When a relay is used for hot switching, and particularly when that relay is switching DC signals, erosion of the relay contact materials occurs. The precious metals that cover the contact area and ensure reliably low contact resistance are steadily damaged by hot switching and eventually the contact resistance starts to rise until the relay is deemed to be at the end of its life.
However, another effect also occurs. The contact resistance can become dependent on the voltage and current that is switched
- it becomes non linear. The usual cause of this is that the contact materials start to oxidize, or pick up other contaminants
in the hot switch process. The effect on relay specifications is that some types of EMR, particularly the higher current
types with ratings above 2A, have to specify a MINIMUM switching capacity. The minimum switching capacity is dependent on
the way the EMR works (for example how much wiping action on contact closure) and the metals used in the contacts. Often
the relays will have no non linear behavior when they are new, the nonlinearities will appear after the relay has been in
use for some time or in cases where the relay has not been operated for a long time.
Fault InsertionHot switching is common in fault insertion applications, so where EMR's are used for switching the minimum switching capacity becomes an issue. If the relays are used for applications where both high capacity and low level switching need to be supported it may well be that the first time the fault insertion relays fails is because of the minimum switching capacity not allowing a relay to pass a low level signal accurately because of high contact resistance. or even an apparent open circuit.
The 40-193 fault insertion switch specifies its minimum capacity to be 1A at 12V because it uses relays that are specified for high current automotive applications. The relay can sustain high inrush currents, but the same materials that are used to get the high inrush capacity also result in a high minimum capacity.
The 40-194 fault insertion switch overcomes this problem by using a composite switching system. Every power relay is supported by a lower current relay placed in parallel with it. The path resistances of the low power and the high power relays are carefully balanced so that the power relay takes the high signal currents while the low power relay ensures that at low signal levels it carries the signal without an excessive path resistance. The low power relay is protected by a thermally operated polyfuse—essentially a resistor with a very high positive temperature coefficient once it reaches a certain temperature and restores it low resistance when that temperature falls again. The software driver automatically sequences the two relays to protect the low power relay, it closes the high power relay first and releases the high power relay last. The sequencing operation is embedded in the driver, so the only change the user notices is that the operating time for 40-194 is slightly extended compared to 40-193.
Through this composite switching system and sequencing of the relays the 40-194 allows the use of both low level signals and high level signals through the fault insertion channel. The 40-193 supports just high level signals.