Am working on caterpillar gas engine 3516.
The unit will start very well, put it on load.After about four hours, the unit will dropped, registring on HMI. High RPM.
What could be the cause?

I guess you are trying to say that after about four hours, the unit "trips" (the engine shuts down because of some fault), and that according to the HMI the speed increases very quickly.

USUALLY, there are alarms from the engine control system and/or the electrical system to which the unit is synchronized to indicate why the unit tripped. A trip usually occurs when the fuel is shut off very quickly because of some dangerous fault condition that has been detected--and alarms on the HMI usually alert a conscious operator to the particular problem. (Often times there are several alarms that all seem to occur very close to each other, but it should be possible to determine which one of those conditions actually resulted in the trip of the unit. It may take some analysis and a process of elimination--but often troubleshooting is just that: a process of elimination.)

When a prime mover (such as a CAT engine) has it's fuel shut off very quickly because of some fault condition, the generator breaker also opens. If there is a time delay between the opening of the generator breaker and the shut-off of fuel flow then because the load on the unit goes away very quickly (when the generator breaker opens) the extra fuel flow will cause the prime mover speed to increase very quickly. So, it's likely that's what you are trying to describe: the generator breaker opening (removing the load from the engine) and then for some reason the fuel doesn't get shut off very quickly and the engine speed increases very quickly--maybe even to the point that the overspeed protection function finally shuts off the fuel flow-rate to the engine.

If the engine is not connected to some kind of distribution system with other generators and their prime movers and is driving a load or loads independent of other prime movers and their generators and something in that distribution system causes the load to either decrease very quickly OR to disappear entirely without the engine control system knowing that the load decreased or disappeared then it's very likely that this is a problem with the controls of the power generation system.

This is the most likely reason for the engine overspeeding: a sudden decrease of load or a sudden loss of load without an immediate reduction in fuel flow-rate or an shut-off of fuel flow to the engine. This would explain how the speed could increase so quickly.

Please write back to let us know what you find. If you can post the alarm list from just prior to the even in question to a few seconds after the alarm event we can maybe help to determine what happened--presuming the engine control system is "aware" of all load changes caused by breaker(s) opening in the distribution system.

Hope this helps!!!