Arc Flash Methods 2

[JBUDA54]

I am working with a Client who has given me a reference to a table from NFPA 70E Table 130.7(C) (15) (a).

The line item from that table states that the task when opening hinged covers (to expose bare, energized electrical conductors and circuit parts) in Metal Clad Switchgear, 1kV through 38kV maximum of 35kA (12 cylcle clearing) minimum of 36 in working clearance results in a category 3 risk. Our equipment is a SQ D MasterClad 5kV SWGR main breaker compartment. I would also offer that the SQ D rep states that when the 5kV breaker is racked in that the bus or stabs are not exposed inside the cell.

A senior Power system Analysis engineer stated that once an Arc Flash Calculation has been performed that the Calculation method supersedes any table method. The Calculation method put the equipment in a Category 4 risk.

The task is to simply open the door and inspect a power meter for a Ethernet or Modbus connection. Is using the table method for opening the door just to inspect a legitimate procedure, or are we bound to the Label that the equipment is listed as based on the Arc Flash Calculation method.

Please in your responses list the code reference that defends your answer. I have read briefly through NFPA 70E and can not find this reference that my senior engineer spoke of. He said that the reference is somewhat vague but exists.

Thanks in advance,
JBUDA54

 

[RonShap]

130.5(B) gives you the choice of doing the calculation or using the Table.

First, be sure your short circuit current and clearing time is within the listed criteria of that section of the table (<35kA and 12 cycle clearing time).

Second, review the data of the engineered calculation to be sure it isn't GIGO.

Then decide how you want to establish compliance with 130.5(B) which gives you the option.

 

[dpc]

What is your goal? To protect workers appropriately, or use the lowest PPE you feel you can get away with? If there is fault while you have the door open, the fault isn't going to care what you were doing. The risk is low, but the hazard doesn't change. That breaker could trip due to an external fault while you have your head stuck inside the cubicle. In reality, there won't be much difference between HRC #3 PPE and HRC #4 PPE. If you are just opening a door for quick inspection, just put on the #4 suit and get it over with. If you really want to use PPE of lower value than is required by the warning label, this can be handled through an Energized Work Permit.

NFPA 70E-2012 allows either method to be used, but to use the table, the system fault current and clearing times must be within the ranges specified. BTW, NFPA 70E-2015 will be out in a few weeks.

Also, the fact that there are no exposed live parts doesn't buy you much. See "Informational Note 2" at the bottom of Page 70E-31.

 

[JBUDA54]

Thanks RonShap & dpc for your valuable responses!

dpc, the overall goal should be to protect the worker at all cost. My request was sent to better understand what NFPA 70E says about circumstances where the worker will have very limited working tasks. I realize that when an electrical fault occurs it is a bad situation if the fault occurs when the inspection is performed. I was not familiar with the Energized work permit process, and I intend to research this to see what it involves. It didn't dawn on me that there may be a setting on the Multilin 760 relay to put the breaker into a maintenance mode where we can temporarily disable breaker coordination, and lower the Cal/Cm^2 while the worker takes his pictures and then resets it when the task is completed. This lineup was installed back in 2009 so I am not sure that it is equipped with a maintenance bypass switch. From the pictures I have of the 5kV Gear it doesn't appear they have this feature. I will talk to the client to see if this is something that want to entertain.

 

[mls1]

"It didn't dawn on me that there may be a setting on the Multilin 760 relay to put the breaker into a maintenance mode where we can temporarily disable breaker coordination, and lower the Cal/Cm^2 while the worker takes his pictures and then resets it when the task is completed"

Hopefully you mean the upstream breaker and not the one in the cell. You might also take another look at the overall coordination and arc-flash analysis and see if it was optimized. You might be able to make adjustments that improve clearing times without sacrificing selectivity.

 

[stevenal]

No exposed conductors after the door is opened? No interaction with the circuit? I fail to see the hazard here. Have you looked at the definition of Arc Flash Hazard?

If it is still a concern, relocate the meter.