Abstract: The present invention provides a pumped helium circuit comprising a compressor (14) with a high pressure port (16) and a low pressure port (18) each connected to a supplied equipment (61,63,65,67) to respectively supply compressed helium to, and receive compressed helium from, the supplied equipment; a pressure relief valve (12) operable to link the high pressure port to the low pressure port in response to a predetermined pressure differential; a non-return valve (13) located between a low pressure side of the pressure relief valve and the supplied equipment; and means for preventing oil carry-over from the compressor to the supplied equipment.