Arc Flash Analysis

[zog]

Unlkess you have major changes to a large system your PPE requirements will not likely to change.

Sure, lets just ASSUME it dosent change anything, lets ASSUME no one has changed any pickup settings because a breaker tripped.

 

[don_resqcapt19]

Unlkess you have major changes to a large system your PPE requirements will not likely to change.

According to Bussman, if you are using a 400 amp LPS-RK-SP fuse and the fault current drops from 12kA to 10kA the incident energy goes from 1.7 cal/cm^2 to over 100 cal/cm^2. I don't see that drop in fault current at a major change in the power system.

 

[coulter]

Sure, lets just ASSUME it dosent change anything, lets ASSUME no one has changed any pickup settings because a breaker tripped.

All places I've worked check the relay settings and verify operation on a regular PM. I don't have any anecdotal evidence that I should be concerned about a midnight settings changer.

carl

 

[zog]

All places I've worked check the relay settings and verify operation on a regular PM. I don't have any anecdotal evidence that I should be concerned about a midnight settings changer.

carl

Well that is rare, I have responded to many outages that were caused maintenence electricians cranking up settings on feeder breakers. That is the first thing an untrained person does when a breaker trips, they set it higher and reclose the breaker, or replace the blown fuse with a bigger fuse, or the same amp fuse but a different type with different characteristics.

If you check your settings as part of a regular PM, I dont think you need to worry either, but most plants do not do that.

 

[coulter]

According to Bussman, if you are using a 400 amp LPS-RK-SP fuse and the fault current drops from 12kA to 10kA the incident energy goes from 1.7 cal/cm^2 to over 100 cal/cm^2. ...

Wow - that is interesting. Let's check the math:

Current dropped to 10ka/12ka = .83
Energy delivered (compared to the 12ka SCC) ~ i^2t = .69t
For the example given, the energy changed from 1.7 to 100cal/cm^2 = 59 times
So, with the current at 10ka, the time to trip increase over the 12ka time is:
t(10) = 85 x t(12)

I'm not saying this is not true, but it smells like they are cooking the books, as far as they can without lying, to prove some point. I'd be interested to read the Bussman reference.

carl

 

[jim dungar]

I'm not saying this is not true, but it smells like they are cooking the books, as far as they can without lying, to prove some point. I'd be interested to read the Bussman reference.

At 10kA the Bussmann 400A fuse does not enter its current limiting range, therefore the its clearing time and let through current are both significant. I have done several studies where the recommendation was to replace fuses with breakers due to the low available fault current.

 

[coulter]

... That is the first thing an untrained person does when a breaker trips, they set it higher and reclose the breaker, or replace the blown fuse with a bigger fuse, or the same amp fuse but a different type with different characteristics. ...

Bummer. I'm a lot luckier than that. Anytime I have found a non-specified fuse change, or a mag-only cb set up, it was a trained person that did it:-?

I'm not particularly concerned about the arc-flash change if a 10A fuse gets changed out with a 30A. Nor am I concerned about arcflash change if a 10HP mag-only gets set up from 120A to say 200A. Even so, I have found very few of these.

My thinking changes significantly if we are dealing with undocumented changes to a 600A - 3000A feeder/xfm secondary. Some of these are set with pretty tight coordination just to get the arcflash down

I'm open to getting educated here if my thinking is in error

carl

 

[jim dungar]

Well that is rare, I have responded to many outages that were caused maintenence electricians cranking up settings on feeder breakers. That is the first thing an untrained person does when a breaker trips, they set it higher and reclose the breaker, or replace the blown fuse with a bigger fuse, or the same amp fuse but a different type with different characteristics.

If you check your settings as part of a regular PM, I dont think you need to worry either, but most plants do not do that.

If the breaker settings have been changed, for no reason, they need to be reset as opposed to having a new study performed and new labels applied. All device changes must be reviewed for their impact on a study. I have seen some studies where "upgrading" a fuse from an FRS-R style to an LPS-RK style increased the HRC.

ESWP is not a one-time event. PM must not be ignored. Changes must be documented. Study updates must be performed "regularly".

 

[coulter]

At 10kA the Bussmann 400A fuse does not enter its current limiting range, ...

That soulds reasonable. Makes one wonder why a current limiting fuse would have ever been specified for a 10KA or 12kA system.

carl

 

[don_resqcapt19]

Carl,
I was just trying to make a point that in some cases a decrease in the available fault current can have a huge impact on the incident energy. The same Bussmann chart has showes the same thing can happen with breakers.

 

[weressl]

Sure, lets just ASSUME it dosent change anything, lets ASSUME no one has changed any pickup settings because a breaker tripped.

You are changing the base of the argument.

 

[weressl]

Carl,
I was just trying to make a point that in some cases a decrease in the available fault current can have a huge impact on the incident energy. The same Bussmann chart has showes the same thing can happen with breakers.

The main problem is that the variable with the largest impact - the Utility Contribution - is the most difficult to obtain and ascertain with any accuracy. The 1584 calcuations make allowance to calculate to the worst potetnial condiciton, but it is based on an arbitrary number. It is a conservative assumption, but it does not and can not exclude even worse conditions to exist. Utilities need the flexibility to allow for switching for may reasons and they also continously grow and change. They are not forced to notify the consumer, even large ones, of any of these changes unless they change protective settings and they need the customer to change theirs.

 

[zog]

You are changing the base of the argument.

Dont see why you think that, I was responding to your ststement that implied you dont have to re-evaluate your arc flash study unless you make MAJOR changes to your system .

 

[jim dungar]

That soulds reasonable. Makes one wonder why a current limiting fuse would have ever been specified for a 10KA or 12kA system.

Several years back the major fuse manufacturers started the practice of convincing customers to change all of their different fuses to yellow, or orange, or green ones, this way they could reduce their fuse inventories and never worry that the wrong fuse type was installed.

 

[weressl]

Dont see why you think that, I was responding to your ststement that implied you dont have to re-evaluate your arc flash study unless you make MAJOR changes to your system .

Sure, lets just ASSUME it dosent change anything, lets ASSUME no one has changed any pickup settings because a breaker tripped.

A simple change is not a MAJOR change and changing pickup settings because breaker tripping is changing the system and should not be done without verifying the original coordination study. Having said that, adding and 15 or 20HP motor to a 1500kVA transformer will not change the contribution to any significant degree that would result in change of protective equipment levels.

 

[zog]

A simple change is not a MAJOR change and changing pickup settings because breaker tripping is changing the system and should not be done without verifying the original coordination study. Having said that, adding and 15 or 20HP motor to a 1500kVA transformer will not change the contribution to any significant degree that would result in change of protective equipment levels.

But changing a setting may cause a major change to the PPE requirements, and if you think the coordination study is referenced before adjusting breaker settings or replacing a fuse every time, you are living a sheltered life. Most induatrial electricians have never even heard of a coordination study, let alone understand the implications of not referencing it before making changes.

 

[weressl]

But changing a setting may cause a major change to the PPE requirements, and if you think the coordination study is referenced before adjusting breaker settings or replacing a fuse every time, you are living a sheltered life. Most induatrial electricians have never even heard of a coordination study, let alone understand the implications of not referencing it before making changes.

If you accept that then you also accept that they have no clue what the arc-flash hazard means and how their actions may change the level of hazard. Based on that none of these people are qualified to do what they do according to OSHA definitions.

 

[zog]

If you accept that then you also accept that they have no clue what the arc-flash hazard means and how their actions may change the level of hazard. Based on that none of these people are qualified to do what they do according to OSHA definitions.

Very true, they are not qualified. But that dosent mean they are not operating the equipment, most are not qualified per the OSHA and NFPA 70E definition, they still sre doing the work and they can still become injured. Most industrial facillities have done some oversimplified arc flash training, that was done by someone in house that had no idea what they were talking about. The companies "think" they have qualified thier poeple when if fact they only know enough to be dangerous.

 

[weressl]

Very true, they are not qualified. But that dosent mean they are not operating the equipment, most are not qualified per the OSHA and NFPA 70E definition, they still sre doing the work and they can still become injured. Most industrial facillities have done some oversimplified arc flash training, that was done by someone in house that had no idea what they were talking about. The companies "think" they have qualified thier poeple when if fact they only know enough to be dangerous.

So they just need a smart lawyer to show that the Company or their Union did not provide proficcient training. On the other hand I don't understand how can such people be called 'electricians' in the first place.

 

[don_resqcapt19]

The main problem is that the variable with the largest impact - the Utility Contribution - is the most difficult to obtain and ascertain with any accuracy. The 1584 calcuations make allowance to calculate to the worst potetnial condiciton, but it is based on an arbitrary number. It is a conservative assumption, but it does not and can not exclude even worse conditions to exist. Utilities need the flexibility to allow for switching for may reasons and they also continously grow and change. They are not forced to notify the consumer, even large ones, of any of these changes unless they change protective settings and they need the customer to change theirs.

That is my problem with these calculations when you are working at or very close to the service equipment. The worst case will always be far in excess of the 40 cal/cm^2 that appears to be the maximum that you can work with PPE. How can anyone put a label on equipment like this when they know that the information does not cover all cases of utility operations and may result in injury or death to the person who relied on the label information?