The pressure relief valve opens when the pressure inside the transformer’s tank increases over the set operating value and, by venting oil, reduces the pressure in the tank. All models have electrical contacts which can disconnect the transformer, in some cases faster than the Buchholz relay, in the case of major faults.
An optical indicator provides visible confirmation of operation.

Commonly stocked models are NB 150 mm with pressures 0.4, 0.5 and 0.7 bar and NB 80 mm with pressures 0.3, 0.5 and 0.7 bar, see drawing 10.150.30

Available switches are Telemecanique 1 N/O, 1 N/C (4 pole) and 2 N/O, 2 N/C (8 pole) and the switches latch on operation, see drawing 10.080.30

Directed flow versions are available, type VST080 for the 80 mm and type VQT150 for the 150 mm valves, with drawings 10.T80.41 and 10T15.40 on pages 23 and 24 of the full catalogue.

Full 24 page catalogue is available here:  Series VS