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	Subject - Arcing when 3 way sw changed to silent type kringles When a friend changed his three way switches to the 'silent' (mercury) type he imediately began experiencing problems. When throwing either switch there was usually arcing/sparking, often the circuit breaker would trip. Replacing switches with standard three ways corrected the problem. Investigation revealed nothing wrong with the new three way switches. But futher investigation revealed that the way the circuit was wiring inconjunction with the new switches was the problem. Have attached somewhat crude drawing of the circuit as found and after corrected. The original wiring also had the light off but both the shell and center pin hot, an unsafe condition. The reason for the arcing etc was that the silent switches apparently were not positive break before make type. That is there was some connection between the contacts as the switch was thrown. Since the contacts were connected to the hot and neutral any connection resulted in arcing andshorts with the likely tripping of the breaker. John A. Peters If it was an old house and knob and tube wiring, you may have a "Hollywood threeway" system. The symptoms is that the voltage across the "travelers" is 110V. If so let me know, as there is more to it. kringles Sorry could not get the picture to be added, tried a couple of ways but no luck. Still working on it. kringles John, It was a 1950's house, not old enough for knob & tube. Has plaster lathe walls and older BX cable wiring. Have posted the diagram on my wife's craft website bacause I could not figure out how to include on the previous post, the top diagram is the original, the bottom the rewired configuration; http://www.kountrykringles.com/sys-tmpl/newhousepictures/view.nhtml?profile=newhousepictures&UID=10008 Hope my crude drawing clarifies the picture. As you can see the voltage across the travelers was 110v in the original connection. All seems to work per standard after I rewired it. Checked the house and found two other three ways wired the same screwy way. Rewired both and all seems good now. BTW the 'silent' switches now work fine, as they should, with the new wiring connection. Was this the "Hollywood threeway' system that you mentioned? lctrc789 If I am seeing your drawing right,(and I may not be) does the neutral wire attach to the switches, Many older homes were switched neutrals it was not that un common they use to fuse the neutral as well at the main panel. And if the hot and neutral were clashing because the mercury switch does not operate like a snap switch maybe could have been problem or just not wired properly for a 3 way switch. kringles Pat, Yes you are seeing the diagram correctly, the neutral was connected to one pole of each switch and the hot was connected to the other pole of each switch. The light could have both shell and center button connected to hot or both connected to neutral, in each case the light would be off. In the other two conditions the shell would be neutral and the center button hot or vice versa, in these cases the light would be on. Since the mercury switches allowed for momentary connection between the poles (one hot the other neutral) it would cause the arcing/sparking that was seen. Note that normal type (snap) switches had no problem with the original wiring but did not think it was correct to leave it that way. All wiring is now per the second diagram (unbroken neutral, switched hot) and works regardless of the type of switch used. regards kringles BASE How many wires in each switch box? Are you sure they are nut using 2 14-2's in the switch box and letting the white (not neutral) be a traveler? Hot would be coming in at the light box. kringles Hi BASE, The first box contained 5 wires (hot & neutral feed plus 3 in the 14/3 to the light ceiling box. The other switch box contained three wires (from the 14/3 cable from the ceiling light box). Check out the diagram link in my earlier post it should clarify the situation. I understand how it works and why there was sparking in the original set up. The top diagram is as it was and the second was after the rewiring.