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Old as Dirt!
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This past week we had an unfortunate experience with our AC system that others might want to know of. Essentially, because of a failed cable tie, the 3-lead power cable to the fan assembly came in contact with the very small copper tube that directs hot compressed gas from the compressor to the heat exchanger. Some may be surprised to learn, as was I, that the temperature of that gas line can be great enough to melt the plastic insulation on a power cable and, in our case, did. With that, the copper tube itself acted as a conductor connecting the hot lead in the cable to the ground lead, which activated the fan even when the AC system was "off" (so long as power was supplied to the AC circuit).

I discovered the issue when I arrived at the boat and found the AC fan running even though the AC system was off. Originally I thought there was a defect in the control module on the circuit board but, after a technician from the manufacturer swapped out the board, the problem continued, indicating a short circuit. A careful, tedious, inspection of the wiring finally revealed the damaged cable that was shorting out. Fortunately, the cable was easily enough repaired and wiring re-secured (with better cable ties), and the position of the tube to the heat exchanger re-aligned somewhat to prevent a possible repeat. Moreover, although our system is "out of warranty", the manufacturer waived any charges for the service call (which took rather a good deal of time and included a new circuit board in the control module) declaring that the problem "..should not have happened".

N'any case, for those with air conditioning, one might want to know of the potential issue and might want to double check one's own wiring where movement of the yacht over time might wear on wire and cable ties.

FWIW...
 
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It's hard to believe that the "hot wire to copper pipe" resulted in the motor running continuously. You would think it would result in a short circuit and blown fuse or breaker.
 

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Old as Dirt!
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Discussion Starter #3
It's hard to believe that the "hot wire to copper pipe" resulted in the motor running continuously. You would think it would result in a short circuit and blown fuse or breaker.
The tubing merely fulfilled the function that a closed switch would have served. In operation, the fan pulled only a few amps.
 

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One of None
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Omph... glad the line didn't short enough to make a burn hole in the tube. Loosing the charge and having the line brazed then replacing filter dryer, evacuate and recharge would not have be fun to pay for. Nice Save SVH!
 

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The tubing merely fulfilled the function that a closed switch would have served. In operation, the fan pulled only a few amps.
Understood, but usually an appliance is grounded such that the breaker would trip as soon as the hot wire's insulation is compromised and the conductor makes contact with any metal part of the equipment. It sounds like you may still have an issue with the appliance. It sounds like the equipment ground (green) is not connected. This is very hazardous as it could cause someone to be electrocuted.
 

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Discussion Starter #6
Understood, but usually an appliance is grounded such that the breaker would trip as soon as the hot wire's insulation is compromised and the conductor makes contact with any metal part of the equipment. It sounds like you may still have an issue with the appliance. It sounds like the equipment ground (green) is not connected. This is very hazardous as it could cause someone to be electrocuted.
I appreciate the observation and I shall put a call in to our AC manufacturer about this to be sure but I believe the short was between the white and black leads in the cable itself and I do not know if the case of the compressor from which the tube originated is, itself, connected to the neutral ground. (Unfortunately, my knowledge of matters electrical is rather sparse.)
 

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I am assuming that it was a 12v fan circuit that shorted. The only way I could see such a short to cause the fan to run continuously would be if the system was switching the negative to bring the fan on, and the negative was the compromised conductor. If it was the "hot" lead that shorted it certainly should have blown a fuse.
For what it's worth, under normal circumstances the discharge line of an a/c system should not get hot enough to melt insulation. More likely the damage would have been caused by chaffe. If the circuit had been 110v it probably would have blown a hole in the discharge line and dumped the charge.
 

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The power to an A/C is 120 volts and powers both the fan and the compressor. The control circuit which turns the unit on is typically 24 volts. With an A/C only unit there are three wires from the thermostat to the A/C unit - power, compressor, and fan. If the tubing connected the power to the fan unit, the fan would run constantly. The control circuit is not grounded and so no short circuit would occur.
 

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The power to an A/C is 120 volts and powers both the fan and the compressor. The control circuit which turns the unit on is typically 24 volts. With an A/C only unit there are three wires from the thermostat to the A/C unit - power, compressor, and fan. If the tubing connected the power to the fan unit, the fan would run constantly. The control circuit is not grounded and so no short circuit would occur.
I would be curious to see a wiring diagram for the unit, and see which conductor was shorted out. Regardless of the voltage, the only way the short could have caused the fan to run is if the motor is supplied with a constant 120v hot, and they are switching the neutral to cycle the fan on and off. If it was indeed a 110v HOT lead that contacted the refrigerant piping and the breaker did not trip then the unit wiring is in a very unsafe condition.

The OP stated that the cable that was damaged was the 3-lead power cable to the fan assembly. If he was mistaken, and it was actually the 24v thermostat wire that shorted between R and G then the fan would run continuously, however, if the R wire contacted the refrigerant piping it still should have blown the control fuse because the control transformer should be connected to the same ground as the rest of the unit.

Of course the marine industry seems to do some pretty goofy things with electrical, so I guess goofy things can happen as a result....
 

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We are not talking a short here but a closed circuit on the control side. The control wire closes the fan relay and the fan will then get constant power just the same as switching the fan switch to on on the thermostat. Short circuits cause breakers to trip not fans to run. 24 volt control circuits do not have fuses nor is the control transformer grounded on the load side.

The neutral is never switched.
 

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We are not talking a short here but a closed circuit on the control side.
Of course we are talking about a short circuit. The question is why the discharge line that the wires shorted against wasn't grounded.
The control wire closes the fan relay and the fan will then get constant power just the same as switching the fan switch to on on the thermostat.
Yes, if we were talking about a short between R and G on a thermostat circuit that would be the result, assuming that the metal that completed the circuit wasn't grounded. My understanding from the original post was that it was a power cable that was damaged not a thermostat wire.
Short circuits cause breakers to trip not fans to run. 24 volt control circuits do not have fuses nor is the control transformer grounded on the load side.

The neutral is never switched.
A short circuit is something that causes current to flow through a path other than it's intended one. If the short takes a path that allows excessive current flow, such as to ground, then breakers trip, fuses blow or wires burn up.
24v control circuits usually do have fuses, or a breaker on the transformer, at least on land! If they don't, then in an over-current situation the transformer and/or the wiring will burn up. (usually the transformer) If the control voltage is being switched through a circuit board it will destroy the board. That is why there will usually be a fuse on the control board.

I am aware that the neutral should never be switched but I have seen it done due to incorrect field wiring and general incompetence.

The scenario that you put forth that the stat wire hot shorted to the fan wire is a plausible one, and makes more sense than the idea that the line voltage wires shorted together, as the OP stated.

My point is that the copper discharge line that apparently provided the new circuit path SHOULD have been grounded, and whether you are talking about 24v or 115v the current should have taken that path to ground as the path of least resistance. If that pipe is not grounded then it is extremely dangerous because in the event of an electrical failure such as the one we are talking about, that piece of equipment could become energized and stay that way until some unsuspecting person touches it and creates a path to ground through their body! That is no big deal if we are talking about 24v, but with 120v it could be fatal!
 

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SchockT, your definition of a short circuit is wrong. A short circuit bypasses the load by providing a minimal resistance path. In this case the load was not bypassed. The load side of 24 volt transformer is rarely if ever grounded. Therefore there can be no path to ground to complete the circuit if one of the conductors, in this case R, touches the copper tubing. Even if the transformer were fused it would not blow the fuse as no over current situation existed.

While you may have seen neutrals switched, it's note a very likely scenario in a manufactured product such as a marine air conditioner as they would not receive UPL or CSA approval.
 

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Is this wiring issue from the install or is it factory and integral to the unit? If it is factory, what brand is the unit so other owners can check this issue? If it from the install, maybe a sleeve over the wiring would do the trick to prevent the chafe/melt problem.
 

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While I don't make the assumption that ShockT mades about it being DC voltage, nor a neutral switch, I was thinking something similar about the 3-wire. That is either a hot, neutral and ground or has been set up to switch one side of the the circuit, typically the hot side.

If a switching setup, I can see the a short would close the circuit and power whatever it was switching. Although, it seems odd it would only be switching the fan and not the entire unit. Perhaps a closed thermostat in this scenario is why only the fan powered up.

If it was the AC power supply, I don't see how a short would have powered anything and more likely would have tripped a breaker/fuse.
 

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Discussion Starter #15 (Edited)
Guys--

Firstly, thanks for the interest. Secondly, keep in mind that I am "challenged" in matters electrical so my use of terminology or descriptions may leave something to be desired. N'any case, to clarify, the power cable to the fan assembly is 120v. The green wire in that cable is connected to the fan assembly on one end, and to a neutral green bus board on the Air Conditioner ("AC") controller circuit board. The black and white wires in that cable are connected to either side of a variable voltage relay (kind of a digital rheostat) on the circuit board in the control module that, depending upon the difference between the ambient air temperature and the thermostat setting, varies the fan speed from very slow (when there is little temperature difference) to full bore (when there is a 10º or greater difference). The 1/8" dia (or so) compressed gas line that melted the insulation (and one of my slip neighbors, a mechanical engineer who owns a sizable AC contracting firm assures me that such a line can get hot enough to melt insulation) merely acted to complete the circuit between the black and white wires in the cable, activating the fan, just as the switch on the circuit board does/would when it is enabled. (I believe there is a separate "hot" red colored lead to the fan assembly as well.)

The air conditioning system on our boat is identical to its predecessor, which we had for 16+ years, in all respects save for the control module which is now digital verses the earlier analog system--i.e. manual controls of fan speed, heat or cooling selection, etc. (Frankly I prefer the old arrangement to all of this digital BS). The old unit worked extremely well so when it died of old age, we replaced it with it's newer, supposedly more up-to-date sibling.

The power cable to the fan assembly came in contact with the hot coolant tube because of the failure of a wire tie that was used to route the cable around and past the compressor assembly; and, because the coolant tube was not enclosed in an insulating cover (although my slip neighbor indicated that wasn't unusual either). The foregoing, and the compactness of the system which brings potentially conflicting mechanisms in close proximity to one another gave rise to our problem. The make of the machine is irrelevant as I have since observed the same proximities and prospective conflicts/hazards in two other systems, made by two other, different, manufacturers, while helping neighbors check their systems as I suggested others do of theirs in my original post. And, the builder of our machine is issuing a technical bulletin on the matter that is being sent to the owners and installers that handle its machines so that corrective/preventive measures and be taken, if necessary. I am afraid naming the manufacturer would simply cast it in an undeserved bad light, which I prefer not to do, especially considering the lengths the company went to helping me resolve the issue at no charge and with only moderate inconvenience.

FWIW...

Addendum: Since posting the above message I have learned that the red wire referred to in the parenthetical phrase at the end of my first paragraph, above, has nothing to do with the operation of the fan and is merely coincidental to the 3-lead power cable. Please see my follow-up post, #20, below.
 

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Svhylyte,

I agree that the manufacturer should not be blamed for such a failure. That kind of thing can happen on any make of equipment. If anyone is to blame for the problem it would be the installer who didn't make sure the wiring was correctly routed and secured before walking away from the unit.
The only reason I would like to know the make and model of the unit is so I can dig up a wiring diagram to satisfy my own curiosity about how that happened without blowing a breaker, or causing more damage.
 

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SchockT, your definition of a short circuit is wrong. A short circuit bypasses the load by providing a minimal resistance path. In this case the load was not bypassed. The load side of 24 volt transformer is rarely if ever grounded. Therefore there can be no path to ground to complete the circuit if one of the conductors, in this case R, touches the copper tubing. Even if the transformer were fused it would not blow the fuse as no over current situation existed.

While you may have seen neutrals switched, it's note a very likely scenario in a manufactured product such as a marine air conditioner as they would not receive UPL or CSA approval.
No, my definition of a short circuit is not wrong. 24v control transformers are almost always grounded, but even if they aren't, if you touched the hot side to ground the electrons would take that current path.( We are not talking about a battery circuit here!) And you could easily end up with a scenario where the neutral side of a circuit is being switched. It could be as simple as the installer reversing the wires when he connected the power to the unit.

But what do I know, I have only been working in the Refrigeration/HVAC trade for the past 20 years! ;)
 

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Looks in... reads... is thankful there hasn't been any recent boat fires on Tampa Bay... goes back to her woodworking bench project.

guys and their shorts.... :)
 

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No, my definition of a short circuit is not wrong. 24v control transformers are almost always grounded, but even if they aren't, if you touched the hot side to ground the electrons would take that current path.( We are not talking about a battery circuit here!) And you could easily end up with a scenario where the neutral side of a circuit is being switched. It could be as simple as the installer reversing the wires when he connected the power to the unit.

But what do I know, I have only been working in the Refrigeration/HVAC trade for the past 20 years! ;)


The only explanation that makes sense is that the fan relay was energized powering on the fan.

From the OP's explanation above it sure sounds like he is talking about the control circuit with red, green, black and white wires mentioned as going to the fan assembly. The black wire coming in contact with the copper would cause a short circuit if it was 120 volts.

BTW here is the definition of a short circuit from the on line free dictionary

(Electronics) a faulty or accidental connection between two points of different potential in an electric circuit, bypassing the load and establishing a path of low resistance through which an excessive current can flow. It can cause damage to the components if the circuit is not protected by a fuse.

I was unable to find anywhere that said a short circuit was just an alternate path. Maybe HVAC guys have their own definition that is different than the rest of the world.

X1 or X2 of the 24 volt transformer has to be grounded in order for the hot to cause a short to ground. You are without question wrong in that regard.
 

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Discussion Starter #20
The only explanation that makes sense is that the fan relay was energized powering on the fan.

From the OP's explanation above it sure sounds like he is talking about the control circuit with red, green, black and white wires mentioned as going to the fan assembly. The black wire coming in contact with the copper would cause a short circuit if it was 120 volts.

BTW here is the definition of a short circuit from the on line free dictionary

(Electronics) a faulty or accidental connection between two points of different potential in an electric circuit, bypassing the load and establishing a path of low resistance through which an excessive current can flow. It can cause damage to the components if the circuit is not protected by a fuse.

I was unable to find anywhere that said a short circuit was just an alternate path. Maybe HVAC guys have their own definition that is different than the rest of the world.

X1 or X2 of the 24 volt transformer has to be grounded in order for the hot to cause a short to ground. You are without question wrong in that regard.
Subsequent to my last post, I have spoken with the engineers at the AC Maker. The red wire I observed is near to, but has nothing to do with the fan assembly. The white and black cables are 120v and power the fan. The copper tube that temporarily connected these wires merely completed the circuit in place of the connection normally made by the fan speed control module on the circuit board which was inactive in the event. The insulation on the green "neutral ground" cable was damaged but not fully compromised and so not involved in the event.

FWIW...
 
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