Overcurrent Protection Assumption

[xptpcrewx]

Whenever the code requires overcurrent protection, is it assumed that these devices must have long-time (LT), short-time (ST) and instantaneous time-current characteristics?

I would think so since the definition for Overcurrent Protective Device, Branch-Circuit in Article 100 mentions the capability for providing protection over the full range of overcurrent between the OCPD's rated current and interrupting rating; but I cant find anything linking LT, ST or I characteristics to the terminology the NEC uses, i.e. Overload, Short-Circuit, Ground-Fault.

Why doesn't the NEC just simply state when LT, ST, I protection is required instead of stating short-circuit protection is required?
Note: The term short-circuit is ambiguous because it could be a combination of ST, I and/or GF time-current characteristics, leaving things open to interpretation...

Thanks in advance.

 

[ron]

Can you provide some context to your question?

There are times when an OCPD will not have instantaneous protection in order to provide selectivity with downstream OCPDs and thus that downstream equipment will need to be able to withstand 30 cycles of fault current.

There are times when OCPD will have GF and some times it will not depending on the application.

Sometimes the LT and ST will be adjustable and sometimes fixed.

It depends.

The definition for Overcurrent Protective Device, Branch-Circuit in Article 100 ​is a good start.

 

[Jraef]

LT, ST and I are terms used only when describing adjustable circuit breakers with electronic trip units (ETUs). Most "plain old" circuit breakers are just Thermal-Magnetic, not electronic. Thermal is the Over Current protection, Magnetic is the Short Circuit protection.

For the ETUs, the NEC concept of "Over Current" is only the LT setting, "Short Circuit" is I setting. The NEC does not call for (or bother with) ST, that is a strictly voluntary setting on the part of the person willing to pay extra for an ETU.

 

[xptpcrewx]

Response

Response

LT, ST and I are terms used only when describing adjustable circuit breakers with electronic trip units (ETUs). Most "plain old" circuit breakers are just Thermal-Magnetic, not electronic. Thermal is the Over Current protection, Magnetic is the Short Circuit protection.

Thanks for your response but I disagree. LT, ST, and I are regions/characteristics of the time-current curve an not necessarily a feature associated with overcurrent protection.

Consider that LT, ST and I are not limited to adjustable trip circuit breakers. Sometimes these parameters are fixed and non-adjustable. For example, it is common to have a fixed shot-time and instantaneous portion of the curve in most applications.

For the ETUs, the NEC concept of "Over Current" is only the LT setting, "Short Circuit" is I setting. The NEC does not call for (or bother with) ST, that is a strictly voluntary setting on the part of the person willing to pay extra for an ETU.

Do you have any reference(s) you can provide to back this statement? Thanks in advance.

 

[xptpcrewx]

Response

Response

Can you provide some context to your question?

Not sure what more I can add... I want to understand what protection the NEC considers to be adequate when requiring "Overcurrent Protection"; i.e. LT. ST, I, all of these parameters, certain combinations of these parameters, etc.

Also, The NEC is specific about when GF protection is required but does it say how we may achieve this? For example can I just use short-time and instantaneous if this method is capable of detecting all ground-faults for a particular installation?

There are times when an OCPD will not have instantaneous protection in order to provide selectivity with downstream OCPDs and thus that downstream equipment will need to be able to withstand 30 cycles of fault current.

Sounds to me like this is an example of something that wouldn't count as an "overcurrent protective device" since it does not cover the whole range of overcurrent as defined in Article 100 (Overcurrent Protective Device, Branch-Circuit).

 

[ron]

Sounds to me like this is an example of something that wouldn't count as an "overcurrent protective device" since it does not cover the whole range of overcurrent as defined in Article 100 (Overcurrent Protective Device, Branch-Circuit).

For sure no branch circuit OCPD will have an option for you to disable the instantaneous. Nor will there be 30 cycle rated final use equipment that would be fed from a branch circuit. These are for feeders.

Power circuit breakers (associated with UL 1558 Switchgear) have the ability to use trip units that can disable the instantaneous characteristic. You will not be feeding a branch circuit from one of those breakers, especially not one that has a 30 cycle rating.

 

[xptpcrewx]

For sure no branch circuit OCPD will have an option for you to disable the instantaneous. Nor will there be 30 cycle rated final use equipment that would be fed from a branch circuit. These are for feeders.

Power circuit breakers (associated with UL 1558 Switchgear) have the ability to use trip units that can disable the instantaneous characteristic. You will not be feeding a branch circuit from one of those breakers, especially not one that has a 30 cycle rating.

I have definitely seen power circuit breakers used for branch circuits in industrial applications, where instantaneous is capable of being disabled. At any rate, this doesn't really answer my question...

 

[iceworm]

Not answered in the order you asked:

...; but I cant find anything linking LT, ST or I characteristics to the terminology the NEC uses, i.e. Overload, Short-Circuit, Ground-Fault.

Why doesn't the NEC just simply state when LT, ST, I protection is required instead of stating short-circuit protection is required? ....

The NEC is not a design guide. LSI, LSIG may not fit the requirements.

Whenever the code requires overcurrent protection, is it assumed that these devices must have long-time (LT), short-time (ST) and instantaneous time-current characteristics? ....

Nope. That assumption is nowhere in the code and shouldn't be. Consider there are trip curves other than LSI, LSIG. For example:

I^2t
I^4t
IEEE moderately inverse
IEEE Very inverse
IEEE Extremely Inverse
And a whole bunch of IEC curves​

As jraef and ron discussed, ordinary molded case are thermal-magnetic. The thermal part is I^2t (I may not be exactly right, but it is close) with a mag trip (instantaneous) - both fixed. If you are working on Grandpa's cottage by the sea - that is just fine. The only thing the NEC cares about is if the number on the handle meets number specified by 240 (or 430, or 450 or ...)

However, Grandma's A1 Abrams factory may require different trip curves. For example LSI fits well with starting large motors. And I think (that's code meaning I don't know exactly) some of the IEEE curves match fuse characteristics - which wouldn't matter much for low voltage, mostly medium voltage.
The NEC is not going to care that you selected a screwie curve that bent around a transformer damage curve and slid inside of the cable damage curve and still would hold long enough to get past the inrush. Again, the NEC just wants to see the number on the dial meets the minimums.


So, why doesn't the NEC tell you ? Look at 90.1.A and B. The code just sets the minimums that will keep it from catching fire - maybe. It's up to you to specify equipment/settings that meets the owners requirements.

the worm

 

[iceworm]

I have definitely seen power circuit breakers used for branch circuits in industrial applications, where instantaneous is capable of being disabled. At any rate, this doesn't really answer my question...

"Branch" means the last CB in-line. The only place I have see that is when used as motor starter. I don't know why one would disable the instantaneous for this application. That strikes me as an arc-flash disaster waiting to happen.

 

[xptpcrewx]

"Branch" means the last CB in-line. The only place I have see that is when used as motor starter. I don't know why one would disable the instantaneous for this application. That strikes me as an arc-flash disaster waiting to happen.

I don't disagree that disabling instantaneous is a bad idea. But this is besides the point. There are many applications where the last circuit breaker in line has adjustable parameters via trip unit including LT, ST, I and GF.

 

[iceworm]

.... There are many applications where the last circuit breaker in line has adjustable parameters via trip unit including LT, ST, I and GF.

Google search on "many":

A couple means two. A handful is five.
Anything more than a couple but less than a handful is a few.
Some is more than a handful but less than a dozen.
Several is more than a dozen but less than a score, which is 20.
Many is any number more than 20.
What is the upper limit? Well, according to the Bible, “Many are called, but few are chosen.” Your guess is as good as mine.​

Educate me - And I'll give you a break - just give us five (other than a large motor starter)

 

[xptpcrewx]

The NEC is not a design guide. LSI, LSIG may not fit the requirements.

I understand the NEC is not a design guide, but it needs to define the installation requirements. Note: I'm not talking about optimizing protection here...
LSIG DOES cover the full range of overcurrent as it relates to the NEC (when set correctly) - namely because overload, short-circuit and ground fault are the failure modes LSIG is intended to protect against.

Nope. That assumption is nowhere in the code and shouldn't be. Consider there are trip curves other than LSI, LSIG. For example:

I^2t
I^4t
IEEE moderately inverse
IEEE Very inverse
IEEE Extremely Inverse
And a whole bunch of IEC curves​

I disagree. LSIG are curve characteristics specific to the failure modes described above.
I^2t and I^4t are curves that can either apply to LT or ST and really only describe the "shape" of LT or ST.
As for the rest, you are describing conventional variations of LT protection available in the industry. (Again, these are just "shapes" mostly/historically defined by electro-mechanical relay characteristics)

As jraef and ron discussed, ordinary molded case are thermal-magnetic. The thermal part is I^2t (I may not be exactly right, but it is close) with a mag trip (instantaneous) - both fixed.

My guess is not exactly right, but thermal is a form of LT protection and mag can sometimes be ST and I simultaneously.

Again, the NEC just wants to see the number on the dial meets the minimums.

What you are describing here is the LT pickup setting.

So, why doesn't the NEC tell you ? Look at 90.1.A and B. The code just sets the minimums that will keep it from catching fire - maybe. It's up to you to specify equipment/settings that meets the owners requirements.

My guess is because the NEC has a dumbed down sense of protection and fails to see the ambiguity in its own requirements. Consider there is no correlation with setting protection to the minimum dial and preventing fire.

 

[xptpcrewx]

Google search on "many":

A couple means two. A handful is five.
Anything more than a couple but less than a handful is a few.
Some is more than a handful but less than a dozen.
Several is more than a dozen but less than a score, which is 20.
Many is any number more than 20.
What is the upper limit? Well, according to the Bible, “Many are called, but few are chosen.” Your guess is as good as mine.​

Educate me - And I'll give you a break - just give us five (other than a large motor starter)

You mean just give "you" five (other than a large motor starter)

Branch Circuit: The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s)

1. Recto-former applications (mining)
2. MV Capacitor(s)
3. MV Reactor(s)
4. Some load-banks
5. Industrial equipment; i.e. magnetic particle inspection equipment
6. Bonus - Test equipment; i.e. primary injection test set

 

[iceworm]

You mean just give "you" five (other than a large motor starter)

Branch Circuit: The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s)

1. Recto-former applications (mining)
2. MV Capacitor(s)
3. MV Reactor(s)
4. Some load-banks
5. Industrial equipment; i.e. magnetic particle inspection equipment
6. Bonus - Test equipment; i.e. primary injection test set

All are good. 2, 3, 4 are great. I had not thought of those

 

[xptpcrewx]

Educate me - And I'll give you a break - just give us five (other than a large motor starter)

Now that we have that settled.... What exactly is it you are giving me a break on?

 

[iceworm]

Now that we have that settled.... What exactly is it you are giving me a break on?

I didn't ask you for 20.

I'm working on a response to your long post, but I got caught up working for a living. Soon

carl

 

[xptpcrewx]

Overcurrent Protection Assumption

I didn't ask you for 20.

You basically did ask for 20, but were a little too confident that there weren’t even 5 and went with that instead.

Regardless, I assure you there are definitely more than 20 examples in the world. Just think about any application that requires high-power utilization/end-use.

Could I have chosen a better word than “many” in the context of electrical branch-circuit applications? Yea, maybe but we should stick to debating engineering/code instead of the merriam-webster dictionary.

I'm working on a response to your long post

My post isn’t really very long at all. Let’s not get sidetracked. I encourage you to go back to the original post. Thanks.

 

[iceworm]

I think we have a failure to communicate. And maybe I see it. I'll try

... As for the rest, you are describing conventional variations of LT protection available in the industry. (Again, these are just "shapes" mostly/historically defined by electro-mechanical relay characteristics) ....

Yes, that is true. However, the molded case thermal magnetic is trip curve is still very much alive. And, yes, I see why you would call it an I^2t. It nearly is. And yes, the LSI LT delay and ST delay are both I^2t

Attachment 1 generic TM (SQD FA)
Attachment 2 shows a generic representation of an electronic LSI
Attachment 3 Standard LSI
Attachment 4 Moderately Inverse
​

I understand the NEC is not a design guide, but it needs to define the installation requirements.

.... LSIG DOES cover the full range of overcurrent as it relates to the NEC (when set correctly) - namely because overload, short-circuit and ground fault are the failure modes LSIG is intended to protect against. ...

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. Again, once the NEC gets past the 90.1.A, B minimums, the rest is a design decision. Anytime the NEC dabbles in design, it generally is a mess. Although there is one exception (in this area) I can think of, 240 part VIII in general. That one they did good.

... My guess is because the NEC has a dumbed down sense of protection and fails to see the ambiguity in its own requirements. ....

My guess is the NEC chose to stay out of an area where they knew they could not put together a cookbook. That in itself is surprising - but welcome.

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

I got no idea what this means

 

[iceworm]

... Regardless, I assure you there are definitely more than 20 examples in the world. Just think about any application that requires high-power utilization/end-use. ....

And I did (think about it). I scraped through brain cobwebs back to the 70s. Yuck (pulling the cobwebs off me), but thank you, it was interesting.

Early 70s - Navy reactor coolant pumps 400hp - 700hp, 480V across the line start
80s, 90s - Injection pumps 1200hp - 3000hp, 4160V across the line
00s - Injection pump 400Hp, 480V auto transformer reduced voltage
10s - Load bank, 1MW, 13.8KV This one had a lot of fuses and contactors internally, but that would be supplementary, so the ACB would be a branch CB. I had not thought of it.
10s - VFDs, 200hp - 400hp, 480V, molded case feeder CB Remote operated CB not needed.
10s - VFD, 3000hp, 13.8KV​

As you can see, my experience is power circuit breakers as branch CBs are Motor Starters. Remote controls required. And I had it stuck in my head you were looking at application where remote control was needed. However you never restricted the issue to that area.
But I don't deal with retro-cabulators
Or magnetic alien containment fields
So those I don't know about.

However, you are correct, "any application that requires high-power utilization/end-use" would use a power circuit breaker, or even a motor operator on a molded case.

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.

You basically did ask for 20, but were a little too confident that there weren’t even 5 and went with that instead.

Regardless, I assure you there are definitely more than 20 examples in the world. ....

You certainly put me in my place.

.... My post isn’t really very long at all. Let’s not get sidetracked. I encourage you to go back to the original post. Thanks.

Nope. I have to get to work. Got somebody that wants to pay. Have a good day.

 

[iceworm]

Bummer - did not intend to continue, but a thought did occur to me:

... Why doesn't the NEC just simply state when LT, ST, I protection is required instead of stating short-circuit protection is required? ...

You see a need for the NEC to be more specific in this area:

... My guess is because the NEC has a dumbed down sense of protection and fails to see the ambiguity in its own requirements.

Here is your chance for fame. Consider submitting a change request. 50 years rom now you will be referred to in the same hushed tones as the 25ohms-or-less guy.:roll:

I'm poking fun - not poking.

Still ...................?