How do I select only line features in one layer that are connected to line features in another layer?

[MichaelHeing]

To explain further, I am working with a gas distrbution network with ArcFM. I have a Gas Service Line layer with an attribute field Pressure Class and a Gas Main Line layer with an attribute field Pressure Class. I need to select all of the Service Lines of a specific pressure class (either 2, 5, 6, 7, or 😎 that are connected to a Main Line of a different pressure class.