Fuses and Breakers

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Karl H

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My boss has repeatedly made a statement to our crew.While i've been present.That has always bothered me."Fuses and breakers just protect the wire."Until i'm corrected by this forum.I will always disagree with this statement.My understanding,Fuses and Breakers are for short ckt and ground fault protection.Because,fuses can be sized well beyond the conductors rating.For starting load current.IMO Fuses and Breakers do not primarily protect the conductor."Electricians" protect the insulation of the wire by sizing per the NEC.Totally dependant on the load being served.Now i do see how "Heaters" or thermal overload protection would do both.Protect the conductor\insulation and equitment.Am i correct?If not please educate me on my errors of thinking.
 
While breakers and fuses do provide shortcircuit protection, they also protect the conductor from overheating in an overload condition by shutting of power. If te wiring is installed per the nec, it should be protected from physical damge.
 
Yes but a Breaker/fuse will allow currnet in excess of it's rating to contiune operation without opening until a certain threshold is exceeded.Ie. a 20I breaker can allow 25I and not trip even for days,weeks,months.I've seen the insulation of conductors melted together in raceways and the breaker never opened.I'm not talking about a FPE i'm talking about a new 20I Siemens BL.
 
Fuses and breakers, properly sized, provide some amount of protection for the conductors. But they _cannot_ provide absolute protection.

If there is an anomalous heat source, for example a loose connection, then the insulation could overheat without tripping the OCPD.

If there is excessive thermal insulation holding the generated heat in, then the insulation could overheat without tripping the OCPD.

If there is excessive ambient heating, then the insulation could overheat without tripping the OCPD.

If the breaker is properly sized under the NEC, but oversized for the ampacity of the conductor using rules that expect overload protection elsewhere (feeder for a motor with OL protection at the motor, for example), then the breaker explicitly does not provide thermal overload protection.

But if the breaker is correctly chosen for the conductors, and the conductors are properly run in acceptable ambient conditions, then the 'trip curve' of the breaker does provide protection from thermal overload. Even though a 20A breaker will permit >20A to flow for a significant period of time, a conductor with a thermal ampacity of 20A continuous can tolerate >20A for a significant period of time. As long as the 'trip curve' of the breaker falls below the 'damage curve' of the conductor, the breaker is providing thermal protection for the conductor.

-Jon
 
Karl would you be happy to know that your kind of right for some specific circuits?

For the majority of circuits, both branch circuits and feeders the fuse or breaker does in fact protect the conductor from overload damage along with short circuit and ground fault protection.

Take a look at 240.4, it specifically requires that we protect the conductors from overcurrent unless otherwise permitted or required by 240.4(A) through (G).

If we think of a 20 amp circuit supplying ten 20 amp duplex receptacles we really have no idea what may be plugged into the circuit at any given time. Someone might plug in a toaster, a space heater and try to iron all at the same time, the circuit will be overloaded and if not for the breaker shutting off the circuit before long we could have a fire.

NOW...on the other hand if we are thinking of a circuit supplying a motor load then what you said is true, the breaker can be sized very large for start up current and the breaker will only provide short circuit and ground fault protection of the conductors.

However that is not the end of the story, the conductors still need to be protected from overcurrent. That protection will be provided by the same motor overloads that the NEC requires to protect the motor. If the motor becomes overloaded and starts to draw to much current the motor overloads will open the circuit saving the motor and the conductors supplying it from overcurrent damage.

The NEC requires conductor overcurrent protection in one form or another for almost every conceivable circuit.

Even service conductors are protected from overcurrent by the overcurrent protection with the service disconnect.
 
Thank you Bob!
Everytime i start on a rampage i always forget to address the situation.Yes we are talking about MOTORS or any piece of equitment with continious start stop cycles.I was'nt talking about OCPD for branch ckts. I was talking about Motors,UPS,Chillers,VFDs,Transformers etc. When will i learn that only I know what i'm talking about without situation or communication skills. lol
 
Karl H said:
Thank you Bob!
Everytime i start on a rampage i always forget to address the situation.Yes we are talking about MOTORS or any piece of equitment with continious start stop cycles.I was'nt talking about OCPD for branch ckts. I was talking about Motors,UPS,Chillers,VFDs,Transformers etc. When will i learn that only I know what i'm talking about without situation or communication skills. lol
Click to expand...

Communication is key. I have problems getting my point across most of the time. :roll: Itall makes sense in your head!
 
Karl H said:
...."Fuses and breakers just protect the wire."...
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The boss is somewhat correct. And so are you.

As IW and Jon said, you're right in that the overloads protect the conductors from, well, overloads. The also do a marginallly poor job at protecting the motor from overloads (an opinion). Repeated overloading of a motor with proper NEC sized overloads will lead to motor catastrophic failure. The conductors will likely be fine.

As IW said, the CB protects the wire from Short Circuit and Ground fault. The CB also protects the structure and the people.

It the motor develops a short or a ground fault, there is no motor to save - it's dead. The conductors however may still be good. The CB should be sized to open before the wire is damaged. This is what your boss is talking about. Most of the stuff I work are not laid out to the NEC. Rather there is a coordination scheme. The generator damage curve, Gen CB protective relays, feeder CB, ovld, wire damage curves are carefully laid out to insure the protective relays trip with minimum equipment damage. Notice I didn't say no equipment damage. Same goal as the NEC, just some different equipment to protect.

Now, about protecting the structure and people: If the motor dies attempting to turn into a molten pile of slag, or if the conductors die in a backhoe attack and turn into an arcwelder, the job of the CB is to put the fire out - save the structure, save the people inside of the structure.

Philosophy time (where's my brother the philosopher when I need him?)
The mission of the protective devices is always the same:
1. Protect the people
2. Protect the structure
3. Protect the conductors
4. Maybe Protect the device (last on the list)
5. Keep the system making money as long as possible
 
Karl H said:
My boss has repeatedly made a statement to our crew.While i've been present.That has always bothered me."Fuses and breakers just protect the wire."Until i'm corrected by this forum.I will always disagree with this statement.My understanding,Fuses and Breakers are for short ckt and ground fault protection.Because,fuses can be sized well beyond the conductors rating.For starting load current.IMO Fuses and Breakers do not primarily protect the conductor."Electricians" protect the insulation of the wire by sizing per the NEC.Totally dependant on the load being served.Now i do see how "Heaters" or thermal overload protection would do both.Protect the conductor\insulation and equitment.Am i correct?If not please educate me on my errors of thinking.
Click to expand...

I think the point your boss is trying to drive home is that fuses and breakers do not protect people.
 
LawnGuyLandSparky said:
I think the point your boss is trying to drive home is that fuses and breakers do not protect people.
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I think it's worth pointing out the obvious. A major function of fuses and breakers is to protect the switch gear.
 
Bob,
Even service conductors are protected from overcurrent by the overcurrent protection with the service disconnect.
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No, the service conductors are only protected from overload. They are not protected from overcurrent as the Article 100 definition of overcurrent includes short circuit and ground fault currents. You cannot provide short circuit and ground fault protection for a conductor by installing the protective device on the load end of that conductor. It appears, based on the title to Part VII of Article 230 that CMP 4 does not understand this point.
Don
 
don_resqcapt19 said:
Bob,

No, the service conductors are only protected from overload. They are not protected from overcurrent as the Article 100 definition of overcurrent includes short circuit and ground fault currents.
Click to expand...

Sorry:rolleyes: ....I did not read the definition of "Overcurrent".

In that case I agree I should have used the word overload.
 
Siemens Panel

Siemens Panel

Hi Everyone.......a Square D 200A main panel is feeding a Siemens EQ12 100 A sub-panel.....the Siemens panel is approx. 12-13 yrs old......the main breaker in the Siemens 16 space panel is overheating, and tripping.......voltages were tested and everything looks ok...has anyone ever had this problem with this panel before?......has there ever been a recall on this panel?............Thanks Guys
 
HotWire367 said:
the main breaker in the Siemens 16 space panel is overheating, and tripping.......voltages were tested and everything looks ok...
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I would take current readings on each line of the Siemens panel. Either the main is tripping because it should be, or its developing heat because of a defect or a high-resistance contact.
 
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