Can I touch an exposed main ground terminal?

Answer

You can just touch the main ground terminal. If the grounding terminal were dangerous to touch, it should not be exposed.

If the installation has been carried out in accordance with legislation, the main earthing terminal can never be subject to a dangerous voltage. Compliant means: sufficiently low earthing resistance and presence of a differential (in case of TT net)

A broken toaster can cause the metal exterior of that toaster to come under a dangerous electrical voltage. We call this an insulation failure. However, the metal exterior of the toaster must be grounded, which means it is connected to the main ground terminal. This dangerous voltage is therefore also applied to the main grounding terminal. However, because the main earth terminal is connected to the earth potential (via a necessarily small enough earth resistance), the potential will remain sufficiently low to still be safe. In the case of a TT network (as the vast majority of domestic installations are), the differential will ensure that the fault current is switched off sufficiently quickly. In all cases, the outside of the toaster and the main earth terminal were still safe to touch, thanks to the presence of the earth (with sufficiently low earth resistance) in combination with differential.