# Ac loop impedance testing



## Sprink

I am not from the UK, but I am part Irish.

It appears you only test below 50 amps. Then anything over 50 amps you calculate.

http://www.w.mylocalelectrician.co.uk/news/2012/jun/when-measuring-low-zs-values-does-your-meter-lie


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## Frank Mc

chicken steve said:


> Ac loop impedance testing
> 
> Methinks this article is a tad confusing, if not outright misleading
> 
> I'd like to ask some of you UK sparks to unfuzz us, as i'm aware you're far more up on the topic
> 
> thx
> 
> ~CS~


Hi Steve
Fault loop impedence testing ,is. Just measuring the. Fault current paths resistance to ensure its low enough for the protection device ie circuit breaker trips.
Here in Oz we have. Two tables which we reference to, one is for live testing using a fault loop impedence tester and the other table is for dead testing. Which youncan use a digital multimeter. You lift the earth of. Say at the load end and short it to the hot wire and back at the switchboard you measure the resistance between the hot wire and earth. The reason for lifting the earth of. Is to ensure no parallel paths exist..hope that makes sense.
There are some good videos UK based on youtube by a guy called Chris Kitcher check them out.
HTH
Frank


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## chicken steve

https://www.youtube.com/watch?v=d9gJzNNV5PY

Thx Frank

So what would be the dif if we lift our GEC's , and used a megger here?

~CS~


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## Frank Mc

chicken steve said:


> https://www.youtube.com/watch?v=d9gJzNNV5PY
> 
> Thx Frank
> 
> So what would be the dif if we lift our GEC's , and used a megger here?
> 
> ~CS~


Hi Steve
your meggar would be used to measure the insulation resistance not fault loop impedence,unless your. Meggar can also measure low resistance???
The video you linked to was.a Fault loop impedence of the mains cable back to the substation known as Ze ( I believe)
HTH
Frank


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## Meadow

Ze would be correct (e I think stands to 'external')


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## mrmarksparks

Earth loop testing has become more troublesome with the massive rollout of rcbo type breakers this means the loop test is carried out using only 15ma so as not to trip a 30ma rcbo. Imagine pushing 15ma round from the outlet you are testing from all the way to the transformer and back, the readings can vary terribly by as much as 1 ohm. 
with no rcbo in place and just an mcb we use a different scale which pushes more current through and is more accurate and quicker. 1-2 seconds instead of 10seconds.


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## Meadow

High current test is always more accurate from what Ive heard. Cant you get away with 30ma RCBOs?


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## mrmarksparks

Yep the rcbos are 30 ma it's the tester that only puts 15 ma down the cable I have a Megger 1720 multi function tester they cost about £700 including our VAT. 
What sort of testers are common in the U.S. is there a brand that is most popular. We tend to use Megger/avo or fluke.


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## Meadow

mrmarksparks said:


> Yep the rcbos are 30 ma it's the tester that only puts 15 ma down the cable I have a Megger 1720 multi function tester they cost about £700 including our VAT.
> What sort of testers are common in the U.S. is there a brand that is most popular. We tend to use Megger/avo or fluke.



Earth fault loop impedance testing isnt done in the US, but when it comes to volt meters/clamp on amp meters ect Fluke is very common. 


Megger seems to be the most common when insulation resistance testing is done.

Do you have a link to your Fluke multi function testers?


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## mrmarksparks

http://isswww.co.uk/fluke-1652c-17t...EQsSQyX4WiVi-hDvSciXIoodsK_sbJyxQ9RoCRB_w_wcB

This is the instrument our engineers use.


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## Meadow

mrmarksparks said:


> http://isswww.co.uk/fluke-1652c-17t...EQsSQyX4WiVi-hDvSciXIoodsK_sbJyxQ9RoCRB_w_wcB
> 
> This is the instrument our engineers use.



Dont sparkies also use those day to day? I've been thinking of getting one but unsure how those tester will like 120 volts 60Hz.


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## mrmarksparks

We all have a multifunction tester each of like that or the Megger equivalent used mainly for testing our own new installations or fault finding. 
We also carry out a lot of electrical installation condition reports (5 yearly)
Inspections on commercial property's schools , offices etc. 

http://www.eyre-electrical-mechanical.co.uk

The link is to the website of the company I work for.


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## chicken steve

mrmarksparks said:


> Earth loop testing has become more troublesome with the massive rollout of rcbo type breakers this means the loop test is carried out using only 15ma so as not to trip a 30ma rcbo. Imagine pushing 15ma round from the outlet you are testing from all the way to the transformer and back, the readings can vary terribly by as much as 1 ohm.
> with no rcbo in place and just an mcb we use a different scale which pushes more current through and is more accurate and quicker. 1-2 seconds instead of 10seconds.



Well i see there are many different abbreivations for types of breakers manufactured and installed off my turf>>>


http://www.rcd-rcbo.com/



> Ezitown,founed in 2005,A Professional manufacturer of Circuit breaker,*mcb,mccb,elcb,rccb,rcd,rcbo* etc. which are exported to all over theworld. As an ISO 9001:2008 certified company we are always keeping our supply with high quality products at competitive price


If I can i take it they all employ some sort of torodial coil , i get why 1 ohm can be a problem 

As we _(American NEC & Canadian CEC)_ seem to be stumbling along with our our version of this coil philosophy _(however hackish it may be)_ , i would imagine the testing(s) would _follow suit_ as well.....?:blink:








Best i can do, am trained in & know is to meg out circuitry.....
Why aren't we doing EFL testing here?

~CS~


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## Southeast Power

24 string temporary light set,
24 100 watt lamps
=
20amp load test.


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## Meadow

chicken steve said:


> Well i see there are many different abbreivations for types of breakers manufactured and installed off my turf>>>
> 
> 
> http://www.rcd-rcbo.com/
> 
> 
> 
> If I can i take it they all employ some sort of torodial coil , i get why 1 ohm can be a problem
> 
> As we _(American NEC & Canadian CEC)_ seem to be stumbling along with our our version of this coil philosophy _(however hackish it may be)_ , i would imagine the testing(s) would _follow suit_ as well.....?:blink:
> 
> Best i can do, am trained in & know is to meg out circuitry.....
> Why aren't we doing EFL testing here?
> 
> ~CS~


Its interesting, earth fault loop impedance testing and mag trip provide arc fault protection as claimed by NRTLs and the CMP:







> *The UL report states “breakers can be effective at mitigating arcing faults, provided the available fault current can be guaranteed to exceed the magnetic trip level of the circuit breaker by a factor of 1.25.”*




The function of UL489e (supplemental arc breaker):





> (b)The branch circuit breaker shall be listed and marked as having an
> instantaneous trip not exceeding 300 amperes.​






> This proposal establishes a circuit breaker
> listing and marking requirement for the magnetic trip level at or below 300A in order to ensure the breaker will protect the circuit from a parallel arcing fault when at least 500A of available fault current is present as required in the first
> parameter.






> LAROCCA, R.: While we support the panel action, continued support is
> dependent upon review of additional data that would confirm the availability of
> sufficient short circuit current capability at the panel of a typical installation.
> The arc fault protection of the branch circuit will be provided by a system
> that includes an outlet branch circuit AFCI, a circuit breaker having a knowninstantaneous trip current and a branch circuit of a limited length and resistance to ensure that the fault current is sufficient to trip the breaker during a parallel arcing fault at the installation point of the outlet branch circuit AFCI. The latest UL Research Report takes into consideration the impact of the available current at the panel on the acceptable length of the branch circuit home run to the first outlet. Calculation shows that as the available current at the origin of the branch circuit varies, so does the allowable length of the home run. Additional study is needed to provide data regarding the current available at the origin of the branch circuit in a typical installation. From this data, the panel will be able to determine if modification of the panel action should be considered at the ROC.





> The report focused on providing data on the performance of conventional circuit breakers with respect to arc faults in the home run portion of the branch circuit and identified the parameters that must be met and controlled for this tohappen. As long as these parameters are controlled, it can be concluded that an outlet branch circuit type arc-fault circuit interrupter could possibly be used in conjunction with a low-magnetic type circuit breaker to protect the branchcircuit. The critical parameters summarized in the report include: a minimum available fault current, a maximum magnetic trip level for the circuit breaker, impedance of the conductor, the actual voltage and the length of the conductor.This proposal is based on utilizing the parameters set forth by the UL Report to revise 210.12 to permit using an outlet branch circuit arc-fault circuit interrupter in conjunction with a low magnetic trip circuit breaker.








> (2) A listed outlet branch circuit type arc-fault circuit interrupter installed at the first outlet on the branch circuit where all of the following conditions aremet:
> 
> (a) The branch circuit over current protection device shall be a listed circuitbreaker having an instantaneous trip not exceeding 300 amperes.​







> (2) A listed outlet branch circuit type arc-fault circuit interrupter installed at the
> first outlet on the branch circuit where all of the following conditions are met :
> (a) The available fault current at the branch circuit overcurrent device shall not
> be less than 500A and the ambient temperature shall not be less than 20°C
> (68°F).
> (b)The branch circuit breaker shall be listed and marked as having an
> instantaneous trip not exceeding 300 amperes.
> © The branch circuit wiring shall be continuous from the branch circuit
> overcurrent device to the outlet branch circuit arc-fault circuit interrupter.
> (d) The maximum length of the branch circuit wiring from the branch circuit
> overcurrent device to the first outlet shall be determined using the following:
> L = (0.4×Vrms) / (1.25×300×pL)
> L is the maximum length of the “home run” in feet;
> pL is the resistivity per unit foot of each conductor of the NM cable gauge
> being used; and
> Vrms is the actual supply voltage.







Can be found here starting at page 70-129 (139 in the viewer):


https://www.nfpa.org...0-A2013-ROP.pdf


Somehow I sense that the British take this for granted.

Mind you we are talking proposals for 300amps. An established 150 amp OCPD will provide arc fault protection to 125 feet of NM in most applications. Its ironic they rejected this same concept for branch feeder AFCIs in 1999, yet revisiting it only when the same function is needed after series arc protection is already made mandatory.


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## chicken steve

Meadow,
I'm compelled to ask.....

what is the _advantage_ of lower mag trips ? :blink:

in fact...

what are the disadvantages ? :blink:

~CS~


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## Meadow

chicken steve said:


> Meadow,
> I'm compelled to ask.....
> 
> what is the _advantage_ of lower mag trips ? :blink:
> 
> in fact...
> 
> what are the disadvantages ? :blink:
> 
> ~CS~


Low mag trip is cheaper, far more reliable and contains no electronics to fail. It drastically reduces the incident energy at any fault, and NRTL testing has shown it mitigate arc faults better then electronics. 

Two disadvantages:

1. Mag trip that is to low can trip on large motors or high inrush appliances.

2. In some cases it maybe necessary to consider fault current in the circuit if you want to grantee ever point in the circuit can trip the breaker magnetically. (Earth fault loop impedance). 

Any way, just to give you an idea of what magnetic trip vs thermal trip looks like on a fault consider the breaker trip times here:


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## chicken steve

meadow said:


> Low mag trip is cheaper, far more reliable and contains no electronics to fail. It drastically reduces the incident energy at any fault, and NRTL testing has shown it mitigate arc faults better then electronics.
> 
> 
> 
> 
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> 
> Two disadvantages:
> 
> 1. Mag trip that is to low can trip on large motors or high inrush appliances.
> 
> 2. In some cases it maybe necessary to consider fault current in the circuit if you want to grantee ever point in the circuit can trip the breaker magnetically. (Earth fault loop impedance).
> 
> Click to expand...
> 
> 
> So if fault currents in a system with unknown impedance can cause incendiary events , lowering the mag trip levels AND verifying said impedance would be prudent, _am i close_....?
> 
> 
> 
> 
> 
> 
> 
> Any way, just to give you an idea of what magnetic trip vs thermal trip looks like on a fault consider the breaker trip times here:
> 
> Click to expand...
> 
> The thermal smoked @ 11 seconds there. I guess that _also_ serves as a good Ac/Dc example , for any of us delving into photovoltaics.
> 
> ~CS~
Click to expand...


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## Meadow

chicken steve said:


> So if fault currents in a system with unknown impedance can cause incendiary events , lowering the mag trip levels AND verifying said impedance would be prudent, _am i close_....?


Thats correct. Anytime a breaker can trip magnetically during a fault incident energy and wire stress (thermal and magnetic) is greatly reduced. 






> The thermal smoked @ 11 seconds there. I guess that _also_ serves as a good Ac/Dc example , for any of us delving into photovoltaics.
> 
> ~CS~


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:

https://www.google.com/search?q=ins...67y0-jixwIVglKICh2HwQ-9#imgrc=xDoQaRcqLq3s1M:


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## chicken steve

meadow said:


> 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>
> 
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> 
> 
> 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:
> 
> Click to expand...
> 
> 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~
Click to expand...


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## Meadow

chicken steve said:


> 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. 




> 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 *****. 

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). 






> 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). 









> 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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## chicken steve

meadow said:


> 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 ?
> 
> 
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> 
> 
> 
> 
> A short circuit that does not trip magnetically will have enormous incident energy. Picture cutting into live NM.
> 
> Click to expand...
> 
> errruhm....i really don't _need _to use my imagination....:whistling2:
> 
> 
> 
> 
> 
> 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 *****
> 
> Click to expand...
> 
> .
> 
> and then they're a ****/stripper combo tool.....:no:
> 
> 
> 
> 
> 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).
> 
> Click to expand...
> 
> 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....:001_huh:
> 
> 
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> 
> 
> 
> 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)
> 
> Click to expand...
> 
> .
> 
> I stand corrected then, it should rightly have gone over to the BadOne's fall of potential thread....
> 
> 
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> 
> 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
> 
> Click to expand...
> 
> Understood and thank you for you clarification here Mr Meadow :thumbsup:
> 
> 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~
Click to expand...


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## Meadow

chicken steve said:


> 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. 






> errruhm....i really don't _need _to use my imagination....:whistling2:


I figured, you know this stuff well. 




> and then they're a ****/stripper combo tool.....:no:


 Those singe easily. 





> 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....:001_huh:



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. 





> 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. 







> Understood and thank you for you clarification here Mr Meadow :thumbsup:
> 
> 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. 





> 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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## chicken steve

meadow said:


> 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.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 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.
> 
> Click to expand...
> 
> Methinks my simplistic concept of OCPD functionality just got thrown under the theory bus here....
> 
> 
> 
> 
> 
> 
> 
> 
> 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.
> 
> Click to expand...
> 
> 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?
> 
> 
> 
> 
> 
> 
> 
> 
> 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.
> 
> Click to expand...
> 
> 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~
Click to expand...


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## Meadow

chicken steve said:


> I suppose it would_ regardless _of impedance Meadow.


The concept applies to all short circuit current levels.







> 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. 



> 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.





> 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. 







> 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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## Meadow

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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## Meadow

This explains the difference between time trip and mag trip:


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## Meadow

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.


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## Meadow

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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