Arc Flash vs Gear Interrupting Ratings vs Labeling

[sparkyguy545]

Difficult to answer question.
I'm a Consulting Engineer. Virtually 99% of the time, POCO's won't give REAL available fault currents. The numbers they give us "surprisingly" work out to nothing more than the xfmr Z with infinite primary bus reflected to the secondary (our SE). OK, they want to cover their butts so that we adequately provide interrupting gear that exceeds the worst-case. However, a very different (dangerous) thing happens when you do an Arc Flash Study. It is totally counter-intuitive, but often a higher supply fault current will CLEAR faster than a much-smaller service. The opposite is that a lower fault current will often result in a HIGHER hazard than a larger fault current because it will burn-longer (take longer to clear). The formula's are only as good as what data they are provided. However, when we base ALL our computations and ALL our labeling on an unrealistically (but unverifiable) figure from a utility EVERYTHING we calculate and label will ultimately be wrong. The result is that a higher hazard may actually exist because the utility gave us an unrealistically large fault current. After all it NEVER is actually an infinite primary bus.

How do others reconcile this conundrum when calculating Arc-Flash data and labeling?

 

[cornbread]

We use e-tap for our studies and we run about 10 different scenarios, one of which is based on 80% rating of the POCO fault current. We then compare the diffenrt runs and pick the worse case for our arc flash.

 

[RoberteFuhr]

This is one of the most difficult things about doing an Arc Flash Study. Utilities tend to want to give you the highest value of fault current. They do not understand how lower fault currens can make a protective device operate longer and drive up the arc flash energy values.

We actually ask the utility company what their fault currents are and the kVA and impedance of their transformer(s). Many times they will provide infinite available fault current for the primary side. However, if they give you the transformer kVA and impedance and use it in your study, you'll find that it will be pretty close to real world conditions. You can vary the primary fault current quite a bit, and it has a small impact on the secondary side.

We sometimes will call the utility engineers and ask them what their "ball park" values of fault current are for the primary side. Usually, they are reluctant to put it in writing buy they will tell us over the phone. For example, knowing the primary fault current is around 13,000 for a 15 kV system is better than using infinite.

The following is a link to the form that we send to the utility companies. Some will fill it out and some will not. It is hit and miss with utility data.

http://www.powerstudies.com/documents/Utility Data Form.pdf

 

[RoberteFuhr]

This is one of the most difficult things about doing an Arc Flash Study. Utilities tend to want to give you the highest value of fault current. They do not understand how lower fault currens can make a protective device operate longer and drive up the arc flash energy values.

We actually ask the utility company what their fault currents are and the kVA and impedance of their transformer(s). Many times they will provide infinite available fault current for the primary side. However, if they give you the transformer kVA and impedance and use it in your study, you'll find that it will be pretty close to real world conditions. You can vary the primary fault current quite a bit, and it has a small impact on the secondary side.

We sometimes will call the utility engineers and ask them what their "ball park" values of fault current are for the primary side. Usually, they are reluctant to put it in writing buy they will tell us over the phone. For example, knowing the primary fault current is around 13,000 for a 15 kV system is better than using infinite.

The following is a link to the form that we send to the utility companies. Some will fill it out and some will not. It is hit and miss with utility data.

http://www.powerstudies.com/documents/Utility Data Form.pdf

Sorry....The correct link is http://www.powerstudies.com/documents/Utility_Data_Form.pdf

 

[mtfallsmikey]

No hijack intended, but...

No hijack intended, but...

Has anyone used this device?

http://www.electroswitch.com/electr...chnical_publications/ES TD-CSR Data Sheet.pdf

 

[RoberteFuhr]

I have not but it makes a lot of sense. For medium voltage switchgear, many times the breaker switch is mounted on the breaker cubicle door. Having a delay action switch will allow the operator to initiate the breaker close action and then walk away. In arc flash energy calculations, distance is your friend. Increasing the distance between you and the hazard will the lower the energy.

I like it. I am going to use this in my Arc Flash Training Class - Electrical Safety in the Workplace.

 

[zog]

I have not either but I like it, thanks for sharing.

 

[ron]

IEEE 1584 requires calculations of incident energy at 100% and 85% of arcing fault current (in my opinion, for that reason of unknown utility values). Keep in mind that arcing fault current is already less than bolted fault current.
A high impedance arcing fault will have some value less than the theoretical calculated value and will hopefully not even support the continuous event. It is an unknown, and the utility availability is just another unknown to think about, but nothing we can do.
The utility fault current is changing, in some cases, throughout the day depending on network grid activities. It is something that we can only estimate, so as long as you are following the standards, it is the best you can do.
I think that IEEE 1584 is in its infancy and will develop over time where we wont recognize what we are doing today as compared to the methods in the future.