Why aren't resistance loads shorts?

JoeNorm

Senior Member
Location
WA
Basic electrical question that I cannot seem to get a good answer from the internet.

Why isn't an incandescent lightbulb a short? Or any heating element? On a similar thought, why doesn't a motor short through its brushes(I admittedly have a narrow understanding of motors)?



thanks
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I think that you have to start with what is a short circuit and compare that to a resistive load. In the resistive load the resistance of the load is limiting the amount of current as calculated by Ohm's law. For example 2400 watt heater @ 240 volts = 10 amps.

For a short circuit the only resistance is that of the conductors which is very low and allows for a large amount of current to flow. Short together the two 240 volt hot legs and calculate the current using only the resistance of the conductors. That would create a large amount of current that will open the OCPD.
 

JoeNorm

Senior Member
Location
WA
I think that you have to start with what is a short circuit and compare that to a resistive load. In the resistive load the resistance of the load is limiting the amount of current as calculated by Ohm's law. For example 2400 watt heater @ 240 volts = 10 amps.

For a short circuit the only resistance is that of the conductors which is very low and allows for a large amount of current to flow. Short together the two 240 volt hot legs and calculate the current using only the resistance of the conductors. That would create a large amount of current that will open the OCPD.
This is what I thought, thanks for clarifying.
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
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Electrical Contractor
IMO, it is technically a short and I see every circuit with a load as a short. That doesn't mean it is a bad thing....LOL . As trevor said the resistor changes the resistance to prevent the short from blowing the circuit. A light bulb glows instead of tripping the breaker.

I will get hell for this but that is how I would describe it to someone just for simplicity sake.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
Basic electrical question that I cannot seem to get a good answer from the internet.

Why isn't an incandescent lightbulb a short? Or any heating element? On a similar thought, why doesn't a motor short through its brushes(I admittedly have a narrow understanding of motors)?

thanks
Then I'll I get hell for this?....................:)
For a typical 100W incandescent lamp isn't a short. It is roughly ten times the instantaneous current falling to the continuous level. Roughly 4A to 0.4A.
So no, it isn't a short.

Same with electric motors. DC motors can be about 10-30 initial starting. AC cage motors have a typical starting of typically six times.
 

480sparky

Senior Member
Location
Iowegia
If a load doesn't have some resistance, current cannot flow. A load... any load.... with infinite resistance is, by definition, an insulator. And if current cannot flow, no work can be done. Work being a motor turning, an element heating etc.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Technically, if there is no load, there is no circuit.

A circuit is a circular path. It could be the circuit of an electric current or the route of the Indianapolis Motor Speedway.

The NEC doesn't define the word circuit, but uses it as a term, branch circuit, which by their definition is just the conductors.

I think Dennis is confusing 'circuit' with 'short circuit'. The NEC definition doesn't help much in that respect.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
In a normal circuit, the current flows through the intended load.
If a path is created (normally unintentionally) that allows the current to bypass the load (take a “shortcut”), that is a “short”.

Simplistic, I know.
I had been taught that as well, simple yes but that definition would also include a ground fault which IMO is not the same as a short circuit. I think of a short circuit as a shortcut between normal current carrying conductors.

So I guess we call both a ground fault and a short circuit a short?
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I had been taught that as well, simple yes but that definition would also include a ground fault which IMO is not the same as a short circuit. I think of a short circuit as a shortcut between normal current carrying conductors.

So I guess we call both a ground fault and a short circuit a short?
Short to ground, but yes, I agree.
 

kwired

Electron manager
Location
NE Nebraska
If a load doesn't have some resistance, current cannot flow. A load... any load.... with infinite resistance is, by definition, an insulator. And if current cannot flow, no work can be done. Work being a motor turning, an element heating etc.
How about if it doesn't have some continuity.

Infinite resistance is just resistance that is higher than what is capable of being measured or what is higher than some other maximum threshold for the application.

Really high resistance may conduct current too low to measure with a certain measuring device if source is only 10 volts, but increase voltage to 10kV through same circuit and maybe measurable current does flow.
 

480sparky

Senior Member
Location
Iowegia
How about if it doesn't have some continuity.

Infinite resistance is just resistance that is higher than what is capable of being measured or what is higher than some other maximum threshold for the application.

Really high resistance may conduct current too low to measure with a certain measuring device if source is only 10 volts, but increase voltage to 10kV through same circuit and maybe measurable current does flow.

We can talk about 'there is no such thing as infinite resistance' and 'there is no such thing as a perfect conductor'. Yes, technically all insulation is in reality a semi-conductor. But that's theory, not in actual practice.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Engineer
One of the strange things I learned in school electronics was all of the current sources. Problems were given with specified currents. In the real world, we typically have voltage sources. The amount of current that flows from the voltage source is limited by the circuit resistance or impedance. Resistance is typically our "valve" to regulate current flow since voltage is fixed.

100' of #10 copper has a resistance of .124 ohms. Put that wire across a 120V voltage source, and 967 amps will flow. Most people would call that a short circuit.
Put a 9.6 ohm heating element across a 120V voltage source, and 12.5 amps will flow from that 120V source. This will dissipate 1500 watts which is hopefully what was desired by choosing a 9.6 ohm element.

So a short circuit is just a resistive load with a very low resistance.
 

Carultch

Senior Member
Location
Massachusetts
If a load doesn't have some resistance, current cannot flow. A load... any load.... with infinite resistance is, by definition, an insulator. And if current cannot flow, no work can be done. Work being a motor turning, an element heating etc.
Part of your post is mixing up resistance and conductance.

It takes conductance for a current to flow, not necessarily resistance. Zero resistance is infinite conductance, and vice versa. Infinite resistance is an ideal insulator, zero resistance is a superconductor. For a pure resistive load, conductance is 1/resistance.

If a load has strictly zero resistance, and no other form of impedance to go with it, infinite current would flow. The charges would accelerate indefinitely as a consequence of the voltage like a body in free fall. In real world circuits, this short circuit really means that the circuit will draw more current than the source was designed to supply, and overheat it to amounts not intended in its design.
 

480sparky

Senior Member
Location
Iowegia
Part of your post is mixing up resistance and conductance.

It takes conductance for a current to flow, not necessarily resistance. Zero resistance is infinite conductance, and vice versa. Infinite resistance is an ideal insulator, zero resistance is a superconductor. For a pure resistive load, conductance is 1/resistance.

If a load has strictly zero resistance, and no other form of impedance to go with it, infinite current would flow. The charges would accelerate indefinitely as a consequence of the voltage like a body in free fall. In real world circuits, this short circuit really means that the circuit will draw more current than the source was designed to supply, and overheat it to amounts not intended in its design.
Where are you going to find infinite current?
 

Carultch

Senior Member
Location
Massachusetts
Where are you going to find infinite current?
In the zero gravity shop, in the next aisle after the massless inextensible ropes, and frictionless pulleys.

Joking aside, that's what would happen on a theoretical circuit with an ideal DC voltage source and a superconductor to short circuit it. By ideal voltage source, what I mean is a voltage source that always produces a constant fixed DC voltage output, independent of the load current drawn, and that has no internal resistance. No such voltage source exists in reality. This is just the way it is represented as a theoretical example to simplify the math and physics needed to solve the problem.
 
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