# Magnetic trip questions



## chicken steve (Mar 22, 2011)

What are _our_ NRTL specifics for magnetic trip levels?

How does , or would, this _differ_ from an electronic or digital trip?

How do other _non_-NEC documents address this?

How important is it to be aware of sizing for this in any given install?

~CS~


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## Bird dog (Oct 27, 2015)

chicken steve said:


> What are _our_ NRTL specifics for magnetic trip levels?
> 
> How does , or would, this _differ_ from an electronic or digital trip?
> 
> ...


Years back in a code class, the instructor said we size motor feed according to the NEC table because out of 30(?) motors the chart showed the highest current draw of the most poorly designed motor(worst case). Then when the motor went bad, it was replaced and only the heaters had to be replaced according to the motor nameplate. It might be also good to ask about manufacturer specs.
Are you referring to running overload or short circuit/ground fault protection?


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## Bogart (Jul 20, 2015)

Normally I choose the OL based upon the FLA rating of the motor. But you also have to take ambient temperatures into consideration and the location of the controller compared to the motor...

Say the controller is going to be in a nice air conditioned control room while the motor is pumping fuel to a boiler in a nice hot boiler room....then I would down size the motor OL

15-20F temp difference is where you start to play with changing OL sizes based upon ambient temperature differences

But as always....whenever in doubt...Contact the Manufacturer and tell them your situation and they will recommend what you should do


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## don_resqcapt19 (Jul 18, 2010)

As I recall, there is no requirement that a breaker have a magnetic or instantaneous trip device.


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## Big John (May 23, 2010)

I can't reference an actual standard, but MCCBs seem to pretty commonly have an instantaneous pickup at 10X the nameplate rating.

As far as requirements for trip settings, the only thing I am familiar with is that switchgear must be protected by either a short-time or instantantous function as long as short-circuit fault clears withing 30 cycles or less. And switchboards must be protected by an instantaneous function where a short-circuit fault clears within 3 cycles or less.

For that reason, on most--or possibly all--electronic trip units, it's actually impossible to turn off both instantaneous and short-time protection at the same time.


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## Meadow (Jan 14, 2011)

Big John said:


> I can't reference an actual standard, but MCCBs seem to pretty commonly have an instantaneous pickup at 10X the nameplate rating.
> 
> As far as requirements for trip settings, the only thing I am familiar with is that switchgear must be protected by either a short-time or instantantous function as long as short-circuit fault clears withing 30 cycles or less. And switchboards must be protected by an instantaneous function where a short-circuit fault clears within 3 cycles or less.
> 
> For that reason, on most--or possibly all--electronic trip units, it's actually impossible to turn off both instantaneous and short-time protection at the same time.



Why the 30 and 3 cycle trip requirements? Is this like the GFP requirement for 1000 amp and over services?


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## chicken steve (Mar 22, 2011)

don_resqcapt19 said:


> As I recall, there is no requirement that a breaker have a magnetic or instantaneous trip device.


From my reading(s) i'd say that true Don

From the manufacturers info , it appears one can overlay mag trips from breakers with 10X difference , only to reveal similar magnitudes

It just doesn't make sense to me.....why not insist on cascading values ?

What would be the harm in upgrading/downgrading values, say for a dedicated motor load?

~CS~


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## Big John (May 23, 2010)

AcidTrip said:


> Why the 30 and 3 cycle trip requirements...?


 Two separate construction standards and two separate UL listenings. 

Switchgear is designed to withstand fault current forces for a much longer duration than switchboards, so gear can be protected under "short" or "ground" bands which won't exceed 0.5 second trip times. 

More fragile switchboards must clear faster to avoid potential destruction during rated short-circuit conditions.


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## Meadow (Jan 14, 2011)

chicken steve said:


> From my reading(s) i'd say that true Don
> 
> From the manufacturers info , it appears one can overlay mag trips from breakers with 10X difference , only to reveal similar magnitudes
> 
> ...



FWIW, double pole breakers have a higher mag trip then single pole 15 and 20s. Those went down in mag trip over the years. 

Someone who knows more can shed light, Im just as confused as you.


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## Meadow (Jan 14, 2011)

Big John said:


> Two separate construction standards and two separate UL listenings.
> 
> Switchgear is designed to withstand fault current forces for a much longer duration than switchboards, so gear can be protected under "short" or "ground" bands which won't exceed 0.5 second trip times.
> 
> More fragile switchboards must clear faster to avoid potential destruction during rated short-circuit conditions.



Question, what is the difference between a switchboard and switch-gear? I see these terms sometimes used interchangeably.


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## chicken steve (Mar 22, 2011)

AcidTrip said:


> FWIW, double pole breakers have a higher mag trip then single pole 15 and 20s. Those went down in mag trip over the years.
> 
> Someone who knows more can shed light, Im just as confused as you.


Perhaps this is of some help>>>>

Low Voltage Circuit Breakers, working with trip characteristic curves


For purposes of discussion toward my OP query(s) , i would like to address the resi mini breaker market, which appears _(please correct me if i'm wrong)_to be all the following classification >>>>_(from the glossary)_



> *TMF* Thermal magnetic trip unit with _fixed_ thermal and magnetic
> thresholds


~CS~


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## Meadow (Jan 14, 2011)

^ Yes


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## don_resqcapt19 (Jul 18, 2010)

chicken steve said:


> ......
> What would be the harm in upgrading/downgrading values, say for a dedicated motor load?
> 
> ~CS~


The short circuit and ground fault protective devices for most MCC buckets that I see are instantaneous only, and you do set the trip based on the motor.


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## Meadow (Jan 14, 2011)

chicken steve said:


> Low Voltage Circuit Breakers, working with trip characteristic curves



:thumbup::thumbup::thumbup::thumbup::thumbup: Very good read to help me grasp the concept. 

I must ask the experts, there is a long time delay and an instantaneous trip, why a short time delay? :blink:


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## emtnut (Mar 1, 2015)

Didn't read the above article (yet) ... From what I remember back in the days, was we had to do a breaker coordination study.

We would look at the whole system, and based on individual O/Ls and S/C's 
time current graphs, you would have to adjust the time delays so that the upstream switchgear would not trip ahead of the fused/breaker disconnects.


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## JRaef (Mar 23, 2009)

Big John said:


> Two separate construction standards and two separate UL listenings.
> 
> Switchgear is designed to withstand fault current forces for a much longer duration than switchboards, so gear can be protected under "short" or "ground" bands which won't exceed 0.5 second trip times.
> 
> More fragile switchboards must clear faster to avoid potential destruction during rated short-circuit conditions.


As I was taught, it's not necessarily _based_ on the "fragility" of gear, it's based on the expected use. SwitchGEAR is expected to be used as primary distribution gear, the "last line of defense" in a facility where the incoming power comes in off of the transformer(s). So if a breaker trips in the SwitchGEAR, most or a very large part of a facility goes down. Therefor the breakers in the gear must be expected to hold in longer, to give devices down stream the time to clear the fault closer to the problem, limiting the outage in a facility. That then means that the gear holding them will be subjected to much much stronger magnetic forces trying to force things apart. That's why SwitchGEAR is so much heavier and more expensive. 

So down stream of the SwitchGEAR, you have SwitchBOARDS and Panelboards. Those contain breakers or fuses that have shorter trip times, so they do not need to hold in for as long; they are EXPECTED to clear the fault first. Because of that, the gear holding them doesn't need to be quite as strong. There are therefore different ANSI design and UL testing standard for each type of gear. 

So yes, the SwitchBOARDS and Panelboards end up being more "fragile", but it's an indirect result of what they are designed to do.


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## Meadow (Jan 14, 2011)

Question of just as much mystery to me... why dont NRTLs require mag trip yet manufacturers add it anyways? Whats the advantage.


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## chicken steve (Mar 22, 2011)

AcidTrip said:


> Question of just as much mystery to me... why dont NRTLs require mag trip yet manufacturers add it anyways? Whats the advantage.



Add to that why manufacturers set the limits they do......?

~CS~


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## Meadow (Jan 14, 2011)

chicken steve said:


> Add to that why manufacturers set the limits they do......?
> 
> ~CS~



I think it has to do with tripping on inrush if it was any lower then 7x on standard 120 volts circuits. 

I will crack open some old GE breakers where in the late 80s they actually went to the trouble of adding copper loops to bring down the mag trip.


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## Meadow (Jan 14, 2011)

*Whats this do?*

Whats does this coil of wire do?


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## chicken steve (Mar 22, 2011)

It appears to be some sort of addition to the '_magnetic pole piece_' seen here>









~CS~


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## Big John (May 23, 2010)

Looks like part of the electromagnet that trips the breaker on instantaneous. I'm actually more surprised that the one posyed by CS doesn't need a coil to operate.


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## chicken steve (Mar 22, 2011)

Big John said:


> Looks like part of the electromagnet that trips the breaker on instantaneous. I'm actually more surprised that the one posyed by CS doesn't need a coil to operate.



i'm googling images for the simplicity of comparison , as my knowledge of ocpd innards couldn't fill a thimble BJ....

why is it some have it/some do not.....???










~CS~


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## guest (Feb 21, 2009)

AcidTrip said:


> Question of just as much mystery to me... why dont NRTLs require mag trip yet manufacturers add it anyways? _*Whats the advantage.*_


Simple. 

Mag trip is the primary "sensor" of short-circuit current faults. The high magnetic forces generated by a short will cause the magnetic trip elements to respond immediately (well within a few cycles anyways.) EDIT: _*That is the purpose of the coils in the above pics...*_

If you relied on ONLY the thermal trip elements (Cough... FPE... cough) a short would be allowed to persist until the thermal element heated up enough to finally trip the breaker (which as we know from experience rarely happens with FPE). 

Why thermal trip at all? Because a mag trip would have to have the current exceed a set threshold over time before it tripped, which could allow too much heating on the circuit conductors to occur. 

The thermal element, however, will heat up over time on sustained overloads (mimicking, loosely, the time and degree of heating of the conductors) until the heat becomes critical then it will trip. 

Best of both worlds. :thumbsup:


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## JRaef (Mar 23, 2009)

Generally the way mag trips work is using the coils around a rod as a plunger of sorts. If the current increases slowly within the range of the device, the plunger moves only a little but not enough to trip anything. If the current increase really high really fast, as in a short circuit, the plunger jerks and over travels, hitting the trip bar. The other type in the photo from Chicken Steve is different, but similar. Instead of a plunger, it's slider mechanism. But still, a slow increase is OK, a rapid increase makes it over travel and hit the trip release.


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## chicken steve (Mar 22, 2011)

My understanding of thermal vs. magnetic is fledgling at best JR

In my mind _(easily subject to scrutiny) _ anything thermal is best elicited proximal vs. distal if only for the obvious loss of thermal transmission.

That which is of magnetic qualities differs in that it can be assessed at a distal point relevant to incendiary and/or explosive elements 

Which brings me to that split second burst energy via fault , which mitigates that?

~CS~


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## chicken steve (Mar 22, 2011)

mxslick said:


> Simple.
> 
> Mag trip is the primary "sensor" of short-circuit current faults. The high magnetic forces generated by a short will cause the magnetic trip elements to respond immediately (well within a few cycles anyways.) EDIT: _*That is the purpose of the coils in the above pics...*_


Having only googled images to rely on,_ all _appear to have a magnetic trip lever or similar mechanism, but _not all_ appear to have the aforementioned *coil* inclusive Slick.

It would seem it is an _addition_ to the function.....

~CS~


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## Meadow (Jan 14, 2011)

chicken steve said:


> Having only googled images to rely on,_ all _appear to have a magnetic trip lever or similar mechanism, but _not all_ appear to have the aforementioned *coil* inclusive Slick.
> 
> It would seem it is an _addition_ to the function.....
> 
> ~CS~



Even more interesting, why doesn't this double pole breaker of the same vintage and company not have it? (Yes I know the handle tie broke off)


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## chicken steve (Mar 22, 2011)

AcidTrip said:


> Even more interesting, why doesn't this double pole breaker of the same vintage and company not have it? (Yes I know the handle tie broke off)


_eggggzactly_ Acidic one....:thumbsup:

your first round of pix shows >>>









this small 3 turn copper wire appears to be paralleled with the magnetic trip lever _(not sure if lever is the right term, but they all seem to look like one)_

so why in some and not others.....?

and why would any ONE manufacturer have it in/out of the same product?

~CS~


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## Meadow (Jan 14, 2011)

chicken steve said:


> _eggggzactly_ Acidic one....:thumbsup:
> 
> your first round of pix shows >>>
> 
> ...



Magnetic pole piece actually from the help of Google. :thumbsup:

Exactly my question though. Why does one 20amp breaker have it and another does not? What does it change? Im guessing it makes the breaker trip at a lower level, but why?


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## guest (Feb 21, 2009)

AcidTrip said:


> Magnetic pole piece actually from the help of Google. :thumbsup:
> 
> Exactly my question though. *Why does one 20amp breaker have it and another does not? What does it change? Im guessing it makes the breaker trip at a lower level, but why?*


It probably falls under the disclaimer that some products carry:

"This device may be subject to design changes or improvements at any time, and may not look exactly like the product in the package.":laughing:

IIRC the early mag trips were just a metal strip, sometimes with a small permanent magnet located somewhere nearby (been a long time since I opened up MCCB's) so the coil may be a later addition... OR the reverse could be true, first coils were used then they went to the strip/magnet method to save costs...

Best way to get to the bottom of it would be to research the patents on breakers of both styles to see what the patent citations have to say about the purpose/benefits of each method.

Yet another thought:

MAYBE the "SWD" (Switch Duty) and/or "HACR" (Heating/Air Conditioning) rated styles use different styles to handle inrush better? I can see the strip style having better inrush withstand (as a coil will generate a stronger magnetic field than a flat conductor (strip).


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## Meadow (Jan 14, 2011)

mxslick said:


> It probably falls under the disclaimer that some products carry:
> 
> "This device may be subject to design changes or improvements at any time, and may not look exactly like the product in the package.":laughing:
> 
> ...


I came across this today  I think this is what did it:

http://paceforensic.com/pdfs/newsletter/KeepingPace-37.pdf


So is that why the coil came about? :blink:


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## JRaef (Mar 23, 2009)

chicken steve said:


> It appears to be some sort of addition to the '_magnetic pole piece_' seen here>
> 
> 
> 
> ...


In this type of mechanism, the current PATH is part of the mag trip function and it operates based on the "left hand rule" of current and magnetic forces. Think about this image:









Now imagine the same image, but upside down, with the current flowing the other direction. When in close proximity, the magnetic forces will oppose each other, they repel. If you follow line to load in that breaker photo, the current path becomes a horseshoe shaped path, where the current goes from the line lug up toward the top, then down to the bottom. When it is going up to the top, the current flow has a magnetic field, then when it goes down along the same path, the magnetic force is opposing the mag field from when it flowed up. If current is low, the deflection this causes is less than even the deflection caused by the thermal trip bending. But if the current is very high, the magnetic forces create even MORE deflection and the mechanism slides off of the latch point.


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## Meadow (Jan 14, 2011)

JRaef said:


> In this type of mechanism, the current PATH is part of the mag trip function and it operates based on the "left hand rule" of current and magnetic forces. Think about this image:
> 
> 
> 
> ...



Question, since the power is AC, why dont we get vibrating instead of unlatching? Does something provide phase displacement?


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