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 Post subject: What to do line side of Main?
PostPosted: Wed Jan 30, 2008 9:37 pm 
Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 498
Location: New England
480V 3200A Switchgear.
I can handle everything load side of the main, but how do you analyze the Main. The only upstream clearing device is going to be the primary device of the transformer. That's going to be slow to clear. I haven't pulled the curve yet, but does the Table 130 PPE/Task represent a typical system with Draw out primary breaker at 13.8kv-480v and a 2000KVA transformer.

I'll pull all the info, I just wanted to see if someone has done a similar set up. My thinking is that the Main Breaker compartment will be at a much higher IE than the branch breakers.

Another observation, is that Mains typically don't come with Instantaneous trips as they don't coordinate well with the branch breakers. That will mean long clearing times on a line side fault of a branch breaker. When the table lists Switchgear, did they take this into consideration.


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PostPosted: Thu Jan 31, 2008 7:47 am 

Joined: Mon Aug 06, 2007 10:21 am
Posts: 35
Location: Ellijay, GA
Do NOT open

What you're seeing is a common issue. With regards to your service main, there is no clearing device between the utility transformer secondary and the line side of the main. The primary fuse cannot be used to calculate your incident energy rating on the secondary, this is an IEEE 1584 issue.

Due to impedance differences and such, the primary side fuse cannot be depended upon to clear in the event of an arcing fault on the secondary side. Unless there are fuses on the secondary, between the transformer and the service main, no calculation can be made of the incident energy. Therefore, the service main must be labeled "Do Not Open, No PPE level found".


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PostPosted: Thu Jan 31, 2008 12:41 pm 
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Joined: Thu Oct 18, 2007 7:10 pm
Posts: 261
Location: NW USA
You can calculate the incident energy available, based on primary fuses or other protection. If these devices will not reliably clear a secondary fault, THEN the equipment will be deemed unworkable or do not open energized. But you do not need to assume that is what the calculations will result in.

Standard software will include arc resistance and calculate incident energy available with clearing curves based on lower than max available fault current.

In many cases the calculations will result in a high exposure, however, we have also found some that can be worked on, with appropriate PPE.

To avoid a Man trap, we have used the worst case exposure on a lineup for the entire lineup. This adoption has shown the benefit of secondary main breakers located apart from the lineups, not a bad way to design new systems.

Gary B


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PostPosted: Thu Jan 31, 2008 4:13 pm 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 498
Location: New England
Okay, with that understood, can you still use the table PPE to 'operate' and 'rack' the main?


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PostPosted: Thu Jan 31, 2008 5:56 pm 
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Joined: Thu Jan 10, 2008 8:49 pm
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Location: New England
Let me ask another question, how would you take a voltage measurement on the secondary of a pad mount transformer. Seems hard to believe something this simple and common can no longer be done. What if you want to check rotation on a new installation, or record the rotation on existing. As long as the primary is in a seperate compartment I usually think nothing excessive in doing these tasks.


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PostPosted: Fri Feb 01, 2008 2:15 pm 
Sparks Level

Joined: Thu Oct 18, 2007 7:10 pm
Posts: 261
Location: NW USA
You are raising valid concerns that seem to indicate the committees don't spend much time in the field.

Whether racking breakers or taking voltage checks, you are technically messing with the electrical field in a fashion that could start an arc flash incident. (IF your rotation meter has inductive pickup that goes around the insulated part of the cable, I would say you are not disturbing the electric field).

The table PPE for many cases is based on 2 cycle clearing time, that would indicate a fusible secondary main to me. A fusible main is not something that is typical where I work, it is usually a breaker (in the panel) that would be 6 cycle clearing time.

We have run a bunch of calculations on 400KVA, 225KVA, 150KVA transformers, 208V 3 phase secondary, typical available fault current,typical impedance. Because there is no secondary main, except in the panels themselves, we are only using the primary device as clearing. The calculations indicate risk categories that require higher PPE than what the tables recommend. I'm not finished with this work so I don't have any recommendation except to be cautious..........

NFPA 70E seems to raise more questions than give answers, but at least we are considering arc flash PPE now.


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PostPosted: Mon Jun 02, 2008 8:45 pm 
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Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
haze10 wrote:
Okay, with that understood, can you still use the table PPE to 'operate' and 'rack' the main?


No you are outside the linitations of the tables


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PostPosted: Mon Jun 02, 2008 8:50 pm 
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Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
haze10 wrote:
480V 3200A Switchgear.
I can handle everything load side of the main, but how do you analyze the Main. The only upstream clearing device is going to be the primary device of the transformer. That's going to be slow to clear. I haven't pulled the curve yet, but does the Table 130 PPE/Task represent a typical system with Draw out primary breaker at 13.8kv-480v and a 2000KVA transformer.

I'll pull all the info, I just wanted to see if someone has done a similar set up. My thinking is that the Main Breaker compartment will be at a much higher IE than the branch breakers.


I have retorfitted the fuse compartment in the primary switch with a mini vacuum breaker that has CT's on both primary and seconday sides of the transformer, in some case light sensing relays on the secondary (Depending on budget), that has typically taken 40+ cal.cm2 down to <8 cal/cm2 on the seconday side.

haze10 wrote:
Another observation, is that Mains typically don't come with Instantaneous trips as they don't coordinate well with the branch breakers. That will mean long clearing times on a line side fault of a branch breaker. When the table lists Switchgear, did they take this into consideration.



Arc flash redustion switches, the best one IMO is the Quick Trip system from URC for the AC PRO tri units, easy upgrade, do hundreds a year.


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