Arc Flash Mitigation
Arc Flash Mitigation
(OP)
Hello,
I had a question with regards to utilizing the arc flash mitigation maintenance mode switch on a low voltage circuit breaker. Should the maintenance mode setting take into consideration the energization of a large load downstream? For example, the MCC main breaker has the maintenance mode switch enabled and a feeder breaker on the MCC feeding a large transformer is energized. Should the maintenance mode setting on the main breaker take into consideration the inrush current of the downstream transformer for energization purposes? Or, is the maintenance mode only intended for hot work, such as racking in a MCC cubicle or carrying out work on a live bus etc.? If the downstream transformer inrush current is to be considered then the maintenance mode pickup level would have to be increased. Is this typically taken into consideration?
Please let me know your thoughts.
Thanks
I had a question with regards to utilizing the arc flash mitigation maintenance mode switch on a low voltage circuit breaker. Should the maintenance mode setting take into consideration the energization of a large load downstream? For example, the MCC main breaker has the maintenance mode switch enabled and a feeder breaker on the MCC feeding a large transformer is energized. Should the maintenance mode setting on the main breaker take into consideration the inrush current of the downstream transformer for energization purposes? Or, is the maintenance mode only intended for hot work, such as racking in a MCC cubicle or carrying out work on a live bus etc.? If the downstream transformer inrush current is to be considered then the maintenance mode pickup level would have to be increased. Is this typically taken into consideration?
Please let me know your thoughts.
Thanks






RE: Arc Flash Mitigation
RE: Arc Flash Mitigation
RE: Arc Flash Mitigation
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Arc Flash Mitigation
RE: Arc Flash Mitigation
RE: Arc Flash Mitigation
RE: Arc Flash Mitigation
Now that it has been explained a little better, there is another consideration. Typically unless the main breaker is completely isolated from the rest of the MCC/Panel, that cannot be used to determine the incident energy if the arc flash event involves the buswork or a breaker installed in that configuration. In this case the next upstream protective device is used in the determination of the incident energy. (IEEE 1584b-2011, Section 4.9 Step 8). Most likely the main breaker is integral to the rest of the MCC/Panel, so it cannot be used to mitigate an event on a breaker located in that MCC/Panel regardless of a maintenance mode. This is because an internal arcing fault is considered to escalate to engulf the line side of the main breaker, thereby rendering it useless to interrupt the fault and the fault would be interrupted by the upstream device. Therefore, if the study was done correctly, the incident value of that MCC is with the upstream breaker, NOT THE MCC MAIN BREAKER, interrupting the fault current and determining the incident energy on that MCC.
The other item to be considered is a risk analysis of operating a breaker and the need for arc rated PPE for such a task. In NFPA 70E-2015, Table 130.7(C)(15)(A)(a) provides examples of tasks and equipment condition to determine if arc flash PPE is required. For normal operation of a breaker, switch, contactor or starter, if the following conditions are met, no arc flash PPE is needed:
1. The equipment is properly installed.
2. The equipment is properly maintained.
3. All equipment doors are closed and secured.
4. All equipment covers are in place and secured.
5. There is no evidence of impending failure.
RE: Arc Flash Mitigation
Under NFPA 70E, is it still acceptable to utilize the table in NFPA if the incident energy analysis/calculation method is used? Or is the intent that when the incident energies are calculated that it is up to the end user's safety program to establish when PPE is required for certain tasks?
RE: Arc Flash Mitigation
Additionally, the table method can leave a person underprotected. I have attached a write up I came across on another forum that shows some of the pitfalls of using the tables. Another point of a study would show any overdutied equipment that cannot be used in the analysis for incident energy. I have found a number of those in studies, particularly in older facilities.
RE: Arc Flash Mitigation
A nearby plant has similar situation. The primary 15 kV feed is through a normally open double ended switchgear where breakers on each half of the bus feed remote 15 kV/480V substations ranging from 6 MVA to 0.3 MVA. For process reasons, a plant operator may want to transfer load from one side to the other using a break-before-make kirk key interlock. From the advice above I gather, that the plant electrician would have to lock out every single remote substation before putting the primary switchgear into maintenance mode.
RE: Arc Flash Mitigation