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Rotary Switches ...if you like the smell of melting plastic!
The Great Meltdown
It's been over 10 years since David Smead identified the battery selector switch as a failure waiting to happen. Since 1994, Ample Power system diagrams do not show a battery selector switch that is used to conduct charge current from the alternator. There are two reasons for this.
First, if the battery selector is turned off when the alternator is charging, there will be bad consequences. The alternator and alternator regulator are likely to be destroyed, along with other electronics which happen to be turned on at the time. Electrical transient can reach several hundred volts for a few hundred milliseconds ...enough to fry all onboard electrical equipment.
However, it doesn't take a human to turn off the switch to get the switch to open circuit. With high charge currents through the switch, a build-up of heat begins. As the switch warms up, spring tension on the moving wiper decreases. Lessening spring tension increases the resistance of the switch, which in turn causes more heat. If the charge current persists for any length of time, the switch open circuits and the effect is the same as if it had been turned off by a human.
A rotary battery switch can be used to select which battery supplies the load distribution panel, as long as there will not be high currents for any duration. An inverter should probably not be connected through a rotary type battery selector switch if the inverter is used to power refrigeration compressors.
The best solution is to get rid of all rotary selector switches and used the key type on-off switch in its place. Those type of switches have a greater and more consistent contact pressure.
Rotary Motion versus Rotary Contacts
Since this article was first posted, several readers have become confused by the term rotary switch, and questioned the fact that key operated on-off switches operate by a rotary motion.
Indeed, the key rotates to engage the contacts in the keyswitch, but the pressure is linear against the movable bar that shorts the two contacts. The contacts do not rotate, and the bar is held against the two contacts with several pounds of force.
In the typical 1-2-both switch, there is a center wiper contact that rotates across the contacts. That wiper is pressed against the contacts with much less force than the shorting bar of the keyswitch. The wiper is also a lever, and it's spring tension weakens with heat.
Don't confuse rotary motion with rotary contacts!
In the photo above, notice the bubble around the top left lug. Here's a switch that left its DNA at the crime scene. At the time of failure, alternator current was about 150 Amps. The switch is rated at 250 Amps continuous, as molded into the body of the switch.
In the photo below, note the crack that runs between the two terminals. It's hard to see ...in fact the tech who pulled the switch apart missed seeing it until it was pointed out by someone observing the photo.
These are not an isolated failures. We've seen three, and heard of a few more. Circumstantial evidence has long pointed to these switches. When someone reports widescale frying of onboard electronics, they either report a lightning strike or have a rotary switch in the charging path.
Don't wait until a switch costs you thousands of dollars. Get rid of it!