changes. Also, SSRs have both a minimum and a maximum amount of current they can switch. Too little, and there is not enough power to switch on the SSR, and too much can destroy the SSR by creating too much heat. None of this negates the many advantages of the solid-state relay, but I wanted to illustrate how it might not always be a no-brainer when deciding to replace mechanical relays.

The most interesting twist comes when you swap out all the old logic with a PAC, then use the PAC's high-density digital I/O (low-power capability) while leaving the existing mechanicals in place for their higher current-carrying capability. Other advantages of trying it this way would be the added communications abilities and the state-of the-art logic that comes built-in with the PAC. Plus, there are the savings of not having to replace and rewire the existing system.

Tom Edwards, Senior Technical Advisor,
Opto 22,

Strengths and Weaknesses

In most control systems, PLCs and relays are not mutually exclusive. They both have their strengths and weaknesses, depending on the level and types of functions needed in each control system.

Relays are traditionally easy-to-install, economical solutions for simple logic and standard control functions. In these types of applications, relays provide cost savings because of their relative low price and ease of installation and setup compared to PLCs. Wiring a relay for simple, non-variable logic functions is a standard job for an electrical technician, whereas PLCs often require specific programming skills that add cost and time for installation and commissioning. This situation is reversed, however, when logic functions vary, such as in a flexible production environment where the PLC could have different schemes or processes stored in memory and can be implemented instantly by an operator.

While mechanical relays can be sensitive to pickup of EMI on both the control voltage and the output, solid-state relays using opto or pulse transformer technologies provide excellent solutions for high-EMI applications through the use of integral protection on their control and the output circuits. In fact, PLCs in many control systems depend on relays for isolation and protection against EMI on their inputs and outputs, especially in applications where inductive loads are driven. In these cases, solid-state relays can protect the PLC from high-voltage transient feedback when switching off the load.

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