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Thread: Service Disconnect Fuse Contacts

  1. #1

    Default Service Disconnect Fuse Contacts

    The service disconnect for our air handler has two 60A time-delay fuses. The fuse contacts (where the fuses plug into the disconnect) are oxidized. This is causing fuse failure. Two questions:

    1. What is the best way to clean the oxidation off the contacts? A wire brush didn't do much good.

    2. P=I^2xR. So as the resistance across the fuse increases because of the oxidation the heat is increasing across the fuse leading to premature fuse failure. Am I understanding this correctly?

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Default fuse

    You may also be incorrect as the the cause of the problem. If that situation has occurred, the contacts inside the switch have probably been overheated so they no longer have the ability to make a tight connection to the blades, and that is what is causing the heating condition.

  3. #3

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    Quote Originally Posted by hj View Post
    You may also be incorrect as the the cause of the problem. If that situation has occurred, the contacts inside the switch have probably been overheated so they no longer have the ability to make a tight connection to the blades, and that is what is causing the heating condition.
    Unless you're talking about a switch inside the unit itself I'm not sure I understand. There isn't a switch at the disconnect.

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    DIY Senior Member Probedude's Avatar
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    Default

    Quote Originally Posted by dave9600 View Post
    The service disconnect for our air handler has two 60A time-delay fuses. The fuse contacts (where the fuses plug into the disconnect) are oxidized. This is causing fuse failure. Two questions:

    1. What is the best way to clean the oxidation off the contacts? A wire brush didn't do much good.

    2. P=I^2xR. So as the resistance across the fuse increases because of the oxidation the heat is increasing across the fuse leading to premature fuse failure. Am I understanding this correctly?
    Your equation is correct and your assumption is likely correct too. What may be a complicating problem is the springy-ness of the fuse clips in your disconnect box is now fatiqued from heat. If so they likely are not applying enough force to keep intimate contact with the new fuse.

  5. #5

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    Quote Originally Posted by Probedude View Post
    Your equation is correct and your assumption is likely correct too. What may be a complicating problem is the springy-ness of the fuse clips in your disconnect box is now fatiqued from heat. If so they likely are not applying enough force to keep intimate contact with the new fuse.
    So I would be better off just replacing the disconnect altogether. That's kinda what I figured.

  6. #6
    DIY Senior Member Probedude's Avatar
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    Quote Originally Posted by dave9600 View Post
    So I would be better off just replacing the disconnect altogether. That's kinda what I figured.
    It would solve the problem if this is truly where your failure is.

    Have you confirmed that the fuse clips are heating up when the air handler is in operation (using an IR camera or thermometer)?

    Has anything changed on the air handler? (new motor, new blower, change in pulleys or pulley ratios).

    Dave

  7. #7
    Electrician Chris75's Avatar
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    Default

    Quote Originally Posted by dave9600 View Post
    Unless you're talking about a switch inside the unit itself I'm not sure I understand. There isn't a switch at the disconnect.
    Of course there is, why do you think they call it a disconnect?

  8. #8
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by Chris75 View Post
    Of course there is, why do you think they call it a disconnect?
    Because the I squred R losses are making the fuses blow?

    I would think that the best avenue would be to put an ammeter on the conductors and see what the unit is drawing but then again what do I know?

  9. #9
    Moderator & Master Plumber hj's Avatar
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    Default disconnect

    "Disconnect" implies a switch at the fuse site, either a rotating switch or a pull out fuse holder. Either of these will lose their temper and tension if overheated by poor contact.

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