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 Post subject: Underground electrical lines hazard analysis 110.5
PostPosted: Thu Feb 09, 2012 11:41 am 
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Is anyone using some sort of form to document proof of 110.5? Sure we can say we have the electrical lines on a drawing, but what kind of proof do we need? Do utility companies (if the are the owners) really provide specific documentation for this? What kinds of safe work practices do we need to list? This portion of the standards seems a lot less specific than others.....
Thank you!


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PostPosted: Thu Feb 09, 2012 6:36 pm 
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The utilities that are doing modeling can give you a pretty good estimate of X/R and available short circuit fault energy at least.


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PostPosted: Mon Feb 13, 2012 7:00 am 
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My experiences with utilities is getting info is like pulling teeth. Hello my name is Peggy..... transfer.


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PostPosted: Tue Feb 14, 2012 5:12 pm 
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in our area of the country, the utility (Xcel) will oftentimes only quote out of their bluebook, and they won't reply to your question.


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PostPosted: Wed Feb 15, 2012 4:25 am 
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It is possible to do about 99% of your model without utility X/R or SCCR. It's also possible to operate a utility without that kind of data. The REA's and EMC's for instance usually don't have that kind of modelling. They go by some assumptions that someone set up for them. It's a bit vague but for your purposes might work OK.

The problem is that from the incoming line to your transformer, you won't have accurate data. You will have to assume worst case analysis. This is actually not that bad. A good way to get close is to look at the allowable ampacity for the incoming feeder. That gives you the available fault current. Then make reasonable assumptions about X/R such as plugging it in at around 3 or 4 for a utility. That jump starts your model beyond the transformer. On the primary/service side, there are tables in both 70E and also in the NESC which provide reasonable numbers. As the instructions say, these are reasonable guesses when no other data is available. You can't use them if the actual values exceed the values given in the tables but in your case, you wouldn't have that data.

This does come up quite often. We have a lot of >15 kV equipment where I work. There are only two models I'm aware of "valid" at that voltage; ArcPro, and Lee method. Lee is known to be incorrect and gets worse as the voltage increases especially above 600 V. ArcPro isn't published. Either way, nobody knows if any of the models are right above 15 kV because no tests have been run at that voltage.


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PostPosted: Tue Feb 21, 2012 5:48 pm 
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PaulEngr wrote:
A good way to get close is to look at the allowable ampacity for the incoming feeder. That gives you the available fault current.


I'm confused. How can you relate available fault current to feeder load ampacity?

But 110.5 doesn't deal with available fault current, it deals with the locating of underground electrical facilities prior to excavation. Some areas have adopted a one call system that will notify all the affected utilities, in others you will need to contact each separately. For proof, ask for a copy of the locate ticket or take pictures of the locate paint prior to excavation.

Getting a response to a call for locates should not be difficult, since the cost to the utility for repairs is very much higher than the cost of having the facilities located.


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PostPosted: Tue Feb 21, 2012 8:27 pm 
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I'm confused. How can you relate available fault current to feeder load ampacity?

Good point. It can't. I was thinking of when I don't have design current information but I can get wire size and at least guess at what the maximum (potential, assuming everything was done to code) current is under normal, non--fault conditions.

I know only one way to find out available fault current without design data but it's a little messy. Set up a way to measure current. Take a bare copper wire about 1/4 of the size of the condcutors to be tested. I'll leave it to the reader to determine what to do with the wire but let's just say that the end result is a measured fault current. Oh, and be prepared to an outage when you run the test. And be prepared for a fairly substantial arcing fault.


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PostPosted: Wed Feb 22, 2012 5:44 am 
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uriah1 wrote:
Is anyone using some sort of form to document proof of 110.5? Sure we can say we have the electrical lines on a drawing, but what kind of proof do we need? Do utility companies (if the are the owners) really provide specific documentation for this? What kinds of safe work practices do we need to list? This portion of the standards seems a lot less specific than others.....
Thank you!

As Stevenal said 110.5 deals will having the underground cables located, the one-call usually satisfies this. The other part of the article is that once the cable location has been identified, a hazard analysis needs to be done. In other words, what other hazards exist in the area that the excavation is being done and how are you going to mitigate them. This is part of the normal process for all electrical work, to know, remove, mitigate or protect against the hazards in the work area.


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