testing mast head light circuit

[bob mitchell]

This information is from "Boatowners Mechanical and Electrical Manuel" by Nigel Calder

Using OHMMETER (a setting selection on many voltmeters)

1) disconnect circuit at base of mast and test accross the two mast leads.
(refering to the MAST LIGHT circuit)

---a very low (close to 0 ohms) reading (unusual) indicates a short inside the mast or at the masthead. (like two wires touching that shouldn't)

----a low ohms reading (typically 10 to 20 ohms) indicates that the in mast wiring and the bulb are good. the problem lies elsewhere. (maybe the mast head circuit fuse, or connection from main panel, or subpanel, or on/off switch?)

----an infinite ohms reading indicates a break inside the mast or at the masthead.
(like a wire is disconnected or broken, or a bad bulb)

If the ohmeter gives an infinite reading twist the two wires together , go to the masthead,and remove the bulb (haha piece of cake) Test the bulb across its two terminals. An infinite reading shows a blown bulb, a low reading indicates the bulb is working. (might as well put in a new bulb while your there? but this is more about knowing what is wrong) . If the bulb is working (low reading) a test is made across the two wire terminals at the masthead. Since these wires have been shorted together (where we twisted the two wires together ) at the base of the mast, this last test should show a low ohms reading---IF the reading is infinite there is a break in the mast circuit. Check for dirty or broken terminals at the light fixture before condemning the in mast wiring.

If you happen to be wondering or trying to visualize how the Newporter masthead wiring runs through the mast you can look at the NEWPORTER POSTMORTUM MAST ANALYSIS and see the cable in its tunnel (approximately 5/8 by 5/8") inside the mast. So it enters at the base in a solid core, then up through the hollow until it enters another foot of solid core tunnel at lower frontstay tang, and then up to masthead. I am not sure if it exits before the solid core of the masthead or not. (the last few feet of the mast - the masthead- is also solid.)

[Justine]

Me and Ohm meters don't get along. :) I keep hoping it is the bulbs but getting up there is a little hard. We did finally notice that the main mast does have an anchor light but it is so small you can't really see it unless you look really hard. Good advice keep it coming.