TRANSFORMERS
Large three-phase transformers were made from 1931 onwards, starting with 30 000 and 45 000 kVA units for 132/33-kV service on the 'grid' and 75,000-kVA U/66-kV units for the new Fulham power station. A 40,000-kVA 33/11-kV unit built for Sheffield in 1932 was fitted with banks of radiators and is still one of the largest self-cooled transformers. A 35,700-kVA 11/33-kV transformer for Clarence Dock, Liverpool, was constructed in five-leg form to suit the limited headroom.

The scope of on-load tap changers was extended in 1938 by the development of the first British equipment for use with high voltage transformers having fully insulated neutral points and subject to the full test of twice the system voltage. This design now established for systems up to 250 kV, has been used on transformers made by M-V and others for New Zealand, India, Russia and elsewhere m Europe.

With the increasing use of high voltage systems, investigations into insulation materials, the impulse voltage strength of windings, and the ability of transformers to withstand short-circuit stresses were intensified. The principal insulating material was and still is high grade pressboard, manufactured under the control of the research department; this material is now supplied not only to most British transformer makers but also to other parts of the world. Closer cooperation with the manufacturers of transformer steel was successful in producing core steels having lower losses.

For installation underground in collieries the Company developed in 1932 (on behalf of B.E.A.M.A.) a small flameproof tank for 5-kVA single-phase and 3-kVA three-phase transformers; this became a national standard. In the following year came an important innovation: the mining Transwitch equipment, which as its name implies is a combination of transformer and flameproof switchgear and is designed to be moved forward as the coal face advances.

SWITCHGEAR
The cross-jet explosion pot took some time to perfect, and when 66-kV metalclad gear was ordered for Battersea A station it was decided to use circuit breakers with double quick-break contacts. This installation of 1932 set the modern fashion of placing breakers, isolators and busbars in separate fireproof rooms. Another innovation was the use of armoured condenser bushings for all busbars and interconnections. Thus the conductors in each phase were encased in earthed metal, and terminals on each bushing enabled the condition of the dielectric to be tested while the gear was alive. The control board had a mimic diagram, which was completely illuminated, the changing colours giving a continuous indication of the state of the apparatus and circuits.

Extensive tests on the cross-jet pot confirmed the theory that one of the two breaks in a conventional breaker did most of the work. This led D. R. Dayies to introduce single-break metalclad gear, the first example of which was installed in the London Power Company's 66-kV substation at Grove Road in 1934. The