Relays for Logic – Current and debouncing

The relays you're using aren't sealed, and over time - depending on the environment in which they're being stored/used - their contacts can become oxidized or contaminated with chlorine or sulfur, causing their closed resistance to become higher than specified on the data sheet.

If light, the corrosion can usually be blasted away and the native silver alloy exposed by hot-switching the contacts at their maximum rated voltage and current. If that doesn't work, then the contacts must be mechanically cleaned.

The reason these relays are failing for you is because their contacts are made from a silver alloy especially formulated to switch highish power, and the low power levels you're using don't have enough "oomph" to blast off the contact contamination and get down to the bare metal. For reliable operation and maximum life of the relay you should switch to a relay designed to handle signal-level power, and it should be hermetically sealed.

For your perusal:

The DC voltage rating of relays and switches is roughly 10% of the AC rating. When you start getting to higher currents the DC rating is even worse.

This is because of arcing when the contacts are broken. With AC the arc is automatically extinguished when the waveform crosses the zero point. Of course, DC doesn't do this, so the arc doesn't extinguish by itself. So you can only support much lower voltages for the same contact distance. And of course, because of this the arcs can last longer, which means the maximum current must be reduced as well.

In general relays are break-before-make since they work by a single leaf moving between two contacts. To get a make-before-break you effectively have to have two completely separate contact systems which operate at different times. It is more normal to use two separate relays and control them at different times if you really need make-before-break.

"Power" relays which handle very high currents are often just on-off, since they have very large contacts which travel over a greater distance.

Another consideration with the relays is the type of load. You have an electromagnet. That is an inductive load, not just a resistive load. The rating is for purely resistive, and doesn't take into account any back-EMF (back electromotive force) from an inductive load. The EMF induced by the collapsing magnetic field can be many times higher than the voltage you are operating at, and this EMF can cause more arcs in the relay contacts further degrading the relay. So for an inductive load you need a higher rated relay than for a resistive load.