Control Relay Ratings

Control relays are mostly used to switch Power contactor coils, usually AC. The inrush current of a contactor coil is 10 times the steady-state current. That is why relays are rated like this.

This is actually AC15 rating in IEC60947-5. There are other ratings also, depending on application. Maybe interesting for you....

As the relay contacts approach closure (make) there is no arc, so they can handle higher current. As the contacts begin to separate (break), there is an arc. Excessive current would burn the contacts, so the break capability is less.

(AC arcs are also easier to break than DC arcs, because the AC voltage goes to zero within half a cycle. Thus relays often have separate AC and DC break ratings, with the DC capability being less.)

But specifically as to "why" (although ultimately there are no absolute answers to "why" questions. Contacts keep current from flowing by haveing a gap. That gap provides a dielectric separation of the potential for the current to flow. In air, the amount of dielectric strength increases with distance and decreases as things get closer. As a set of contacts begin to close, at some point the air gap separating them becomes so small that the current jumps the gap slightly in advance of the final closure and when it does, an arc happens. That arc will be at extremely high temperatures and essentially melts the contact surface a little. but because the contacts are in the process of closing, the duration of that arc is very very short, sop very little damage is done.

But when a set of contacts is opened in order to again interrupt the flow of current, the gap distance increases more slowly and once the arc starts, it ionizes the air and vaporized contact material, which helps to maintain it until eventually the distance increases the dielectric again to an amount that will extinguish it. So the damaging heat of the arc is sustained for a much longer time and to prevent significant damage and premature failure, the contacts must have as much mass as economically possible. One way to accomplish that is to select contacts based upon a rating of what the contacts can interrupt, or "break" since it is a lot easier to "make" the circuit.