Relays fail for a variety of reasons (see article: What Causes a Relay to Fail). Some are accidents, some are caused by manufacturing defects, and some are simply end-of-life failures.
Different relays fail in different ways. Mechanical relays, such as electromechanical relays and reed relays have shorter lifetimes than solid-state relays. The reason for this is that they simply wear out after millions of operations. Solid state relays, on the other hand, have a longer life because there is no mechanical action when they open or close. Even so, electro-mechanical relays are still a very popular choice because they offer lower path resistance and higher operating voltage and operating current.
End-of life failures are the most common type of failure, but using a relay to switch voltages and currents beyond its rated specifications can also cause them to fail. There are also some failures that are related to the length of time that a relay is in operation (particularly when a relay is switching very low signal levels or when a relay is not operated very often and oxidation forms on the contacts), but these are relatively uncommon.
Whatever the cause of failure, it's important that Automatic Test Equipment (ATE) users be able to quickly find and replace failed relays. To make this easier for our customers, we provide not only the information they need, but software tools that allow them to zero in on a failed relay quickly.
When designing a switching system, one of the most important things that you need to know is how long your system will operate without failure. On all relay data sheets, you will find two relay lifetime specifications:
- Mechanical life. This is the life of the relay when operated under low-level switching conditions and specifies the minimum number of operations that you can expect without a mechanical failure from contact wear, coil damage, or an actuation mechanism failure. The reed relays that we use in our switching systems are manufactured by our Reed Relay Division, Pickering Electronics, and have a lifetime of more than one billion operations. The lifetime of electromechanical relays (EMRs) varies widely depending on the relay construction, with the longest lifetimes being around 100 million operations.
- Full load life. This is the life of the relay when hot switching a load at the maximum current, voltage, or power rating. When operated at full load, a relay is said to have failed when its contacts fail to operate (often because they weld together) or when contact material erosion results in an unacceptable high path resistance. When asked to hot switch a load, a relay may only be good for 100,000 operations. On the other hand, if a signal is cold-switched, i.e. is applied only after the relay contacts have been operated, the relay's lifetime will be much longer and approach the relay's mechanical life.
In real switching systems, don't count on relays to last as long as specified. Even in low-level signal applications, accidents and faulty UUTs can cause relay failures, and inrush currents, caused by hot-switching capacitive loads, and voltage spikes, caused by hot-switching inductive loads, accelerate relay aging. The bottom line is that there is no way of accurately predicting failures from the number of relay operations. Below are examples of failed relays.
Some switching systems include relay operation counters to attempt to predict when a relay will fail. Although it may be helpful to know how intensively a relay is being used, it is not a good indicator on its own. Load conditions alone can impact a relay's operating life by more than three orders of magnitude.
Using the number of operations as a predictive maintenance tool, and replacing relays when they have operated a pre-determined number of times, can easily degrade the reliability of a switching system. Replacing a relay can disturb adjacent devices (not just relays), especially if the board uses surface-mount devices. Pickering only uses surface-mount devices when the switching characteristics demand it (for example for RF applications). In addition, there's always a chance that the replacement relay will experience an “infant mortality” failure.
Locating a Defective RelayTo quickly find faulty relays in some of our switching systems, we offer diagnostic test tools:
eBIRST is a set of USB-controlled tools that allow you to find relay failures in almost any Pickering Interfaces' switching product that uses relays with consistent resistance at low signal levels. It performs a test by measuring the path resistance between different pins of a switching system's connectors. This tool is external to the switching system, unlike BIRST, which is built in to the system. Because the tool is external it can be applied to any DC coupled switching architecture and it will test the matrix connector(s).
BIRST is a built-in self-test feature found in many of our high-density PXI and LXI switching matrix systems. These modules include BIRST hardware on the module that can measure resistance and access the switching system via isolation relays. To perform a self-test, the user disconnects the UUT from the module or device and invokes the BIRST software utility. The software utility runs an algorithm which works through the switching system, identifying paths that have an unacceptably high resistance (for closed paths) or low resistance (for open paths). The test results are compared to a set of acceptable reference values for that module or device and identifies marginal or major failures. Using this information, BIRST can identify the specific relay that has failed.
It should be noted that BIRST does not test the integrity of the connector(s) on the front of the matrix. While it is rare, if BIRST passes and problems still persist, you may need to verify the integrity of the connector(s).
For more information on BIRST & eBIRST, take a look at these additional Knowledgebase articles
High Current Relays
Users can also use a DMM to check for relay failures, but some caution needs to be exercised on high current relays. These relays typically have a minimum operating current/voltage that is needed to overcome surface films that can accumulate on the contacts. The contacts have to be designed for their robustness when hot switching signals, so this limits the material choices available to the relay manufacturer. If the relay has not been used for a while, or if it has been used for hot switching loads, the contacts at low current/voltage may exhibit variable or even open circuit values. A DMM typically generates a relatively low output voltage and current when measuring a path resistance, so can return a value that is not representative of the resistance when used with the intended load. A DMM may indicate a faulty relay when in fact the relay meets the manufacturers specification. If in doubt the user should check using the recommended minimum load condition for voltage or current. Pickering Interfaces states the manufacturers specified minimum load data on its data sheets where appropriate, however, the manufacturers caution these numbers can be influenced by various factors and we do so significant variations under some conditions.
Be Wary of High Current Relays
Users can also use a DMM to check for relay failures, but when testing high current relays keep in mind that these relays typically have a minimum operating current/voltage that is needed to overcome surface films that can accumulate on the contacts. If the relay has not been used for a while, or if it has been used for hot switching loads, the contacts at low current/voltage may exhibit variable or even open circuit values.
A DMM typically generates a relatively low output voltage and current when measuring a path resistance, and the contact resistance measured with a low test current may not be equal to the contact resistance under a high current load. A DMM may indicate a faulty relay when in fact the relay meets the manufacturers specification. If in doubt, the user should check using the recommended minimum load condition for voltage or current. We specify the minimum load current on our data sheets where appropriate. Relay manufacturers, however, caution that these numbers can be influenced by various factors, such as testing environment or wear over a relay's lifetime.
Replacing Faulty Relays
Having found a faulty relay, the user then needs to replace it. To help users do this, all Pickering Interfaces devices are supplied with PCB layout information (normally in the manual) that identifies the location of every relay on the PCB. In many instances, a Pickering Interfaces switch module may have a spare relay included on the PCB. This will be noted in the PCB layout information.
Be careful when replacing relays to avoid damaging the PCB or other components. Most Pickering Interfaces switching modules use thru-hole mounted relays to make this easier. Frequent replacement of surface mounted relays carry a considerably higher risk of damaging both the PCB and surrounding components.
Working with System Tools
Neither the eBIRST or BIRST diagnostic tools are intended to entirely replace the use of self-test tools that are built into some ATE systems. System-level self-test tools typically use an external DMM and loopback mechanisms that check for switching and cable harness faults. Since the cable harness is disconnected when running BIRST or eBIRST, these tools will not find faults in the cable or test fixture.
Having a diagnostic tool dedicated to a switching module or device does, however, allow the system tools to concentrate on providing cable and connector diagnostics rather than trying to test the relays and the switching system architecture. As a result, system level self-test tools and BIRST/eBIRST complement one another, and the availability of self-test or diagnostic tools at the switch level saves time and effort in developing system-level diagnostic tools.