Data from utility

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jim dungar

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Is there any way to get a utility to provide information so that NFPA70E can be followed. Maximum fault values are not sufficient for performing arc-flash calculations.

If the primary side is so dynamic that it is unpredictable, why can't they simply provide maximum and actual fault current and X/R values at the secondary of the transformer? That data would be steady, until the transformer is replaced.

An engineered (like IEEE 1584) arc-flash study can take into the fact the utility will never clear the fault. The customer can then deal with the fact no PPE is adequate for service entrance equipment. But, without valid information from the utility (the same data the utility uses for their own calculations) it is not possible to determine the actual fault levels at other locations in the facility.

A local POCO says they do not believe in any engineered arc-flash study and advises their customers to simply follow the tables in NFPA 70E. But, the first note to the tables says the information is only valid for "25kA short circuit available, .03 second (2 cyle) fault clearing time". What is the customer to do when they are told the maximum [future] fault level is 57kA?
 
Re: Data from utility

Is there any way to get a utility to provide information so that NFPA70E can be followed. Maximum fault values are not sufficient for performing arc-flash calculations.

In my opinion, no.

If the primary side is so dynamic that it is unpredictable, why can't they simply provide maximum and actual fault current and X/R values at the secondary of the transformer? That data would be steady, until the transformer is replaced.

We have to start somewhere also. Where do we start? Our choice is at the substation and work with the configuration of the lines, conductor size, etc. until we get to the point where you are located. Now we have the problem with the amount of time it will take the transformer fuses to open. That is dependant on the loading on the bank, the type of fault, the fuse sizes, the impedance of the transformers, etc. We don't know how long the fuses will hold and can't know. Assume the bank is well loaded and a fault occurs, the fuse will open quickly since the fuse is already warm. Assume the bank is not loaded much on a weekend and maintenance is performed. The fuse is cold and the fault is of a lower level. The incident energy is going through the roof because the fuse will hold for minutes instead of seconds.

An engineered (like IEEE 1584) arc-flash study can take into the fact the utility will never clear the fault. The customer can then deal with the fact no PPE is adequate for service entrance equipment. But, without valid information from the utility (the same data the utility uses for their own calculations) it is not possible to determine the actual fault levels at other locations in the facility.

I agree.

A local POCO says they do not believe in any engineered arc-flash study and advises their customers to simply follow the tables in NFPA 70E. But, the first note to the tables says the information is only valid for "25kA short circuit available, .03 second (2 cyle) fault clearing time". What is the customer to do when they are told the maximum [future] fault level is 57kA?

I don't know the answer to that question. That is at the heart of this whole problem. :confused:
 
Re: Data from utility

Isn't all of the system information available except for the clearing time of devices?

If I have data like %Z, X/R, and actual fault level I can calculate how fast the customer's main(s) will operate to clear any in-plant fault. This will allow information in NFPA70E to be used at all points in the facility past the POCO connection.

As it is now, customers are being prevented from following OSHA regulations because of the POCOs.
 
Re: Data from utility

As it is now, customers are being prevented from following OSHA regulations because of the POCOs.
Click to expand...
I don't know much about this but I'm thinking that it's just as much an issue of the regulations being unrealistic.

You can't necessarily have stuff just cause you tell someone they have to do something.
 
Re: Data from utility

If I have data like %Z, X/R, and actual fault level I can calculate how fast the customer's main(s) will operate to clear any in-plant fault.

Sorry, you can't have it. You also need the clearing time of the fuses on our bank. From the time I go through the calculations to the time I give them to you, our dispatcher has switched you to a different substation and all the calculations were for naught.

By the way, what do you do for your system from the service point to your service overcurrent device? If you can't do anything with that, how do you do anything inside the service equipment at all? :D
 
Re: Data from utility

I agree that the regulation seems unrealistic ( from what little I know) but from a legal standpoint there must be a way to limit liability. Has anyone ever directly confronted OSHA with the problem. How could they expect a company to comply with a regulation without the means to do so. I would approach OSHA and ask them for the means to supply the variable to the equation. If they can't do so, at least you could document their failure.
 
Re: Data from utility

Jim,
I think you stole my show on the subject of available fault current, but you couldn't of said it better and I agree. How do we solve this hams and eggs dilemma so that we can safely install the right size breakers?
 
Re: Data from utility

If the primary side is so dynamic that it is unpredictable, why can't they simply provide maximum and actual fault current and X/R values at the secondary of the transformer? That data would be steady, until the transformer is replaced.
Click to expand...
You could be given this information and you engineer the job base on this info. Six months later a large usage customer moves in next door and the bank that was 3 25kva is now 3 333 kva.
Also due to loading the primary feeder is switched to a larger sub. What do you do about the results from the first engineered job? If this were to happen even the present method of supplying the available fault at the transformer terminals would not be adequate.
I agree this is a problem. The info given by the utility gives the max fault current with a infinite bus. This data is usually ok for the fault current rating of the electrical equipment but it is not accurate for the arc flash caculation.

[ July 01, 2005, 02:30 PM: Message edited by: bob ]
 
Re: Data from utility

Sparks,

It is very easy to select the correct breaker rating, simply use the design fault level supplied by the utility. Maximum short circuit fault levels are usually readily available. This is not the same problem as determing incident energy levels.

Charlie,

I don't care how long it takes the utility equipment to clear a fault. Maybe the term "actual fault level" is the problem. Everyone agrees the fault at any one point in time is variable. However, it is possible to have a minimum fault level, a maximum fault level, and a "installed" fault level. With out these levels how can you select your own protective devices?

One utility provided:
Design fault level, 10,000A, X/R = 20
Actual (simulated) 3PH fault = 2,144.1A also they gave Z+,Z0, and 1PH L-G values.

Then they said you must use the design level for arc-flash and the "actual" levels for short circuit analyses.
 
Re: Data from utility

Jim,
How can the utility even begin to know what the actual available fault currents are at any given time and point on their system, given the dynamics of todays power distribution system? This will only get worse as more distributed generation is brought online. Take a look at what could be happening on a second to second basis on any part of a distribution system close to a wind farm. The output of wind farm changes with the wind, and would have to have some effect on the available fault current.
Don
 
Re: Data from utility

Don,

How does the utility protect the primary feeds to it's transformers? Every time I have had to engineer a medium voltage service, the utility has asked for my device data to see if it will coordinate with their devices. How do they do this without a target fault level?

Any electrical calculation is a snapshot in time, the argument that it might change therefore do nothing is not always supportable. The same thing holds true for service sizing, add a load, recalculate your service needs.

If a customer requests system information it is possible for the utility to run a simulation and provide something definitive. The utility then simply needs to notify the customer if the "bank that was 3 25kva is now 3 333 kva". At that time the customer will need to re-run the arc-flash study. But if the customer changes something internal to their faciltiy (i.e. 450A fuse upgraded to 600A or add a new 100HP motor) they also need to re-run their study.

At least give me today's data. Today's fault level is probably lower than the maximum design (otherwise we have no argument) and so statistically it will have a higher incident energy.
 
Re: Data from utility

Jim,
I don't think that the utility has any real idea of what the fault current is at any given time. There are just too many variables. Its not just the changing of the actual equipment, but the day to day switching of the substations, and feeders. There are an infinite number of available fault currents and some of those currents will be low enough to affect the trip times of the devices on the load side of the service, and cause an increase in the incident energy. I guess that you can do the calculations and prove that the incident energy cannot exceed 40 cal/cm^2 and use level 4 PPE.
I just don't see any way to select the correct PPE for live work at this time. The target fault currents may be good enough to protect equipment, but I don't think they are close enough to select PPE.
Don
 
Re: Data from utility

With the advent of the SCADA (supervisory control and data acquisition) system and remote radio controlled and automated switches, I could take notes on a circuit feeding your plant and the available fault current change while standing there. When I tell you that our systems are dynamic, I really mean that they are indeed dynamic. The overcurrent protection we set up is based on the substation and circuit configuration we intend for the circuit to be most of the time. I have even seen multiple primary circuits from several substations all tied together in order to keep the lights on during especially hot days and overloaded substations and circuits.

It is guess work on our part and there is no overcurrent protection on our main line circuits except at the substation switchgear. Our protection is based on protecting the primary taps first from the transformers. We then protect the large taps from the smaller taps. Then, we protect the main line from the taps.

The point I am making is that we are keeping the most meters turning as possible and always protecting the upstream line. Even the substation breaker will reclose on the fault to try to burn free or open a fuse or recloser. With all that, we can design a line to work but we can't give you what you are asking for. :D
 
Re: Data from utility

Originally posted by jim dungar:
[QB] Sparks,

It is very easy to select the correct breaker rating, simply use the design fault level supplied by the utility. Maximum short circuit fault levels are usually readily available. This is not the same problem as determing incident energy levels.


My point is this, how can the poco provide the available fault current on the secondary side to determine breakers when the primary side and grid is so dynamic?

How do we deal with this incident energy on the secondary side?
Click to expand...
 
Re: Data from utility

By Charlie:

Even the substation breaker will reclose on the fault to try to burn free or open a fuse or recloser
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This electricity stuff must be pretty important.

Isn't that a rather complete disregard for life and property?
 
Re: Data from utility

Isn't that a rather complete disregard for life and property?

In my opinion, no. If we have an underground cable fault, the fuse will let go fairly quickly and a cable fault is always a permanent fault.

If it is overhead and on the mainline primary, it is almost always a tree limb or an animal that has started the fault and needs time to burn free. Once the arc has started, the ionized air will provide the path for the arc to continue unless we open the circuit and then close it again. We open the circuit, and close it twice and then open it the third time for good and wait for a trouble man to clear the problem before closing it again.

If it is a tap that is close to the substation and lots of fault current is available, we will use a sectionalizer to count the substation breaker operations and will open after it cycles twice. That will permit the substation circuit breaker to act as the overcurrent device for the tap and the tap sectionalizer will not open under load. :D
 
Re: Data from utility

I didn't figure the operation was as calous as I made it sound. And I actually like the idea of the auto reclose thing and "burning free". And opening ionized connections.

But I was thinking of things like car accidents knocking down power lines and big, mean, nasty breakers trying to burn through people, automobiles and buildings.

On another note, we were talking about ionized air in another thread about Bryan's arc machine. The best info we could come up with on the subject is that ionized air can pretty much short out an electrical supply. It looks like that's true from what you've said.

Edit: Oh and, Hi Charlie.

[ July 02, 2005, 05:31 PM: Message edited by: physis ]
 
Re: Data from utility

Originally posted by physis:
But I was thinking of things like car accidents knocking down power lines and big, mean, nasty breakers trying to burn through people, automobiles and buildings.
Click to expand...
That will teach not to run your car into the power companies property. :(
 
Re: Data from utility

Gee Bob, usually I can find a way to argue with you but I'm having too much fun imagining all the Darwin people wandering around in the dark. :D
 
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