Sparkie Question

[Eric in IL]

I read an article in one of the farm magazines a while back that said if you had an electric motor wired to a circuit that was protected by a circuit breaker, you also needed the old screw-in melting type fuse/s in the line before the motor load.

I think they said the melting fuse was to protect the motor NOT the circuit.

Can anyone educate me on what they were thinking ?

Thanks Eric
 

[old]

Sounds wrong to me since a breaker or a fuse does the same thing. Only difference in the 2 is one burns out and you throw it away the other you just flip it back on but either way either one burns out or flips due to an over loaded circuit
 

[Dalex]

Could he have meant the "Heaters " in the magnetic starter ? I'll let someone more knowlegable explain them ....
 

[fixerupper]

I have heard that too. The characteristics are different in how the two react to an overload. Seems like an overloaded motor will melt a fuse before it will throw a breaker of the same amperage, but you have to use the right kind of fuse. I could be wrong. Jim
 

[John T]

Eric, my very first electrical engineering job fresh out of Purdue in the late sixties was with Century Electric Motor Company, so I will try my best to "educate" you as you asked.

1) The purpose of a fuze orrrrrrrrrrrrr circuit breaker is to protect the branch circuit feeder wires from short circuit protection and overload. If you have a branch circuit that requires wire rated for 30 amps, then you use a 30 amp fuze or circuit breaker to PROTECT THE BRANCH CIRCUIT FEED WIRES...

2) Next if you use a motor you also want it protected against over current and overheat so to protect the MOTOR you use some sort of a thermal overload device which may be a device called a "thermal" or a "heater" in a starter or perhaps an internal thermal overload (red button you depress to reset)

3) The fuze or circuit breaker is to protect the BRANCH CIRCUIT FEED WIRES,,,,,,,,,The purpose of a thermal overload or heaters or thermals is to protect the MOTOR.

Perhaps what you heard refers to the engineering design fact that the branch circuit may be protected by EITHER a fuze or circuit breaker while the motor itself is protected by a device such as a heater or thermal which can melt open a link (if excess current) to protect the MOTOR i.e. you need BOTH branch circuit protection PLUS motor protection which is often a heater or thermal similar to a fuze in that a link actually melts and opens the circuit to prevent motor damage from excess current.

YOU NEED BOTH,,,,,,,BREAKER OR FUZE TO PROTECT THE BRANCH CIRCUIT FEED WIRES PLUSSSSSSS A THERMAL OVERLOAD DEVICE (heater or thermal acts like a fuze) TO PROTECT THE MOTOR

Got it????????????????

John T BSEE, JD
 

[John T]

May "sound wrong" but actually its right, EITHER a fuze or circuit breaker is there to protect the branch circuit feeder wires,,,,,,While a thermal overload device (called heaters or thermals in a magnetic motor starter) is there to protect the MOTOR and its a device thats sorta like a fuze in that a link melts open to open the circuit so the motor cant over current or overheat.

There are TWO seperate protective functions involved with powering a motor, 1) A fuze or circuit breaker to protect the branch circuit feed wires, 2) A thermal overload device (thermals or heaters if in a magnetic starter act sorta like a fuze) to protect the motor and you need BOTH for best protection....

Hope this helps

John T
 

[Greg K]

Maybe depends which kind of fuse or breaker. There are instantaneus trip, nontime delay, time delay, and inverse time fuses. Now days most all breakers are dual rated as HACR and whatever the other rating is. However there are also breakers made specifically for HID lighting that are better designed to handle the inrush current of High Intensity Discharge lighting.

That being said, John T is corect in that now, the fuse or breaker protects the branch circuit and to a point the motor. The motor overloads thermally protect the motor.
 

[John T]

Duel element or time delay or K5 etc fuzes are to allow a motor to start since that may be like 5 times the run current!!!!!!! Yeppers, youre right on, the breaker or fuze in the panel is to protect the branch circuit feeders while the thermal overload device is to protect the motor, two seperate and distinct functions....Its NOT rocket scient but to lay persons it may be hard to understand???

John T
 

[Greg K]

It's sometimes difficult for me to understand! It is the same principle on that the breakers protect property and Ground Fault Circuit Interrupters (GFI's) protect people. If you don't understand the function it makes the rest of it confusing.
 

[JMOR]

[wisbaker]

Not to be the grammar police but a FUZE goes in a bomb, its the first thing that goes to initiate an explosive train. A FUSE is a current limiting device placed in an electrical circuit.

Next week we'll discuss the difference between WELDER and a WELDOR.
 

[John T]

Good analogy to a point Id say,,,,,,the breaker protects the branch circuit feed wires from overheating and shorts and that can prevent a fire that could destroy LIFE ORRRRRRRRR PROPERTY

The GFCI is indeed aimed more towards saving a life!!!!!!

The thermals or heaters or internal thermal overload device is to protect the motor

Fun chat

John T
 

[buickanddeere]

You are talking a about two of many kinds of motor and circuit protection.
There is short circuit protection which being either fuses and or breakers . That protects the supply circuit and wiring.
There is thermal protection which is the heaters inside the motor starter. Or on occasion a built in heater and reset button on the motor. This protects the motor.
 

[barnE]

Used to work with Edison engineers. Their oddest statement was that the "Safest house was the house with a fuse box." They insisted fuses were safer than circuit breakers because their calibration of breakers is questionable and they also can stick closed when they should open. Also more reasons I don't recall. Dave the absent minded electrician after 50 years.
 

[John T]

As a retired electrical distribution design engineer I can verify some truth in your statement. The breaker is a mechanical device with springs and other mechanical devices while s fuse is a SIMPLE short piece of calibrated metal which simply melts and opens the circuit at given amperages.....While still not perfect, GOTTA BE MORE FOOLPROOF AND LESS PRONE TO MECHANICAL PROBLEMS.

John T