In outdoor switchgear the 165-kV circuit breakers with oil-blast pots supplied in 1932 for the Dnieprostroi system in Russia gave a further lead to British manufacturers. However, these very high voltage breakers used a great quantity of oil, and shorter opening times were becoming essential for maintaining stability on large interconnected systems. Both objections were met by C. H. Flurscheim's impulse circuit breaker, which was used on an installation at Lydney in 1938 and again at Littlebrook in 1940. Here a single break was mounted at one end of an oil-immersed condenser bushing, and the oil was mechanically forced across the contacts at the instant of operation. Although this design was later replaced by the simpler airblast breaker, it is still one of the fastest 132-kV interrupting devices ever built.

None of these ventures could have been attempted with only limited access to generators that happened to be on test. Towards the end of 1932 a welcome palliative was provided by two old 25-c/s generators, which were installed on dynamo test for the use of the switchgear engineers, and in 1934 came a great advance— the building of an A.E.I, switchgear testing station on land adjoining the west works. Designed by the Group companies concerned and owned and operated by an A.E.I, subsidiary—the Switchgear Testing Company Limited—the station is one of those approved by the National Physical Laboratory as competent to issue certificates of circuit-breaker performance. It has enabled the Company to check all new designs of switchgear and thus ensure the high degree of reliability that is essential on the interconnected systems of today.

The equipment of this testing station had to be specially designed to withstand severe electrical and mechanical stresses due to repeated short circuits. That the special generator and a 25,000-kVA 260,000-A testing transformer originally supplied by M-V are still in use is fair evidence of good design and workmanship.

INSTRUMENTS AND METERS
Instrument and meter developments were headed by the redesign of the whole range of switchboard instruments to fit into sheet steel cases, a departure from British practice that resulted in large sales of instruments to other switchgear manufacturers. Next came a revised design of watt-hour meter having an improved electromagnetic system and at first a cast iron frame, case and cover; by 1934 bakelite cases and covers had been developed, and large moulding machines were installed.

Soon afterwards came a die-casting section for making instrument parts from zinc-base alloys, using automatic plant developed for the purpose. Between 1934 and 1938 new prepayment mechanisms, voltage regulators, and protective relays were introduced, and the house service meter was provided with refinements such as micrometer adjustments and accuracy over an increased range.