arc flash worst case scenario

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All possible scenarios should be investigated and the worst arc hazard case chosen. The worst case may be the highest fault current (high current short time) or the lowest (low current long time). Programs such as SKM allow defining multiple scenarios and can automatically report the worst case for each bus.
 
Lower fault current does not always mean lower arc flash energy. Calculations take into account the amount fault current and the amount of time it takes for devices to clear a fault. So if lower fault current is flowing because motors are not feeding into the fault, the time it takes for an OCPD to clear the fault may be longer, which in some cases can mean more energy, which is power integrated over time.
 
If a motor load is removed by for example by a starter and there is no way to add resistance to a circuit for example operating a disconnect after a stop is applied then wouldn't the possibility of an arc be almost nonexistent because of no resistance?
 
pistolpigg said:
If a motor load is removed by for example by a starter and there is no way to add resistance to a circuit for example operating a disconnect after a stop is applied then wouldn't the possibility of an arc be almost nonexistent because of no resistance?
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Why don't you explain exactlly what situation you are trying to analyze, rather than feeding us information piece by piece in the hopes we will give you an answer you want.

Arc flash incident energy is a function of the amount of current flowing and length of time it flows. Decrease one and the incident energy will decrease, increase one and the incident energy will increase. Sounds simple, the issue comes in that the length of time is usually an inverse function of the protective device and the amount current. This means decreasing the current may increase the time. The result is there is no simple 'always this way' answer.
 
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I don?t think you are describing an arc flash scenario correctly. The onset of an arc flash event is not related to whether any particular motor or resistive load is running. It begins when some internal failure (perhaps as simple as a loose wire or an accidentally dropped tool) causes a spark. That ionizes the surrounding air, which in turn becomes a conductive path with very low resistance, and that is when the ?flash? takes place. This event will not be terminated by a motor starter turning off a motor, or a disconnect switch opening. It terminates when an upstream overcurrent device opens. During the flash event, energy will be fed through the arc by the utility source and by any motors that had been running when the event began. The total amount of energy released during the event will depend, as has already been said, on the amount of fault current flowing and the time it takes for the overcurrent device to open.
 
pistolpigg said:
Wouldn' t a motor or circuit being turned off lower arc flash cal and possibly ppe and could this be put on the arc flash notification stickers?
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The amount of load has nothing to do with arc flash, the only way a motor is a factor is the motor contribution that is added to the fault current.
 
I'm trying to figure out why and how it would logical to have to put a hrc four requirement to operate a disconnect for a motor when its load is disconnected by a motor starter or a drive. Seems like a lot of overkill for operating a device that has no load for example.
 
pistolpigg said:
I'm trying to figure out why and how it would logical to have to put a hrc four requirement to operate a disconnect for a motor when its load is disconnected by a motor starter or a drive. Seems like a lot of overkill for operating a device that has no load for example.
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Stop thinking about the load, it does not matter, it is the source and the OCPD protecting that disconnect that determine the hazard level. Chances are that disconnect is fed from a breaker without an INST function so the clearing time is long.
 
Ok that makes sense about the clearing time and the arc flash calculation but the thing that seems to be missing from all the arc flash calculation is probability of an event in relation to the task performed. In other words would it be a good comparison to buying a lottery ticket ?
 
pistolpigg said:
Ok that makes sense about the clearing time and the arc flash calculation but the thing that seems to be missing from all the arc flash calculation is probability of an event in relation to the task performed. In other words would it be a good comparison to buying a lottery ticket ?
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Incident energy calculations are not intended to include any factor of risk or probability. To use your analogy, the calculations only give you the payout not the odds.
It is up to you to 'determine' the odds.
 
So would it be possible to compare the arc flash notifications to a sign warning that there is a drop off ahead and tell how many feet drop? Kinda a stretch but an interesting comparison for perspective. So what I mean is the calculations are more warning of can happen like if you were to fall from an unguarded edge?
 
pistolpigg said:
Ok that makes sense about the clearing time and the arc flash calculation but the thing that seems to be missing from all the arc flash calculation is probability of an event in relation to the task performed. In other words would it be a good comparison to buying a lottery ticket ?
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That all depends on your work practices. Many companies, specifically industrial, have a zero hot work policy due to the large incident energies you find in 480V systems. So you chances are slim to none if you adhere to those policies.

Guys who use unsafe practices because they don't understand the hazards have a much greater chance.
 
So do most places require the ppe on the arc flash sticker for operation of disconnects and verification of no power with a meter to get it to a safe work position?
 
pistolpigg said:
So do most places require the ppe on the arc flash sticker for operation of disconnects and verification of no power with a meter to get it to a safe work position?
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Facilities and companies that comply with NFPA 70E and OSHA requirements do.

Chris
 
Going back to original question then should we not factor in probability in when the chance of something happening such as an arc flash incident during non contact procedures such as operation of a disconnect or Mcc bucket with power off and or load disconnected? Seems like trying to drive a nail with a sledge hammer to me.
 
pistolpigg said:
So do most places require the ppe on the arc flash sticker for operation of disconnects and verification of no power with a meter to get it to a safe work position?
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Many companies have developed their own safe work practices addressing the operation of disconnects and circuit breakers which accomodate the Informational Note #1 to the definition of Arc Flash Hazard in NFPA70E-2012 and include some risk assessment.

I know of no company with a safe work practice that does not consider 'verification of no power' as one of the riskiest tasks that can be performed and therefore requires the use of the 'worst case' PPE.
 
pistolpigg said:
Going back to original question then should we not factor in probability in when the chance of something happening such as an arc flash incident during non contact procedures such as operation of a disconnect or Mcc bucket with power off and or load disconnected? Seems like trying to drive a nail with a sledge hammer to me.
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You are confusing the Arc Flash Hazard Analysis with the Hazard Identification and Risk Assessment.

The Arc Flash Hazard Analysis will determine the arc flash boundary, the incident energy at the working distance, and the PPE equipment that people with the arc flash boundary shall use.

The Hazard Identification and Risk Assessment is used before work is started within the limited approach boundary or within the arc flash boundary. The procedure shall identify the process to be used by the employee before work is started to identify hazards and assess risks, including potential mitigation strategies. As part of this assessment process the probability of occurrence of an incident is brought in the equation.

There is a huge difference between operating a disconnect when the power is verified to be off and operating a disconnect when the load is off. A disconnect can fail while being operated while still energized and the load disconnect.

Chris
 
jim dungar said:
Many companies have developed their own safe work practices addressing the operation of disconnects and circuit breakers which accomodate the Informational Note #1 to the definition of Arc Flash Hazard in NFPA70E-2012 and include some risk assessment.

I know of no company with a safe work practice that does not consider 'verification of no power' as one of the riskiest tasks that can be performed and therefore requires the use of the 'worst case' PPE.
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Agreed, creating an electrically safe work condition (Deenergized) and the verification of no power is considered to be a hazardous task that exposes an employee to both arc flash hazards and shock hazards so the appropriate PPE is needed to perform the task.

Chris
 
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