Ring Main Units

An extensible system is one whose internal structure and power flow are minimally or not affected by new or modified functionality...an increase in rated voltage is handled by an increase in the insulating gas pressure.... three-position design, having the possibility to close or open or earth the feeder in question....

http://electrical-engineering-portal.com/ring-main-unit-rmu-as-an-important-part-of-secondary-distribution-substations

http://www.schneider-electric.com/en/product-range-presentation/968-ringmaster/

but i was asking for 3 way extensible, four way extensible, and five way extensible - and not 'three position design' - I am sorry but i could not understand it - the difference is not clear to me.

Best Regards,

The 1-5 way are the number of load breaking switches and wiring configurations possible with that unit....Extensible means it can be expanded or added to...You can have no Vacuum breaker or 1 Vacuum breaker or two Vacuum breakers with the 3 way switch option, then you have the Load breaking switches all in one box or mounted separately....

https://www.breconenergy.com/titan