Interrupts power immediately in a low water condition. Burner circuit locks-out if water remains below probe for 30 seconds. Manual reset will not trip due to power failures. Test button checks burner circuit to ensure proper control operation and lock-out function without lowering the water level. Note: Can also be used as a secondary cut-off on steam boilers. Hydrolevel Safgard™ Low Water Cut-Off, Series: 500, 24 V AC, 160 psi, Manual Reset, 2 VA