There appears to be some joint responsibility here. I assume you are contractors/suppliers/designers of the installed equipment? I would expect some form of brief/specification requirement from your client so that you could select and design the correct equipment that is suitable for the DSEAR application/location. e.g. probably/possibly working under CDM - design risk assessments should have been produced, which would include zone classification, correct group classification/T rating etc etc. Once the equipment is installed and up and running, the equipment is then your clients and under their control. Your company is then not the employer of the people working/operating on/the equipment. Your client then takes on the 'employers' DSEAR duties If you were involved in future maintenance - you would be 'contractors' so far as your client is concerned, so would have to work to their requirements. Again, joint responsibility for safety between the 2 organisations. Your client can't wash their hands of the maintenance activities - see case law of 'Associated Octel', which appears relevant.