RV generator circuit protection

Discussion in 'Electrical Forum discussion & Blog' started by moisheh, Oct 21, 2007.

  1. moisheh

    moisheh New Member

    Messages:
    42
    Location:
    Mexico and Canada
    I am currently working on an older ONAN diesel generator. Everything works fine. However the RV mfr. failed to do a proper install. There are 2 cooling fans that were installed by the RV mfr. One is a small squirell cage to exhaust hot air. It is 110 volt ( I have to remove it to read the label for amps draw, but it is very little). The other is a huge fan to cool the radiator. It too is a squirell cage and looks like a common furnace fan except it is 208 volt. Neither of these fans are protected by a circuit breaker. They are wired direct to the genset. I would just get a common 4 breaker box but there is no room in the compartment for anything that large. I am looking for ideas on some sort of small circuit breaker, maybe push button, that I could use. I did find some 110 volt small PB breakers but dont know if they would be suitable. Would the heat in the compartment affect the breakers? Any ideas ?

    Thanks:

    Moisheh
  2. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,542
    Location:
    North Carolina
    If these motors were installed by the company that built the genset then do nothing.

    I do question a 208 motor on a genset for a RV. The genset can’t produce both 208 and 240 volts. Are there any appliances in the RV rated at 208 or 240?
  3. abikerboy

    abikerboy DIY Senior Member

    Messages:
    202
    Location:
    VA
    I would leave these alone. I imagine the motors are thermal protected. If you add breakers, one or both of two things could happen. The radiator cooling fan trips a breaker, you dont know it has stopped, the engine overheats, and you say bye bye to that expensive gen-set. The other thing, your hot air exhaust fan trips the breaker. The compartment that the genny is in gets extremely hot, then you wake up in flames one hot night. With the thermally protected motors, if something goes wrong, the motor will stop for a short time, cool down a little, and restart, and you're fairly safe until you notice something not quite right within a short time. If it's a dead short, the regulator in the genny burns out, and without power, the whole thing shuts down. Cheaper to replace a regulator than to buy a gen-set or a new rv.
  4. moisheh

    moisheh New Member

    Messages:
    42
    Location:
    Mexico and Canada
    I was wrong on the motor. It is 208/230 volt. There are no appliances in the RV that are 230 volt but most RV gensets produce 230 volts. They usually have a 50 amp 230 volt service thus there are 2 legs of 110 each. My terminology may not be technically correct but the above is what is installed in current RV's. And yes the service entrance has a 4 wire cable to go the power pole outlet. Thank you for the advice. So if a motor has thermal protection and it shorts out will the thermal breaker protect the genset? That is why I thought it should have breakers.


    Moisheh
  5. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    There may be an interlock that shuts down the generator engine if the fan goes off or if the temperature rises in the generator area. There would probably be a time delay if it depends on the fan running because it would be necessary to start the generator to make the fan run.

    You might test it by momentarily disconnecting the fan to see if it shuts down the generator engine.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,180
    Location:
    New England
    You could use an in-line fuse to protect the motors. Keep a box of spares handy. If either fan were to overload, as mentioned, unless there is an interlock to shut the generator off, it could get very expensive. If you wanted to get fancy, you could put in current sensing relays and build your own interlock, but I would hope that the manufacturer already has some sort of safety circuits in there.
  7. abikerboy

    abikerboy DIY Senior Member

    Messages:
    202
    Location:
    VA
    Trust me on this please, from someone who knows what he's saying, and has worked with and on these units.... Yes, if a motor shorts out, the thermal protectors will protect the system until you notice something wrong, for instance, steam from an overheated engine, or dim lights from a pending electrical failure. Even at worse, a failing motor will stop when it gets hot, cool down, restart, and provide its normal function on a reduced level. Even if a motor should fail drastically and instantly (not very likely), at most, youve fried a $60 to $100 voltage regulator, and maybe a few feet of cable, plus an hour or two labor for the fix. On a diesel engine (and most gas engines also have protection), if you fry the regulator, the diesel throttle/fuel solenoid loses power (or the electric fuel pump on a gas engine...thats why they use electric pumps now), and the engine shuts down. If you install a breaker or fuse on the fan circuit, and a fan motor fails, then the breaker trips or the fuse blows, the regulator is protected, BUT the engine keeps running, and the overheated engine burns up, or the overheated compartment catches fire, and thats all she wrote! Could cost you between $5000 and $7000 for an engine, or $6000 to $10,000 for a gen-set depending on the size, OR it could cost you the coach, plus your safety. On a side note, if your gen-set is anywhere from about the mid 80's up, it will have electronic controls on the engine, which will shut everything down for any malfunction, including a short circuit or overheated engine, and the control board may even sense the extra resistance from an added fuse or circuit breaker, causing no starts or shutdown. I would leave everything alone, and leave it just as it is. These units are very well tested, and safety is number one. If the manufacturer had problems, and safety wasnt of primary concern, they wouldnt be in the business very long, especially with the number of RV fires and deaths that occur each year. The way it is designed, nothing that could happen to this very short and brief circuit will ever cost you more than a couple of hundred bucks, but to modify it or mess with it could cost you your life!
    Last edited: Oct 22, 2007
  8. moisheh

    moisheh New Member

    Messages:
    42
    Location:
    Mexico and Canada
    abikerboy:

    Thanks for the info. This Onan is from the mid 80's. Diesel power. It does not have the modern electronics but has a low oil pressure and an overheat shut down function. The schematic and parts manual does show a breaker for the fan motor but the hole in the control box has been filled with a screw (factory). The installation manual does state that the motor should be protected. BUT this genset is in a tight compartment and the control box is at the top of the unit almost touching the top. There is a circulating fan that brings in outside air but I bet it gets darn hot in there. Maybe the mfr. felt that in this application the breaker would not work properly. I am installing 2 new fans. If the gennie has gone almost 20 years with no problems and dirty rusty fan motors I should be good for another 20 years ( the gennie not me!) Thanks for your insight.


    Moisheh
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