I should have added this...
look at www.abb.com/substationautomation
pick the tab 'transmission products'
Once there, highlight COMBIFLEX.
you will find the RXMVB relays on the COMBIFLEX home page.
an adder to my previous msg
acc to page 139 of PRAG ( where you can see the curves) the formula for the extremely inverse curve is t=80/(Isquare-1) where t is op time in sec and I is current (multiple of plug setting)
There is some amount of documentation available.
these relays typically belong to an old GSX generator protection from ABB/BBC.
Where are you located? send me a note & I will help you out
rgds