Reducing arc flash chances

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unsaint33

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In a MCC panel, I had a line fuse blown. The fuse was 15A fuse for three phase 480V motor circuit. After replacing the fuse, I manually tripped the MCP overload unit. Then I closed the fuse holder by pushing and pivoting the door up. My coworker asked me why I tripped the MCP manually before engaging the fuse. I always did that when I install fuses for motor circuits because if the starter coil was energized when the fuse was added to the circuit, the current draw will spike causing the danger of arc flash to be much greater. The MCP overload unit's ON-switch is spring-loaded so it completing the circuit is very quick so the arc danger level is relatively low. However, my hand closing the fuse holder door is relatively very slow posing high arc flash danger. Am I correct on this thinking?
 
Why do you think there will be an arc flash with the fuse? An arc flash will occur when a person makes contact with energized terminals or buses with a conducting object, among other causes. So for the fuse in the mcc cubicle, your proper procedure should be:
1. Open the breaker with the cubicle door closed
2. Put on the appropriate arc rated PPE (as determined by the arc flash label on the MCC) and open the cubicle door
3. Replace fuse using fuse pullers
4. Close cubicle door and re-energize.

It is not clear on what the fuse does from your description. There is an MCP so I assume there are overloads and then is this the fuse for the control power transformer?
 
wbdvt said:
Why do you think there will be an arc flash with the fuse? An arc flash will occur when a person makes contact with energized terminals or buses with a conducting object, among other causes. So for the fuse in the mcc cubicle, your proper procedure should be:
1. Open the breaker with the cubicle door closed
2. Put on the appropriate arc rated PPE (as determined by the arc flash label on the MCC) and open the cubicle door
3. Replace fuse using fuse pullers
4. Close cubicle door and re-energize.

It is not clear on what the fuse does from your description. There is an MCP so I assume there are overloads and then is this the fuse for the control power transformer?
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Sounds like he is just trying to minimize some risk involved with closing the switch (after replacing a blown fuse) into a fault. Another step that should be included in his procedure is to check for any obvious faults before replacing the fuse and closing the circuit again.
 
If the fuse is inserted quickly it will clear the fault. The natural reaction is to remove a fuse under fault, the worse thing you can possibly do. As the fuse is removed it will draw an arc, the results will not be pretty.

Most MV equipment nowadays is fitted with anti-reflex mechanisms to stop the operator changing their mind during a switching operation.
 
unsaint33 said:
In a MCC panel, I had a line fuse blown. The fuse was 15A fuse for three phase 480V motor circuit. After replacing the fuse, I manually tripped the MCP overload unit. Then I closed the fuse holder by pushing and pivoting the door up. My coworker asked me why I tripped the MCP manually before engaging the fuse. I always did that when I install fuses for motor circuits because if the starter coil was energized when the fuse was added to the circuit, the current draw will spike causing the danger of arc flash to be much greater. The MCP overload unit's ON-switch is spring-loaded so it completing the circuit is very quick so the arc danger level is relatively low. However, my hand closing the fuse holder door is relatively very slow posing high arc flash danger. Am I correct on this thinking?
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Wait, I don't understand why you have a 15A fuse AND an MCP in the same cubicle... MCP = Motor Circuit Protector, a magnetic-only circuit breaker, used INSTEAD of a fuse. Most don't even have an external trip button.

If you are instead referring to the OVERLOAD relay that has a test button that trips it externally, then that makes more sense. Personally I never do that when installing a new fuse, but I see no harm in it. In an MCC bucket or a properly designed combo starter, you should not be able to open the door to replace a fuse before opening the line side disconnect, so manually tripping the OL before installing the fuse is totally redundant. If on the other hand you are defeating the door interlocks and replacing line fuses without opening the disconnect switch, you have some SERIOUS procedural problems here
 
Jraef said:
Wait, I don't understand why you have a 15A fuse AND an MCP in the same cubicle... MCP = Motor Circuit Protector, a magnetic-only circuit breaker, used INSTEAD of a fuse. Most don't even have an external trip button.

If you are instead referring to the OVERLOAD relay that has a test button that trips it externally, then that makes more sense. Personally I never do that when installing a new fuse, but I see no harm in it. In an MCC bucket or a properly designed combo starter, you should not be able to open the door to replace a fuse before opening the line side disconnect, so manually tripping the OL before installing the fuse is totally redundant. If on the other hand you are defeating the door interlocks and replacing line fuses without opening the disconnect switch, you have some SERIOUS procedural problems here
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From my understanding of OP he isn't changing the fuse with live fuseholder, but rather is concerned about closing the disconnect into a fault after having changed a fuse, should the motor controller be closed/closing nearly simultaneously and his hand is still in front of the MCC unit. A problem I see if that is the logic is most cases you still would have your hand in front of the MCC unit when you do reset the overload and the motor would automatically attempt restarting.
 
kwired said:
From my understanding of OP he isn't changing the fuse with live fuseholder, but rather is concerned about closing the disconnect into a fault after having changed a fuse, should the motor controller be closed/closing nearly simultaneously and his hand is still in front of the MCC unit. A problem I see if that is the logic is most cases you still would have your hand in front of the MCC unit when you do reset the overload and the motor would automatically attempt restarting.
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Ok. But what is the difference between closing the disconnect into a fault vs closing a contactor into a fault in terms of arc flash safety? If the unit door is open or closed, whatever risk exists will exist no matter which device closes into a pre-existing fault.

By the way, replacing a fuse without first determining the CAUSE of the fault and eliminating it is a way to create an arc flash incident... smoke testing with fuses is never a good way to troubleshoot.
 
Jraef said:
Ok. But what is the difference between closing the disconnect into a fault vs closing a contactor into a fault in terms of arc flash safety? If the unit door is open or closed, whatever risk exists will exist no matter which device closes into a pre-existing fault.

By the way, replacing a fuse without first determining the CAUSE of the fault and eliminating it is a way to create an arc flash incident... smoke testing with fuses is never a good way to troubleshoot.
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I thought I kind of already said same things, but.. yes.
 
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