Hi,Kagiso
Normally we use extensible RMU in two cases :
1. The future network structure is unknown and the client
expects that in will be 1-3 additional feedes in RMU but
the client doesn't know how many.
2. Since each manufacturer of RMU has limited line of non-extensible RMU and the max.number of feeders (functions)in one RMU is usually 4,in case of the client needs for example 6 feeders in one RMU it has two possibility
a) to order two 4-functional RMU and to connect them,loosing
two function in connection or
b) to order one 4-functional RMU + two extentions.
The last option is normally cheaper.
We have the experience with Merlin-Gerin RMU RM6 with SF6
insulation. I havn't enough information about reliability
of extensible since we deliver only few of them whereas
non-extensible - some hundreed.
B.r.