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  1. #1
    2Slow's Avatar
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    Now that my RPC is working great, I figure I should start screwing with it again ...

    I was thinking of replacing the two fuse blocks shown with 30A overlaod relays because I am not sure I like some of the failure modes if 1 fuse blows.



    I think that if I replace the fuse blocks with overload relays if one leg draws too much current, it kills all three legs. Is that true? Will the overload relays allow the "overcurrent" during starting that the slow blow fuses allow?

    -Joe

    [ 02-24-2007, 11:24 AM: Message edited by: 2Slow ]

  2. #2
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    After reading some more, it seems the fuses are there to protect the wire and the overloads are there to protect the motor, so do I need both?

    If the overloads are set at 28.5 amps (my idler motor's FLA) wouldn't they protect the wire to the motor which is #10 and good for 30 amps? Are overloads not OK to protect the wire?

    I don't know if I have enough room in my panel for both overloads and fuses, so if you had to choose only one what would you do?

    The individual machines have overloads in them to protect their motors, but I was going to replace the RPC output fuse block with 30 amp overlaods (to protect the #10 output wire) The only reason I want to swap the fuses with overloads is so if one leg overloads, power to all legs are removed to prevent "single phasing".



    Thanks,
    -Joe

  3. #3
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    "The only reason I want to swap the fuses with overloads is so if one leg overloads, power to all legs are removed to prevent 'single phasing'."

    These days, fuses are mainly used in fusible safety switches.

    You could use a "common trip" circuit breaker, should you want an overload in one phase to trip all three.

  4. #4
    2Slow's Avatar
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    What is the difference between a common trip breaker and an overload relay?

    Thanks,
    -Joe

  5. #5
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    Jraef is online now Titanium
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    First off, realize that Circuit breakers and Fuses perform the task of protecting the CIRCUIT, i.e. the wires, from fast acting short circuits such as a wire going to ground or one motor winding melting into another one. For this reason, CBs and fuses collectively are referred to as SCPDs, Short Circuit Protective Devices.

    The Overload Relay protects the MOTOR from a long-term running overload. It does not see the current from a short or ground fault until long after a fire will have started. It also does not interrupt current, it just senses it and trips a little relay contact. You would then wire that relay contact into the coil circuit of a magnetic contactor to drop out the magnetic coil power if the Overload Relay trips, and it is the contactor that interrupts the current flow to the motor. The Overload Relay works by sensing the heat produced by the current flowing through the circuit and uses the I^2t (Inverse Current / Time) calculation method to determine that heat. So an overload Relay is referred to as a Thermal Over Load (TOL) device.

    According to the NEC every motor circuit, however it is used, must contain a SCPD, a TOL and a switching / controlling device, as well as a means of safely disconnecting the motor from power and locking it off.

    Technically fuses can theoretically only be used as the TOL on motors 1HP and below, but only if they are specifically sized to a narrow range of the motor FLA, 115% max. I believe. The problem is, fuses only come in certain sizes so it is not really practical to find the exact right size, plus you have the problem of what happens when only 1 fuse blows. This however does not apply to your situation.

    A circuit breaker is similar to the contactor in that it has all 3 power elements linked together and opens all 3 at once, but it is not automatic in that it does not (typically) have a "coil" circuit that can be operated by the Overload Relay.

    So in your case, I would replace the 30A fuse block feeding the idler with an Overload Relay, and then use the NC contact of the OL relay in the 24VAC coil circuit of that 90A relay to drop it out if the idler gets overloaded. The caveat to that is the size of wire you have used. Since your upstream fused disconnect is sized at 50A, you can use that as the SCPD for the entire circuit only as long as all of the wires downstream are rated for 50A. If not (which is your case), the OL relay is considered to provide long term protection of the wires when combined with upstream fuses, just as long as 175% of the motor FLA must does exceed that fuse rating. So since your motor is rated 28.5 FLA, 175% is 49.875A, tantalizingly close to the 50A fuse rating. I personally would go for it, but you have to take your risks as you see fit. The safest route would be leaving the 30A fuses in there and just adding the OL relay downstream.

  6. #6
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    Here's the danger I see.

    A classic overload heater "section" is a "signaling" device. Classically it signals a contactor's coil to open the contacts.

    However, there's a failure mode out there where the contacts weld together and despite the coil "releasing" the contacts can't open.

    So then you have small to very severe problems with an uniterrupted current flow. A fuse on the other hand cannnot weld closed. It "welds" open, which is a great safety feature.

  7. #7
    2Slow's Avatar
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    Jraef,

    Excellent explination!!! I will order the overloads and do my best to squeeze them in along with the fuses.

    Since I think I can squeeze in only 1 overload as well as the two fuse blocks. I would put the overload on the idler, and leave the fuses for the output side of the RPC since each individual machine has it's own overload set at values way under 30 amps.

    Thanks for taking the time to write that up...

    Matt,

    I did not see the failure mode that you were talking about. But I didn't know the overload was just a signaling device either...

    This group always has some wise people...
    Thanks,

    -Joe

    [ 02-25-2007, 06:51 PM: Message edited by: 2Slow ]

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