Power Transformer with two Buchholz relays
According to this pdf, (see page 16) that would be a relay that signals when the seal that is used to prevent air from getting in contact with the oil is ruptured.

So, you have a Buchholz relay to detect arcing in an oil filled transformer, and then you have the other thing to tell you when the seal around the oil container is bad and you've got air in contact with the oil - which can lead to moisture in the oil and therefore to arcing.