FIRERESCUE
Member
Question: Industrial application - is there a way to open a thermal OL device prior to blowing branch circuit fuses during a locked rotor condition or motor stall?
We use 20A "General Use" 480V receptacles and have BUSSMAN LP-CC-20 FUSES. The OL device for this motor is set at 9.6 Amps. According to Table 430.52 fuse size on Class CC can be up to 300% of nameplate FLA which in this case would be 28.8Amps (9.6X300%). Locked Rotor current conversion calculations per Table 430.151 (B) show design code E as 91.5Amps. The only way I can find to detect a locked rotor condition is to upgrade the OL devices in question to a new unit designed to open for such conditions. I assume the amperage increase in time vs. the heat build up in time is the crutial difference as to why this happens.
Anyway, my supervisor (boss) is wondering if there is a way to detect this fault in order to reduce the number of blown fuse replacments. One of our plants switched over from 20A fuses to 20A circuit breakers but I felt that this was not an "answer" to our problem - just a bandaid (per say).
We got in an argument over the theory of how an OL device works opposed to a fuse (I am referencing when a fuse is not intended to be used for overload protection) but he is not willing to accept the fact that OL devices work on heat build up over time and are not intended to be used for short circuit or ground fault protection. He even went as far as calling a local reputable contractor during a plant wide maintenance meeting and in front of everyone asked the contracor if OLs work on voltage spikes caused by stalled motor conditions......... he was trying to embarrase me........ their reply was "an OL device is not intended to be used for voltage spikes and if it is continually opened under such stalled conditions it would eventually fail".
Hmmm, and all this time I was under the impression that heat build up causes thermal OL devices to open and that they are desinged and intended to be resetable for just that - resetting a fault condition during a pre-determined motor overload.......
Voltage spike? pre-mature failures due to overload conditions? where did he come up with that one??? Anyway, my Boss is out to get me and is trying to make me look bad in front of all my fellow maintenance team members. I am one of the only licensed electricians that our company has .......... and my boss is not one of them.
Any feedback or help on where I can find any further information or answers would be greatly appreciated.
INJECTION MOLDING REGRINDER MOTOR - 7.5HP/480V/9.6A 3 phase
CONDITION - Locked Rotor
BRANCH CIRCUIT - 4 General Use 480V 3 phase Receptacles each independantly fused at 20Amps using LP-CC-20 fuses fed from a 60Amp Fused Disconnect. All wiring is THHN and sized accordingly. OL devices are Allen Bradley - properly rated for this motors HP and Current and are set for each motors Name Plate FLA.
Brian G.
Michigan
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We use 20A "General Use" 480V receptacles and have BUSSMAN LP-CC-20 FUSES. The OL device for this motor is set at 9.6 Amps. According to Table 430.52 fuse size on Class CC can be up to 300% of nameplate FLA which in this case would be 28.8Amps (9.6X300%). Locked Rotor current conversion calculations per Table 430.151 (B) show design code E as 91.5Amps. The only way I can find to detect a locked rotor condition is to upgrade the OL devices in question to a new unit designed to open for such conditions. I assume the amperage increase in time vs. the heat build up in time is the crutial difference as to why this happens.
Anyway, my supervisor (boss) is wondering if there is a way to detect this fault in order to reduce the number of blown fuse replacments. One of our plants switched over from 20A fuses to 20A circuit breakers but I felt that this was not an "answer" to our problem - just a bandaid (per say).
We got in an argument over the theory of how an OL device works opposed to a fuse (I am referencing when a fuse is not intended to be used for overload protection) but he is not willing to accept the fact that OL devices work on heat build up over time and are not intended to be used for short circuit or ground fault protection. He even went as far as calling a local reputable contractor during a plant wide maintenance meeting and in front of everyone asked the contracor if OLs work on voltage spikes caused by stalled motor conditions......... he was trying to embarrase me........ their reply was "an OL device is not intended to be used for voltage spikes and if it is continually opened under such stalled conditions it would eventually fail".
Hmmm, and all this time I was under the impression that heat build up causes thermal OL devices to open and that they are desinged and intended to be resetable for just that - resetting a fault condition during a pre-determined motor overload.......
Voltage spike? pre-mature failures due to overload conditions? where did he come up with that one??? Anyway, my Boss is out to get me and is trying to make me look bad in front of all my fellow maintenance team members. I am one of the only licensed electricians that our company has .......... and my boss is not one of them.
Any feedback or help on where I can find any further information or answers would be greatly appreciated.
INJECTION MOLDING REGRINDER MOTOR - 7.5HP/480V/9.6A 3 phase
CONDITION - Locked Rotor
BRANCH CIRCUIT - 4 General Use 480V 3 phase Receptacles each independantly fused at 20Amps using LP-CC-20 fuses fed from a 60Amp Fused Disconnect. All wiring is THHN and sized accordingly. OL devices are Allen Bradley - properly rated for this motors HP and Current and are set for each motors Name Plate FLA.
Brian G.
Michigan
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