# what is the differences between inverse time circuit breaker and regular breaker?



## joebanana (Dec 21, 2010)

An inverse time breaker works on the principle of the higher the fault current the faster it trips, as opposed to an ordinary overload situation.


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## Big John (May 23, 2010)

With very rare exceptions, pretty much all breakers are inverse time. They work just like Joebanana described: As current goes up, trip time goes down.

There are protective functions that don't work like that. Instantaneous trip (the magnetic pick up function in thermal-magnetic breakers) is not inverse time: Once the breaker decides to trip on instantaneous, it is not going to operate any faster even if you double or triple the fault current.

On breakers with electronic trip units there is a point where short pickup and ground fault pick up work the same way: Once the current reaches that point the breaker no longer trips faster as the current increases; it stops being inverse-time.


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## JRaef (Mar 23, 2009)

As some people know, the origin of the old "ITE" line of circuit breakers (now Siemens) was an acronym for Inverse Time Element. They had been the inventors of that technology back in the 1920s. Before that, circuit breakers used many other technologies that had varying degrees of accuracy. The ITE concept was many more times reliable and repeatable, which basically supplanted the other types for all intents and purposes. But there are still hydraulic circuit breakers out there.


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## Big John (May 23, 2010)

JRaef said:


> ...But there are still hydraulic circuit breakers out there.


 We still run into a the old series-overcurrent dashpot style air breakers, but more often than not they have marginal to no protective ability; they're just big switches.

You can actually buy tiny little hydraulic molded case breakers for use in environments where temperature would cause a prohibitive amount of derating to a thermal element.


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## chicken steve (Mar 22, 2011)

Hoe does inverse _differ_ from instantaneous?

~CS~


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## Big John (May 23, 2010)

chicken steve said:


> Hoe does inverse _differ_ from instantaneous?


 Instantaneous has no time current curve. It is either on or off.

All other functions follow a curve where the relationship between time and current is inverse meaning each does the opposite of the other. If current goes up, trip time goes down. If current goes down, time goes up.


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## chicken steve (Mar 22, 2011)

But how does it operate Big John? low mag trip?

~CS~


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## AK_sparky (Aug 13, 2013)

chicken steve said:


> But how does it operate Big John? low mag trip?
> 
> ~CS~


Ya, just a mag trip. Not sure if it is relatively lower, higher, or similar.

Best would be to compare some trip curves.


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## JRaef (Mar 23, 2009)

The Inverse Time Element is a bi-metal strip, exactly the same as in an motor thermal overload relay or the old temperature switches used in electric heaters. As current goes up, the strip heats up, but one side of the strip expands faster than the other in the same heat, which makes it bend. 








In a breaker, it bends into a latch on a ratchet mechanism that is holding a spring that keeps the breaker contact closed. Release the latch, the springs force the breaker to open.

By manipulating the alloys in the metals, they can design a thermal "trip curve", called an "I squared t" curve (I = current, t = time) that essentially mimics the thermal damage curve of the devices down stream. Of course, the protection curve needs to be a little LOWER than the thermal damage curve. That's why repeatability is so important. 

But the drawback of a thermal-_*only *_breaker is that it can take too long to react to a dead short. In that Inverse Time Element curve, the "fast" time was 10 seconds at 600% of rated current. In a dead short you can go right to the available fault current, way more than 600%, and that can start a fire much faster than the inverse time element will react.

In an old hydraulic breaker, a closed little container (called a "dash pot") of mineral oil is in the current path, and the heat of the current expands the oil. There is a cylinder attached to it with a calibrated orifice that lets the heated oil flow into the cylinder and when it fills up, it moves into that same sort of latch release. The problem was, the more you heated the fluid up, the more the chemistry and viscosity changed permanently until they didn't work as well any more and the fluid had to be changed. Nobody wants to have to do that.

Another type of "automatic circuit breakers" before ITE came out were strictly magnetic. People liked them because they didn't care about temperature, so you could have them right next to hot machinery or foundry equipment and it didn't matter. But they nuisance tripped a lot from momentary surge current if they were sized to try to protect base on continuous current and if sized for surge current, would not open on lower continuous current. When AC motors became more popular than DC motors in the 30s, magnetic breakers became a liability, except as what we now call a Motor Circuit Protector (MCP) that MUST be used with an overload relay.

What ITE innovated really was to take the thermal bi-metal Inverse Time Element element that could be sized closer to continuous current, and marry it with a magnetic trip element that could take care of the instant short circuits.

The marine industry still likes to use those little Heinemann "magnetic-hydraulic" mini breakers, because they work in wide temperature extremes. When your crab boat engine is smoking but you have to get out of the way of that cargo ship, you don't want the heat of the engine to trip your breakers on you. It's called "magnetic-hydraulic", but it's really still just a magnetic breaker with a hydraulic dampening system to help try to avoid most of the nuisance trips. They're expensive though.


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## chicken steve (Mar 22, 2011)

This is intriguing .....probably some grand historical electrical revelations most are not aware of JR:thumbsup:

Here's one for you _(et all)_ , why is it the IEC focuses on mag trips _(i believe they've an A,B,C,D selection)_ *vs.* the NEC that does not?:blink:

~CS~


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## AK_sparky (Aug 13, 2013)

chicken steve said:


> Here's one for you _(et all)_ , why is it the IEC focuses on mag trips _(i believe they've an A,B,C,D selection)_ *vs.* the NEC that does not?:blink:
> 
> ~CS~


I'm pretty sure those IEC A,B,C,D selections are trip curves, which would indicate not a magnetic trip point. Someone feel free to correct me if I'm wrong.


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## Meadow (Jan 14, 2011)

AK_sparky said:


> I'm pretty sure those IEC A,B,C,D selections are trip curves, which would indicate not a magnetic trip point. Someone feel free to correct me if I'm wrong.



Nope, magnetic pickups:

http://www.allinterview.com/showans...nce-between-c-curve-and-d-curve-for-mcbs.html


So the question is, why does the IEC offer different magnetic trip values for all their breakers, even the domestic ones?


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## mike883 (Mar 22, 2016)

JRaef said:


> The Inverse Time Element is a bi-metal strip, exactly the same as in an motor thermal overload relay or the old temperature switches used in electric heaters. As current goes up, the strip heats up, but one side of the strip expands faster than the other in the same heat, which makes it bend.
> 
> 
> 
> ...


 
good information

and if we are holding a circuit breaker on our hands
how can we tell if it is a Instantaneous trip circuit breaker or a inverse time circuit breaker ?

do they have some kind of label on description on them?


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## AK_sparky (Aug 13, 2013)

AcidTrip said:


> Nope, magnetic pickups:
> 
> http://www.allinterview.com/showans...nce-between-c-curve-and-d-curve-for-mcbs.html
> 
> ...


The breakers are still thermal-magnetic. They have a thermal trip curve as well as an instantaneous magnetic trip. The thermal part of the trip curve is basically the same for all of them.

You are right the specifications (b,c,d,k) are indicative of the instantaneous trip point. The reason is that for some loads you want a lower instantaneous trip, for some you want higher. For example a motor with high inrush you would want something with a trip at 10 or 20 times the MCB rating. For a power supply you'd want something lower like 2-3 times the rating.


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## Meadow (Jan 14, 2011)

AK_sparky said:


> ]The breakers are still thermal-magnetic. They have a thermal trip curve as well as an instantaneous magnetic trip. The thermal part of the trip curve is basically the same for all of them.



Yup, what I was thinking. 



> You are right the specifications (b,c,d,k) are indicative of the instantaneous trip point. The reason is that for some loads you want a lower instantaneous trip, for some you want higher. For example a motor with high inrush you would want something with a trip at 10 or 20 times the MCB rating. For a power supply you'd want something lower like 2-3 times the rating.


But why not have a type D and cover all installations?


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## AK_sparky (Aug 13, 2013)

AcidTrip said:


> But why not have a type D and cover all installations?


Take a look at the chart on this page for some guidelines for application.
http://www.electricalclassroom.com/2015/04/what-is-meant-by-b-c-d-k-and-z-curves.html

A trip curve C or D can be used for most general stuff. I think most resi circuit breakers are close to a C curve if I remember correctly.

Using a lower trip can allow a 'feeder' conductor to be sized down, which we have done on projects not restrained by NEC/CEC. The Z curves are a safer bet for sensitive controllers in some cases.

To me it seems like IEC systems are more precisely designed and installed, and nothing is a lot bigger than it needs to be.

NEMA and NEC/CEC type system seem to be over-engineered and more general, so using the same trip curve for everything can work effectively. It is also much simpler to install in that sense.


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## Meadow (Jan 14, 2011)

AK_sparky said:


> Take a look at the chart on this page for some guidelines for application.
> http://www.electricalclassroom.com/2015/04/what-is-meant-by-b-c-d-k-and-z-curves.html
> 
> A trip curve C or D can be used for most general stuff. I think most resi circuit breakers are close to a C curve if I remember correctly.
> ...



Great info! 

But I am wondering now, how does a lower mag trip let you down size the cable? :001_huh: Don't hold back, I am learning a great deal here


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## kringga (Oct 11, 2020)

An inverse-time breakers function is to respond to overloads caused by say, plugging in too many appliances. The instantaneous function is designed to trip immediately in the event of a bolted short. It would surprise most people that breakers wont trip if you simply touch the two sides of a circuit together. That will instead result in an arc-fault that won’t trip a breaker which is the reason why the NEC now requires AFCI breakers.


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## paulengr (Oct 8, 2017)

kringga said:


> An inverse-time breakers function is to respond to overloads caused by say, plugging in too many appliances. The instantaneous function is designed to trip immediately in the event of a bolted short. It would surprise most people that breakers wont trip if you simply touch the two sides of a circuit together. That will instead result in an arc-fault that won’t trip a breaker which is the reason why the NEC now requires AFCI breakers.


A sustained shorting type arcing fault is about 40-80% of the bolted fault current. It becomes 100% as voltage increases (over 1000 V). The calculations are in IEEE 1584.

The fixed magnetic trips in UL 489 breakers are 6-10x the ultimate trip rating in inverse time. So a 100 A breaker has a magnetic trip of 6000-10000 A.

The concept is good but the somewhat arbitrary 6-10 times is intended to avoid tripping on motor or transformer startups while capturing “everything” else. But if your wiring is long enough and/or the transformer is small enough, it won’t trip on short circuit. Ground faults which are really just another type of bolted fault are lower, too. It can also trip on arcing faults in the same kind of way, most of the time. The only way to be assured that this works is to calculate and verify your short circuit current is high enough. On small breakers it always is but in larger ones it isn’t a sure thing.

In industrial plants concerned with not just arcing faults but arc flash they actually calculate the arcing and short circuit currents. Then it’s a simple matter to verify if the breaker will trip or not and to adjust accordingly. In some cases instantaneous is turned off normally for coordination but turned on temporarily for maintenance with settings calculated to ensure it trips during arcing faults. Because of much higher currents other than the cost of the engineering, dealing with arcing faults is relatively easy.

GFCIs are designed to trip on not only true ground faults but even on leaking current to ground which is very low level. In industrial plants the leakage that a GFCI trips on would detect the natural system charging current that exists as a ground fault even in small 480 V systems so they can’t be used above 120/240 or they are used but with higher settings. Bender is one manufacturer.

The original AFCIs were designed only to catch a shorting type of arcing fault. These are relatively rare and so the AFCI didn’t do much for anybody except make a lot of money for breaker manufacturers. A much more common arcing fault in low voltage wiring is the series type which is a loose but not quite broken connection. If you’ve ever heard an old almost failed light switch crackling, that’s a series arc. This is what the later combination AFCIs address along with GFCI. Since this type of arcing fault does not involve short circuits standard breakers never detect them. Unfortunately with having “one size fits all” the price tags have a one size fits all price.


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