THE FLOW OF CURRENT IN ABOUT ONE TWENTY-FIFTH OF A SECOND IN ORDER TO PREVENT DAMAGE TO GENERATING EQUIPMENT , MOTORS , AND OTHER APPARATUS CONNECTED WITHIN THE SYSTEM . THE DUTY PERFORMED BY THE CIRCUIT BREAKER WHEN A FAULT IS DETECTED IN A HIGH POWERED TRANSMISSION LINE IS EXTREMELY ARDUOUS AND REQUIRES A MOST SOPHISTICATED PIECE OF EQUIPMENT . THE INTERRUPTION OF HEAVY CURRENTS PRODUCES HIGH POWERED ARCS WHICH HAVE CONVENTIONALLY BEEN INTERRUPTED IN OIL , COMPRESSED AIR , OR INERT GASES . THE DEVELOPMENT OF THE VACUUM TYPE INTERRUPTER HAS PROCEEDED IN ORDER TO TAKE ADVANTAGE OF THE FACT THAT AN ARC CANNOT BE SUSTAINED IN A VACUUM , A PHENOMENON WHICH PRODUCES AN EXTREMELY RAPID ARC EXTINCTION AND A NUMBER OF RESIDUAL ADVANTAGES SUCH AS A DECREASED RISK OF FIRE AND THE DURABILITY OF WORKING PARTS .
7 . BOTH ASSOCIATED ELECTRICAL INDUSTRIES LTD AND REYROLLE PARSONS LTD DESIGN , MANUFACTURE AND SELL INDEPENDENTLY OF EACH OTHER SWITCHGEAR APPARATUS FOR ALL VOLTAGES UP TO THE MAXIMUM LEVEL FOR USE IN BOTH INDOOR AND OUTDOOR SUB-STATIONS . VACUUM INTERRUPTERS LTD DEVELOPS , DESIGNS , MANUFACTURES AND SELLS VACUUM INTERRUPTERS FOR INCORPORATION INTO SWITCHES AND CIRCUIT BREAKERS USED WITHIN INDIVIDUAL UNITS OF SWITCHGEAR APPARATUS .
8 . THE DEVELOPMENT OF THIS TYPE OF INTERRUPTER HAS BEEN SLOW . WHILE THE BASIC PRINCIPLES HAVE BEEN KNOWN FOR MANY YEARS , THE ACTUAL CONSTRUCTION AND OPERATION WAS KNOWN TO PRESENT PROBLEMS OF THE MOST COMPLEX AND DIFFICULT TECHNICAL NATURE . THUS THERE WAS THE QUESTION OF ( A ) MAINTAINING A VACUUM ( B ) MAINTAINING THE ELECTRICAL ISOLATION IN SPITE OF THE METAL VAPOUR CREATED BY THE ARC ( C ) AVOIDING EXCESSIVE WEAR ON CONTACTS ( D ) AVOIDING CURRENT " CHOPPING " AND ( E ) AVOIDING THE SITUATION WHERE CONTACT SURFACES WOULD WELD TOGETHER - WHICH CAN HAPPEN RATHER EASILY WHEN TWO SMOOTH AND HIGHLY CLEANED SURFACES ARE PRESSED TOGETHER IN A VACUUM AND ARE HEATED BY THE ACTUAL ELECTRIC CURRENT .
THESE SEVERAL PROBLEMS HAVE BEEN SOLVED IN THEORY DURING THE PAST 20 YEARS , AND MANY PATENTS HAVE BEEN TAKEN OUT IN RESPECT OF THEM , BOTH IN THE UNITED STATES AND THE UNITED KINGDOM , SOME OF WHICH HAVE NOW EXPIRED .
9 . IT HAD BEEN KNOWN FOR MANY YEARS IN THE UNITED KINGDOM THAT RESEARCH INTO VACUUM INTERRUPTERS WAS PROCEEDING IN THE UNITED STATES MAINLY BY THE GENERAL ELECTRIC AND WESTINGHOUSE COMPANIES , ALTHOUGH JENNINGS RADIO AND ALLIS-CHALMERS WERE ALSO BELIEVED TO BE INTERESTED AND ACTIVE .
10 . HOWEVER , RESEARCH INTO VACUUM INTERRUPTERS DID NOT START SERIOUSLY IN THE UNITED KINGDOM UNTIL ABOUT 10 YEARS AGO WHEN ASSOCIATED ELECTRICAL INDUSTRIES LTD , A . REYROLLE AND CO . LTD AND THE ENGLISH ELECTRIC CO . LTD ALL SEPARATELY COMMENCED TECHNICAL INVESTIGATION AND WORK . EACH OF THE COMPANIES FOUND THE COST OF DEVELOPMENT WAS VERY SUBSTANTIAL AND EACH RECOGNIZED THAT IF VACUUM INTERRUPTERS WERE TO BE BROUGHT TO COMMERCIAL USE