Why doesn't the relay de-activate on my Hardy Instrument?

If you are using a HI 2151/30WC-D2, HI 2160RC, HI 3010, HI 3030, or HI 4050  and you have experienced a situation where solid-state relays have been used to activate a relay or solenoid that failed to shut off when de-energized, the problem is usually caused by the internal snubbing network in parallel with the Silicon Controlled Rectifier (SCR) which does the actual switching. This network presents an impedance of 30K ohms, and with 120 vac across it will pass 4ma of AC current, which can prevent the relay from de-energizing.

Please review the attached Tech Note for additional information and a schematic to help you install a shunt resistor.

For the HI 2151/30WC-D2, HI 3030, HI4050 another reason is the setpoint deadband is incorrectly set.  When the deadband point is reached, the relay is deactivated.  See you operations manual for Setpoint configuration for each controller.