The breaker is sized for the cable, and the clock tells you the current
A domestic breaker is a thermal-magnetic device, and the name is the whole explanation. The thermal part is a strip of two bonded metals that bends as it heats: current makes it hot, it bends, and at some point it trips the latch. Because heating takes time, this part is inverse-time — 1.45 times its rating may carry for the best part of an hour, twice its rating for about ten minutes, five times for twenty or thirty seconds. The magnetic part is a coil around a plunger, and above roughly five to ten times the rating the pull is strong enough to trip directly. That happens in milliseconds, long before the bimetal has warmed up.
Which gives the most useful inference on this page: how long the breaker took is a measurement of how big the current was. If it trips the instant you push the lever up, the current is enormous — a short circuit, or a hard earth fault — and no amount of re-closing will change that. If it runs for ten minutes and then trips, the current is merely too big for the circuit: an overload, and the answer is fewer appliances rather than fewer amps. Two very different repairs, and the difference is the stopwatch in your head.
The third device on the panel measures something else entirely. An earth-leakage trip — the one with the test button and, usually, the widest body — passes both the live and the neutral through a small toroidal transformer and compares them. Current out must equal current back; anything that leaves by another route, through a person or through wet masonry, shows up as a difference. At 30 mA and within a tenth of a second it trips. That threshold is chosen for people, not for cables: 30 mA is about the largest shock a healthy adult can let go of. It is also why an RCD has no useful "try again" — it is not reporting a big load, it is reporting a path where there should not be one.
Three faults, three answers
Trips after a while, and the lever is warm is an overload. Do the arithmetic once: on a single-phase 230 V circuit, 16 A is about 3.5 kW, and that is the whole circuit, not the socket. A 2 kW heater and a 1.8 kW kettle together are 3.8 kW, so one of them has to go somewhere else. Spread the big loads across ways, keep high-power appliances out of daisy-chained extension leads, and check the plugs afterwards: a plug that is warm to the touch is a contact going high-resistance, which is a fire risk in its own right and worth more attention than the tripping.
Trips instantly, and will not stay up is a short. Unplug everything on that circuit — not switch off, unplug — and try once. If it now holds, plug them back one at a time until it trips, and the last one is your culprit: look at its plug and flex for scorching, cracking or water. If it will not hold even with the circuit empty, the fault is in the fixed wiring, and that is the end of the DIY part.
The main switch with the test button has fallen is earth leakage. Re-close it properly — many types need the small reset button pressed first, and pushing the lever alone feels blocked. Then look for water: a bathroom or outdoor socket, a washing machine, a water heater, a kettle that boiled over, a wall that has been getting damp. Unplug the suspect and try again. If it trips with everything unplugged, the insulation itself has dropped and needs a megger test — an electrician, not a longer extension lead.