The PBwel was established in 1937 to manufacture electrical instruments.
In the mid-1940s, the company identified a requirement for gas and oil operated (Buchholz) relays to protect oil filled transformers and the first production units were installed at that time. A continual development programme, in conjunction with the Central Electricity Generating Board, led to the production of the Mk10 Buchholz relay.
All double element relays are adjusted so that their performances lie within the limit specified. Alternative values may be available upon specific request. These switches have Rhodium contacts, located midway along the length of their glass tubes. The tubes contain anatmosphere of Nitrogen.
Connected in series with each reed switch, and mounted within the terminal box, is an inductor of approximately 30 microhenries and 0.04ohm. These inductors are intended to protect the reed switch contacts from the effects of capacitive loads, such as are imposed by long leads or pilot cable, and must not be removed from relays in service.
Protection of reed of reed switch contacts against the effect of inductive loads are imposed by Tripping Relays which is achieved by means of a diode wired across each load. This diode must be rated with forward current at least be rated as the steady load current and connected observing polarity so as to absorb the back e.m.f. A protection unit, designated D2, fitted with suitably rated
diodes for this purpose.
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