Arc Flash Mitigation

[elmatador]

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

 

[wbd]

The maintenance mode is only intended to be used for a short period of time during any necessary hot work. Typically enabling it will result in lack of coordination with the downstream protective devices as it is set for a low instantaneous value to reduce the arc flash hazard. If you have a maintenance mode setting on the breaker then I assume that an arc flash study has been done to determine the setting. If not and was set randomly, you are rolling the dice so to speak that you have the setting where the arc flash is reduced.

 

[elmatador]

Wbd, yes an arc flash study has been done and the incident energies have been calculated. There is room to increase the pickup level during maintenance mode and still be within acceptable incident energy values.

 

[jraef]

But, Maintenance Mode is INTENDED for being able to lower the Incident Energy on a system while someone is PERFORMING MAINTENANCE TASKS ON ENERGIZED EQUIPMENT. That then presumes the equipment could NOT be shut down, which would of course be inherently the safest. So if the equipment is not shut down, why is inrush current a factor? Is it because it is a feeder? So your maintenance procedures would allow the closing of a feeder WHILE someone is servicing the up stream equipment? Perhaps that is an easier fix than worrying about the settings of the breaker. If the primary gear is being worked on, no energizing of down stream systems, problem solved. Remember, Arc Flash mitigation is supposed to be part of your program of PROCEDURES to maintain a safe workplace with regard to electrical workers. It starts with the procedures, the features of the equipment can work into those procedures when inherently unsafe conditions cannot be avoided. Avoiding them is paramount however.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[davidbeach]

You can't do switching while in maintenance more. Get everybody clear, get out of maintenance more, and do the switching. Then, if necessary, go back into maintenance mode.

 

[elmatador]

The end user has the perception that the main breaker on the MCC can be set to maintenance mode when the downstream feeder breakers on the MCC is being operated. Their take on it is that they should be able to set the main breaker to maintenance mode while energizing feeder breakers on the MCC. Their take on "live" or "hot" work includes closing of the feeder breakers on the MCC such that the operator can wear the lower PPE value gear based on the maintenance mode pickup setting instead since the non-maintenance mode is a Category 2 level. Is there any documentation available from IEEE or any other industry standard which defines what type of activities is taken into consideration when a breaker is in maintenance mode? i.e. switching of feeder breakers including inrush does not need to be included? If we start considering inrush we end up having to increase pickup levels on the maintenance mode settings to allow for the inrush which at times will defeat the purpose of the maintenance mode.

 

[davidbeach]

The end user may well have a misperception. The setting need to be what they need to be, and inrush is what it is. We do something essentially the same on our substation transformers; we can switch in the first feeder with the low-set instantaneous (equivalent of maintenance mode) enabled, but a second feeder will trip the transformer. Have to turn off the low-set to get the whole load back on to the transformer.

 

[wbd]

The end user has the perception that the main breaker on the MCC can be set to maintenance mode when the downstream feeder breakers on the MCC is being operated. Their take on it is that they should be able to set the main breaker to maintenance mode while energizing feeder breakers on the MCC. Their take on "live" or "hot" work includes closing of the feeder breakers on the MCC such that the operator can wear the lower PPE value gear based on the maintenance mode pickup setting instead since the non-maintenance mode is a Category 2 level. Is there any documentation available from IEEE or any other industry standard which defines what type of activities is taken into consideration when a breaker is in maintenance mode? i.e. switching of feeder breakers including inrush does not need to be included? If we start considering inrush we end up having to increase pickup levels on the maintenance mode settings to allow for the inrush which at times will defeat the purpose of the maintenance mode.

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.

 

[elmatador]

wbd, thank you kindly for your insight on this subject. I managed to find the equivalent verbiage to NFPA 70E-2015, Table 130.7 (C)(15)(A)(a) in the Canadian CSA Z462 which is derived from NFPA 70E. The CSA document states the two methods, (1) Incident Energy Analysis, (2) Arc Flash PPE Category Method. The Table 4A is equivalent in nature to the 70E Table mentioned above, however it suggests that the table can only be used when using method 2, i.e. Arc Flash PPE Category Method per "4.3.7.3.15.1 When the arc flash PPE category method specified in Clause 4.3.5.4.3 is selected in lieu of the incident energy analysis method specified in Clause 4.3.5.4.2, Table 4A shall be used to identify when arc flash PPE is required."

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?

 

[wbd]

Basically it is one or the other as Art 130.1 mentions utilizing the tables in lieu of an incident energy analysis. Art 130.7(15) also states that if the tables are being used and there are work tasks that need to be performed that are not in the tables, then an incident energy analysis needs to be performed.

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.

 

[bacon4life]

Although inspection or breaker operation does not strictly require rated PPE, is maintenance mode used these activities as a way to further increase safety? I thought I had heard discussion of tying maintenance mode directly to motion sensors or to the electrical room access doors.

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.

 

[davidbeach]

I'm sorry, but sh!t happens and events occur regardless of who's where or doing what. The "rules" may make a distinction between "doing work" and "otherwise", but the random bitchiness of nature doesn't. Had the rear door of a switchgear section bounce off a cap bank rack 60 feet away earlier this year while nobody was doing anything except observing. Substations are inherently dangerous places; the risk is simply being there. One easy way to have an event in switchgear is an animal induced event; one easy way to have an animal induced event is to open the door on the switchgear enclosure and startle said animal. Sure, it almost never happens. Protection, and personnel safety, is all about that narrow little sliver between it almost never happens and it never happens. Don't bet your lift on "it hasn't happened yet".