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 Post subject: Ran up against it...
PostPosted: Wed Jan 14, 2009 12:39 pm 
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Location: St. Louis, MO
Ok, here’s the situation. We have two old unitsubs, Federal Pacific, 1000kVA transformers in one building. The unitsubs face each other, with a narrow, ~3ft walkway between them.
Both of the subs are rated >40 cal on the low voltage (480V) sections. This includes the Main breakers, which will probably not operate (FP HD50B, 1600A) as well as the load breakers.
The MV switch, which is also included, is below 40cal (~3 cal), but is overdutied, and cannot be operated by company policy.

I've been over the calulations a number of times, and cannot reduce the AF hazard below 40 cal. (We've adopted the 40 cal cutoff for hot work.)

Now, both sets of switchgear are scheduled to be replaced in the next year. But, until then, if a breaker trips, we have no plan in place on how to reset the breaker. In order to shut down the subs in this scenario, about a block worth of buildings on our facility will also have to be shut down in order to cut off the MV feeds and kill the power to the subs. This is unlikely to happen.

How to Proceed? Any ideas?


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PostPosted: Wed Jan 14, 2009 12:53 pm 
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Joined: Mon Jun 02, 2008 11:58 am
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Location: Charlotte, NC
WDeanN wrote:
Ok, here’s the situation. We have two old unitsubs, Federal Pacific, 1000kVA transformers in one building. The unitsubs face each other, with a narrow, ~3ft walkway between them.
Both of the subs are rated >40 cal on the low voltage (480V) sections. This includes the Main breakers, which will probably not operate (FP HD50B, 1600A) as well as the load breakers.
The MV switch, which is also included, is below 40cal (~3 cal), but is overdutied, and cannot be operated by company policy.

I've been over the calulations a number of times, and cannot reduce the AF hazard below 40 cal. (We've adopted the 40 cal cutoff for hot work.)

Now, both sets of switchgear are scheduled to be replaced in the next year. But, until then, if a breaker trips, we have no plan in place on how to reset the breaker. In order to shut down the subs in this scenario, about a block worth of buildings on our facility will also have to be shut down in order to cut off the MV feeds and kill the power to the subs. This is unlikely to happen.

How to Proceed? Any ideas?



Easy one, retrofit the Main breaker with AC PRO and quick trip, should take your switchgear (load side of Main) to HRC 1 or 2. We do these every day.


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PostPosted: Wed Jan 14, 2009 1:16 pm 
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Location: St. Louis, MO
Zog wrote:
Easy one, retrofit the Main breaker with AC PRO and quick trip, should take your switchgear (load side of Main) to HRC 1 or 2. We do these every day.


Zog,
The switchgear is already slated for replacement.
In the meantime, however, there is a high probablility that one of the Federal Pacific breakers will trip (In addition to age, they have had very little maintenance performed on them, the dashpots have probably leaked out, and they are known to be very unreliable - we've had problems with them in the past.)

If they trip before the switchgear gets replaced (highly likely, given past performance) we have no plan in place to reset them, besides a hot stick, and there's very little room even for this in the room where they are located.


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PostPosted: Wed Jan 14, 2009 1:19 pm 
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Location: St. Louis, MO
Oh Zog,
As a matter of fact, I am expecting to get back another FPE HD50B 1600A breaker (yes, we have more of these) from refurbishment with an AC Pro trip unit soon.

So how do I get it put in without taking the whole block down? Remember, the MV switch feeding the unit sub is overdutied, and cannot be operated.


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PostPosted: Wed Jan 14, 2009 1:33 pm 
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WDeanN wrote:
Oh Zog,
As a matter of fact, I am expecting to get back another FPE HD50B 1600A breaker (yes, we have more of these) from refurbishment with an AC Pro trip unit soon.

So how do I get it put in without taking the whole block down? Remember, the MV switch feeding the unit sub is overdutied, and cannot be operated.


Well you dont have to denergize the bus to rack in the breaker (It is just safer that way), suit up and rack it in. Remote racking works well here also.


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PostPosted: Thu Jan 15, 2009 7:28 am 
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Zog wrote:
Well you dont have to denergize the bus to rack in the breaker (It is just safer that way), suit up and rack it in. Remote racking works well here also.


But it's over 40 cal, so we cannot even operate the breaker at the moment. We cannot shut the circuit off or operate the breaker, because the whole low voltage section is over 40 cal, and the MV switch is overdutied, and is not load interrupting rated.

I think this is one reason that NFPA 70E stops just short of a hard and fast 40 cal limit...


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PostPosted: Thu Jan 15, 2009 7:41 am 
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WDeanN wrote:
I think this is one reason that NFPA 70E stops just short of a hard and fast 40 cal limit...


I agree with that, sometimes there are few options


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