Questions regarding question on elevator max available fault current on primary and secondary side of fused disconnect??

Status
Not open for further replies.
J

JohnnyPElec

Member
Location
Portsmouth, VA
Occupation
EE
Hey all,
I just started a job this month at a company here in the US and I've encountered a question on a project I got from my predecessor that has me stumped. A client on a project in CA is saying that the elevator manufacturer is requesting several things regarding the elevator electrical system. They want labels for the elevator displaying the max available fault current at both the primary and secondary sides of the elevator's disconnect switch. I haven't been able to locate this in any of the submittals left by the guy who was here before me, and it seems like he was dodging the requests for several months. I'm really not that familiar with short circuit calculations and even how to go about doing them in a competent manner and I'm a week into research and coming up with nothing. The client also wants the short circuit rating of the building the elevator is going in.

Has anyone encountered a request like this before? Are there resources for this kind of thing that could aid in me finding the information the client is requesting? And around what year in terms of my engineering career should I be familiar with short circuit characteristics? (~2 years of experience currently).
 
augie47 said:
There are a number of guidelines on line.'
Here is a decent one to start


Step 1 normally would be to obtain an available fault current from your utility
Click to expand...
They have a free app that can do the calculations, but as you said, you need a starting point and that is typically from the utility.
The app is FC² and is available for both Apple and Android.
 
I am confused by the term 'primary
JohnnyPElec said:
They want labels for the elevator displaying the max available fault current at both the primary and secondary sides of the elevator's disconnect switch.
Click to expand...
This doesn't make sense. A switch does not have a primary and a secondary side.

Is there a transformer involved?
Do they want to know the fault on the line side lugs of the switch and then on the load side of any fuses installed in that switch?
 
jim dungar said:
I am confused by the term 'primary

This doesn't make sense. A switch does not have a primary and a secondary side.

Is there a transformer involved?
Do they want to know the fault on the line side lugs of the switch and then on the load side of any fuses installed in that switch?
Click to expand...
Yes, "primary" is a bit confusing, but I also took the term to mean what you have, in that by "primary" they mean the fault current at the line side and by "secondary" they are looking for the fault current at the load side of the fuses

They also asked for the info and supporting data on the model of the fuses being used so the context supports the above assumption
 
JohnnyPElec said:
They also asked for the info and supporting data on the model of the fuses being used so the context supports the above assumption
Click to expand...
The fault current on the load side of a branch circuit style protective device will be the same as the line side, except in the rare situation where the device is in its current limiting range. If this is the case the reduced value is pretty much useless unless the downstream equipment is rated or listed in combination with the protective device (research series-ratings for more information).

I would not trust any so-called power engineer that refers to a switch as having primary and secondary sides.
 
jim dungar said:
The fault current on the load side of a branch circuit style protective device will be the same as the line side, except in the rare situation where the device is in its current limiting range. If this is the case the reduced value is pretty much useless unless the downstream equipment is rated or listed in combination with the protective device (research series-ratings for more information).

I would not trust any so-called power engineer that refers to a switch as having primary and secondary sides.
Click to expand...

First the OP said the manufacturer not the engineer. Second the IP said the primary and secondary side of the disconnect switch, which in my experience always has overcurrent protection. The purpose of the question is likely that the SCCR rating of the elevator controller is 5000. In Florida the code requires the disconnect be marked with the fault current and that it be below 5000. I expect the OP has a similar situation.
 
JohnnyPElec said:
Hey all,
I just started a job this month at a company here in the US and I've encountered a question on a project I got from my predecessor that has me stumped. A client on a project in CA is saying that the elevator manufacturer is requesting several things regarding the elevator electrical system. They want labels for the elevator displaying the max available fault current at both the primary and secondary sides of the elevator's disconnect switch. I haven't been able to locate this in any of the submittals left by the guy who was here before me, and it seems like he was dodging the requests for several months. I'm really not that familiar with short circuit calculations and even how to go about doing them in a competent manner and I'm a week into research and coming up with nothing. The client also wants the short circuit rating of the building the elevator is going in.

Has anyone encountered a request like this before? Are there resources for this kind of thing that could aid in me finding the information the client is requesting? And around what year in terms of my engineering career should I be familiar with short circuit characteristics? (~2 years of experience currently).
Click to expand...
I would try to claim that the primary fault current is the design engineers responsibility. The secondary is available from the fuses manufacturer. You will use the RMS value not the peak. '
 
jim dungar said:
I am confused by the term 'primary

This doesn't make sense. A switch does not have a primary and a secondary side.

Is there a transformer involved?
Do they want to know the fault on the line side lugs of the switch and then on the load side of any fuses installed in that switch?
Click to expand...

Yes I was going to say this as well. Primary and secondary maximum available fault current refers to the transformer.

The NEC code requires service equipment other than those for dwellings to be labeled with “maximum available fault current,” computed fault current value (secondary) and The Date it was calculated.

I posted a question to this last year before I had to come up with something for an industrial load center.

What I could find was the formula for maximum available fault current for final transformer to service prior to first point of disconnect, which is if I remember?:

Transformer KVA rating on nameplate divided by secondary voltage (for single phase)

Or KVA divided by secondary voltage x 1.732 (square root of 3) for 3 phase

Then divide total by transformer name plate percent of impedance (3% would = 0.03 in calculation don’t input just 3)

This would yield max fault current at transformer but max current would actually be less depending on distance from transformer to breaker panel and secondary conductor size but not much typically. This is because of secondary service/ feeder conductor resistance. There is more calculations to this but I wish someone could show me this part.

However the calculation I gave provided the worst case scenario for selecting equipment sizes based on maximum Available short circuit Current at the transformer and will at least give you a baseline of worst case fault current.


The primary is done the same way just substitute voltage Into the equation
 
Strathead said:
First the OP said the manufacturer not the engineer. Second the IP said the primary and secondary side of the disconnect switch, which in my experience always has overcurrent protection. The purpose of the question is likely that the SCCR rating of the elevator controller is 5000. In Florida the code requires the disconnect be marked with the fault current and that it be below 5000. I expect the OP has a similar situation.
Click to expand...
Makes sense, but at very least OP should only need to find supply side available fault current, fuse type and the design engineer should determine whether that will work with his equipment. As Jim mentioned the downstream equipment needs to be evaluated as to whether it is acceptable to be used in conjunction with any current limiting feature of the fuse.
 
kwired said:
Makes sense, but at very least OP should only need to find supply side available fault current, fuse type and the design engineer should determine whether that will work with his equipment. As Jim mentioned the downstream equipment needs to be evaluated as to whether it is acceptable to be used in conjunction with any current limiting feature of the fuse.
Click to expand...
In both California and Florida, (the 2 places I have worked) the elevator inspectors have been all powerful and arbitrary. Recently in Florida, I had an elevator. The new code requires it be marked as stated by the OP. Yhe manufacturer required the elevator to be protected by J type fuses, which reduce 12000 AFC to about 3000 AFC. I assume this is part of the reason the fuses were required.

On a related note, the NEC requires equipment to be rated for the AFC, but we rarely even think about this. I came to the conclusion some engineers actually know this and that is why they just spec out fused disconnects even though they cost more.
 
Strathead said:
In both California and Florida, (the 2 places I have worked) the elevator inspectors have been all powerful and arbitrary. Recently in Florida, I had an elevator. The new code requires it be marked as stated by the OP. Yhe manufacturer required the elevator to be protected by J type fuses, which reduce 12000 AFC to about 3000 AFC. I assume this is part of the reason the fuses were required.

On a related note, the NEC requires equipment to be rated for the AFC, but we rarely even think about this. I came to the conclusion some engineers actually know this and that is why they just spec out fused disconnects even though they cost more.
Click to expand...
Which they probably have series tested their equipment with that fuse type.

You can't just put in a type J fuse and then say it now has 3000 AFC as a general rule.

Shouldn't really need any specialty fuse if the AFC were already 5000 or less. Some maybe don't look into such details (laziness or something) and figure you should use those specialty fuses in all cases.
 
jim dungar said:
The fault current on the load side of a branch circuit style protective device will be the same as the line side, except in the rare situation where the device is in its current limiting range. If this is the case the reduced value is pretty much useless unless the downstream equipment is rated or listed in combination with the protective device (research series-ratings for more information).

I would not trust any so-called power engineer that refers to a switch as having primary and secondary sides.
Click to expand...
Exactly. The entire premise that there is a difference on either side of a fuse is incorrect from the standpoint of it being useful information in the field.

IF the equipment for the elevator has a “series listing” at a higher AFC value downstream of fuses, the equipment supplier would have needed to document that up front AND it would be done with SPECIFIC fuses that would be part of THEIR documentation. This cannot be done willy-nilly in the field. The referenced documents from Eaton and such are for use by whomever is doing that series listing procedure, NOT for contractors in the field. This concept gets misused a lot, but that doesn’t make it correct.

Ergo, the correct solution for this issue is that the AFC should be shown as the same on either side, in the absense of any other documentation from the equipment supplier. And if they have that information, it would be in the form of THEIR “SCCR” listing (Short Circuit Current Rating), not a “difference” in the AFC. The AFC itself does not change (unless there is a transformer in between).
 
Last edited:
kwired said:
Which they probably have series tested their equipment with that fuse type.

You can't just put in a type J fuse and then say it now has 3000 AFC as a general rule.

Shouldn't really need any specialty fuse if the AFC were already 5000 or less. Some maybe don't look into such details (laziness or something) and figure you should use those specialty fuses in all cases.
Click to expand...
My education is sketchy, but I was lead to believe by the PE who did the job’s coordination study that this says otherwise. It has nothing to do with the equipment. As you can read from the first paragraph, the fuse will let through x amount of current before tripping. That limits the rating to below the sccr of the equipment. https://www.eaton.com/content/dam/e...through-charts-current-limitations-charts.pdf
 
Strathead said:
My education is sketchy, but I was lead to believe by the PE who did the job’s coordination study that this says otherwise. It has nothing to do with the equipment. As you can read from the first paragraph, the fuse will let through x amount of current before tripping. That limits the rating to below the sccr of the equipment. https://www.eaton.com/content/dam/e...through-charts-current-limitations-charts.pdf
Click to expand...
Again, you can use let-through curves from current limiting fuses in the process of applying for an SCCR on a piece of equipment to a listing agency (such as UL), or a registered PE can use them in a coordination study that they are going to sign off on. A contractor in the field cannot use that method alone, nor can an equipment manufacturer rely on that to get around their lack of an SCCR (or the “courtesy” untested level of 5kA which is all but useless in real life). So what that let-through curve says is irrelevant to this situation.
 
And even using the let thru curves as allowed by UL508a, you don't gain as much as you might think. It is hard to get anywhere useful with fuses above 30 or 60 A.
 
Status
Not open for further replies.
Top