This article details the reasons we felt we needed a Built-In Relay Self-Test tool in our switching matrices and what it can do.
Switching systems are a vital part of any test system, they allow users to connect the tests equipment to the unit under test (UUT) in different ways, ensuring that a common set of equipment can be easily connected to different parts of the UUT during the test process and therefore reducing the quantity of test equipment required to carry out the test.
It is true that switching based on mechanical devices has a limited life, but the life of modern relays is very high. Typical EMR's often have lifetimes quoted of the order of 100 million operations under light load conditions, and instrument grade reed relays have lifetimes in excess of 1 billion operations.
The key factor that effects test systems is the load characteristics and the conditions under which the relays are operated. Whatever the type of mechanical device, as soon as the relay has to close or open, signal connections that have significant voltage or currents present the operation of the relay becomes a "hot switch" event—at the point the contacts close or open they are carrying a signal that generates an arc inside the relay which erodes the precious metal contact materials. Relay life is strongly influenced by the load present during these hot switch events, a variation of three orders of magnitude (1000) is a common change in life time between a light load and a full load. Designers try to avoid hot switching, but the reality is that some tests require hot switching of the signals to keep test times low, avoid having to restart systems between tests or to simulate conditions such as intermittent faults. Hot switching is a compromise that designers have to manage in their test systems.
Even so, many failures in test systems are not caused by the relays reaching their normal end of life, a few are due to infant mortality caused by manufacturing defects that are not detectable by testing. Many more though are caused by accidental events that occur in the system. A common source of accidental events is the integration of the test system—cabling and software errors can accidentally connect parts together that were never intended to be connected, for example, accidental shorts on power supplies or accidental hot switch events into capacitive loads. The relay may withstand these accidents, but the relay could have partial damage to its contacts which will shorten its operational life as the system is used and ages. Even when the system is operating correctly an attempt to test a faulty UUT can stress the switching system, forcing operation beyond the specification for the relays.
The vulnerability of the switching system has resulted in users requiring a system check that includes a way of testing the switching system. Some platforms, such as VXI, have historically offered a self-test facility for the relays. The degree of coverage such a self-test gives though is patchy, sometimes it simply tests the control system and not the relay contacts (the most likely part to fail), some products test the relay contacts but are unable to test isolation relays. VXI products have included this capability because the prime customers (usually military or aerospace) have demanded it and there is enough room in the product to allocate space to self-test harder.
Smaller footprint products, such as PXI, have not provided self-test. The burden on the design to include self-test has been high in terms of space, which reduces the density that can be achieved in the module and increases cost.
For these lower cost and space constrained products, vendors have added perhaps one of the most misguided tools as a way of managing the issue—relay operation counting. In relay counting the software counts how many times a relay has been operated, the intent then is to replace the relay when the number of operations reaches some threshold. As a tool this method is deeply flawed for a number of reasons:
- Relay life changes by three orders of magnitude according to the load present. The software has little or more commonly no knowledge of the load; it is simply a counting system
- It takes no account of the accidents that happen in systems, such as UUT failures, that shorten relay life
- Real relays are subject to significant variations by batch in their life, and the datasheet estimates should reflect the worst batches rather than the best batches
The situation gets worse if the user then performs maintenance based on relay operation counting. Modern devices always work better and longer the fewer the operations carried out on the assembly. Early "preventative" replacements risk disturbance of other parts and damage to tracks, particularly in the case of surface mount parts. There is much to be said for a philosophy that says "if it's working leave it alone until you have good reason to suspect it is going to or has a problem".
The status on self-test has changed. Pickering Interfaces launched the first of a new generation of PXI matrix solutions in 2009 which includes a built-in self-test facility, called BIRST or Built-In Relay Self-Test and eventually introduced it to some additional PXI matrices as well as some of their LXI solutions (the product web pages and datasheets will note whether BIRST is present). BIRST is implemented as a very compact hardware addition to some of Pickering's PXI and LXI modules that allows the supplied software to explore a matrix switch, measuring each path in the matrix for path resistance with repeatability measured in a few milliohms. Every relay is checked for its operation so welded closed and open relays can be quickly found, the resolution of the measuring hardware allows variable contacts to be identified. The location of individual faulty relays is identified rapidly, in most cases without ambiguity or to within a very few devices.
All the user needs to do is to disconnect the PXI module from the test system and then run the supplied program. The test process is fast, each relay requiring just a few 10's of milliseconds to test fully. The software then displays the test results as a graphical representation of the matrix, highlighting the relays that are faulty and allowing the user to quickly identify the physical position of the faulty device within the PXI module. The use of thru-hole relay components allows users to simple service the module with commonly available tools and get the module back in service after running BIRST again to check that all faults have been cleared.
The BIRST tool addresses the shortcomings of the older self-test systems and the relay counting methods. It allows users
to get the maximum out their relays in and the modules that contain them with little additional investment, only identifies
those relays which need attention for maintenance and avoids unnecessary disturbance and system downtime. The tool can identify
relays with higher than expected path resistance, allowing users to change those relays before they fail.
The BIRST tool also compliments system level tools. When running diagnostic tests the system level tools can concentrate on the testing of the cable interfaces and switching systems without BIRST, leaving exploration of the BIRST enabled matrices to the BIRST tool.
Some care should be taken in trying to relate BIRST results to the datasheet specifications. BIRST measures the switching system by accessing it through a different path to that which a user would use (via the front panel) and in exploring the switching system, it uses different paths to probe and discover faults. These paths often have much longer connection lengths than the user based connections and include many more relay contacts, so often the resistance results can appear to be higher.
In addition, the datasheet specifies the path resistance at the point of manufacture. As the relays age the resistance may increase depending on the switching conditions experienced and the type of relay, though in many cases an increasing resistance may also mark the early onset of end of life issues.
In 2015 a new set of external test tools called eBIRST was added to provide a much higher degree of cover that includes non-matrix parts. For more information see eBIRST and BIRST.