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Switches - Line and Load

SmittysRV

SmittysRV

Well Known Member
In Van's wiring harness instructions it talks about putting the 1/2" copper buss bar across the switches, connecting one post on each switch to the single copper bar. The copper bar provides the positive charge to each switch. Each switch has "Line" and "Load" stamped on it next to the posts. I can't see in the instructions where which one gets the copper bar attached to it. In the drawing, it looks like the "Load" terminal get the copper bar and the "Line" goes to the device being powered. Anyone know which is which?
 
Sounds backwards. Bus should probably be line side (if it matters).
P.S. I love exposed busses, how bout you Dan?
 
W31 Switch

Smitty,
You're right, Van's direction has the copper bus bar on the "load" terminal, the switched device on the line side. Yes, backwards from what was intended by the manufactuer. Two electrical engineers and a real smart guy I trust assured me in this case it makes no difference.
 
Thanks Dave. From all the things I read on the internet and some friends who came over to the garage/hangar, the whole setup seems backward. But I think you're correct, the copper buss bar goes on the "load" terminal. Here's the drawing I went by, showing the switch with the "line" going out to power that lights, avionics, etc: (sorry for the fuzzy pic)

lineload.jpg
 
Since it's a DC switch it's not clear why it matters which way around it goes, and you may want to have different switches oriented different ways. Why does orientation matter?
 
It doesn't

Snowflake said:
Since it's a DC switch it's not clear why it matters which way around it goes, and you may want to have different switches oriented different ways. Why does orientation matter?
Click to expand...

As long as "up" is "on" (closed). I suspect the buss is put on that side for physical reasons, not electrical.
 
Understand that when you "daisy-chain" several switches to one feed wire, that any voltage drop on the feed wire will be felt by each switch and passed on to their loads; this is know as common-mode interference. So if you have one of the switches controlling a load that draws several amps, and especially if the load is a pulsating load such as strobe lights, the intermittent voltage drop on the feed wire will get passed to the other loads. So if you have a load that is sensitive to pulsating voltage, such as an intercom, radio, or music system, it's really best to feed each switch independently from the main buss. This is even more-so on the return (ground) side of the circuit. To never have mutual interference from one device to another, always use individual wires from the unit's return to a single-point return.
 
I guess I'm a little confussed, if you use a buss bar across all the switches, how do you get individual circuit protection? Would you put fuses or breakers between the switchs and the devices rather than putting it above the switches. I'm familar with using a buss to link your breakers but not for the switches.
 
I am not a fan of connecting the copper buss across breakers because of what Eclipse mentioned. I have never heard of anyone doing it for switches. I am not sure how that would work. Gordo is correct, you would have to put the breaker on the load side of the switch. That's not a good idea. It's early, maybe my brain is still asleep. Someone chime in if I am not correct on this.

For trouble shooting I like to have the ability to isolate circuits. When you buss things together it become more complicated (IMO).
 
Because I've got a plastic airplane, I use only twisted-pair, TP, or shielded-twisted-pair, STP, wiring to eliminate electromagnetic and electrostatic interference. I use a TP from the buss to the CB and back to a terminal board. That way any current flowing from the buss to the CB on one wire flows back on the other wire to the TB, cancelling any magnetic field. Then from the TB and return, ground, I have an STP with the shield connected to the return also at the distribution end only, not at the load. this seems to work for me! Be careful using the metal airframe as your circuit return for over a long time, corrosion can occur at the joints in the current flow path, especially on planes in wetter climates. For pulsating loads such as strobes, it's usually advisable to use STP from the source to the lamps.
 
gordodavis said:
I guess I'm a little confussed, if you use a buss bar across all the switches, how do you get individual circuit protection? Would you put fuses or breakers between the switchs and the devices rather than putting it above the switches. I'm familar with using a buss to link your breakers but not for the switches.
Click to expand...

I got confused by that, too. I'm guessing that these are dual-purpose "circuit breaker switches?"

I have a copper bar across all of my MAIN bus CBs, and another copper bar across all of my AVIONICS CBs. I, too, have considered that that COULD be inviting a risk of short circuit, but since my dash is fiberglass and far away from the ground bar it seems pretty unlikely.

There is no copper bar connecting my switches, which are NOT the CB type.
 
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