Overcurrent Protection Assumption

[Wire-Smith]

I think what you are missing is how overcurrent protection is looked at,

something to note, 240.4 lets you round up a size for most applications.

another thing to remember is that many overload protection devices are thermal based, so your ocpd could be outdoors in the winter in northern Minnesota while your circuit is inside a foundry.

it's not as precise as other things we deal with, and if used correctly doesn't need to be. usually when there is a problem the overcurrent is not just a little above the circuits rated current, it goes a lot higher. if it is just a little above, there is usually some play in the material/equipment limits and if not everything should still fail safe whether it is a melting wire shorting to a conduit or equipment melting down, the NEC is more of just a minimum fail safe based standard, not a fail without damage.

look at the conductor tables, i'm not sure if you are concerned with protecting equipment or conductors more but look at the tables, they are based on an ambient temperature, the circuits should never have been designed to be expected to be exposed to ambient temperature above what was used for design.

so now consider if your ambient is lower than that then you can run your current above that ambient temp rating, now consider the likelihood of the overcurrent situation happening during the times of the higher ambient temp, some situations it will be very likely some not.

do you really want a code book with all that would be required for what you are asking? i don't. we slightly overbuild in some situations to simplify the design phase, the code gives provisions to allow under engineer supervision to use a calculation to value engineer a conductor size if you don't want to use the dumb tables and corrections like the rest of us

its a dumbed down code yeah, but 80% of the industry still can't apply it properly, so........


there is a listing standard for a ocpd to be able to be rated at a ocpd rating it has to be within a trip curve, so you would have to check the manufacturers trip curves but i would guess the device being looked at that the slowest curve is within the allowable curve for the devices ocpd rating

and i don't have my codebook with me but i think it is over 1KA that a relay engineer has to decide on the curves

 

[xptpcrewx]

Where do I begin...

First let me say, if you have attempted to answer my OP, thank you. I appreciate it even if it may seem like I disagree with just about every point made. I am just looking for a specific/suitable answer.

So far there is just too much misinformation, conjecture, selective responses and opinions for this thread to really be useful to me or, as I imagine, for anyone else to follow logically/chronologically. I believe the question is simple and straight forward, but so far is sounds like either:

1. No one really understands what is fundamentally being asked; or
2. They understand, but are deliberately choosing to answer a different question altogether

Perhaps I will try to ask/articulate the question differently (in an upcoming post).

All I ask is to please just stay on topic and know that the answer will be in the form of a standard and/or reference (maybe even with some history attached), so please spare me and this forum of personal stories, irrelevant experiences, conjecture and drawn-out responses... They are not necessary or insightful. Be direct and thorough.

Note: If you are an engineer or PE, and you want to chime-in, I will have a higher standard for the quality/completeness of your response. Otherwise, without data you’re just another person with an (unwanted) opinion – xptpcrewx

Well, perhaps the NEC does define the requirements enough. Consider that if the application is a standard application, then the SQD FA generic T-M curve is fine. It is a $300 CB. However, if the application is one that requires careful coordination, then an electronic LSI is a great choice. That is a $3000 CB.

Stop thinking of LSIG only as a trip-unit feature!

I got no idea what this means

It means there is no data to support your claim that code minimum requirements will keep anything from catching fire.

I will call you primary injection test set. I can remember three, maybe up to 3000A. The biggest one was a 3ph, 480V, 15A, cord and plug. Power circuit breaker - really? Maybe you have seen bigger ones than I have.

480-V Single Phase, 350A input.

I think what you are missing is how overcurrent protection is looked at,

something to note, 240.4 lets you round up a size for most applications.

another thing to remember is that many overload protection devices are thermal based, so your ocpd could be outdoors in the winter in northern Minnesota while your circuit is inside a foundry.

it's not as precise as other things we deal with, and if used correctly doesn't need to be. usually when there is a problem the overcurrent is not just a little above the circuits rated current, it goes a lot higher. if it is just a little above, there is usually some play in the material/equipment limits and if not everything should still fail safe whether it is a melting wire shorting to a conduit or equipment melting down, the NEC is more of just a minimum fail safe based standard, not a fail without damage.

look at the conductor tables, i'm not sure if you are concerned with protecting equipment or conductors more but look at the tables, they are based on an ambient temperature, the circuits should never have been designed to be expected to be exposed to ambient temperature above what was used for design.

so now consider if your ambient is lower than that then you can run your current above that ambient temp rating, now consider the likelihood of the overcurrent situation happening during the times of the higher ambient temp, some situations it will be very likely some not.

do you really want a code book with all that would be required for what you are asking? i don't. we slightly overbuild in some situations to simplify the design phase, the code gives provisions to allow under engineer supervision to use a calculation to value engineer a conductor size if you don't want to use the dumb tables and corrections like the rest of us

its a dumbed down code yeah, but 80% of the industry still can't apply it properly, so........


there is a listing standard for a ocpd to be able to be rated at a ocpd rating it has to be within a trip curve, so you would have to check the manufacturers trip curves but i would guess the device being looked at that the slowest curve is within the allowable curve for the devices ocpd rating

and i don't have my codebook with me but i think it is over 1KA that a relay engineer has to decide on the curves

All of this is irrelevant...

 

[Wire-Smith]

you probably already know, but i think what you are specifically looking for is the listing standard. try ansi, UL, etc. i thought you were asking more about the code side of things.

 

[xptpcrewx]

you probably already know, but i think what you are specifically looking for is the listing standard. try ansi, UL, etc. i thought you were asking more about the code side of things.

Wire-Smith I am definitely not looking for the listing standard because I’m not talking about products here...

Yes, I am asking about about the “code side of things”. Refer to post #1 and follow-up post #5.


Sent from my iPhone using Tapatalk

 

[Wire-Smith]

You appear to know quite a lot about this subject, please don't take my next statement as me thinking otherwise. You may want to read NEC 110.1 through 110.3. Maybe you already know these sections by heart but the way you are looking at all of this just seams off to me. I'm just trying to help, not that i'm sure i can but no one else seems to want to talk to you. I just think you are looking at the NEC with some assumptions probably bred from reading tons of IEEE standards before you really dug into the NEC. Just a suggestion though, if you don't find your answer anywhere else i think the listing standard might clear it up for you.

 

[xptpcrewx]

You appear to know quite a lot about this subject, please don't take my next statement as me thinking otherwise. You may want to read NEC 110.1 through 110.3. Maybe you already know these sections by heart but the way you are looking at all of this just seams off to me. I'm just trying to help, not that i'm sure i can but no one else seems to want to talk to you. I just think you are looking at the NEC with some assumptions probably bred from reading tons of IEEE standards before you really dug into the NEC. Just a suggestion though, if you don't find your answer anywhere else i think the listing standard might clear it up for you.

Wire-Smith, I am definitely not a code guru by any means. I did take a look at 110.1 through 110.3 again but I’m having difficulty seeing the connection with my question. I will say I definitely have some assumptions (and so does the NEC it seems) and yes they are based on engineering standards (some IEEE) and yes, it’s before I dug deep into the NEC.

 

[xptpcrewx]

Just a suggestion though, if you don't find your answer anywhere else i think the listing standard might clear it up for you.

As for listing standards, I’m open for suggestions. Just remember I’m not talking about a specific product or device. Many people assume that LSIG refers to trip-unit adjustable features and therefore automatically think of products. LSIG are unique curve parameters/characteristics that protection & control engineers use to detect overcurrent (overload, short-circuit, and ground-fault).

 

[xptpcrewx]

Overcurrent Protection Assumption

This might help...

The general idea is:
LT is for overload (inverse-time characteristic)
ST is for locked-rotor and short-circuits (more aggressive inverse-time characteristic than LT)
I is for severe short-circuits (definite-time or no international delay characteristic)
G characteristic is for ground-faults (low pick-up, inverse-time characteristic)

An example of where a thermal-magnetic breaker would fit into this is:

LT characteristic - Thermal
Instantaneous characteristic - Magnetic

An example of where a dual-element fuse would fit into this is:

LT characteristic - Overload element
ST characteristic - Short-Circuit element

 

[iceworm]

... Stop thinking of LSIG only as a trip-unit feature! ......

No
It would be rare that it wouldn't be

Originally Posted by Xptpcrewx ..... Consider there is no correlation with setting protection to the minimum dial and preventing fire.

I got no idea what this means

... It means there is no data to support your claim that code minimum requirements will keep anything from catching fire. ...

I certainly don't have any data at my fingertips.
You certainly don't want to have any.
I'm going to be surprised if the code panels don't think that they do.

 

[jaggedben]

I have zero technical expertise on the subject, but I have enough knowledge about how the NEC gets made to wager that you won't find what you're looking for.

Questions that begin with "Why doesn't the NEC just simply state..." can only be solidly answered by referring to the revision history or by talking to someone who was on the code making panel when the relevant language was adopted. I doubt these will be fruitful avenues for answering your question, since it's quite technically narrow and I'd wager that for the latter option they're all dead. It's possible that the phrase "over the full range of overcurrent" was thought up many decades ago by someone less expert than you, perhaps even before all the relevant technical concepts were developed. This is why iceworm compared it to the decades old and arbitrary 25ohm requirement for grounding electrodes.

I think Wire-Smith referred you to 110.1-110.3 as a way of pointing out that, where the code lacks the relevant specificity, it's entirely up to an AHJ to decide what is 'approved.'

 

[xptpcrewx]

No
It would be rare that it wouldn't be

Rare like the 5 branch-circuit examples?
I just provided two simple examples: dual-element fuses and thermal-magnetic breakers. These are certainly not rare and do not use trip-units nor do “many” other single/multi function protection elements in existence. LSIG doesn’t have to be an adjustable trip-unit feature. Again it’s a curve characteristic/parameter... for protective elements.

I certainly don't have any data at my fingertips.

The irony of your signature...

You certainly don't want to have any.

Quite the contrary.

I'm going to be surprised if the code panels don't think that they do.

So will I. Until then I am giving them and the forum the benefit of the doubt to provide a reasonable answer.

 

[Wire-Smith]

I'll try stating things a little differently.
The NEC refers to overcurrent protection, short circuit protection and ground fault protection because that is what you are protecting against, they are not specifying how to protect against them(in terms of trip curves anyway). The listing standards would have the criteria to do that.
Does that help at all?
And that's why I referenced what I have, NEC requires some things to meet listing requirements.

Any of the three st, lt, or I could protect any and all three of those problems, I'm confident but assuming the listing standards have maximum allowable curves for the three problems. There are other people on here that I know would better explain this but I think you have come off a bit abrasive for them to care, I will try to help if I can.

 

[xptpcrewx]

Overcurrent Protection Assumption

I have zero technical expertise on the subject, but I have enough knowledge about how the NEC gets made to wager that you won't find what you're looking for.

Questions that begin with "Why doesn't the NEC just simply state..." can only be solidly answered by referring to the revision history or by talking to someone who was on the code making panel when the relevant language was adopted. I doubt these will be fruitful avenues for answering your question, since it's quite technically narrow and I'd wager that for the latter option they're all dead. It's possible that the phrase "over the full range of overcurrent" was thought up many decades ago by someone less expert than you, perhaps even before all the relevant technical concepts were developed.

Thanks. This is the best point made thus far. I will say however that the technical concepts of LSIG were probably developed long before the code language/phrase was thought up and added.

 

[Wire-Smith]

It may be the "over full range" part that may be tripping you up, I don't see how this could mean anything other than the device automatically opening in that range, if it didn't say that you could have a 50 amp breaker that would be able to just trip in the range of say 50 to 1k but still survive 10ka sc, just not trip

 

[xptpcrewx]

Overcurrent Protection Assumption

I'll try stating things a little differently.
The NEC refers to overcurrent protection, short circuit protection and ground fault protection because that is what you are protecting against, they are not specifying how to protect against them(in terms of trip curves anyway). The listing standards would have the criteria to do that.
Does that help at all?

Yes! You put things into perspective. Thanks.

And that's why I referenced what I have, NEC requires some things to meet listing requirements.

Some things?

Any of the three st, lt, or I could protect any and all three of those problems, I'm confident but assuming the listing standards have maximum allowable curves for the three problems.

This seems fundamentally wrong. Do you have some examples where this is the case?

There are other people on here that I know would better explain this but I think you have come off a bit abrasive for them to care, I will try to help if I can.

I apologize if I have offended anyone. I’m not intentionally trying to be abrasive as much as I am trying to sift through all the noise.

 

[Wire-Smith]

Yes! You put things into perspective. Thanks.

Some things?
not everything is required to be listed
This seems fundamentally wrong. Do you have some examples where this is the case?look at the three things we are talking about protecting against, they are problems arising in the electrical system, would either of those three characteristics open the protective device during any of the three types of faults? Yes, they would just take different amounts of time to open

I apologize if I have offended anyone. I’m not intentionally trying to be abrasive as much as I am trying to sift through all the noise.

See responses above in red

 

[xptpcrewx]

It may be the "over full range" part that may be tripping you up, I don't see how this could mean anything other than the device automatically opening in that range, if it didn't say that you could have a 50 amp breaker that would be able to just trip in the range of say 50 to 1k but still survive 10ka sc, just not trip

The way I interpret “over the full range” is based on the nature of load variations and it’s failure modes (its entirely circuit/load specific). For example, there’s no reason to protect against overloads when dealing with lighting, so overloading is not part of its full range. Short-circuits and ground-faults are however which requires protection in the range of what a short-circuit or ground-fault would look like for that circuit.

 

[Wire-Smith]

The way I interpret “over the full range” is based on the nature of load variations and it’s failure modes (its entirely circuit/load specific). For example, there’s no reason to protect against overloads when dealing with lighting, so overloading is not part of its full range. Short-circuits and ground-faults are however which requires protection in the range of what a short-circuit or ground-fault would look like for that circuit.

I believe your interpretation is wrong, although I'm not thinking of a good argument against it.
I don't know of where the NEC discusses the different faults current characteristic's. Nor can I think of where that would be needed.

Let me say this, I don't know if it will help.
When you adjust an instantaneous setting to say 8x plug, you are not providing code compliant overload protection with that instantaneous protection( if plug is max allowable size for circuit anyway). To provide required overload protection with the instantaneous protection it could only be x1 or less.

 

[xptpcrewx]

Not everything is required to be listed

Are there instances where the NEC requires overcurrent protection but doesn’t require the devices to be listed?

Yes, they would just take different amounts of time to open

There are instances where some of the three characteristics would actually fail to open the protective device for faults. Think about long distribution lines, high impedance faults, and systems with lower ground fault current than the load such as impedance grounded and ungrounded systems.





Sent from my iPhone using Tapatalk

 

[xptpcrewx]

I believe your interpretation is wrong, although I'm not thinking of a good argument against it.
I don't know of where the NEC discusses the different faults current characteristic's. Nor can I think of where that would be needed.

This is a start, but if I am wrong I will need a good argument against it and a substitute idea to take its place... This interpretation that I have is just basic protection and controls philosophy.

Let me say this, I don't know if it will help.
When you adjust an instantaneous setting to say 8x plug, you are not providing code compliant overload protection with that instantaneous protection( if plug is max allowable size for circuit anyway). To provide required overload protection with the instantaneous protection it could only be x1 or less.

Agreed but this approach isn’t allowed for motor protection.