ROPS (Roll-over protective structures) are designed to protect the operator in the event of a roll over. Factory built ROPS are designed using physical testing; large rams are used to bend and break the ROPS while the forces and energies required to do so are measured with load cells. ROPS designed by this method are considered to be Grade 1.
Testing this way accounts for every little bit of strength. It accounts for the strength in the windows and the energy required to shatter them; it accounts for the complex way the door handle and the door transfers load to the cab through friction. The key factor is that it allows external elements to break, metal can tear, welds can crack and bolts can yield as long as the space surrounding the operator is protected.
Copies of Grade 1 ROPS that are then certified by a CPEng (Chartered Professional Engineer) are called Grade 2.
Physically testing each ROPS design is great if you are a large manufacturer pumping out thousands of ROPS each year, but it doesn’t make sense for one off custom builds often encountered in New Zealand. Typically, locally built ROPS are designed by simulating a physical test using FEA (Finite Element Analysis). This is a conservative method of working out the strength of a frame or cab. Any complex elements that can not be modeled but may add strength are removed.
Most importantly, since there is (currently) no way to model cracking or tearing material once steel reaches this threshold, it is deemed to have failed. ROPS designed using FEA inevitably turn out larger than physically tested specimens. These ROPS are called Grade 3.
Grade 4 ROPS are a special category reserved for machines that do not meet Grade 1, 2 or 3. It is only used in special circumstances and generally doesn’t exist.
An example of one of these that I have seen was a half height ROPS designed to fit under houses.