Only half of main tripped - how?

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lakee911

Senior Member
Location
Columbus, OH
So, I got called out to do some electrical troubleshooting today. 240/120V (maybe 208/120, don't know). Turns out a bolted fault on a 1P-20A branch circuit caused the main breaker (2P-100A) to trip and open only one leg of the 3w service when the 20A branch circuit was closed. The main was a Square D QO so I saw the orange indicator was on, but it wasn't in the tripped position. I turned it off and then back on. All was well. Wierd?

I suggested she contact the condo association and see if they would investigate. I told her that it was wired wrong or maybe the main breaker was bad.

Any ideas?
 
lakee911 said:
So, I got called out to do some electrical troubleshooting today. 240/120V (maybe 208/120, don't know). Turns out a bolted fault on a 1P-20A branch circuit caused the main breaker (2P-100A) to trip and open only one leg of the 3w service when the 20A branch circuit was closed. The main was a Square D QO so I saw the orange indicator was on, but it wasn't in the tripped position. I turned it off and then back on. All was well. Wierd?

I suggested she contact the condo association and see if they would investigate. I told her that it was wired wrong or maybe the main breaker was bad.

Any ideas?
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Bad breaker if the internal trip did not open both poles.
 
It may not have been that one pole tripped. There may be bad contacts on one pole and resetting and closing the breaker allowed those contacts to make contact again.
One of the things that happen is that breakers are not exercised and never are opened and closed which both distributes the lubrication in the mechanical mechanism and reseats the contacts when closed as the moving and stationary contacts rub against one another.
On a trouble shooting service call the customer complained that one side of their apartment was dead and when the electric baseboard heating turned on the lights on the dead side glowed dimly.
I found that the energized side was being back feed through the 240v heaters to the none-energized side completing the circuit to the neutral. The contacts on one pole of the 2-pole breaker were not making contact. Exercising the breaker allowed that pole of the breaker to make contact again and restore power to that side of the circuit.
However, I considered that simply restoring power was only a temporary fix until I was able to source a replacement breaker and to schedule an outage for it to be installed.
One thing that I noted was that the side of the breaker with the defective contacts was noticeably warmer than the wither side indicating that the contact resistance was generating heat. I verified it with an infrared temperature sensor.
 
This happens all to often. I see plenty of 2 pole breakers that on trip on one pole. Many of the standard resi branch and main breakers today are only Common Trip not internal common trip. The only thing that makes both poles trip is the handle tie. If the handle tie is not snug or poorly designed it will not trip all poles. I don't know what UL requires but It appears that as long as you turn of one it will move the other pole in the off position. Unfortunately it may not trip both sides.

I don't know how the QO is made. It may have a internal common trip mechanisim. I'll have to tear one open to see!
 
I'll let her know to replace the main breaker. It's outside so maybe its just the lube issue from being in weather and getting little use.

QO uses an internal common trip mechanism--no tie handle on the 2P.

Thanks everyone.
 
Sierrasparky said:
This happens all to often. I see plenty of 2 pole breakers that on trip on one pole. Many of the standard resi branch and main breakers today are only Common Trip not internal common trip. The only thing that makes both poles trip is the handle tie. If the handle tie is not snug or poorly designed it will not trip all poles. I don't know what UL requires but It appears that as long as you turn of one it will move the other pole in the off position. Unfortunately it may not trip both sides.

I don't know how the QO is made. It may have a internal common trip mechanisim. I'll have to tear one open to see!
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I gotta look into this, I thought they were all internal common trip.
 
Handle ties are not UL listed nor are you allowed to tie together (2) 1p breakers as common trip.
The common trip breaker ties together the trip mechanisms of both poles should one pole trip it will trip and open the other.
You know all of those extra breakers you have laying around, the ones that are either cheep or ready to throw away? Take one apart so you can see what makes them work. See if you can identify the thermal and the magnetic element and how they trip the breaker. Take a single pole apart and then a 2-pole from the same manufacturer.
Then you may want to compare them with another manufacturer.

And if you happen to replace that 2P100AT main instead of scrapping it take if apart and compare the contacts.
 
Handle ties are not UL listed nor are you allowed to tie together (2) 1p breakers as common trip.
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I really want to argue this one point but have no UL backing otherwise!

... Take one apart so you can see what makes them work. See if you can identify the thermal and the magnetic element and how they trip the breaker. Take a single pole apart and then a 2-pole from the same manufacturer.
Then you may want to compare them with another manufacturer.
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I really have problems with this statement because it's not our place or "deal" to worry about the construction of a two pole braker, it's approved!

And if you happen to replace that 2P100AT main instead of scrapping it take if apart and compare the contacts.
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Nor do I have the time to worry about all that,I have no answer as to this one part as to a specific breaker per a specific application. Let me say well sure meet the spec's. and confirm that a breaker is meeting the "load".

I've seem many main breaker trip ahead of a single circuit, if thats the case your breakers on the circuit aren't matched for the application! Or your overloading the circuit as to what it can handle ~ ! IE not the right breaker for the job.
 
templdl said:
And if you happen to replace that 2P100AT main instead of scrapping it take if apart and compare the contacts.
Click to expand...

I can not do it because one I'm not an electrician (I'm an engineer), and two I can't break and make the seal on the meter base (where the main is). I'll let the customer know that she'll need to contact her condo association (maybe they will take care of it), or if that doesn't work, then an electrician. So, that being said, I'll probably not get it back in my hands to examine. :(
 
lakee911 said:
So, I got called out to do some electrical troubleshooting today. 240/120V (maybe 208/120, don't know).

Any ideas?
Click to expand...

lakee911 said:
I can not do it because one I'm not an electrician (I'm an engineer), and two I can't break and make the seal on the meter base (where the main is). (
Click to expand...



I don't mean to get picky but who calls an engineer to troubleshoot a residential electrical problem? You did call this lady a customer.

If you are just doing this for a friend I guess it OK but if you are taking service calls as business then it's better just to have them get a service electrician.

Most PEs I know won't touch the equipment, it's not because they don't know about electrical but it's the liability issue.
 
cadpoint said:
I really want to argue this one point but have no UL backing otherwise!



I really have problems with this statement because it's not our place or "deal" to worry about the construction of a two pole braker, it's approved!



Nor do I have the time to worry about all that,I have no answer as to this one part as to a specific breaker per a specific application. Let me say well sure meet the spec's. and confirm that a breaker is meeting the "load".

I've seem many main breaker trip ahead of a single circuit, if thats the case your breakers on the circuit aren't matched for the application! Or your overloading the circuit as to what it can handle ~ ! IE not the right breaker for the job.
Click to expand...

SQuare D QO yes has internal trip mechanism

Homeline , Murray resi stuff, Ge and similar only have a buit in hadle tie. NO INTERNAL TRIP!
they are UL and classified breakers
 
Sierrasparky said:
Homeline , Murray resi stuff, Ge and similar only have a buit in hadle tie. NO INTERNAL TRIP!
they are UL and classified breakers
Click to expand...

Wrong. You need to look at the breaker not at the handle.

Homeline definitely has internal common trip as well as external handle tie.
Even the old ITE QP breaker had common trip as far back as 1982.
 
I guess I never noticed the little slide that trips the other pole. I can see why they fail. The darn thing is open to the elements dust and crap. Don't see why they just don't enclose the whole thing like a SD -QO
 
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