LH36400 ( 400 amp breaker trip's on motor fault ) Molded case circuit breaker

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Davebones

Senior Member
We had a 400 amp ( LH36400 ) trip due to a short in a 150 hp motor . I know this breaker has seen two ( 2 ) hard shorts that caused it to trip . I say we need to replace this breaker due to the fact we can't open it up even if we get it tested . Have a supervisor that doesn't like the idea of spending money to replace it . Looking for some opinions on this ?
 

kwired

Electron manager
Location
NE Nebraska
We had a 400 amp ( LH36400 ) trip due to a short in a 150 hp motor . I know this breaker has seen two ( 2 ) hard shorts that caused it to trip . I say we need to replace this breaker due to the fact we can't open it up even if we get it tested . Have a supervisor that doesn't like the idea of spending money to replace it . Looking for some opinions on this ?

other conditions that would be a clue as to how hard that short may have been could change some people's answers.

As far as testing, how much does testing this particular breaker cost compared to replacement cost? This is still a small breaker to some people but is definitely not in the "miniature breaker" class either.
 

Davebones

Senior Member
Received a quote to do a primary injection test for $ 600 . I ask about replacing due to 2 fault's since I have seen on this forum reference to " IEEE 1015 " which talks about replacing a breaker should it see two high short circuit current faults .
 

Davebones

Senior Member
Replacement breaker from manufacture would run about $ 4500.00 . Now since we have a high amperage bussway in that area would be considered better to replace a breaker with a fusible bus switch ? This would then allow us to replace a fuse should a fault occur . With the type of testing we do there is a good chance this breaker will see another hard fault ...
 

kwired

Electron manager
Location
NE Nebraska
Replacement breaker from manufacture would run about $ 4500.00 . Now since we have a high amperage bussway in that area would be considered better to replace a breaker with a fusible bus switch ? This would then allow us to replace a fuse should a fault occur . With the type of testing we do there is a good chance this breaker will see another hard fault ...

Are we talking Square D LH breaker or someone else's? Don't know how much higher LH would be from LA but I have gotten a Square D LA main plus panelboard and panel full of branch breakers for less then $4500, that is how they want to sell those type items, buy an individual and they honk you hard on price.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
How much fault current flowed through the breaker?

The blanket breaker replacement is based on it clearing a bolted fault at the maximum AIC of the breaker.

But, a breaker is designed to interrupt 10x fault current for maybe hundreds of operations
 

Davebones

Senior Member
I know the rated fault at the panel was 33,000 amps . This motor is only about 60 ft away from the main switchgear in that area . I think the motor rewind shop said it was phase to phase shorted .
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I know the rated fault at the panel was 33,000 amps . This motor is only about 60 ft away from the main switchgear in that area . I think the motor rewind shop said it was phase to phase shorted .
Knowing the length and size of the branch wire to the motor and assuming a bolted fault at the terminals (very conservative) you can calculate the maximum fault current ignoring the source (bus) impedance. Do that and see what you get.
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I don't get that far into it . I'm just the plant electrician . Not sure how accurate I would be if I tried to calculate that .
It is just Ohm's law as applied to the resistance you can get from the Southwire VD calculator. No complex computations or simulator programs needed.
But if you would like to get a sanity check on your results here, that would be fine too.
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Replacement breaker from manufacture would run about $ 4500.00 . Now since we have a high amperage bussway in that area would be considered better to replace a breaker with a fusible bus switch ? This would then allow us to replace a fuse should a fault occur . With the type of testing we do there is a good chance this breaker will see another hard fault ...

There is no way this breaker costs $4500 to replace. That may be what the list price is but anyone paying even half of that is not shopping at all. Based on buying similar breakers I would bet it is available for $1000-1500, maybe even less.

But, if you expect shorts to happen, fuses might well be a better choice, although fuse blocks and fuses in that size are not cheap either.
 
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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Fuses add the possibility of a single phasing condition, so adding fuses to replace a CB should then also include the addition of a phase loss monitor down stream if there is not already something there to do that.

Bottom line, your employer must provide a safe working environment. That includes having all electrical protection equipment maintained to be in proper working order. That implies not only periodic testing, but following the manufacturer's recommendations for testing or replacement after serious faults. So they must do something, ignoring this is not an option. If testing is performed and it passes, I would not worry about forcing a presumptive replacement. But if it fails, then someone will have to evaluate the replace vs rebuild equation.

Rather than just automatically deferring to Sq. D, I would get a quote and suggestions from a NETA certified apparatus service shop in your area. Chances are, Sq. D Services is just going to take it to one anyway and mark it up to you. That's the way it works around here.
 
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