AC help. Compressor Sporadically Trips Breaker (air conditioning, load, replacing, duct)

[mathjak107]

Because different fuses and breakers have different time curves . also some fuses are designed to blow only if they see short circuits not some over current . a time curve is how much over current and for how long before a fuse blows or breaker trips .

some fuses will not trip under just over current , they only trip when they see huge amperage draws like a short .

if you give me a fuse number i can tell you what type it is and i can get you a curve , you can compare it to the curve of the bqc breaker .

http://electrical-engineering-portal...current-curves

page 2 is for the bqc breaker

http://www.eaton.com/ecm/groups/publ...tc003002en.pdf

[snebarekim]

Originally Posted by counterclockwise

I don't understand why the 60 amp breaker in the breaker box trips and the 60 amp fuses in the pull out box don't blow. Ordinarily the fuses would blow before the breaker trips. At least you should try replacing the breaker. You can't really properly test a breaker acting like this. I agree with most of the input here you have a weak breaker and if you can't find one to replace it with go ahead and replace the AC unit and see if the breaker still trips.

I got a chuckle from that!

[snebarekim]

Originally Posted by counterclockwise

If the 60 amp circuit breaker circuit feeds the unit through similar rated fuses in the pull out box why do the fuses never blow?

Slo-Blo fuses.

[mathjak107]

Originally Posted by counterclockwise

If the 60 amp circuit breaker circuit feeds the unit through similar rated fuses in the pull out box why do the fuses never blow?

need the part number of the fuse if you want to see a comparison as to what will trip and at what point . .

i retired from the business but i still teach motor controls one day a week , so here is a very quick lesson on fuses and breakers :

the term slo-blow , time delay or dual element fuse only means that it has over current protection as well as short circuit protection .

over current means a slow rising curve that is above what should be drawn but no where near the currents of a short which can be explosive and devastating .

but the term slo-blo means nothing as far as the time curve . within those terms are all kinds of different slo-blo fuses .

in fact i used to do fuse co-ordination studies for my customers .

the difference in amperage between various amperage fuses has to be a certain span or you risk a short down stream on a branch taking out the larger power fuses up stream .

with standard time delay fuses like bussmans frn or frs you need at least 4x the sizes .

so if you have 800 amp fuses up stream , the next size tapped off for a branch has to be less than 200 amp fuses .

there are slo-blow time delay fuses that are called low peak fuses that allow 2:1 . so next stop would allow you a 400 amp switch .

so my point is the term slo-blow really means little as far as fuse time curves .

breakers and fuses will never match each other's curve regardless .one will always trip or blow sooner or later when it comes to over current .

when i design my control panels i use circuit breakers that are short circuit protection only. no motor over current protection at all .

standard breakers are called thermal magnetic.the thermal part refers to the action of the breaker due to motor over current and the magnetic part refers to it's short circuit ability. i use only magnetic trip breakers in my designs as i want the breakers to provide only short circuit protection , no over current protection .

i have overload blocks on my starters to protect the motors from over current . i don't want the breaker making its own determination as to when to trip because of over current because then you can get nuisance trips .

i carefully select the overload blocks to sustain either 500% overload for 10,20 or 30 seconds depending on the motor and application . a breaker has it's own built in time curve for over current that is different and non adjustable. some larger breakers can have the short circuit protection adjustable but not motor over current generally .

on the other hand if i am sizing standard time delay fuses i size them for 125% of the motor nameplate full load amperage .

so breakers and fuses are sized very differently .

sqd has a very good tutorial on the differences .

http://static.schneider-electric.us/...0600DB0601.pdf

[mathjak107]

if my eyes are reading these very tiny numbers correctly , looking at the time curve for a 2 pole 60 bqc and a 60 amp frn fuse

a standard frn type fusetron fuse trips in 10 seconds at a 500% over current .

a bqc breaker trips in about 2 seconds at 500% , so this is why i never mix breakers with over current protection and starters with overload protection .

a standard class 20 overload relay in comparison allows 500% for 20 seconds . some constant torque motors can take as long as 20 seconds to reach operating speed.

there are some that can go as long as 30 seconds so we use class 30 overloads . you can see a breaker would trip way before if it had over current protection too and not just short circuit protection .

european standards are a bit different , they go by 500% over current for 10 seconds .

500% is used because motors generally draw about 5x what they do starting up and then they level off once up to speed .

so there is your answer if you have frn fuses

[Joe33]

Originally Posted by mathjak107

if my eyes are reading these very tiny numbers correctly , looking at the time curve for a 2 pole 60 bqc and a 60 amp frn fuse

a standard frn type fusetron fuse trips in 10 seconds at a 500% over current .

a bqc breaker trips in about 2 seconds at 500% , so this is why i never mix breakers with over current protection and starters with overload protection .

a standard class 20 overload relay in comparison allows 500% for 20 seconds . some constant torque motors can take as long as 20 seconds to reach operating speed.

there are some that can go as long as 30 seconds so we use class 30 overloads . you can see a breaker would trip way before if it had over current protection too and not just short circuit protection .

european standards are a bit different , they go by 500% over current for 10 seconds .

500% is used because motors generally draw about 5x what they do starting up and then they level off once up to speed .

so there is your answer if you have frn fuses

That is what they are supposed to trip at. However usage and time degrade and warp the internal elements.

[mathjak107]

Originally Posted by Joe33

That is what they are supposed to trip at. However usage and time degrade and warp the internal elements.

there is no exact point in time a trip is supposed to happen .

there is an inverse time curve for every breaker and it is totally different than the inverse time curve for a fuse of the same amperage . the two will not trip at the same point in time even if rated for the same amperage .

the time it takes is totally dependent on how much over current there is . it can run in to minutes of time with only slight overages in both cases . standard thermal magnetic load center breakers tend to allow shorter times than a class rk5 dual element time delay fuse.

there is no exact point in time where a breaker or fuse blows or trips. it is a variable time line with variable over current . the greater the percentage over the rating the faster the trip time. it is called an inverse time curve. but even so a breaker and fuse will have different times for the same overage . a bqc breaker trips at 500% in 2 seconds , an rk5 fuse 10 seconds . change the over current percentage and you will get different trip times .

[counterclockwise]

Interesting information and I have been reviewing it but I am not the one with the problem so I don't know what kind of fuses he has. If tripping breaker was my problem OP describes I would try a new breaker and go on to something else. A slower response breaker might allow whatever it is to go ahead and fail completely.

[mathjak107]

the panel type determines the breaker type. you can get breakers with higher short circuit interrupting capacity that will fit (22k aic vs 10k aic ) but the time curve for over current is usually the same . most panels are locked in to a particular breaker type .

you can get that type from various manufacturer's as some will interchange but you lose ul approval mixing and matching since the panel was never submitted for testing with another manufacturers breakers .

i would try a new breaker . the parts do wear .

[Joe33]

Originally Posted by mathjak107

there is no exact point in time a trip is supposed to happen .

there is an inverse time curve for every breaker and it is totally different than the inverse time curve for a fuse of the same amperage . the two will not trip at the same point in time even if rated for the same amperage .

the time it takes is totally dependent on how much over current there is . it can run in to minutes of time with only slight overages in both cases . standard thermal magnetic load center breakers tend to allow shorter times than a class rk5 dual element time delay fuse.

there is no exact point in time where a breaker or fuse blows or trips. it is a variable time line with variable over current . the greater the percentage over the rating the faster the trip time. it is called an inverse time curve. but even so a breaker and fuse will have different times for the same overage . a bqc breaker trips at 500% in 2 seconds , an rk5 fuse 10 seconds . change the over current percentage and you will get different trip times .

Unfortunately you can only use the circuit breaker that fits your panel. Fuses are a little different story but still time and usage on inductive loads tends to degrade them.