A positive-acting, unfuzable current interrupt switch, rated at 400 amperes must be in the welding circuit. This switch protects the diver by breaking the electric circuit, thereby stopping the current supply to the electrode holder. A typical double-pole safety switch setup is shown in Figure 4-8.
The safety switch, more often referred to as a knife switch, is the most important safety item included in the underwater cutting or welding equipment inventory. The diver is fully isolated from electric current when the knife switch is open. The tender (phone talker) should only be instructed to close the switch when the electrode is poised for cutting or welding.
Both double-pole and single-pole safety switches are authorized, however a double-pole is the most often used because both the working and ground lead are opened or closed simultaneously (see Figure 4-8).
To ensure safety switch effectiveness, the following guidelines must be followed:
a. When using a single-pole knife switch, it should be located in the welding-lead side of the electric circuit and should be able to handle the maximum welding current.
b. The safety switch must be mounted vertically on a non-conducting (wooden, plastic, etc.) stand. The switch has an open circuit potential of approximately 80 volts across the poles. To prevent accidents, the safety switch should be fitted with a non-conducting slotted cover.
c. When reverse polarity is required, the safety switch must be placed in the cutting or welding lead side of the circuit.
d. Never operate a knife switch in a combustible atmosphere.
e. The knife switch must be in proper working order. Additionally, the switch contact surfaces should be periodically checked for verdigris accumulation.
f. The current shall be off (knife switch open) at all times, except when poised for or actually cutting or welding.
When a single-pole knife switch is used, special care must be taken to ensure that the safety switch is not shunted out between the switch and welding machine. Wet, bruised or worn cables can be shorted by rubbing against the welding machine frame, hatch combings or by lying on a steel deck. This creates a potential source of danger. Inspect the cable thoroughly and wrap any questionable spots in the insulation with rubber tape, followed by an additional layer of electrical tape or heat shrink. Periodic inspection should be made to ensure that the insulation is not damaged.
The safety switch shall be located in such a position that enables the phone talker or designated tender to operate or oversee the operation of the switch during the entire time the diver is in the water. The switch shall not be closed unless specifically directed by the diver to do so. The phone talker shall confirm each change to the diver via the intercommunications system.
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