It's not with me, but I have a very fat book about relays. One of the big points very often missed is that, depending on contact material, relays can have a mi

It's not with me, but I have a very fat book about relays. One of the big points very often missed is that, depending on contact material, relays can have a minimum current rating, which can get you in trouble when using a contact to switch a PLC input for example. This to ensure the tarnish gets wiped. You can get around this by using gold flashing, but this is also extremely easy to burn off with unexpected over-current, eg capacitive or inductive switching, which can result from just a cable run. There are also recommendations for things like contacts that are open but wetted with DC for long periods of time, like some protection circuits, eg contact material and which face should be more negative/positive. The optimum DC control voltage is 48VDC, but it is not the most practical due to availability. Look around at metal specific industry groups that often publish highly specific useful material, eg Copper Development Association and same for Silver, MIL specs. I have found you pick up bits and pieces over the years and you just have to build up your personal library over time. Final one, not metallurgy but depending on your industry might save you one day, dangers of AC control voltages with distance. https://www.newark.com/wcsstore/ExtendedSitesCatalogAssetStore/cms/asset/pdf/americas/common/storefront/schneider-electric/RemoteContactorControl.pdf https://www.newark.com/wcsstore/ExtendedSitesCatalogAssetSto... I got a bunch more, if you have a specific problem then DM me.