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    #16
    Let me get this straight. An EE is advocating NOT using a motor starter on 3 phase 480 motors in excess of X HP. WOW is this "value engineering" gone way wrong?

    First time you blow one fuse, single phase that motor and ruin it, your value engineering has cost me a lot of downtime, labor, and money.

    Spec motor starters with electronic overload relays. Spec fuse blocks with class J, RK1, or CC fuses ahead of these motor starters. Size fuses in accordance with overload relay setting, motor HP, and maker of starter recommendations to provide type 2 level of protection. In my experience this is generally around 175% of motor FLC. Small fans and pumps can often be closer to 125% as they never run at motor FLC much less SF amps. The branch circuit breaker protects the conductors. The fuses protect the starter and limit current to a level of SCCR the starter can survive. The overload relay protects the motor. Done right all you do is replace a fuse, reset an OL relay, reset a breaker, or maybe replace a motor. Done wrong and it can get real ugly.

    Or better yet, spec a factory built MCC and let them do the work.

    There are so many good reasons to have a motor starter with current limiting fuses/MCCBs. The only reason to not have them is "penny wise pound foolish" mentality. That type of engineering will get you a very bad name in a hurry. IMHO what you are suggesting is downright dangerous.

    Okay rant off and a I humbly apologize if I misunderstood what you are proposing.

    Comment


      #17
      Originally posted by mbrooke View Post

      But- I am embarrassed to admit that I don't know the inrush properties of 3 phase induction motors for various horse powers when switched on at various points in the sine wave, when there is a momentary "blink" and when there is residual magnetism still in the iron.

      I know transformer inrush varies based on those 3 factors, and, I'd imagine its the same for motors.
      It much the same MBrooke

      OL's are sized 430.32, OCPD's 430.52

      OL's are slower than OCPD's , so they are a tighter tolerance to FLA

      It's all constructed so we don't have to run 'fat' wires

      ~RJ~

      Comment


        #18
        Originally posted by Russs57 View Post
        Let me get this straight. An EE is advocating NOT using a motor starter on 3 phase 480 motors in excess of X HP. WOW is this "value engineering" gone way wrong?
        Yup, its even been done in old building MCCs.

        First time you blow one fuse, single phase that motor and ruin it, your value engineering has cost me a lot of downtime, labor, and money.

        I'd think that once the motor goes single phase, it draws more current that causes the other fuses to blow.

        Of course there will be single phase protection, if the feed into the MCC drops a phase the power is cut to the banging contactor.

        Spec motor starters with electronic overload relays. Spec fuse blocks with class J, RK1, or CC fuses ahead of these motor starters. Size fuses in accordance with overload relay setting, motor HP, and maker of starter recommendations to provide type 2 level of protection. In my experience this is generally around 175% of motor FLC. Small fans and pumps can often be closer to 125% as they never run at motor FLC much less SF amps. The branch circuit breaker protects the conductors. The fuses protect the starter and limit current to a level of SCCR the starter can survive. The overload relay protects the motor. Done right all you do is replace a fuse, reset an OL relay, reset a breaker, or maybe replace a motor. Done wrong and it can get real ugly.
        The little fuse pdf says RK fuses alone will work.


        Or better yet, spec a factory built MCC and let them do the work.

        There are so many good reasons to have a motor starter with current limiting fuses/MCCBs. The only reason to not have them is "penny wise pound foolish" mentality. That type of engineering will get you a very bad name in a hurry. IMHO what you are suggesting is downright dangerous.

        Okay rant off and a I humbly apologize if I misunderstood what you are proposing.


        Why dangerous? The fuse will protect the wire and the motor from thermal damage.

        Doing more with less has many advantages.

        Kindly showing my green light
        Attached Files
        Last edited by mbrooke; 10-21-19, 06:51 PM.

        Comment


          #19
          Originally posted by romex jockey View Post

          It much the same MBrooke

          OL's are sized 430.32, OCPD's 430.52

          OL's are slower than OCPD's , so they are a tighter tolerance to FLA

          It's all constructed so we don't have to run 'fat' wires

          ~RJ~
          True, but I figure with the same wires the fuse can do the same of the overload.

          Comment


            #20
            Originally posted by mbrooke View Post
            I'd think that once the motor goes single phase, it draws more current that causes the other fuses to blow.
            You would typically be mistaken with this thought. For example, if the motor is not loaded substantially, the current in the remaining phases may not increase significantly.
            Some 60 years ago the NEC started requiring running Overload protective devices in each phase, for this very reason.


            Just because you can, doesn't mean you should.

            Comment


              #21
              Originally posted by jim dungar View Post

              You would typically be mistaken with this thought. For example, if the motor is not loaded substantially, the current in the remaining phases may not increase significantly.

              In that case neither would heater catch it. One phase opens- if the remaining two phases pull more current they either cause the soldier to spin the wheel tripping the starter OR the fuse link melts on one phase dropping the motor.

              Some 60 years ago the NEC started requiring running Overload protective devices in each phase, for this very reason.

              [/QUOTE]

              Correct, for open phase conditions outside the structure:

              http://www.cooperindustries.com/cont...le-Phasing.pdf


              I know I'm being a hard head, but in so far I can't find any case against fuses providing overload protection. To be frank with everyone, I've never even fully understood the purpose of heaters. I always wondered why they didn't just sell a standard molded case breaker with the thermal and magnetic trip slightly tweaked at the factory.
              Attached Files

              Comment


                #22
                Originally posted by LarryFine View Post

                Certainly, but will result in larger numbers.
                True, but it gives an acceptable number albeit worst case.

                Comment


                  #23
                  I would never dream of relying on fuses for motor protection.

                  For a start it would be against BS7671 regulations. Secondly, the cost comparison between an overload relay and the cost of a rewind or a new motor.
                  Call it a one off payment insurance policy.

                  Fuses protect the cable, the overload relay protects the motor.
                  [COLOR=#000000]The reports of my death are greatly exaggerated.[/COLOR]

                  Comment


                    #24
                    Originally posted by Russs57 View Post
                    Let me get this straight. An EE is advocating NOT using a motor starter on 3 phase 480 motors in excess of X HP. WOW is this "value engineering" gone way wrong?

                    First time you blow one fuse, single phase that motor and ruin it, your value engineering has cost me a lot of downtime, labor, and money.
                    Totally agree. We mostly used thermal overloads with single phasing protection. When I was "growing up" the practical side of my learning was in a linoleum factory. They had manual star-delta starters. The overloads had oil filled dash pots. The viscosity of the oil gave the delay that thermal elements do these days.

                    Comment


                      #25
                      Originally posted by mbrooke View Post

                      Can we assume infinite bus?
                      Well if the desired selection won't hold during startup you either need a different a different trip curve or something to limit current during starting.

                      I have found many cases where you can get by with lesser fuses or circuit breakers than what is recommended as starting current is limited by source and/or the circuit. Also found many cases where you may need to use all 175 or 250% (especially for breakers with no mag trip adjustment) of the general code rules for short circuit/ ground fault protection to allow starting - all depends on conditions of the installation.
                      I live for today, I'm just a day behind.

                      Comment


                        #26
                        Originally posted by kwired View Post

                        Well if the desired selection won't hold during startup you either need a different a different trip curve or something to limit current during starting.

                        I have found many cases where you can get by with lesser fuses or circuit breakers than what is recommended as starting current is limited by source and/or the circuit. Also found many cases where you may need to use all 175 or 250% (especially for breakers with no mag trip adjustment) of the general code rules for short circuit/ ground fault protection to allow starting - all depends on conditions of the installation.
                        So you are saying he could put it all together as he wants and if it works, good? If not, what the heck, try again with a new motor and different fuses.

                        Tom
                        TBLO

                        Comment


                          #27
                          Originally posted by Tony S View Post
                          I would never dream of relying on fuses for motor protection.

                          For a start it would be against BS7671 regulations. Secondly, the cost comparison between an overload relay and the cost of a rewind or a new motor.
                          Call it a one off payment insurance policy.

                          Fuses protect the cable, the overload relay protects the motor.
                          What passage of BS7671 does it violate?

                          Comment


                            #28
                            Mbrooke, please understand I am a maintenance engineer at a hospital. I'm not one to quote code. I'm not interested in the lowest possible first cost solution. I'm interested in a robust, bullet-proof, quick and easy to service system. What is dangerous for me is likely perfectly safe for others. So with that in mind I'll offer the following based of my limited knowledge of code and 40 years of actual experience. All this assumes 480/277/4W/3PH with 1.15 SF motors greater than 1 HP.

                            As I understand it conductors must be sized for 125% of FLC per NEC table. Short circuit/ground fault protection ranges from 150% to 300% of FLC. IIRC it is 300% for non-time delay fuses, 175% for dual element time delay fuses, and 250% for inverse time breaker. For a 1.15 SF motor overload protection is 125% of motor nameplate amps. You can round up to next standard size expect for overload where you have to round down. Of course there are exceptions.

                            All of this can lead to some rather weird looking installations. Things like # 10 THHN on a 60 amp breaker, which is fine because overload protection would be set at 21.9 amps. So IMHO you are going to have a hard time picking fuses that will stay within that 125% requirement of nameplate amps (remembering that you have to round down), and reliably handling starting current. Fuses aren't cheap so you don't want them blowing every time you get a power blink.

                            You mention a 200 HP motor. Lets see what we have. A quick look says nameplate amps of 225 times 1.25 = 281 amps for overload protection. You have to round down so no 300 amp fuses allowed. Bet you have a hard time finding anything other than a dual element 250 amp fuse. You really think that is going to start that motor? Normally that fuse would be 400 amps if you had an overload relay.

                            Now to what I consider dangerous. I won't mention times I have been shocked or things that have caught on fire. This following is a real experience.

                            I had a set of three medical vacuum pumps. IIRC each pump had a 3 phase, 480, 10 HP motor. Each motor had an IEC motor starter with overload relay. Motor starter cabinet was fed from one breaker in an emergency power panel and motor starter cabinet had one MCCB for short circuit protection. One motor shorted to ground, IEC motor starter was blown to pieces with some melted wiring, causing upstream main breaker to trip on ground fault. Next thing you know I have a whole lot of patients without life safety/critical/equipment power (division of service wasn't required back then) and people on beds/stretchers trapped in elevators. All because someone didn't think three 3 phases fuse blocks with current limiting fuses were worth the price. You can bet your bottom dollar your's truly installed a new cabinet with class RK1 fuses and NEMA rated motor starters with electronic overloads the next day. Never had a problem again ever though other motors shorted to ground over the decades.

                            Too bad that sometimes bad things have to happen before the bean counters understand first costs versus life cycle costs.

                            Comment


                              #29
                              Originally posted by ptonsparky View Post

                              So you are saying he could put it all together as he wants and if it works, good? If not, what the heck, try again with a new motor and different fuses.
                              Certainly

                              Or use the more typical motor overload protection devices. Many times you have a contactor as a controller anyway, not too big of a deal to add motor overload that matches up to the contactor or buy it as a complete assembly.
                              I live for today, I'm just a day behind.

                              Comment


                                #30
                                Originally posted by Russs57 View Post
                                Mbrooke, please understand I am a maintenance engineer at a hospital. I'm not one to quote code. I'm not interested in the lowest possible first cost solution. I'm interested in a robust, bullet-proof, quick and easy to service system. What is dangerous for me is likely perfectly safe for others. So with that in mind I'll offer the following based of my limited knowledge of code and 40 years of actual experience. All this assumes 480/277/4W/3PH with 1.15 SF motors greater than 1 HP.


                                As I understand it conductors must be sized for 125% of FLC per NEC table. Short circuit/ground fault protection ranges from 150% to 300% of FLC. IIRC it is 300% for non-time delay fuses, 175% for dual element time delay fuses, and 250% for inverse time breaker. For a 1.15 SF motor overload protection is 125% of motor nameplate amps. You can round up to next standard size expect for overload where you have to round down. Of course there are exceptions.
                                Yup, no disagreement.

                                All of this can lead to some rather weird looking installations. Things like # 10 THHN on a 60 amp breaker, which is fine because overload protection would be set at 21.9 amps. So IMHO you are going to have a hard time picking fuses that will stay within that 125% requirement of nameplate amps (remembering that you have to round down), and reliably handling starting current. Fuses aren't cheap so you don't want them blowing every time you get a power blink.

                                I understand starting current is the crux of the issue- however- several fuse manufacturers are confident that their time delay fuses will handle these inrush currents while simultaneously providing good overload protection.


                                You mention a 200 HP motor. Lets see what we have. A quick look says nameplate amps of 225 times 1.25 = 281 amps for overload protection. You have to round down so no 300 amp fuses allowed. Bet you have a hard time finding anything other than a dual element 250 amp fuse. You really think that is going to start that motor? Normally that fuse would be 400 amps if you had an overload relay.
                                Honestly, I have no idea- but two fuse manufacturers are confident that it will.


                                Now to what I consider dangerous. I won't mention times I have been shocked or things that have caught on fire. This following is a real experience.

                                I had a set of three medical vacuum pumps. IIRC each pump had a 3 phase, 480, 10 HP motor. Each motor had an IEC motor starter with overload relay. Motor starter cabinet was fed from one breaker in an emergency power panel and motor starter cabinet had one MCCB for short circuit protection. One motor shorted to ground, IEC motor starter was blown to pieces with some melted wiring, causing upstream main breaker to trip on ground fault. Next thing you know I have a whole lot of patients without life safety/critical/equipment power (division of service wasn't required back then) and people on beds/stretchers trapped in elevators. All because someone didn't think three 3 phases fuse blocks with current limiting fuses were worth the price. You can bet your bottom dollar your's truly installed a new cabinet with class RK1 fuses and NEMA rated motor starters with electronic overloads the next day. Never had a problem again ever though other motors shorted to ground over the decades.

                                Too bad that sometimes bad things have to happen before the bean counters understand first costs versus life cycle costs.

                                That sounds like a complete lack of selective coordination, and to be honest having individual short circuit protection for each motor does not fix your problem. What if there is a buss fault inside your motor control center or the fuse block melts down? In your case the MCCB and/or the branch circuit breaker should have tripped, not the whole riser or critical power system. Which to be honest is to common in critical application.

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