Clearing Time

[GScollins]

If the power company gives me clearing time on primary of a 750 kva transformer, how does that apply to secondary? He is going to give me fault current on secondary

 

[drktmplr12]

If the power company gives me clearing time on primary of a 750 kva transformer, how does that apply to secondary? He is going to give me fault current on secondary

i would say that is how long you can expect the secondary fault current to be active. you want your secondary or service OCPD to clear the fault before their primary.

 

[ron]

If the power company gives me clearing time on primary of a 750 kva transformer

That is an odd thing for the utility to give you.
The clearing time will depend on lots of stuff. It is usually inversely proportional to the amount of current cause the clearing, so they would have to give you the TCC of the upstream device, which is also odd for such a small transformer.

You will want to be selective on the secondary with their primary protective device so you can reset and not wait for them.

 

[GScollins]

This pipeline pump facility actually has 5 different transformers that feed our different equipment. Each of them feed equipment that either has a breaker or fuses, some 4160 1500 hp motors and some 500 hp 480 motors. When I asked for fault current from utility, I will get it for secondary of each transformer and those are their equipment. The 750 is just one of the 480 ones

 

[wbdvt]

What are you using the information for? They may be providing you with the clearing time for the primary protective device for a fault on the secondary side of the transformer. Since the only protection between the secondary transformer spades and the line side of the secondary protective device is the transformer primary protective device, this could be useful for conductor damage curves or arc flash.

 

[GScollins]

We need to update our arc flash labels on all of our equipment and this is the starting point by contacting the utility to get available fault current. None of it has had a study and the old labels were just a wild guess. Our new procedure has a matrix for ppe selection with no analysis done and it has short circuit current available and clearing time as part of ppe and arc flash boundary. Mostly for loto testing and trouble shooting since we can turn it off and don’t work energized. But we have to fill out jsa and wear proper PPE to comply with our safety department with no arc flash analysis

 

[mivey]

I'm with ron, clearing times vary with fault.

It may be on a timed curve or even a delayed instantaneous trip.

It may not be your worst case exposure.

 

[wbdvt]

We need to update our arc flash labels on all of our equipment and this is the starting point by contacting the utility to get available fault current. None of it has had a study and the old labels were just a wild guess. Our new procedure has a matrix for ppe selection with no analysis done and it has short circuit current available and clearing time as part of ppe and arc flash boundary. Mostly for loto testing and trouble shooting since we can turn it off and don’t work energized. But we have to fill out jsa and wear proper PPE to comply with our safety department with no arc flash analysis

Ok, then to model for the arc flash correctly, you will need from the utility:
1. 3ph available fault current and X/R at the riser fuses
2. SLG available fault current and X/R at the riser fuses
3. Utility transformer - kVA, %Z, primary voltage, primary connection, bay-o-net fuse if any
4. Riser cable - material, size, length, conduit material
5. Riser fuse - size, speed

This way your model can utilize the utility protective device for determining the incident energy at the main service equipment. The system can then be modeled to determine the short circuit current at each point in the system and then TCC's for the upstream protective device can be used to determine the clearing time. Then your table can be used. But if going to all this work, why not calculate the incident energy and then you will know what PPE is needed.

What software are you using?

 

[kingpb]

Likely that the fault current they give you on the secondary is going to be based on there system considered an infinite bus; you can confirm that by calculating the maximum short circuit MVA through the transformer and comparing the two calculated values. If it is not the same, then you will most likely need more info as pointed out in other posts.

 

[GScollins]

Thank you all for your replies. I am just a E&I tech and after they rolled out our new procedures was voluntold to contact utility to ask for fault current. As you know the company is supposed to do an analysis and provide labels for 70e. I know just enough to know that a Qualified Engineer needs to do those calculations. The Utility guy is coming Monday to look at facility and get a list of transformers to their engineering to get some info. This station is our biggest. Most the others are just one drop with 400 or 200 amp 480 disconnect. With what you guys replied, that gives me what to ask for from utility.

 

[wbdvt]

Thank you all for your replies. I am just a E&I tech and after they rolled out our new procedures was voluntold to contact utility to ask for fault current. As you know the company is supposed to do an analysis and provide labels for 70e. I know just enough to know that a Qualified Engineer needs to do those calculations. The Utility guy is coming Monday to look at facility and get a list of transformers to their engineering to get some info. This station is our biggest. Most the others are just one drop with 400 or 200 amp 480 disconnect. With what you guys replied, that gives me what to ask for from utility.

I don't think you quite know what to do for the incident energy. You most likely get the infinite bus secondary current but if you do get the available fault current, then you need to determine the trip time of the protective device upstream from a TCC. Do you have all those? You will have to calculate the fault current at each piece of equipment where you need to determine the clearing time to use the table method. Do you know how to do that? Also, what do you do if there is a protective device that is overdutied? Are you checking for that?

This is not an area to be on a learning curve and make a mistake. This is PPE to protect a fellow employee from serious injury or even a fatality. You should let a qualified engineer experienced in these studies do the work.

 

[GScollins]

I don't think you quite know what to do for the incident energy. You most likely get the infinite bus secondary current but if you do get the available fault current, then you need to determine the trip time of the protective device upstream from a TCC. Do you have all those? You will have to calculate the fault current at each piece of equipment where you need to determine the clearing time to use the table method. Do you know how to do that? Also, what do you do if there is a protective device that is overdutied? Are you checking for that?

This is not an area to be on a learning curve and make a mistake. This is PPE to protect a fellow employee from serious injury or even a fatality. You should let a qualified engineer experienced in these studies do the work.

it looks like they finally see that they are going to hire a 3rd party engineering firm to do an analysis. With the help of your replies I won’t have to show up for work each morning in my cat 4 suit