Which safety device is unnecessary for an electrode boiler where the electrodes are submerged in water?

In the context of an electrode boiler, the safety device labeled as unnecessary is the low water cut-off. This is because, in electrode boilers, the heating element relies on water being present to conduct electricity. If the water level drops low enough to expose the electrodes, the boiler would effectively shut down as there would be insufficient water to maintain electrical conductivity. Therefore, the inherent operational principle of the electrode itself acts as a safeguard against low water conditions.

The other safety devices play critical roles: the safety valve ensures that pressure does not exceed safe limits, the blow-off valve aids in maintaining proper system pressure and can help eliminate impurities from the boiler, and the gauge glass provides visual monitoring of the water level, which is crucial for safe operation. Thus, while all these devices serve important functions, the low water cut-off is rendered unnecessary by the operational characteristics of the electrode boiler.