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Generators with Isolated Neutrals

Most boat electrofishing pulsators (control boxes) are powered with portable generators that provide 240-V, single-phase AC. These generators are available from common commercial sources such as hardware stores or electrical supply houses. (Note: a notable exception is the GPP pulsator made by Smith-Root, Inc., that requires a specially-constructed generator; it will not be discussed further here.) This short blog explains the electrical outlet of a generator, how to test for the safety of a generator if used for boat electrofishing, and what to do if the test indicates that a generator is not safe for electrofishing.

The 240-V outlet on common generators is a four-prong receptacle: two “hot” slots side by side, one L-shaped ground slot on top and one neutral slot on the bottom. The ground is connected to the generator case and frame but is not part of the electrical circuit; it allows one to connect the metal exterior (frame and case) of the generator to earth to prevent an accidental shock in case the frame is electrified by a “short” in the circuit. The neutral is part of the electrical circuit and cuts the circuit in case of an overload (very high current) on the equipment. There is no earth ground in an electrofishing boat, so the frame is connected through the ground to the metal hull of the boat to provide a uniform voltage on all metal surfaces inside the boat (so that personnel in a boat are like a bird on a power line—they can touch any two surfaces inside the boat without being shocked). The neutral may be connected to the case (often called bonded neutral or case neutral); if connected, it must be dis-connected (isolated neutral) to render the generator safe for boat electrofishing.

Most portable generators in the past had a bonded or case neutral as a safety feature because many were being used at construction sites to power tools such as saws, drills and drivers; those sites generally have an earth ground. Perhaps largely due to the recreational vehicle market, where one may not have a path to earth ground, more portable generators with isolated neutrals are now being constructed. That is good news for those of us wanting to use generators for powering electrofishing boats.

The first photo below is the front panel of a generator that featured an isolated neutral as indicated by manufacturer specifications. The oval shows the electrical receptacles. The receptacle on the far right is the one to use for electrofishing and is the one to check for an isolated neutral.

Generator panel

The second photo is a close-up of the 240-V outlet showing the four-prong receptacle. The last photo is a continuity test between the ground and the neutral slots of the 240-volt receptacle; any digital multi-meter may be used for this test. This is done with the generator off, of course, so that no voltage is going to the receptacle. I did flip on the toggle switch for the circuit breaker on the front panel, however. The Fluke T5-600 tester indicates “OL” if the resistance is over 1000 ohms, which indicates an open circuit or no continuity. In this case, an open circuit between the ground and neutral slots indicated the desired condition, an isolated neutral.

Closeup of receptacle

Continuity check

If zero or near-zero resistance (e.g., 0-2) reads on the meter, then the neutral is not isolated. In that case, the generator should not be used until isolation is achieved. The isolation procedure may be electrically complicated. An electrical expert should be engaged to assure isolation of the neutral. Many new generators come with the neutral isolated; these are ideal for boat electrofishing and should be purchased instead of others without this feature.

Dr. Jim Reynolds offered many constructive edits to this blog, and much of the credit goes to him. Also, Shawn Banks of Midwest Lake Electrofishing Systems reviewed the blog and made several useful comments.

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© 2015 Thread One Page. Imagery: Tom Rayner, Alan temple, Richard Pearson, Paul Godfrey, Roger Scott, John Rayner.