Sunday, August 26, 2007

That pesky air conditioning switch

We get questions over here at Our Odyssey, and today I'll answer some about the latest problem -- the air conditioning switch. I'll also share the results of the repairs.

First off, an alert reader asked what a "dead font" was, and, while I am tempted to make lousy jokes about it like "anything with a glyph for long S", or how I am often a "font" of useless knowledge but after touching the live bus bar I was nearly a dead font, the fact is that it was simply a typo -- I meant to write "dead-front," which is something else entirely. (The typo, in Friday's post, has since been corrected.) Of course, font being an actual word, the spell-checker did not catch my error.

As for what a "dead front" is, if that is still a question, it is a metal cover, through which the switch handles protrude, for an electrical panel or switchboard, which is grounded. A dead-front enclosure ensures that the user, while operating a switch or circuit breaker, can not be shocked or burned by, for example, a loose hot wire, or an arcing current. Any dangerous exposed voltages will contact the grounded front of the panel before they can come into contact with the operator. And, while this may seem intuitive or even just common sense, not all electrical systems are built this way. Electrical panels on boats, for example, which follow a different set of codes (the NEC, which governs wiring of RV's, requires dead fronts on all panels above 30 volts), often have plastic circuit breaker panels, or exposed open-frame relay enclosures.

In any case, both our 120/240VAC and our 12/24VDC panels are of the dead-front type, so the arcing that occurred, while alarming, posed no electric shock risk to us. Of course, once I open up the panel doors ("authorized service personnel only"), all bets are off. And, like many electricians, I tend to work panels (of less than 270 volts) "hot." I do remove all my jewelry and try to have only one hand inside at a time, though.

So, what is this "air conditioning selector switch," I hear you ask. First, here's a photo of the switch, which is built into our electrical panel:

Note that, in addition to two "off" positions, there are six other positions, each labeled with a combination of two of our three roof air locations (Front, Center, and Rear). What this switch does is choose which of the three air conditioners can run directly off the inverter -- which means we can run it from the batteries, or from the main engine alternator while we are driving down the road. That's the first word, before the "/". If that's all it did, of course, it would only need three settings -- front, center, and rear. However, the other thing it does is choose which other air conditioner we can run when we are on shore power of 30 amps or less. That's the second word, after the "/".

The switch is shown, in this photo, set to "Rear/Center" (hard to read, because the shadow of the knob fell on the label), which means that the rear unit, which is in the bedroom, can run all the time, even if we have no shore or generator power, and the center unit, which straddles the kitchen and bathroom, can also run if we are plugged in to a 120-volt shore outlet. Of course, if the generator is running, or we have a 50-amp shore connection, then all three of the air conditioners can be running.

This is a big switch -- the switch body is about 3" in diameter, and about that deep, too (without the knob or its shaft). It is a 12-position switch, of which we are using only eight, and is available with any number of poles from 1 to perhaps ten or so. Ours has three, corresponding to the number of air conditioners. For the curious, it is an NKK brand, which I purchased from Allied Electronics for around $80. It's an expensive part, but there isn't really any other convenient or elegant way to implement this selector functionality. The poles, by the way, are rated at 30 amps each @ 250v. Here's what it looked like from the back, mounted in the panel:

The black/red/blue wires on the left of the photo are where the three air conditioners connect, and where the three individual circuit breakers served by three different electrical sources come in to the selector. The black jumper wires to the right are how we made a 3-pole, 12-throw switch do the selecting -- the incoming power is "passed through" on different terminals at every wiper position. The gray "dust" on the panel door above the switch is actually detritus from the arcing.

Here's another photo of the back of the switch:

In this photo, you can see how the air conditioners connect, at lower left, to the "common" terminal, and the incoming power, at top, connects to one set of terminals, and then is passed on to the next set, and so on around the switch. Also notice that the jumper wires on the right have been "flattened" down as close to the body of the switch as possible. I built this switch at home, while the bus was being converted, and then sent it to Infinity to install. I didn't know how much clearance there would be around the switch, so I wanted to make it as compact as possible. This may have contributed to its demise.

Here's what it looked like when I got it out:

You can see where heat has damaged several wires. On one, all the insulation has been completely burned off, leaving only black char. And you can clearly see where one almost bare wire arced to an adjacent wire, melting through the first and taking a "bite" out of the second. Two of the terminals on the switch are also badly scorched -- one, it turns out, so much so that the tinning was melted off.

The badly burned and melted wires turned out to be the ones on the "inverter" circuit, the one which gets the most use. Also, this is the circuit most likely to be running when we are on "marginal" campground power, where the voltage drops below 110, and thus the current (and therefore circuit heating) increases commensurately. The fact that the wires were right next to or on top of each other did not help, as the heat was more concentrated, and the proximity facilitated the arcing once the insulation failed.

This is what the worst three wires looked like when I removed them:

Out of 15 jumper wires, only one was undamaged. I replaced all 15 as a matter of course. This time, I did a few things differently. For one, I used blue wire instead of black. This will allow me to more easily notice if the wires are suffering any heat damage when I open the panel for routine inspections or maintenance. For another, after tightly crimping the ring terminals to the jumper wires, I followed up with some solder, to make a better connection and reduce ohmic heating in the wires (although it is important that the mechanical crimp be tight first, as connections that depend on the solder for mechanical strength will eventually fail as the solder heats up and flows in use). Lastly, I tried to spread the jumpers out a bit, to ensure that there was air space between every wire, both to allow cooling air flow, and to reduce the probability of arcing. I also completely disassembled all three poles of the switch, cleaned all contacts and wipers with contact cleaner, sanded off the carbon deposits and any pitting or rough spots, and reassembled the unit with some dielectric grease to lubricate the mechanism.

Here is the completed, refurbished switch:

We've reinstalled it, and all seems to be working normally. I'll be monitoring the temperature and wire condition closely over the next few days to make certain we are not heading toward a repeat of this problem.


  1. Ah, "dead-front" and not "dead-font". And your explanation made it very clear what was involved. Thank you.

  2. Sean- I know the A/C switch now works, but it seems to me to be unnecessarily complicated to me. Why not just use three (one for each A/C) 20 amp switch-I have one for my radiator blower motor. Then you could just have 3 switches, with each switch having the selection for either: line-off-inverter. You could eliminate all other switches off that board.
    Hope everything is going well-enjoy seeing where you are in the blog. Good Luck, Tom Christman

  3. @Tom --

    One mans complexity is another's simplicity...

    Our switch, while it is "complicated" behind the scenes, is user friendly and fail-safe. There is only one switch, and it's impossible for the user to accidentally put too many air conditioners on the inverter, or too many on the "dry" leg of power. With individual switches, there would first of all be three switches that the user would need to change individually each time to set the AC's up as desired, and forgetting to change a switch could leave two or maybe even all three units running on batteries alone!

    Our system automatically disconnects all but one AC when the shore power is removed. It also automatically adjusts for different levels of shore power, allowing two units on 30 amps or less, and all three on 50 amps.

    Also, we have only three circuit breakers involved -- one for each air conditioner. The switch goes between the breakers and the units. In your scheme, with three double-throw switches, you would need six circuit breakers, two for each air conditioner (one on the inverter side, and one on the "dry" side), remembering, of course, that code requires the overcurrent protection to be ahead of any switches.

    Three double-throw switches and three extra circuit breakers are probably cheaper than the selector switch we used (but only by a little bit -- our circuit breakers run about $15 each), but I would not say the scheme is less "complicated" -- only different. And I would figure the three switches and three breakers to consume about the same amount of real estate on the panel, as well.

    It was worth the few extra dollars and a little extra wiring time to us to have a solution that was less prone to operator error.



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