Let's start with these cautions. The cases of sealed relays should not be removed. Avoid ultrasonic cleaning of all types of relays, due to mechanical and submersion issues. When using automatic soldering techniques (e.g., wave soldering), use a flux resistant, sealed relay.
Derating factors for relays controlling significant amounts of power are typically related to the temperature rise of the relay. The following table gives some reasonable guidelines to follow — unless otherwise stated by the manufacturer.
It is also a common practice to derate by 20°C from the maximum rated temperature limit. Operating control relays at a derated value extends the control relay's operating life.
Inrush current is the maximum instantaneous current that flows with some defined condition. This could be the high initial current due to powering a tungsten (incandescent) lamp or charging a filter capacitor in a power supply. Capacitors and lamps are major factors that cause contact welding.
Relays are not perfect — they have a certain amount of contact resistance. If we assume this resistance is 10mΩ (not an unreasonable assumption), switching 20A will cause 4W to be dissipated in the contacts.
Contacts can weld upon opening or closing, but more likely at closing. The high current, combined with higher resistance as the contacts make connection or seat, creates lots of heat due to the IR drop. That can cause pitting. During opening, arcing across the contacts as they separate is the more likely phenomenon. But at least this means the contacts are separated and no welding occurs — just some amount of degradation. This contact arc is also a powerful high-frequency emitter and causes a large amount of EMI. I prefer not to use relays on my tube amps for plate voltage switching in part because of this.
Regarding the contact closure, it takes time to build up the magnetic field in the coil. Then it takes time for the armature to move. The first connection between the contacts and the final closure can be anywhere from microseconds to milliseconds. Contacts bounce when they make connection, which exacerbates the pitting and arcing problem.
Things get worse when relay contacts are opened and are operated above their derated value. Another issue relates to contact contamination by metallic or nonmetallic material. This can be especially problematic if travel of the armature is restricted or contaminants collect on or get embedded into the contacts.
Mechanical wear — just the action of the contacts banging into each other — will degrade the contacts. Add to that the aforementioned high inrush currents or inductive load induced arcing (high voltage and high current spikes) and contact contamination, and it's easy to see how contact welding can occur.
There are many techniques to avoid welding. The use of tungsten contacts can help. Tungsten is one of the more difficult materials to weld, so it's recommended for use only with heavy inductive loads. Snubber circuits across the contacts can help, too. That's a resistor and capacitor network (devices in series) across the relay contacts.
Let's take a closer look at common contact materials and their characteristics.
The reference in the table above to “self-cleaning” has to do with one of the characteristics of silver contacts. If the contacts develop a thin layer of silver oxide (as silver is wont to do), and if the applied voltage is high enough, that layer will get punched through. Then, if the load draws enough current, the oxide layer gets vaporized (from the heating effects of the current). Hence, the contacts revert to being the silver alloy that we want.
Some relays also will use a surface finish material for the contacts. You will see terms like gold clad, gold plating, gold thin-film plating, and gold flash. These materials have nearly the same characteristics as gold found in switches and connectors. The thickness of the gold affects how long it will last — how many switching cycles it remains effective.
Note that, for power applications, you rarely need gold contacts, but for low-level signal switching, gold is indicated. Low-level switching is sometimes referred to as dry switching, which can mean switching currents of 10mA or lower. This pertains to switching audio, video, and sensor signals.
Let me know how relays are used in your designs and what problems you've seen — and what troubleshooting tips you have.