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 Post subject: Question about workplace
PostPosted: Thu May 10, 2012 11:30 am 
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Hello, I have recently taken a management role in a company that designs and manufactures DC power supplies for a variety of industries. Our supplies are mostly low voltage but can be very high current (eg: 10VDC 10,000ADC). Our larger supplies are also running at 480VAC input and some can have >50A input at 480VAC. After studying the NFPA 70E I have come up with some questions that have sparked some debate at work as to whether or not the standards apply to us.

For example, if somebody is working on an open and energized supply with a 480VAC input should they have to comply with table 130.7(C)(15)(a) and if so would they have to comply with the required PPE for a Hazard Class 2 per table 130.7(C)(16)? The argument is that this would make working on the power supply difficult. But going by the book, it certainly sounds as if the PPE would be required.

The argument that is going around is that most of the standard applies to linemen or people working in the electrical utility sector and not so much to electronic technicians working on power supplies. However, in my opinion there should be no difference as electricity is electricity and it doesn't matter if you're working on 480VAC switch gear or 489VAC input into a power supply the same rules apply.

I would appreciate anybody's opinion on this.

Thank you very much!


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PostPosted: Thu May 10, 2012 4:45 pm 
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REvans wrote:
For example, if somebody is working on an open and energized supply with a 480VAC input should they have to comply with table 130.7(C)(15)(a) and if so would they have to comply with the required PPE for a Hazard Class 2 per table 130.7(C)(16)? The argument is that this would make working on the power supply difficult. But going by the book, it certainly sounds as if the PPE would be required.


Let's get down to what we're really talking about here. First off when it comes to arc flash, we are talking about an arcing fault...basically you manage to get a spark and it heats up the air molecules enough to make it conductive. This arc burns at a fairly high temperature usually stabilizing around 6000+ degrees. This means that radiative heat transfer is very efficient...unfortunately for anyone nearby. At low voltages it becomes difficult for the arc to form or the arc (for AC) may not heat the air sufficiently to sustain the arc for more than a few cycles. At 480 VAC for sure it can initiate and maintain the arc essentially indefinitely.

So...arc FLASH is when the highly efficient heat source transfers heat to surrounding human tissue or clothing and burns it.

Quote:
The argument that is going around is that most of the standard applies to linemen or people working in the electrical utility sector and not so much to electronic technicians working on power supplies. However, in my opinion there should be no difference as electricity is electricity and it doesn't matter if you're working on 480VAC switch gear or 489VAC input into a power supply the same rules apply.


Quite the opposite. Typically linemen are LESS affected by the problem in many cases because the available fault current is not quite so high, or because they are working relatively far from a point of failure if they are doing hot stick work rather than working up close like technicians typically do. I said less...it is highly dependent on a number of variables and this statement is pretty general. For instance where I work, we typically use hot sticks and stay 8-10 feet away, and the available fault currents are generally much lower than similar buses in the same operation which are at 480 VAC. The lineman working on a 25 kV line fuse may have an H/RC of 0 (wear a long sleeve shirt) while the secondary side of the transformer compartment fed by that fuse may have an incident energy well in excess of 40 cal/cm^2, basically fatal irrespective of any PPE available!

It is true that at very low voltages the possibility of initiating an arc in the first place is almost impossible with DC. Look for Hertha Ayrnton's equation for guidance on this. HOWEVER such is not the case for the 480 VAC feed to the power supply you mentioned.


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PostPosted: Sat May 26, 2012 2:46 pm 
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PaulEngr wrote:
Let's get down to what we're really talking about here. First off when it comes to arc flash, we are talking about an arcing fault...basically you manage to get a spark and it heats up the air molecules enough to make it conductive. This arc burns at a fairly high temperature usually stabilizing around 6000+ degrees. This means that radiative heat transfer is very efficient...unfortunately for anyone nearby. At low voltages it becomes difficult for the arc to form or the arc (for AC) may not heat the air sufficiently to sustain the arc for more than a few cycles. At 480 VAC for sure it can initiate and maintain the arc essentially indefinitely.

So...arc FLASH is when the highly efficient heat source transfers heat to surrounding human tissue or clothing and burns it.



Quite the opposite. Typically linemen are LESS affected by the problem in many cases because the available fault current is not quite so high, or because they are working relatively far from a point of failure if they are doing hot stick work rather than working up close like technicians typically do. I said less...it is highly dependent on a number of variables and this statement is pretty general. For instance where I work, we typically use hot sticks and stay 8-10 feet away, and the available fault currents are generally much lower than similar buses in the same operation which are at 480 VAC. The lineman working on a 25 kV line fuse may have an H/RC of 0 (wear a long sleeve shirt) while the secondary side of the transformer compartment fed by that fuse may have an incident energy well in excess of 40 cal/cm^2, basically fatal irrespective of any PPE available!

It is true that at very low voltages the possibility of initiating an arc in the first place is almost impossible with DC. Look for Hertha Ayrnton's equation for guidance on this. HOWEVER such is not the case for the 480 VAC feed to the power supply you mentioned.


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PostPosted: Sat May 26, 2012 2:53 pm 
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[quote="JayWes38"][/quote] One Must remember that utilities work to a different rule book , the National Electical Safety Code, published by the IEEE. This standardwhich includes the lineman safety rules, as well as the construction rules for equipment lines and substations. While there are some simiuilaries with the NFPA documents there are diffences especially for outside line work.

For the record the NFPA does not cover outdoor wiring very well, especially for structural and wind requirements and notes in fine print notes that engineering design is a requirement. The biggest item to be away of is in large industrial power substations the 25 ohm and laess single ground wire is inadquate, ans a though ANSI c80 study of touch and step potentials is needed.


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