Using the 225% Fusing Exception

[gmohr]

My company is one of several companies doing engineering a large (310 loads) water plant (greenfield). The loads are basically pumps pumping water solutions and HVAC type fans. The motors are IEEE-841 version Design B. The chief "Engineer" for the project is insisting on using a technique to size the (time-delay AJT type) fuses for the low voltage motors that I am not comfortable with. Table 430.52 in the NEC gives a limit of 175% of FLA for the maximum fuse size for time-delay dual element. Exception #1 allows you to jump to the next standard fuse size when the 175% doesn't match a standard fuse size. Exception #2(b) allows you to jump to the next standard size, not exceeding 225% of FLA when the 175% fuse or the fuse permitted in Exception 1 is "not sufficient for the starting current of the motor". The clients chief Engineer is citing a paper by Mersen and saying that we can go ahead and jump to the 225% without trying the start the motors with the smaller fuse. My question is can you use an engineering paper like this to determine that the 175% fuse is "not sufficient for the starting current of the motor"?

Here is the link to the Mersen paper: http://ep-us.mersen.com/fileadmin/c...on-Low-Voltage-Fuses-For-Motor-Protection.pdf
See the Table on P12 and the note underneath the table for "Heavy Load".
Actually the facility is in Canada and would be under the CSA. However, I can't find a good forum on the CSA and I would be interested in hearing how this issue would be approached using the NEC. The rules in the CSA seem very similar if not identical to the NEC on this.
I was considering insisting that they have the AHJ "buy in" on this but didn't know if I was being to "hard line"?

 

[petersonra]

Personally, I would be looking at using circuit breakers.

However, I have no issue with the engineer of record making a written declaration that in his experience these motors in these kind of applications tend to occasionally blow fuses at 175% and thus should use the allowed 225% rating, especially given the nature of the equipment. You really do not want this kind of gear shutting down unless absolutely necessary.

Keep in mind that really the fuse is only protecting against short circuits and ground faults. As long as the main conductors and EGC allows for the fuse to open on a SC or GF, the branch circuit wiring is well protected.

I do not believe the cited exception requires that you only use the exception if you try it with lower rated fuses first.

 

[gmohr]

Surprised by only one Reply

Surprised by only one Reply

Personally, I would be looking at using circuit breakers.

However, I have no issue with the engineer of record making a written declaration that in his experience these motors in these kind of applications tend to occasionally blow fuses at 175% and thus should use the allowed 225% rating, especially given the nature of the equipment. You really do not want this kind of gear shutting down unless absolutely necessary.

Keep in mind that really the fuse is only protecting against short circuits and ground faults. As long as the main conductors and EGC allows for the fuse to open on a SC or GF, the branch circuit wiring is well protected.

I do not believe the cited exception requires that you only use the exception if you try it with lower rated fuses first.

Thanks for the reply PETERSONRA! I was a little surprised that I only had one reply to this question. No disrespect mean't but I was hoping that several Senior members agreed one way or the other so I would have a lot of confidence going a direction on this. No more viewpoints on this? If you agree, please chime in! Thx,

 

[iceworm]

Thanks for the reply PETERSONRA! I was a little surprised that I only had one reply to this question. No disrespect mean't but I was hoping that several Senior members agreed one way or the other so I would have a lot of confidence going a direction on this. No more viewpoints on this? If you agree, please chime in! Thx,

gmo -
Hard to answer. I can't tell what your issue is. Are you concerned the larger fuses are illegal to use? Are you concerned the larger fuses will increases risk - unsafe.

If you are looking for a vote, I don't see that does much good - doesn't tell you if it is a good design decision or not.. And I am reluctant to spend the time to write a disertation - You already have one. I'll do my best to explain why I think what I do and keep it short as I can.

...The loads are basically pumps pumping water solutions and HVAC type fans. The motors are IEEE-841 version Design B. The chief "Engineer" for the project is insisting on using a technique to size the (time-delay AJT type) fuses for the low voltage motors that I am not comfortable with....
The clients chief Engineer is citing a paper by Mersen and saying that we can go ahead and jump to the 225% without trying the start the motors with the smaller fuse. My question is can you use an engineering paper like this to determine that the 175% fuse is "not sufficient for the starting current of the motor"?

Personally, I would be looking at using circuit breakers.

However, I have no issue with the engineer of record making a written declaration that in his experience these motors in these kind of applications tend to occasionally blow fuses at 175% and thus should use the allowed 225% rating,

Keep in mind that really the fuse is only protecting against short circuits and ground faults.

I do not believe the cited exception requires that you only use the exception if you try it with lower rated fuses first.

Adding to perersonra comments:

1. I wouldn't need a paper. If the chief engineer says it's okay, and I can't point to a reason why it is unsafe, then it is okay. Unless you have definite evidence the chief engineer is not knowledgable. If you are also not knowledgable - you need to find someone that is. And that is not us. Free Internet Engineering by MH Forum is worth what you paid for it.

2. An industry norm is to use a listed combination starter with a mag only cb. This is 99.99% of what I have worked/specified. My practice is to set the mag-only right up to the NEC max, 1100%, 1300%, 1700%. And since most all the motors are 1.15sf, I set the overloads at 140% (except for the few 1.0sf) I've never had an AHJ tell me this was wrong or a poor design decision. I would do the same if fuses.

... I was considering insisting that they have the AHJ "buy in" ...

I am not a fan of AHJ "buy-in" - for several reasons:
First:
What are you going to ask? "I want to do this, but I want you to tell me it is okay first." What would you expect her to say? "You think it is shakey or you wouldn't ask. And you want me to take responsibility for your design decisions?" That is not a conversation I want to have.

Second:
The crew has had plenty of time to research, ponder, decide. If the AHJ is asked, then she has to spend an equal amount of time to come up to speed to give you a good answer. Where is this time coming from? - they are busy

Third:
You are asking a person, proficient in inspecting to a minimum standard and asking them to do engineering work. Possibly they have the knowledge and training - but they have thier own jobs to do. Why would you ask them to do your engineering work?

None of these work for me.

(way too long - edit to cut into two pieces)

ice

 

[iceworm]

Now lets look at the issue of increased risk.
As noted, the fuses don't protect the motor. 175% fuses don't limit the available SSC any more than 225% fuses do. If the fuses open, you don't have a motor to protect - it's toast. The fuses are in the circuit to protect the conductors. Unless the fuse/cable installation is odd, the cable damage curve is outside of the fuse trip curve. If you are concerned, plot out the fuse trip curve and the cable damage curve and verify the cable is protected.

Additionally, the fuses protect the structure from a fire caused by a backhoe/forklift attack into the cable. 175% fuses is no better at protecting the structure from a fault than 225% fuses are.

I wouldn't count on a document that is self professed to be a minimum standard to be useful in designing a safe, robust, reliable, maintenance friendly system. In short, if one were to build to NEC minimums, the installation is right next to a piece of dung.

As for illegal, petersonra already covered this.

Added extra - not asked:
I don't know why those sections are in the code. Essentially it is saying, "here is the max, unless it needs to be bigger, then more is okay." ???? Good pick for moron-code-section-of-the-month. just an opinion.
Applies to the section on overload selection as well.

My recomendation would be a code section that says, "If you aren't going to bother to design/verify/document system safety, reliability - then here is the max you can use. If you are going to design/verify, then use what you want." That's what happens now.

I've had a few where I set the mag-only way up past 1700% - as I recall, one Design A (small, 1 hp maybe) had to be 40X. NEC is silent on how to handle (not exactly true - but close enough for this example) And if you asked the AHJ the only thing they could say is, "NO". As it was, filed my letter explaining why, AHJ was silent.

... I was considering insisting that they have the AHJ "buy in" on this but didn't know if I was being to "hard line"?

Are you "hard line"?
Don't know - depends on your company culture. Chief engineer is telling you what to do and explaining why she thinks this. That part sounds reasonable. This "insisting" sounds like you are going to call the cops if they don't do it your way. Oooookaaaay.

Generally speaking I feel obligated to tell management when I think they are illegal or unsafe and why I think this - I stick to areas in my expertise and really stick to documented physics. After that, I can either pitch in and help or get out of the way - unless I am really knowledgable and really think it is unsafe. Hasn't happened yet - they either listen or convince me it is okay. But if it did ... well, like I said - hasn't happened yet.

ice