A test rig combines a test stand with significant support hardware that usually sits outside of the cabinet or 19” rack used for the test. This support equipment is usually either too large to fit inside the test enclosure, or needs frequent maintenance access. Examples include tests that need hydraulics, a large electrical power supply, motors to drive the unit under test, vibration tables, and so on.

Sometime this type of support equipment does in fact get put inside the test enclosure because it’s small enough and can be made easily accessible, and this situation makes the boundary between a test stand and a test rig somewhat fuzzy.