The CB52 is sold as a SINGLE BOARD and includes a series of eleven precision low value resistors for verification against the system's calibrated values. The precision resistor values cover a sufficiently large range to ensure a valid calibration. We recommend checking once a quarter. Verify correct 4-wire resistance measurements. Requires Model M4 or HVX Series tester with Advanced Measurement option or 4-Wire Measurement option. The 4-wire measurement option can be added as an upgrade to an existing high voltage tester. Note that this is a Plug < Play interface and does NOT require a 4-wire attachment to operate.