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Old 09-06-2015, 05:23 PM   #21
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[quote=meadow;2240457]
Quote:
Thats correct. Anytime a breaker can trip magnetically during a fault incident energy and wire stress (thermal and magnetic) is greatly reduced.
Alright,
but if i meg out older homes & wiring ,most branch circuits are not the same ,in fact there can be a wide variety of readings.

So i can imagine a high impedance circuit cooking away w/o the ocpd (our american version) being at all bothered

For that matter, seems it can occur right at the ocpd>









Quote:
The breaker did smoke because its interrupting DC, however the DC was able to prevent the solenoid from activating.

Keep in mind IEC miniature breakers have 2 parts, a bimetlic strip for overloads and a solenoid coil for short circuits:
A torodial coil >


juice goes in, juice returns , millionths of a difference (30ma, 5ma depending on setting) trip the incorporated mechanisms

So how does this relate to mag trip settings?

~CS~
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Old 09-06-2015, 09:01 PM   #22
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Quote:
Originally Posted by chicken steve View Post

Alright,
but if i meg out older homes & wiring ,most branch circuits are not the same ,in fact there can be a wide variety of readings.
There is a difference between insulation resistance testing and earth fault loop impedance testing.

Both IR and EFLI will vary in any instillation.


Quote:
So i can imagine a high impedance circuit cooking away w/o the ocpd (our american version) being at all bothered

A short circuit that does not trip magnetically will have enormous incident energy. Picture cutting into live NM. If you cut into NM capable of tripping the breaker magnetically regardless of the fault current the burn mark will be small. Now cut into the same piece without magnetic trip and you will have a large deep gash in your dykes.

A common example is those who have into NM with a QO breaker vs an older FPE or bulldog. In fact the lights will sometimes stay on if the short circuit current was brief enough (30 cycles).




Quote:
For that matter, seems it can occur right at the ocpd>
Id say that is not a short circuit but rather a loose connection. Whole nother animal. A loose or poor connection will not produce high currents or a current leak to ground (unless of course the insulation is burned away and the bare metal then shorts to ground).







Quote:
A torodial coil >


juice goes in, juice returns , millionths of a difference (30ma, 5ma depending on setting) trip the incorporated mechanisms

So how does this relate to mag trip settings?

~CS~

Mag trip is different the a GFCI.

GFCI works on a current imbalance. A GFCI could be short circuited Line to neutral with 10,000amps flowing, because all current passes through the toroid coil (no imbalance) the GFCI will trip.

Magnetic trip is different. A circuit could have 20amps of imbalance, but the solenoid will not pull in. What will cause it to pull in is high current, say 100, 200 or 500amps.

FWIW most standard thermal magnetic breakers on the market sold in the last 30 years have magnetic trip in addition to thermal trip.

Thermal trip relies on RMS I2R heating, meaning a large current will have to heat the bi-metal strip for some time in order to trip. This is done on purpose to let motors start up or inrush loads.

Magnetic trip in the other hand does not have a time delay. As soon as current peaks over a certain point (say 150 amps) the breaker will trip immediately without delay.
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Old 09-07-2015, 06:04 PM   #23
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[quote=meadow;2241417]
Quote:
There is a difference between insulation resistance testing and earth fault loop impedance testing.

Both IR and EFLI will vary in any instillation.
Because one tests impedance back to the Xformer, taking the grounding into consideration, and one does not? Why do we need to worry about that in a TN-S-C , or TN-C system ?





Quote:
A short circuit that does not trip magnetically will have enormous incident energy. Picture cutting into live NM.
errruhm....i really don't need to use my imagination....


Quote:
If you cut into NM capable of tripping the breaker magnetically regardless of the fault current the burn mark will be small. Now cut into the same piece without magnetic trip and you will have a large deep gash in your dykes
.

and then they're a dyke/stripper combo tool.....

Quote:
A common example is those who have into NM with a QO breaker vs an older FPE or bulldog. In fact the lights will sometimes stay on if the short circuit current was brief enough (30 cycles).
I'll refrain from the many stories, but am curious as to what the trip thresholds damage can do in, say, something that is arcing just enough recreating the same scenario....






Quote:
Id say that is not a short circuit but rather a loose connection. Whole nother animal. A loose or poor connection will not produce high currents or a current leak to ground (unless of course the insulation is burned away and the bare metal then shorts to ground)
.

I stand corrected then, it should rightly have gone over to the BadOne's fall of potential thread....










Quote:
Mag trip is different the a GFCI.

GFCI works on a current imbalance. A GFCI could be short circuited Line to neutral with 10,000amps flowing, because all current passes through the toroid coil (no imbalance) the GFCI will trip.

Magnetic trip is different. A circuit could have 20amps of imbalance, but the solenoid will not pull in. What will cause it to pull in is high current, say 100, 200 or 500amps.

FWIW most standard thermal magnetic breakers on the market sold in the last 30 years have magnetic trip in addition to thermal trip.

Thermal trip relies on RMS I2R heating, meaning a large current will have to heat the bi-metal strip for some time in order to trip. This is done on purpose to let motors start up or inrush loads.

Magnetic trip in the other hand does not have a time delay. As soon as current peaks over a certain point (say 150 amps) the breaker will trip immediately without delay
Understood and thank you for you clarification here Mr Meadow

My further queries are two fold.

First, why not allow the American electrical community to install a variety of mag trip level OCPD's according to utilization?

Secondly, all torodial coil workings aside, why is it the afci is manufactures with a lower level, and who or what is responsible FOR it's incorporation into it?

~CS~
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Old 09-07-2015, 07:04 PM   #24
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Quote:
Originally Posted by chicken steve View Post

Because one tests impedance back to the Xformer, taking the grounding into consideration, and one does not? Why do we need to worry about that in a TN-S-C , or TN-C system ?

Even in a TT or IT system short circuit current line to neutral or line to line is uncontrolled, same principal would apply here.




Quote:
errruhm....i really don't need to use my imagination....
I figured, you know this stuff well.


Quote:
and then they're a dyke/stripper combo tool.....
Those singe easily.



Quote:
I'll refrain from the many stories, but am curious as to what the trip thresholds damage can do in, say, something that is arcing just enough recreating the same scenario....

Assuming you carbonized a current path and a sustained arc took place thermal trip alone would take some time to clear the arc, if at all.

Arcing and sputtering short circuits are not a sinusoidal current draw like a load or hard short circuit. Current will often be intermittent and/or high at peaks and troughs but none around the zero crossings. An arc fault will extinguish and re-strike at various points at peaks and troughs.

The result is high peak current, but low RMS current. RMS is what heats the strip, and the lower it is the more time for a breaker to trip thermally.


FPE breakers have no magnetic trip, so it is possible to turn a j-box into a charcoal mess by the time the wire and bi-metal gets warm.



Quote:
I stand corrected then, it should rightly have gone over to the BadOne's fall of potential thread....
Its ok. Good for the convo none the less. Heck everything you say has weight.





Quote:
Understood and thank you for you clarification here Mr Meadow

My further queries are two fold.

First, why not allow the American electrical community to install a variety of mag trip level OCPD's according to utilization?
We do already to a degree. Single pole 15 and 20amp breakers trip at a lot lower magnetic value then 15 and 20amp double poles.

The idea is a good one none the less.



Quote:
Secondly, all torodial coil workings aside, why is it the afci is manufactures with a lower level, and who or what is responsible FOR it's incorporation into it?

~CS~
Most AFCIs have two torrid coils. One looks for a 30/50ma imbalance, the other looks for a current ripple indicative of an arc fault.
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Old 09-08-2015, 08:25 AM   #25
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[quote=meadow;2243337]
Quote:
Even in a TT or IT system short circuit current line to neutral or line to line is uncontrolled, same principal would apply here.
I suppose it would regardless of impedance Meadow.






Quote:
Assuming you carbonized a current path and a sustained arc took place thermal trip alone would take some time to clear the arc, if at all.

Arcing and sputtering short circuits are not a sinusoidal current draw like a load or hard short circuit. Current will often be intermittent and/or high at peaks and troughs but none around the zero crossings. An arc fault will extinguish and re-strike at various points at peaks and troughs.

The result is high peak current, but low RMS current. RMS is what heats the strip, and the lower it is the more time for a breaker to trip thermally.


FPE breakers have no magnetic trip, so it is possible to turn a j-box into a charcoal mess by the time the wire and bi-metal gets warm.
Methinks my simplistic concept of OCPD functionality just got thrown under the theory bus here....





Quote:
We do already to a degree. Single pole 15 and 20amp breakers trip at a lot lower magnetic value then 15 and 20amp double poles.

The idea is a good one none the less.
So are all our mag trip ratings manufactured in ascending order ?, for instance 200A main resi OCDP's having higher ratings ,allowing the lesser branch circuit OCPD's to mitigate faults first?





Quote:
Most AFCIs have two torrid coils. One looks for a 30/50ma imbalance, the other looks for a current ripple indicative of an arc fault.
Yet these tordials have nothing to do with mag trip levels, either here via our afci's or abroad in the rcd's....

So why did afci manufacturers alter the mag trip settings ? This seems a relevant Q to ask here...

~CS~
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Old 09-08-2015, 02:08 PM   #26
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Originally Posted by chicken steve View Post

I suppose it would regardless of impedance Meadow.
The concept applies to all short circuit current levels.





Quote:
Methinks my simplistic concept of OCPD functionality just got thrown under the theory bus here....
OCPDs in theory do more then just prevent thermal damage to the wire itself and surrounding material in contact with the cable.

Quote:
So are all our mag trip ratings manufactured in ascending order ?, for instance 200A main resi OCDP's having higher ratings ,allowing the lesser branch circuit OCPD's to mitigate faults first?
Generally it is that way, mag trips are about 10x the handle rating. For some manufactures like Square D the mag trip is the same over several different handle ratings.

Under most conditions the branch will trip first, but if fault current from the POCO is high enough, and the fault lets through enough current it is possible to trip both the main and branch breaker. This is called misco-ordination, and is very common with magnetic trip. Since magnetic trip has no delay, any fault current over the mag trip will trip the breaker.

Think of this example:

120/240 service with a 8000 amp short circuit capacity.

100amp main breaker. 100 x 10= 1000 amp mag trip start point

20 amp branch breaker. 20 x 10 = 200 amp mag trip start point

A fault occurs close to the panel, the NM run is very short and a large amount of current flows, say 2000 amps.

Because the current is high the magnetic trip is triggered on both breakers and both will trip.

Now, say the fault occurs 125 feet up the branch circuit. The NM impedance will limit the fault current say to 400 amps. In this case the branch will trip magnetically, but not the main.



Quote:
Yet these tordials have nothing to do with mag trip levels, either here via our afci's or abroad in the rcd's...
.


That true. Its interesting though, nearly all short circuits are phase to ground, so in theory a RCD/GFCI/GFP does the same tripping immediately on a fault.





Quote:
So why did afci manufacturers alter the mag trip settings ? This seems a relevant Q to ask here...

~CS~
Million dollar question. The mag trip was altered from 20-32x down to 10x in the 90s for all single pole 15 and 20amp breakers. Clearly something or someone proved a need.

But, in any case its common sense. If you know the highest resi load requiring a 15 or 20amp breaker will not have an inrush more then 180amps, do you really need a breaker that mag trips at 900 amps?

Absolutely nothing is gained by having a higher then needed mag trip or tripping a fault on thermal elements.

Even at the distribution/ transmission level meg trip has always been a concern.
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Old 09-08-2015, 02:14 PM   #27
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MV, HV and UHV relays used in utility and other systems also have two types of trip modes: time over current and instantaneous over current. One is like a what hour meter intentionally having a delay on mild to moderate over current, the other has no delay like a solenoid for high over currents.

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Old 09-08-2015, 02:48 PM   #28
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This explains the difference between time trip and mag trip:

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Old 09-08-2015, 03:18 PM   #29
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Just to give an idea where such relays might be used see 2:30. There is one for each phase (3 in total) plus a ground detector relay which works like an RCD. The auto reclose relay (5th big one) closes the breaker in a few times after a trip in case the fault is temporary. All 3 phase relays and the ground relay have both time dial and instantaneous trip.


Last edited by meadow; 09-08-2015 at 03:27 PM.
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Old 09-08-2015, 03:24 PM   #30
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To give you an idea, starting at around 2:24, the time over current dial in action. It takes its time rather then initiating trip immediately:

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