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  • saw triggering breaker

    why would a 1/2 hp saw trigger the rcd breaker when it stalls?
    Last edited by dian; 03-23-2020, 03:25 PM.

  • #2
    Some RCD's are just switch type and don't react to overload currents and others protect both earth leakage and overload. Yours might be the latter. A motor stall draws high current.
    Helder Ferreira
    Setubal, Portugal

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    • #3
      If the "RCD" (GFCI) is in the outlet, as many are here in the US, then there probably is a fault in either the saw (ground fault) or the RCD (just faulty)..... The RCDs here do go bad eventually, I expect others do as well.

      If it is in the breaker box, then it is a combined type, and it could more likely be an overload, since a stalled saw pulls maybe 6x max curren t.....
      1601

      Keep eye on ball.
      Hashim Khan

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      • #4
        the "GI" breaker is besides the regular breakers (slow 16a/230v) in the box. i have only two "GI"s for the whole house and if this one trips the router is gets offline and all electronics are dead incl. the phone.

        the ground connection on the saw looks good, maybe i should add another one? or add a fast 5amp breaker between the saw and socket (the saw draws around 1.5 amps)?

        any other ideas what to do?

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        • #5
          So, why does the saw stall? That seems to be the easy point of attack.

          Alternately, as you point out, you can "co-ordinate" breakers and limit the fault to the saw itself. That is a reasonable approach, as it is fairly easy to stall saws, and you have a problem with the consequence of doing that.
          1601

          Keep eye on ball.
          Hashim Khan

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          • #6
            the saw stalls from time to time because its a stupid little saw with a coarse blade and doesnt hold the part being cut off. when the part is too thin the blade can jam.

            but im still interested in the reason behind triggering the GI. it could help my understanding of how these devices work. i assume it triggers when there is more current going in than to ground. but im not sure if thats the neutral or the real ground or both maybe? however large the current is, it must be flowing back, right? unless its sparking to the floor or something. so where is the reson for the imbalance? has it to do with the two capacitors?

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            • #7
              If current that goes out on the hot wire comes back on the neutral, everything should be fine. If current goes out on the hot wire, but comes back any other way than the neutral, then it triggers the RCD/GFCI. A balance circuit checks for that, essentially both wires run through a toroidal core coil, and if any of it over about 3 mA does not come back on one of the wires, the imbalance develops a voltage in the coil and trips the circuit. There are other types of check as well, but essentially they detect a leakage to ground.

              Now, because these things are not perfect, it is possible that the balance is not quite exact, and some voltage is developed even when current is not leaking. In that case, with a high current draw, the detector may act simply because the imbalance due to imperfections is enough to develop the "trip voltage" at that current.

              Also, some types are designed to open for an overcurrent as well as an imbalance.

              I do not know what the issue is with yours. Putting a suitably rated breaker in just for the saw may avoid triggering the RCD unnecessarily. The more times they are tripped, the more likely they are to become defective.
              1601

              Keep eye on ball.
              Hashim Khan

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              • #8
                im supposed to trigger them manually twice a year.

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                • #9
                  1. old and tired GFCI. seen once or twice one that would trigger way too easily
                  2. GFCI false triggered with interference from saw, increased current draw could cause more interference. Electronic speed control on saw?
                  3. saw actually faulty, ie something funny mechanically that short-circuits with increased loading

                  hand-held circular saw?
                  On European models most of these build in last 50 years don't even have ground connection on the plug.

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                  • #10
                    I have an older Milwaukee worm drive circular saw that every now and then will trip the breaker of what ever circuit I plug it into, (20 amp.) This happens when I hit the trigger. Doesn't happen every time but enough to become annoying. Once it's running there is no problem and once it gets warmed up it's fine.
                    Since the the current pull is the greatest upon start I'm guessing that dried bearings or armature / brushes is causing the extra load upon start and causing the breaker to trip.
                    One of these days I'll have to take it apart and clean it.

                    JL..................

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                    • #11
                      Originally posted by MattiJ View Post
                      1. old and tired GFCI. seen once or twice one that would trigger way too easily
                      2. GFCI false triggered with interference from saw, increased current draw could cause more interference. Electronic speed control on saw?
                      3. saw actually faulty, ie something funny mechanically that short-circuits with increased loading

                      hand-held circular saw?
                      On European models most of these build in last 50 years don't even have ground connection on the plug.
                      That's because they're double insulated. The armature's and coil metal parts are mounted to the shaft with an extra layer of insulation so it's virtually impossible for any power carrying component touching any metal exposed parts such as drill chucks or saw blades.
                      Helder Ferreira
                      Setubal, Portugal

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                      • #12
                        Yup, just curious what model OP has.

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                        • #13
                          I lead a sheltered life I guess. I had to look up the RCD acronym.

                          If your RCD is in the plug itself, which it sounds like, then it might well be just the plug itself is faulty. I've had two of them go bad on me. One right out of the box and one after about a year of use.

                          It's not just the ground current that'll trip them. It's also any mismatch in the hot to neutral currents. The idea being that the amount of the mismatch might be going to ground through you. But if for some reason that part which senses the difference is a bit wonky then it can cut out when it doesn't need to.

                          On the other hand the ones I've got directly in the panel have always been fine.
                          Chilliwack BC, Canada

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                          • #14
                            Originally posted by BCRider View Post
                            ......

                            If your RCD is in the plug itself, which it sounds like, then it might well be just the plug itself is faulty. ........

                            .
                            Not if it also shuts off other devices like the router and other electronics. The OP has been "very economical" with specifics and information.

                            It would help to get a simple, straight answer as to where the RCD is located, and, in fact, what breakers and other overcurrent devices are in-line between the incoming power source and the saw, along with their ratings and the order in which they are put...
                            1601

                            Keep eye on ball.
                            Hashim Khan

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                            • #15
                              Originally posted by dian View Post
                              the saw stalls from time to time because its a stupid little saw with a coarse blade and doesnt hold the part being cut off. when the part is too thin the blade can jam.

                              but im still interested in the reason behind triggering the GI.
                              I was very surprised to find that the cheap little drill that I bought had no overload protection. The stupid thing smoked itself when i stalled it while drilling a 1/2 inch hole in some green wood. Stall is "locked rotor" situation and the current draw (LRA) goes way up. An online calculator gave as small as 7.1 amps for a 1/2 hp motor and going up from there.

                              Dan
                              At the end of the project, there is a profound difference between spare parts and extra parts.

                              Location: SF East Bay.

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