By Lee Hale, e-Hazard Consultant
A reader posed this question:
In the energized state the fused switch has an arc flash rating of “dangerous.” My question is: If the fusible bus plug is placed in the OFF position, can the door to the bus plug be opened and the fuses replaced without turning off power to the entire busway? All exposed parts within the bus plug are behind a physical barrier. It seems that since there are no exposed parts within the bus plug when the switch is in the OFF position, this would be acceptable. Or does the bus way need to be de-energized to consider the bus plug electrically safe to replace the fuses or attach field conductors?
Any direction on where I can find some documentation to validate that fusible disconnects are in an electrically safe condition when they are switched to the OFF position would be appreciated. Would it be possible to use risk analysis to determine what PPE is appropriate to wear when changing fuses?
Answer: First let me say that as a member of NFPA 70E since 2000 (my comments are not the opinion of NFPA or the 70E committee), I welcome these types of questions, because they really help me understand changes we need to make to add clarity to the standard. I am glad you are asking the questions, as you are determining the hazards to your people as required for the tasks.
The true issue in this scenario is the line-side exposure, as the task in question will be “interacting” with the device and your exposure is to a line-side fault condition. If you are opening the door, you are interacting with the device; it makes no difference if the disconnect or breaker is open or closed, the exposure is the same. If the device were arc flash rated—which it is not, because no one makes one—possibly there would be a different exposure.
OK, maybe not what you wanted to hear, but let’s move on to the question of how to protect your people. You indicated the label says “dangerous.” This is a generic term very loosely extracted from NFPA 70E and IEEE 1584 for any exposure above 40 cal/cm2. If the danger value was something between 40 and 65 cal/cm2, there are suits that can get you to the 65 cal exposure level, so this could be an option.
My most common choice for the situation described is: turn the bus duct off, lock and tag, and test for the absence of voltage. Then change the fuses or perform whatever task you have interacting with the enclosure.
As to your question on the electrically safe condition, I can’t support your statement. Risk analysis will quickly lead you to de-energize. Take a look at the HRC tables in NFPA 70E: In every instance, when we discuss bus ducts or removing an MCC bucket, if you fit the table headers, the HRC value will move from Category 1 or 2 to Category 4. This is a high-risk task. The upcoming 2015 70E tables will take you to Category 4 immediately if the equipment fits the table header requirements. You won’t fit the table header requirements as you already have a “dangerous” label on the bus duct; someone has already done the calculations to get you to this point and it is greater than 40 cal.
There are many other mitigation options available. If the bus duct is fed from fuses and the ampacity is 600 amps or less at 480 volts, let’s look at low-peak, Class J, current- limiting fast-acting fuses to drive the exposure lower. Is it fed from an adjustable trip circuit breaker? Can we adjust the instantaneous value below the arcing fault value? If so, the exposure levels will become lower.
If your production requirements drive you to a point you feel you cannot turn off the bus duct, then you will need to investigate other arc flash reduction techniques, arc flash exposure modeling engineering, arc flash mitigation strategies, and so on.
