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 Post subject: Clearing Time Upstream of Transformer
PostPosted: Tue Mar 25, 2008 1:06 pm 

Joined: Tue Mar 25, 2008 1:00 pm
Posts: 32
Hello,

I am performing and arc flash analysis and have run into a problem figuring out what clearing time to use. I am given the bolted fault current at the transformer secondary and the transformer specifications. The 150Kva transformer has secondary of 208V and a fault current of 32000 A. I have been told there are 100 K fuses upstream of the transformer along with some time delay breakers.

How do I figure out what clearing time to use for an arc at the transformer secondary? The upstream breaker has a delay of 2.5 and the fuse clearing time varies wildly based on the current.


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PostPosted: Tue Mar 25, 2008 1:43 pm 
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Location: Charlotte, NC
deprico,

First of all it appears that the utility gave you the "infinite source" value of fault current. 150 kVA, 208v, and 32000 amps, indicates no source Z other than a 1.3% impedance transformer. I would go back to them and ask for the actual fault duty at the secondary terminals with the MVA base and sequence values. I think your actual values will be much less.

You will also need specifics about the utility protective devices so that you can get the appropriate time current curves to use in your analysis.

Alan


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PostPosted: Tue Mar 25, 2008 7:42 pm 
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Location: New England
Is this a utility supplied transformer. If so, there upstream overcurrent protection could be set quite high and be quite slow. IE levels could be quite high on the secondary. However, the secondaries normally go into the customers main panel that has a main circuit breakers. Is this true for you. If yes, then why would you need to work on the transformer or cables upstream of your MCB. For clearing time you would use your MCB.


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PostPosted: Wed Mar 26, 2008 10:44 am 

Joined: Tue Mar 25, 2008 1:00 pm
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Well say I am doing live work in the panel that houses the main breaker. There still exists an exposure to the secondary on the line side of the breaker inside the panel. This would mean the clearing time of the breaker does not come into play for a fault on the line side of the breaker. For this I would need the clearing time for the protective device upstream of the breaker.


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PostPosted: Fri Mar 28, 2008 10:44 am 
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Location: NW USA
deprico wrote:
Well say I am doing live work in the panel that houses the main breaker. There still exists an exposure to the secondary on the line side of the breaker inside the panel. This would mean the clearing time of the breaker does not come into play for a fault on the line side of the breaker. For this I would need the clearing time for the protective device upstream of the breaker.


It is not unusual, for the situation you describe, for the arc flash exposure to be unworkable.

The transformer primary fuses will provide some protection that may be calculated through the transformer but typically that is too slow to provide arc flash mitigation.

It is a common problem on the sites we've looked at, only official solution is to take an outage.


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PostPosted: Sat Mar 29, 2008 6:48 pm 
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Location: New England
Safeguard the line side with a rubber blanket or suitable shield, and stay out of the Prohibited Approach Boundary - 1" on a 480V. Then you are only concerned with the load side. If you have to work on the line side, then you have to de-energize.


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PostPosted: Wed Apr 23, 2008 6:34 pm 
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Location: Oak Ridge, Tennessee
how to calculate.

Generally, what I do when the upstream device is on the high side of the transformer is use the 2 seconds. If I am labeling the transformer I do this because I am not aware of any testing on arc flash events on the transformer (and how it responds to an arc), and also because the upstream device is often set high (or up on a pole and hard to get).

However, you can do this... If you "arc" the bus where the arc flash is, you want to see what the current is flowing through the upstream device on the high side of the transformer. This current is obviously at a different voltage, but so is the protective device upstream. Most programs should be doing the calculations this way - you might just check with them.

If you are using the IEEE Spreadsheet, this is pretty difficult to do. Given the impedance of the transformer it is a pretty safe bet that a low voltage arc on the low-side may not trigger a trip on the high side very quickly. Use 2 seconds and the 32000 Amps they gave you.

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PostPosted: Tue Apr 29, 2008 6:40 pm 
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Don't you think to some extent we are over analyzing this. Are you seriously going to take the whole panel to Level 4 because of the line side lugs on the main, when the electricians are working on the load side which could be Level 2. Remember that only 'Qualified' individuals can do this work. I think that training includes knowing that the line side mains are different than the load side. Just me but I think we have to enter some degree of practicality and reasonableness.


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PostPosted: Tue May 06, 2008 5:54 am 
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haze,
The problem is that in an unsegregated bus, the flashover could carry over to the line side terminals of the main breaker. In this case, the main is useless. If the bus has a separate compartment for the main breaker, then what you say is true, and that is the way that I calculate it. I put two stickers on equipment with segregated main breakers. One on the Main, and one on the rest of the bus. But panelboards get just one sticker, calculated ahead of the main breaker.


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PostPosted: Fri May 09, 2008 1:40 pm 
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Typically for equipment except for switchgear where the compartments are very segregated, you have exclude the main incoming panel breaker for arc flash hazard analysis. The reasoning is that the main lugs on the incoming breaker or fuse are most likely exposed and any fault inside that panel would cascade rapidly to encompass these lugs. The fault then has to clear from the next upstream breaker or fuse, which is most likely on the high side of the transformer.

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PostPosted: Sat May 10, 2008 7:51 pm 
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On a 400 to 800A MCB panel, the top of the load bus is probably 12" or more to the line lugs on the MCB. So you would be outside the Restricted approach. So we are saying that a load side flash will ionize the air to the point the line lugs are now flashing over. Seems somewhat unlikely to me. I would still think that a cover over the line lugs is all that is needed. I would not be surprised to see the next Square D upgrade to be a bolt on plastic cover over the line lugs.


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