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.
For use in situations subjects to seismic disturbances and mining activities such as blasting. Shock and vibration acting along the tube of a conventional Mercury switch can cause the Mercury within it to move and momentarily bridge the switch electrodes, even though the switch is tilted in the open position.
This is considered to be a mal-operation of the relay, in that it is caused by external influences and not by a fault within the transformer. Consequently, where relays are to be used in situations as described above a more suitable alternative to the usual Mercury switch is required. Magnet operated reed switches were selected specifically for this purpose and this choice is supported by the following type tests which successfully withstood.
In the double element relay collection of has causes the oil level within the relay to fall. This in turn causes the upper element to rotate on its pivots, bringing to magnet it carries into a position where it operates the alarm switch. An oil surge through the relay will cause the lower element to rotate about its pivots and bring its magnet into a position so as to operate the tripping switch.
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