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Are these always the same relays?
Might check contact ratings and loads, especially if switching DC inductive loads like large 24 VDC solenoids etc. Even if the controls are AC, switching DC inductive (coils, small motors etc) puts a lot more arcing and heat on a set of contacts over time when they open as opposed to AC inductive or AC and DC resistive loads.
Back EMF off of these low voltage DC coils can be several hundred volts and scramble a PLC program, even compromising the firmware as well - been there and done that on new microprocessor controls retrofitted on older 24 volt diesel engines with DC fuel and overspeed shutdown solenoids.
Other problems - shorted wiring and/or devices going bad if always the same contacts. AC motor starters hanging up or coil failures will burn a set of relay contacts feeding them, especially with single phasing like this in older NEMA starters with no single phase protection.
Back EMF off of these low voltage DC coils can be several hundred volts and scramble a PLC program, even compromising the firmware as well - been there and done that on new microprocessor controls retrofitted on older 24 volt diesel engines with DC fuel and overspeed shutdown solenoids.
Other problems - shorted wiring and/or devices going bad if always the same contacts. AC motor starters hanging up or coil failures will burn a set of relay contacts feeding them, especially with single phasing like this in older NEMA starters with no single phase protection.
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