Single-channel Series SLD monitors shaft seal of submersible pump to detect leak before pump failure. Leak is detected by sensing status of float or conductivity switch installed in seal cavity. When resistance drops below set sensitivity, output relay energizes and LED indicator illuminates. Utilizing 120 Vac control voltage, relays offer sensitivities of 470 ohm fixed, 470–10 k ohm adjustable, and 4.7–100 k ohm adjustable.