Electromechanical relays are perhaps the most widely used relays in ATE applications today. They are made of a coil, an armature mechanism, and electrical contacts. When the coil is energized, the induced magnetic field moves the armature that opens or closes the contacts. See Figure 1.
Electromechanical relay: Current through the coil creates a magnetic field that moves the armature between the contacts
Electromechanical relays support a wide range of signal characteristics, from low voltage/current to high voltage/current and from DC to GHz frequencies. For this reason, you can almost always find an electromechanical relay with signal characteristics that match given system requirements. The drive circuitry in electromechanical relays is galvanically isolated from the relay contacts, and the contacts themselves are also isolated from one another. This isolation makes electromechanical relays an excellent choice for situations where galvanic isolation is required.
The contacts on electromechanical relays tend to be larger and more robust than some other relay types. The larger contacts give them the ability to withstand unexpected surge currents caused by parasitic capacitances present in your circuit, cables, etc. An unfortunate tradeoff, however, is that the larger contacts require larger package sizes so they cannot be placed as densely on a switch module.
While the mechanical construction of electromechanical relays allows for much flexibility in switching capability, they have one important limitation: speed. When compared to other relays, electromechanical relays are relatively slow devices -- typical models can switch and settle in 5 to 15 ms. This operating speed may be too slow for some applications.
Electromechanical relays typically have shorter mechanical lifetimes than other types. Advances in technology have increased their mechanical lifetime but electromechanical relays still do not have as many possible actuations as a comparable reed relay. As with any relay, the amount of power being switched and other system considerations can have a significant impact on the overall lifetime of the relay. In fact, the mechanical lifetime of an electromechanical relay may be smaller than that of a reed relay, but its electrical lifetime under a similar load (particularly a capacitive load) might decrease at a much slower rate than that of a reed relay. The larger, more robust contacts of an electromechanical relay may often outlast a comparable reed relay.
Electromechanical relays are available in both latching and non-latching varieties. Non latching relays require continuous current flow through the coil to keep the relay actuated. These are often used in applications where the relay must switch back to a safe state in the event of a power failure. Latching relays use permanent magnets to hold the armature in its current position, even after the drive current is removed from the coil. For very low-voltage applications, latching relays are preferable because the lack of coil heating minimizes thermal electromotive force (EMF) which can affect your measurements.
Electromechanical relays are used in a wide variety of switch modules. Their robustness makes them well suited for many applications, particularly where switching speed is not the highest concern, and their versatility means you can use them on all types of switching configurations including general purpose, multiplexers, and matrices.