Interrupting Unit and Interrupter

Perhaps you are only familiar with lower class high voltage switchgear. The dielectric strength of the interrupting medium has a finite limit, once that limit is reached multiple identical modules and/or chambers are used to in effect spread the interrupting duty over a greater dielectric/interrupting paths. You can think of a chamber as a subset of a module, where from one to N chambers comprise a module, N is dependent upon the interrupting duty placed on the module.

Googling gets lots of information, try this as a start http://www.hicoamerica.com/Files/HICO.GIS.pdf There are many others by the major manufacturers of GIS systems, simply look at the pictures to see how they are constructed.

Also remember that GIS (Gas-Insulated Switchgear or Gas-Insulated Substation) is only one type of gas-insulated equipment. There are gas-insulated circuit breakers, circuit switchers, etc., and it is the manufacturer's decision how to design it to meet the standards and the operating requirements. Some might use a single interrupter, some might use multiple.

Even in GIS, just because you have seen only breakers with single interrupters doesn't mean there aren't other, different designs out there. Check out the following article from ABB describing their GIS up to 1100kV, with four (!) interrupters in series.

http://www05.abb.com/global/scot/scot245.nsf/veritydisplay/85111ec6e670beafc125759900570136/$file/ABB%20Review%202009-01_92-98_en.pdf