Diversity Factor applied to Short Circuit - I need help

[mityeltu]

A correction: The xfmr is 1.932MVA. I was looking at the wrong one when I posted that.

I have checked the cables. The secondary cable was not even in the model. I added it and it dropped the SC curren but, I still receive a huge amount (10.4kA) from the loads on the board I'm trying to switch. I attached a screen shot of the board so everyone can see what I'm working with.

 

[rbalex]

It may have been rejected, but this is a pretty standard method of operation. It is usually impractical to size the swgr for a short time event, and most owner's will accept the risk in order to save the cost.

It may be standard practice. It's still an NEC violation and owners accepting the risk is irrelevant - does the AHJ?. That said, I agree it shouldn't be a violation. But my opinion & $?.?? (I don't know the going rate) will get you a cup of coffee.

 

[weressl]

Assuming 40% of the motors are not running might be OK if the process controls are designed so there is never a chance that more than 60% of the motors will be on line at once, even during switch over from Pump A to Pump B. But I would not put my PE stamp on an assumption like that.

Why not? If you have an MCC that has 200HP total small motors connected to it, but has the potential to have several 150-200HP motors to be connected to it, what would you do?

 

[weressl]

I remember now - It was the Standard's number that scared every one.

It amuses me that people have no problem ASSUMING that an ignitable atmosphere only exist for a short period of time and that a coincidental electrical fault will not occur, but ASSUME that during the fractions of a minute - at most - transition time a full fault will occur.

 

[rcwilson]

Why not? If you have an MCC that has 200HP total small motors connected to it, but has the potential to have several 150-200HP motors to be connected to it, what would you do?

I could do a calculation for all possible future loads, but normally I wouldn't.

In the OP's case, I would not make an assumption of only some of the motors being on line based on historical load data. I might accept the diversity factor if I knew enough about the loads and how they operated. But I wouldn't stamp it based on just the assumption of diversity.

I agree with your comment about how our engineering logic doesn't seem consistent between hazardous area classification based on probablity and other aspects of our designs like short circuit or step and touch potentials.

 

[jim dungar]

It amuses me that people have no problem ASSUMING that an ignitable atmosphere only exist for a short period of time and that a coincidental electrical fault will not occur, but ASSUME that during the fractions of a minute - at most - transition time a full fault will occur.

Or how some people assume closed transitions will always be completed within a relatively short time frame. I have seen some installations where the manual "normally open" tie device has been closed for many years.

 

[rbalex]

Or how some people assume closed transitions will always be completed within a relatively short time frame. I have seen some installations where the manual "normally open" tie device has been closed for many years.

Every Proposal from the very first one for four cycles specifically excluded manually operated systems. CMP1's rejections were some of the most contrived I've ever seen. Essentially they wanted AHJs, qualified or not, to make the call.

 

[topgone]

A correction: The xfmr is 1.932MVA. I was looking at the wrong one when I posted that.

I have checked the cables. The secondary cable was not even in the model. I added it and it dropped the SC curren but, I still receive a huge amount (10.4kA) from the loads on the board I'm trying to switch. I attached a screen shot of the board so everyone can see what I'm working with.

So you have worked it out then?

 

[kingpb]

Every Proposal from the very first one for four cycles specifically excluded manually operated systems. CMP1's rejections were some of the most contrived I've ever seen. Essentially they wanted AHJs, qualified or not, to make the call.

Follow the money, in this case probably liability. if it's not in writing, then in case of accident someone else can be blamed.

 

[kingpb]

A correction: The xfmr is 1.932MVA. I was looking at the wrong one when I posted that.

I have checked the cables. The secondary cable was not even in the model. I added it and it dropped the SC curren but, I still receive a huge amount (10.4kA) from the loads on the board I'm trying to switch. I attached a screen shot of the board so everyone can see what I'm working with.

Not sure what this image is telling us? I'm still seeing a 1.5MVA transformer. Where's the 1.932MVA?

Make sure you are looking at the reports, and not just the numbers that pop up on the one-line, they can be deceiving depending on which study you are running.

 

[rbalex]

Follow the money, in this case probably liability. if it's not in writing, then in case of accident someone else can be blamed.

Possibly, but it will never be the AHJ or it's reps.