Relays can fail for a variety of reasons (What causes relay to fail), some are accidents, some are caused by manufacturing defects and others are caused by simply because mechanical relays have reached their end of life.
Despite the fact that solid state relays (Relay Characteristics) can have an operating life which is not sensitive to the number of operations of the relay mechanically based relays are still a very popular choice because they have fewer compromises on path resistance, operating voltage and operating current. Mechanically based relays though do have a limited service life which is usually related to the number of relay operations rather than the length of time they have been used.
There are some failure mechanisms that are related to the length of time of operation (particularly at very low signal levels when a relay is not operated very often and oxidation effects occur on the contacts) but these are relatively uncommon. However, in most cases, end of life failures dominate unless a relay has been used beyond its rated range.
So finding failed relays is a fact of life in ATE systems and users need to be supplied tools that aid their identification. Pickering Interfaces expects its switching solutions to have a very long service life and only use relays that we have independently tested for operation, even so accidents happen (programming errors, UUT faults) and intensively used systems will eventually have failures. So we provide tools and information to help users understand the expected service requirements.
On all data sheets two relay lifetimes for the relays are given.
- Mechanical life. This is the life of the relay when operated under Pickering Electronics low-level switching conditions and describes the minimum number of operations expected without a mechanical failure in the relay. The failure could be due to mechanical wear on the contacts, coil failure or a mechanical failure in the actuation mechanism. For Pickering Interfaces products all reed relay solutions are expected to only have mechanical failures beyond 1 billion operations since we only use reed relays of the highest quality grade manufactured by our sister company Pickering Electronics. EMR's have a very variable mechanical life according to the relay construction - the best being around 100 million operations.
- Full load life. This is the life of the relay when hot switching a load at the limit of the relays rating (current, voltage or power) and the failure mechanism is usually defined as when the contact fails to operate (often because it welds) or contact material erosion results in an unacceptable high path resistance. If a relay is cold switched (rated signal current is applied only after the relay contacts have been operated) lifetime is much less severely impacted, and the relay is more likely to be influenced by is mechanical life. It is common for load life to be stated for 100,000 operations.
In real switching systems, neither of these extreme values are likely to be encountered on all operations. Even in low-level signal applications accidents and faulty UUT's can accelerate failures in the relay, and for hot switching operations inrush currents (caused by capacitive loads), voltage spikes (inductive loads) may accelerate relay aging and switching at below the rated value clearly will increase life. Added to that are variations in relay performance (since these are minimum or typical numbers) with batches so in real systems, there is no way of accurately predicting failures from the number of relay operations.
Relay CountingSome switching solutions include relay operation counters to attempt to predict when a relay will fail. Although it may be helpful to know how intensively a relay might be being used it is not on its own a good indicator. Load conditions alone can impact the relay operating life by more than three orders of magnitude. Using the measure as a predictive maintenance tool (replacing relays when they have operated a number of times) can easily degrade the reliability of a switching system because of the disturbance that relay replacement causes to adjacent devices (not just relays), the risk of introducing a relay with "infant mortality" and even the potential for damaging the PCB when the change is made. This is particularly a problem wen handling surface mount devices - Pickering only use surface mount when the switching characteristics demand it (for example for RF).
Locating a Defective RelaySome of Pickering's switching solutions are supplied with diagnostic test tools to aid the location of faulty relays. Two tools are provided:
These are USB controlled tools that allow relay failures to be found on almost any Pickering Interface switching products that use relays with consistent resistance at low signal levels. It performs a test by measuring the path resistance between different pins of the switching system connector(s). It is a tool which is external to the switching system rather than being built in as BIRST is. Because the tool is external it can be applied to any DC coupled switching architecture.
BIRSTBuilt-in self-test has been a common feature on VXI solutions driven largely by the needs of the defense industry, in PXI the compromise of the board area used for self-test causes a significant reduction in the achievable density and that tended to discourage its inclusion.
PI MXT Diagnostic Tool
The MXT diagnostic tool is available for some PXI modules and requires the purchase of a test interface board that provides a connection between the PXI module and a DMM (not supplied). Like the BIRST tool the user has to disconnect the UUT from the module. A restricted selection of DMM's are supported. A software utility is run that identifies the target module, checks it can be tested with the interface card that has been purchased and then runs a test that identifies the faulty paths and relays using a simple pass or fail criteria.
The functional capability of MXT is dependent on the switching architecture being tested, for designs such as multiplexers and general purpose relays ballast resistors are used to identify paths and that results in the tool being unsuitable for finding high path resistance relays. The test interfaces are also limited to either one or a small number of modules.
MXT are no longer recommended, the eBIRST switching system tools are the preferred test solution and Pickering Interfaces no longer creates new MXT tools.
Be Wary of 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.
Finding the Failed Relay
Having found the faulty relay, the user then needs to replace the relay. To aid 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. Care should be taken when Replacing Relays to avoid damaging PCB or other components. To aid relay replacement most Pickering Interfaces switching modules use thru-hole mounted relays to avoid the support costs of dismounting and inserting relays on an already assembled PCB. Frequent relay replacement on surface mounted components carries a considerably higher risk of damaging both the PCB and surrounding components.