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 Post subject: Motor disconnects and welding disconnects
PostPosted: Tue Mar 03, 2009 6:35 am 
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Now the question is, do we label disconnects for motors <50HP? And do we label welding disconnects?

I will be labeling most of the >50HP (480V) motor disconnects with ratings based on the MCC it is feeding from.

So far here are my rules for labeling. Label all medium voltage breakers with line side ratings and the rest of breakers with load side ratings. One label per compartment.

Label all 480 MCC's with line side ratings. One label per whole MCC.

Label all disconnects based on MCC rating.

Label all <125 kVA panels with PPE level 0 labels.


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PostPosted: Tue Mar 10, 2009 10:50 am 
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Any ideas, anyone?


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PostPosted: Tue Mar 10, 2009 2:00 pm 
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I label all 3 phase disconnects regardless of the hp connected to them, unless they are exempt from the AF study due to being 208 volts or less fed from a single transformer <125 kva.


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PostPosted: Wed Mar 11, 2009 6:09 am 
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What rating do you assign to it? Do you create a separate bus in software, or do you use MCC feeding the motor labels?


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PostPosted: Wed Mar 11, 2009 1:58 pm 
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I create a seperate bus to represent the disconnect and then consider the breaker which feeds the disconnect (inside the MCC bucket) and the conductor from the bucket to the disconnect. If the motor is less than 50 HP, I do not consider the motor contribution.


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PostPosted: Thu Mar 19, 2009 6:15 am 
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Be very careful about labeling a disconnect the same as an MCC value. Let me give you an example....MCC = 7.5cal/cm2. You feed a motor disconnect that is 350ft away. With the impedance of the cable your SC current was dropped drastically and you have a longer clearing time pushing your AF to....9.7cal/cm2. MCC = Level #2 PPE, Motor DS = Level #3 PPE. Big difference.

Also to go along with the original questions. I would label all 480V disconnects.


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PostPosted: Thu Mar 19, 2009 9:04 am 
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McFlash wrote:
Be very careful about labeling a disconnect the same as an MCC value. Let me give you an example....MCC = 7.5cal/cm2. You feed a motor disconnect that is 350ft away. With the impedance of the cable your SC current was dropped drastically and you have a longer clearing time pushing your AF to....9.7cal/cm2. MCC = Level #2 PPE, Motor DS = Level #3 PPE. Big difference.

Also to go along with the original questions. I would label all 480V disconnects.


That's an excellent point McFlash. I am using SKM, do you recommend I add a seperate bus to MCC indicating a primary side of the disconnect?

We have possibly hundreds of disconnects out here. Most, we do not even know where they feed from.


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PostPosted: Thu Mar 19, 2009 10:47 am 
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Unfortunately I would say that is the proper thing to do. It may seem conservative but safety is safety!! You are correct about how to model them I would model a bus to represent the line side of the DS, include the cable data to the DS and the breaker or fuse in the MCC.

Most are going to be low values but it's those hidden dangers lurking where people get injured.


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PostPosted: Fri Mar 20, 2009 7:26 am 
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McFlash wrote:
Unfortunately I would say that is the proper thing to do. It may seem conservative but safety is safety!! You are correct about how to model them I would model a bus to represent the line side of the DS, include the cable data to the DS and the breaker or fuse in the MCC.

Most are going to be low values but it's those hidden dangers lurking where people get injured.


I put in some bogus wire lengths (500'+) for disconnects and so far the numbers haven't budged for any of them.

Does anyone else model disconnects this way, or am I and McFlash too detailed on this level?


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PostPosted: Mon Mar 23, 2009 10:27 am 
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Jnox wrote:
I put in some bogus wire lengths (500'+) for disconnects and so far the numbers haven't budged for any of them.

Does anyone else model disconnects this way, or am I and McFlash too detailed on this level?

I have modelled disconnects in this fashion, however in most of these cases I have included the motor also.

Typically I only include motors that are of 25hp and greater. From experience I have found that of motors of smaller size the impedance introduced by the cable is too high to sustain a fault. However that all changes if you are very close to the MCC in question.

This is one of those, "Use your best judgement" types of situations. Test with your extremes both far and close, large and small and work towards the middle to see if there is any trends, and go from there.


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PostPosted: Tue Mar 24, 2009 8:46 am 
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We have been modeling all disconnects, motors, etc, regardless of size.
Yes, the vast majority are 4 cal or less. There's always that one, though...

I've got a little OCD, though... My goal is to replace the information available in any single-lines with my model...


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PostPosted: Sat Apr 04, 2009 10:17 pm 
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I have done extensive analysis of the AFIE on circuits fed by overcurrent protection devices of less than 100A.

I used Bussmann FRS-R fuses and Square D circuit breakers (at this moment I do not remember the model numbers), keeping voltage drop above the minimum (cable and bus).

The result is that with fuses, you might have an increase in category only in extremely long distances (about 800 feet) to the panel not matter the fuse size. So it is not necessary to survey and calculate the AFIE in panels or disconnects fed by a fuse less than 100A because the HRC will most probably be zero.

For breakers, the shortest length to be concerned is the 60A circuit breaker. For circuits fed by a 60A CB, the category will remain zero for distances of 245 feet or less. For distances larger than 245 feet and fed by a 60A CB, the category my jump to an HRC 2.

For other amperage ratings, the distances for the change in HRC are more than 300 feet.

How often do you find lower amperage circuits with lengths longer than 300 feet?

Something to think about.

RECS


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PostPosted: Mon Apr 20, 2009 9:32 pm 
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Label all discnnects? Not necessary!

I have done an investigation (using ASCC from 10 kA to 100 kA) showing that any circuit fed by a RK5 CL fuse (FRS-R) will NEVER feed an equipent where the AFIE is more than 1.2 cal/cm2. If the OCPD is a non-CL breaker, the shortest length to be concern about is 250 feet for a CB of 60A. So, any equipment fed by an OCPD of less than 100A feeding equipment closer than 250 ft will NEVER have an AFIE of more than 1.2 cal/cm2.

In my model I assumed compliance with NEC requirements for permissible voltage drop, 80% CB loading and proper conductor protection.

So why spending so much money and effort surveying low amperage equipment and calculating and printing labels for 30A diconnects?

I would be interested to hear your opinion.

Have a fantastic day!


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PostPosted: Thu Apr 23, 2009 11:53 am 
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RECS,
You are correct that most small amperage devices will have an IE of less than 1.2 cal/cm2 if installed according to the NEC.

However I don't think we can make the assumption that every installation is according to the NEC. I have found several disconnects that were not installed correctly (wire too small, tap rule violations, etc). This can cause the incident energy to increase.

In one case a #12 AWG wire was added to the load side of a 1200 amp breaker. The wire was 30' long and fed a 20 amp disconnect. The 1200 amp breaker will not clear the fault quickly which resulted in a HRC #4 at the 20 amp disconnect.

We don't know which disconnects are installed according to the NEC until we open them and collect the data. If we have the data we might as well plug the data into the arc flash software and calculate the actual IE for each device.

Thanks.


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PostPosted: Fri May 01, 2009 9:53 am 
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Required Labels

You guys are on the right track. Each installation is different and assumptions can get you into trouble. I place 1 label on an MCC unless it has Main Incoming Protection. I then place an additional label on the Main Incoming Bucket. As for labeling requirements, refer to NEC Article 110.16 2008 and NFPA 70E Article 130.3(C) 2009. It is hard to exclude any disconnect (or other electrical enclosure) from being labeled. Another safety issue is not knowing where the devices are fed from... refer to 29CFR 1910.305.


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PostPosted: Tue May 05, 2009 9:29 pm 
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By previous interpretation it was an fair assumption that a single disconnect switch would not typically require 'hot work'. In absence of work action likely to disturb the electrical field around exposed energized conductors, there would not be an arc flash hazard.

But modern interpretation (2009 NFPA 70E) suggests that even operating devices with the covers on may cause arc flash hazard unless the gear is rated arc resistant. Personally I do not agree with this interpretation, but the manufacturers do not seem willing to defend the integrity of their product, so it is a grey area.

The most conservative interpretation of NFPA 70E would suggest these get labelled with arc flash exposure; and any plant operator (or farmer, or school superintendent) would need to suit up accordingly to operate the disconnect. This is probably not going to happen in the real world and leaves any professional associated with the project with a potential liability.


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