Switching systems are a key part of any test system, they allow users to connect the test equipment to the unit under test (UUT) in different ways, enabling that a core set of equipment to be easily connected to different parts of the UUT during the test process and thereby reducing the quantity of test equipment required to carry out the test.
Users consider the switching system to be perhaps one of the most likely parts of the system to fail - but not because the switching system itself is unreliable. Providing this connectivity function places the switching system is in a vulnerable position which can be subjected to abuse or accidents during development and in use. In this article, we will explore the reasons for concern and a way that one manufacturer has found to diagnose these problems.
It is true that switching based on mechanical devices has a limited life, but the life of modern relays is very high. Typical Electromechanical Relay's (EMR's), often have life times quoted of the order of 100 million operations under light load conditions, instrument grade reed relays have lifetimes in excess of 1 billion operations. The key factor that effects in test systems is the load characteristics and the conditions under which the relays are operated. Whatever the type of mechanical switching device, as soon as the relay has to close or open signal connections that have significant voltage or current present, the operation of the relay becomes a "hot switch" event . At the point that the contacts close or open they are carrying a signal that generates an arc between the relay contacts which erodes or stresses 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. System 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 or changing connectivity. 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 initially detectable; 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 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.