What does the Code require of a disconnecting means installed for a motor and its controller?
- AIt must open the grounded conductor along with all of the ungrounded supply conductors
The grounded conductor is normally not opened by the disconnect. Switching it can leave equipment energized with no return reference and is not what this section requires.
- BIt must be capable of being operated one pole at a time so a technician can isolate phases
Independent pole operation is expressly prohibited. Opening one phase of a running three-phase motor is one of the fastest ways to burn its windings.
- CIt must be located within 6 feet of the motor controller in every installation without exception
The location rules use in sight from the controller and from the motor, not a fixed 6-foot dimension, and they carry their own exceptions.
- It must open all ungrounded supply conductors and plainly indicate whether it is open or closed
Why D is correct
Three ideas in one place: all ungrounded conductors open together, no pole operable by itself, and clear indication of position. The grounded conductor is generally not switched. Distance requirements are covered by separate sections on disconnect location, which use the in sight from concept rather than a fixed 6-foot figure.
What this question is testing
The item tests recall of the basic design requirements for a motor disconnect and whether the candidate recognizes single-phasing as the hazard behind the prohibition on independent poles.
On the job
Single phasing from a partially opened disconnect, a blown fuse, or a loose lug is one of the most common causes of motor failure in plants. The disconnect design rules address the part the electrician controls. In the field the practical companions are checking that the handle position indication is legible after painting, and that the switch is not jammed by a locking device that lets it sit between positions.
Memory technique
All the hots at once, and a handle that tells the truth.
Exam tip
Disconnect questions split into two families: what it must do electrically, and where it must be located. Answer the one being asked.
Where to look it up
Look in the motors article, in the part on disconnecting means, at the sections covering what the disconnect must open and how it indicates position.