Over Speed

Yes, you need both, to cover all situations that could exist. I agree totally with previous posters on this.

Some additions to replies already posted............

Overfrequency protection works from the supply made by the generator(s). It may protect loads connected to supply by opening the load breaker without any association with generator. Also it may trip off generators to protect system/loads by reducing generation quickly to lower system frequency - many smaller generators have no requirement to have frequency governing on-line but connection licence has over-frequency shutdown requirement. Overfrequency detection is usually below 105%.

On turbine-generators, overspeed protection is not always mechanical - often electronic using magnetic speed probes & trip amplifiers. It may be required to act for coupling failure to generator or even shaft failure [requiring "excessive acceleration" detection] and usually acts at 10% overspeed for a 120% design/factory test limit on turbine/generator.

Even with activation at 110%, the final overspeed may be near 120% when detection & fuel/steam shut-off valve delays & residual fuel/steam downstream of valve are considered. Probe frequency is several kiloHertz, because at 50Hz a measure of frequency/period cannot be made in less than 1/2 period, 10 ms, and such delays matter.