Flash Hazard Category Question

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Mike Furlan

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Location
Lemont Il
We have a new installation for which a Flash Hazard study was done.

The results are confusing.

For example.

480Y/277V 2000A switchgear was determined to be Flash Hazard Category 2.

But, a 208y/120V 200A panelboard that is fed from this switchgear was determined to be Flash Hazard Category 3.

Is there a quick explanation for this?
 

charlie b

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Lockport, IL
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For starters, there is the impedance of the transformer and the primary and secondary conductors. But more importantly, the amount of arc flash energy that can be released at any point in an electrical system is very closely related to the settings of the breaker that would trip to terminate the arc flash event. So without knowing the system configuration, including the specific breaker model numbers and setpoints, I can't comment on the results of the analysis.
 

Mike Furlan

Member
Location
Lemont Il
For starters, there is the impedance of the transformer and the primary and secondary conductors. But more importantly, the amount of arc flash energy that can be released at any point in an electrical system is very closely related to the settings of the breaker that would trip to terminate the arc flash event. So without knowing the system configuration, including the specific breaker model numbers and setpoints, I can't comment on the results of the analysis.

Thanks Charlie,

So to restate the issue it is possible to have a higher hazard category rating on the secondary side of a transformer than on the primary side.

A panel feeding a transformer is rated at hazard category 2, and one fed by the transformer is rated as hazard category 3.

It is possible to generate a higher hazard, even if the overall energy is the same or less.

We have another situation where the fused disconnect (480V, 225A) on the load side of a transformer is Hazard Category 0, while the equipment on the load side of the transformer (240V, 330A) is a Hazard Category 4.
 

ron

Senior Member
Thanks Charlie,

So to restate the issue it is possible to have a higher hazard category rating on the secondary side of a transformer than on the primary side.

A panel feeding a transformer is rated at hazard category 2, and one fed by the transformer is rated as hazard category 3.

It is possible to generate a higher hazard, even if the overall energy is the same or less.

We have another situation where the fused disconnect (480V, 225A) on the load side of a transformer is Hazard Category 0, while the equipment on the load side of the transformer (240V, 330A) is a Hazard Category 4.

Those are true statements. It has to do with the performance of the overcurrent protective device relative to the arcing current. The method to calculate the arcing current handles different voltages with different importance, so that will also effect the results.
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
It is possible to generate a higher hazard, even if the overall energy is the same or less.
This is where you are getting confused. The amount of energy going into a transformer from the primary side is almost the same as the energy leaving the transformer's secondary. The energy in is slightly higher than the energy out, because of losses within the transformer. That is true at all times. But in relation to arc flash energy, it is irrelevant. If you have a fault anywhere on the primary side, and if the fault is so severe that it heats the surrounding so much that the air itself becomes a path for current to flow, then this "arc flash event" will continue until some upstream overcurrent protection device opens the circuit. The amount of energy released to the atmosphere (acually, it is energy released within the fire ball) will depend on the strength of the upstream energy source and the amount of time it takes for the overcurrent device to react. We can say the same things about a fault that occurs on the secondary side. So yes, it is possible for the breaker on the secondary side to take longer to trip, and thus release more energy into the arc. More importantly, it is possible that a fault within the first panel served by the transformer will engulf the entire panel in the arc, so that the arc bypasses the panel's main breaker, and so that the arc will continue even if the main breaker succeeds in tripping. In that scenario, the arc could continue for a very long time, and it is unclear what would eventually terminate the event. That would put the secondary size panel's arc flash hazard into the "dangerous" category, even if the hazard category on the primary side was a 1, or even a 0.
 

Mike Furlan

Member
Location
Lemont Il
OK, I understand the issue now a little better.

And this little bit of understanding raises a further question.

If the panel on the load side of a transformer has a higher Hazard Class than the line side, isn't that bad engineering?

More workers will work on the load side, and be exposed to more risk.
 

charlie b

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Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
If the panel on the load side of a transformer has a higher Hazard Class than the line side, isn't that bad engineering?
Not at all. A transformer that steps down the voltage will necessarily increase the current. That is, the secondary side current must be higher than the primary current. Therefore, the amount of fault current available at the secondary side is also higher than at the primary side. That will tend to make the arc flash hazard higher, if all other factors are equal. When I select a main breaker on the first panel downstream of the transformer, I could pick one that has a very fast trip setting, and that would reduce the arc flash hazard. But it would also tend to allow the main breaker to trip earlier than the downstream breakers on a downstream fault (i.e., the "selective coordination" issue). So there are choices to be made, and not all will be made in favor of reducing arc flash categories.
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
Not at all. A transformer that steps down the voltage will necessarily increase the current. That is, the secondary side current must be higher than the primary current. Therefore, the amount of fault current available at the secondary side is also higher than at the primary side. That will tend to make the arc flash hazard higher, if all other factors are equal. When I select a main breaker on the first panel downstream of the transformer, I could pick one that has a very fast trip setting, and that would reduce the arc flash hazard. But it would also tend to allow the main breaker to trip earlier than the downstream breakers on a downstream fault (i.e., the "selective coordination" issue). So there are choices to be made, and not all will be made in favor of reducing arc flash categories.

In all probaility a primary side device, fast enough to lower the secondary incident energy, will trip on the inrush of the transformer.

We have been sucessful in installing Virtual Mains (CTs and a 50/51 relay) on the secondary side which is set up to trip a shunt trip breaker on the primary. No fuss no muss, it just costs $$$.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
anyone looked at some of the nifty new arc flash sensors for reducing hazard levels?
Yes, currently installing 'optical' system.

I find the problem to be more in the area of 'clearing' rather than 'sensing' the arc.

Of course the hardest thing to deal with is the unrealistic expectation of always being able to attain <1.2cal/cm?.
 
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