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They are to be considered a continuous load [427.4]. If you use a controller to turn them on and off every couple of hours, that does not change the characterization.
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Ensure that equipment is physically guarded, isolated, or thermally insulated. The reason is to protect people who may come into contact with it [427.12]. You can't get around this requirement by declaring a section of pipe (or whatever) to be "unlikely" to be in contact with a person. People do stupid things. If there is any risk of contact, follow the requirement.
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The danger is that, since the vessel isn't serving as a heat sink for the equipment, the equipment can overheat. So you must provide a means of preventing overtemperature, unless the design of the heater assembly precludes overheating [427.15].
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You use a dual-winding transformer with a grounded shield between the primary and secondary windings [427.26].
- Yes, the controller must open all ungrounded conductors. And you cannot use a controller as the disconnect means unless it can be locked in the OFF position [427.56(B)].
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