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Old 08-26-2009, 02:09 PM   #1
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Table Saw trips breaker - need troubleshooting help


Here's what I'm dealing with:
- TWO brand new 20AMP AFCI breakers feeding two different unrelated runs of recs in my sunroom. Both circuits are having the same problem.
- They are Square D Breakers QO120AFCI (10kA Issue No AD-8682) Branch/Feeder [not combination type].
- All 12/2
- All recs new COOPER 15 AMP/125 V TR
- No other electrical devices are plugged into the circuit.

My contractor plugged in his Bosch table saw today, upon start up it tripped the breaker. I happen to have the exact same saw - we tested it and same result - flip the switch and as the blade starts to spin up; breaker trips.

My Makita miter saw also did the same (starts to spin - breaker trips). And we did plug this one directly into the outlet (eliminating extension cord from equation).

On other items both circuits work fine. Contractor ran his Dewalt miter saw on it, also his small air compressor. I have also run a space heater and heat gun on both of these circuits with no problem.

Whisky Tango Foxtrot?

I would worry that I had a real arc fault, except this is happening on two completely unrelated circuits and other devices can run just fine.

How can I figure this out? My concern is that I have a REAL problem somewhere in my wiring that I need to fix. How can I diagnose this?

The elephant in the room is nuisance tripping... is there a way to rule in or rule out this possible red herring?

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Last edited by Leah Frances; 08-26-2009 at 02:39 PM.
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Old 08-26-2009, 03:27 PM   #2
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Table Saw trips breaker - need troubleshooting help


Welcome to the world of AFCI false trips
Check & make sure the Square D's were not recalled

http://www.cpsc.gov/cpscpub/prerel/prhtml05/05035.html

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Old 08-26-2009, 04:41 PM   #3
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Table Saw trips breaker - need troubleshooting help


To eliminate excess current draw as the cause, spin up the blade in the correct direction with an elec. drill to maybe half normal speed. This will eliminate the start-up surge.
A blade not cutting wood will draw minimal current, but I can tell you that a motor with a commutator will always spark.
What percent of these sparks, each second, will exceed the trip limit for the signature analysis circuit in the AFCI? If I told you, the AFCI manu's would have to kill me.

Sounds like a new Google search is in order, with the words

patent afci commutator nuisance false trip

WTF, indeed!
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Old 08-26-2009, 05:50 PM   #4
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Table Saw trips breaker - need troubleshooting help


Thanks - They are supposed to be 'good' the ones with the green buttons haven't been recalled, yet.

Grrr. Grrrr.
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Old 08-26-2009, 08:36 PM   #5
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Table Saw trips breaker - need troubleshooting help


I don't know about the current AFIs, but when they first came out, the 'arc detection circuitry' was nothing more than a 30 MA GFI, and a 75 amp instantaneous trip unit.

Every breaker has at least two trip units in it. Some have more.

1) Instantaneous. Also known as short-term, or magnetic. This unit is designed to trip the breaker quickly (usually in less than two cycles of the AC wave) when a short circuit is detected. A typical 20 amp breaker would need to see more than 150 amps to trip this way. An AFI is set at 75 amps.

2) Thermal. Also known as long term. This unit is designed to trip the breaker before the wire connected to it overheats during a sustained overload. A typical 20 amp breaker will trip in about 5 seconds with a 60 amp load.

3) Ground fault. Known as differential protection in technical terms. This unit simply looks at current going out the hot, and compares it to current coming back on the neutral. If there's more than a 5 milliamp difference, it'll trip the breaker. Some GFIs are designed for equipment protection and trip at 30 milliamps.

There are devices that will correctly identify the true characteristics of an actual electrical arc, and trip a breaker. Some of these can even tell how far away the arc is. (With amazing accuracy). Most of these are found in utility switchyards and meterhouses. They cost about $3500.

Most larger power tools will draw more than 75 amps at startup, thus tripping an AFI breaker while a standard breaker will hold.

There's very likely nothing whatsoever wrong with your wiring. In my opinion, you're simply a victim of the requirement of yet another so-called safety device that was lobbied into law by manufacturers using junk science for the sole purpose of producing additional profit.

Rob

P.S. Whiskey...Tango....Foxtrot.....Those are airplane terms. Do you hold a certificate issued by the FAA per chance?

Last edited by micromind; 08-26-2009 at 08:39 PM. Reason: Added P.S.
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Old 08-26-2009, 08:53 PM   #6
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Table Saw trips breaker - need troubleshooting help


As far as your concern about there being a REAL FAULT (arcing between two points) on the circuit. Or is it just NUISANCE TRIPPING, as is the case with many AFCIs. It appears that it's just the latter. Because --as you say (or post)-- that except for your table saw and the Mitre (Miter) saw the breaker does not trip. If you had a real arc fault, the breaker would trip everytime you plug something in. (Now more than ever)Don't Drink and Drive!
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Old 08-26-2009, 10:41 PM   #7
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Table Saw trips breaker - need troubleshooting help


I believe that capacitive loads like capacitor start motors with cause them to trip. The bigger the capacitor the more likely it is to happen.

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