Handbook of Electrical Installation Practice - BeKnowledge

Distribution Transformers 339
Also important is how a material reacts when burning. Silicone liquid forms a
silica crust and this assists in extinguishing flames. Some tests on the burning characteristics
of epoxy resin showed that when heated with a bunsen burner, the material
continued to burn when the flame was removed but did not when a welding
torch was the medium of the heat. Manufacturers of cast-resin transformers claim
that their designs do not make any appreciable contribution to the fire hazard
because of the very high non-flammable filler content. Many have been installed
within the past few years and are giving satisfactory service.
Class H dry-type transformers generally have superior fire resistance to most
other types, the use of suitable flame-retardant grades of insulation giving further
improvements in safety. The utilisation of an open winding construction, in conjunction
with flame-retardant insulation materials, has resulted in the development
of the Securamid design of dry-type transformer (Fig. 13.10). Combustion tests, conducted
on sample windings, have shown this type of transformer to have the best
fire-resistant properties of all the available dry-types.
TAPPINGS AND CONNECTIONS
This chapter, dealing as it does entirely with distribution transformers, will consider
only tappings normally selected with the transformer disconnected from the supply.
Fig. 13.10 A 500kVA dry-type Securamid distribution transformer
(Goodyear Transformers Ltd).