Arc Flash Current Contribution

[tin2779]

I have exhaustyively studied threads on Motor Contribution and they did not answer my question so I am putting forth my concerns.
I am using ETAP for my simulation.

I have few observations and would like a confirmation to these mentioned below.

" Arcing current is less than the three phase bolted Asymmetrical fault current due to the arc resistance. The Arc flash contribution is assessed during 1/2 cycle contribution normally".

The concept of working distance relates to that"How far an electrician will be working" The default option for a medium voltage switchgear is 36". That means this will determine my category level. So, if I assume 18", It will increase.

I just want to make sure that these assumptions are correct.

Thanks

 

[dpc]

If you are using IEEE 1584 equations, the arcing current is calculated from the symmetrical three-phase fault current. The asymmetrical current does not enter into the calculations.

I don't believe there is any guidance in IEEE 1584 regarding inclusion or exclusion of motor contributions. It simply says to calculated the "bolted fault current". Using the 1/2 cycle network and including motor contributions will usually be the most conservative approach, but in reality, most of these contributions are gone within a couple of cycles. I have spoken to one member of the IEEE 1584 committee who normally does NOT include any motor contributions.

"Working Distance" is the minimum distance from the likely source of the arc to the worker's torso or face. Working Distance has a major impact on expected arc energy. If you are using something greater than 18" for enclosed gear, you'd better have a good reason. For metal-clad switchgear, 24" is probably reasonable. For 480 V, we always use 18".

Hope that helps.

 

[whycliffrussell]

Actually IEEE 1584 does speak of motor contributions. It accounts for the current contribution induction machines greater than 50HP. It will use the value of the LRC for the motor or if that info is not supplied it will use the positive sequence sub-transient reactance of the stator to calculate the contribution from the motor (IFF it's over 50HP).

This statement is correct for ETAP and the ieee method WITHIN THE LIMITATIONS OF IEEE 1584:

" Arcing current is less than the three phase bolted Asymmetrical fault current due to the arc resistance. The Arc flash contribution is assessed during 1/2 cycle contribution normally".

**However the second sentence is not ENTIRELY correct because:

i. IEEE used the symetrical component of the fault current....NOT The assymetrical part (which occurs in the first half cycle)
ii. It does add the motor contribution from the first 1/2 cycle.


IFF the constraints for IEEE 1584 are not met (Bolted Fault currnet is outside of 0.7k-107k A or voltage is outside of 0.208kV to 15kV THEN;

The Theoretical Lee Method is used, where in the Lee Method the bolted fault current is yields conservatively...if not ridiculously high rassumed to be equal to the arcing current (The lee method generally esults).

Hope this helps.

 

[rcw retired EE]

"The Theoretical Lee Method is used, where in the Lee Method the bolted fault current is assumed to be equal to the arcing current (The Lee method generally yields conservatively...if not ridiculously high results)."

(edited for typo). Good point on the Lee Method.

 

[dpc]

Where do you find a reference to including motors > 50 hp in IEEE 1584? Just curious.

Also, asymmetrical current is not restricted to the first 1/2 cycle. The peak current will occur in the first 1/2 cycle, but the offset current can last many cycles, depending on the X/R ratio.

 

[dpc]

I found the IEEE 1584 reference to motor contribution for motors >50 hp, so never mind

 

[DanDel]

Keep in mind that in arc flash analysis, very frequently a lower fault current will produce a higher arc flash energy, because the device upstream may open faster on the higher fault.

 

[dpc]

You also have to keep track of two different arcing currents:

The total current at the point of the arc
The current through the upstream protective device

When motor contributions are included, these two will often be different.

 

[tin2779]

Thanks All for your such a quick response.

DPC you mentioned about the distance if greater than 18" I better be have a good reason. My situation is a 13.8 KV switchgear. And I had an impression this distance relates to the distance between the phases of BUS BARS. This is what ETAP considers. So yourcdef. "Working Distance" is the minimum distance from the likely source of the arc to the worker's torso or face"
Where it comes in to picture ? I am considering 36" as the distance.

Thanks

 

[dpc]

The distance between the bus bars relates to the ARC GAP, not the working distance. Two different things.

If someone pulls out a breaker and sticks his head into the compartment to do something, will their torso and head always be at least 36" from the closest live part?

This is a safety-related calculation, so all assumptions should be conservative and consistent with existing work rules and procedures.

 

[tin2779]

So dpc,
this measn ETAP defination of working distance is not correct. When I spoke to these ETAP people, they advised me to consider it as a distance between the pahse of bus bars.Here is a clip from there help file:

" Working Distance "

This group provides information about the working distance to be used for the calculation of the incident energy.

Enter the distance from the possible arc point to the person in inches. This distance is defined as the distance between the arc point and the persons face and torso. This value has a range of 1 to 999.99 in. This is the distance value used to determine the incident energy. The default value for all voltage levels is 18 inches, but you may select a higher distance if required or set the default value accordingly. The table below shows typical working distances for low and medium voltage equipment.

Table 5: Typical Working Distances according to IEEE 1584-2002 Table 3

kV Range Working Distance (inches)
0.208 kV to 1 kV 18
1.1 kV to 5.0 kV 18
5.1 kV to 15 kV 36
15.1 kV and higher 36


Could you advise me, what is my best bet here ?

Thanks

 

[dpc]

The working distance is going to depend on the equipment and the installation. If you're not sure what to use, use a smaller distance. 18" is always more conservative than 36".

Since 5 kV and 15 kV metal-clad switchgear are essentially built on the same dimensions, why would one have an 18" working distance and the other 36". For outdoor exposed bus, this might make some sense.

If you're not given any other guidance, I'd lean toward using 18" through 15 kV. If the results generate a lot of whining, ask them what they recommend using as a MINIMUM working distance. Then have the responsible person put it writing.

There is nothing stopping a maintenance group from developing a work procedure for a specific task with a working distance greater than the minimum you use. They can then prepare an Energized Work Permit per NFPA 70E and compute a lower arc energy level based on the task to be done.

 

[GRP123]

tin...
As per IEEE def of working distance is;

Typical working distance is the sum of the distance between the worker standing in
front of the equipment, and from the front of the equipment to the potential arc source
inside the equipment.
Table 3 from IEEE 1584
Classes of equipment Typical working distance
a
(mm)
15 kV switchgear 910 mm
5 kV switchgear 910 mm

 

[whycliffrussell]

"Also, asymmetrical current is not restricted to the first 1/2 cycle. The peak current will occur in the first 1/2 cycle, but the offset current can last many cycles, depending on the X/R ratio." I know this! Depends on X/R and when in the sine wave the breaker closed...thanks for the text book comment! I didn't say that the assymetical component was LIMITED to the first half cycle, i stated that it OCCURED during the first half cycle.

I would not call upon ETAP for anything other than technical advice on how their software works. Theses guys are a bright bunch of people but they ONLY really understand how their software works. I've been using ETAP extensively for a couple of years now and have spoken with many individuals at the organization. With respect to arc flash IEEE1584 is a good start,and NFPA70E. Some mfgs. also have some good references on arc flash and often give seminars, and they can be informative, minus the sales pitch.

 

[dpc]

whycliffrussell,

You said:

NOT The assymetrical part (which occurs in the first half cycle)

I was just trying to clarify that for the OP.