Why does the amperage go real high if there is a low voltage short?

[sevensages]

TL;DR Summary

Why does the amperage go real high if there is a low voltage short?

I used to be an HVAC technician. One time I had a service call in which there was no power to the thermostat. The thermostat did not have power because the fuse in the air handler was blown. The fuse in the air handler was blown because there was a low voltage short. The rubber coating on one of the thermostat wires was chewed off by a rodent. The exposed metal in the thermostat wire was touching the metal cabinet of the air handler. This was a low voltage short. This low voltage short caused the amp draw to go so high that the fuse blew. Why did the low voltage short of the exposed metal of the thermostat wire touching the metal cabinet of the air handler cause the amp draw to go so high?

I'm not an electrical engineer like you people. Please explain this to me like I am a five year old.

 

[phinds]

That's completely normal.

Volts and amps are related by the expression V=IR, OR to reverse it, I = V/R, SO ... if the resistance is very very low, then the current has to be very very high (given that the power source HAS the power to deliver a lot of current).

 

[sevensages]

That's completely normal.

Volts and amps are related by the expression V=IR, OR to reverse it, I = V/R, SO ... if the resistance is very very low, then the current has to be very very high (given that the power source HAS the power to deliver a lot of current).

What are you telling me? Are you trying to tell me that the reason that the low voltage short caused the fuse to blow is that the low voltage short caused the resistance to be a lot lower, making the amp draw a lot higher?

 

[Baluncore]

... the low voltage short caused the resistance to be a lot lower, making the amp draw a lot higher?

Yes. Ohm's Law. I = V / R. Current is equal to voltage, divided by resistance.

The main AC supply distributes a standard voltage. The circuit presents a resistance, (or an impedance), that will regulate the current drawn, to deliver only the power needed from the AC grid. I = V / R ; Power, W = V * I.

A short circuit, is a fault condition, where a low-resistance path allows more current to flow than the wires can safely carry. A sacrificial (replaceable) fuse is used on each main AC circuit, to protect the wires from high-current fault conditions, by melting first.

 

[phinds]

What are you telling me? Are you trying to tell me that the reason that the low voltage short caused the fuse to blow is that the low voltage short caused the resistance to be a lot lower, making the amp draw a lot higher?

No, I am not TRYING to tell you that, I am TELLING you that. Can you not follow the math?

 

[sevensages]

No, I am not TRYING to tell you that, I am TELLING you that. Can you not follow the math?

I am not an engineer like you. In the OP, I asked you to explain it to me like I am a five year old.

 

[russ_watters]

A short, by definition, is a low resistance path for the electricity to take. Electricity prefers paths of low resistance.

 

[phinds]

I am not an engineer like you. In the OP, I asked you to explain it to me like I am a five year old.

Well, 5 year old, think of it by way of the water analogy. If you have a narrow pipe, only a limited amount of water can flow through it. The pipe resists the water flow by constricting it like the resitance of an electrical circuit resists the flow of electrons. If you replace the narrow pipe with a HUGE pipe (equivalent to a short circuit for current flow) then the amount of water that can flow is basically only limited by the water supply (the voltage/power).

 

[sophiecentaur]

I asked you to explain it to me like I am a five year old.

Not many five years olds would understand even the most basic of EE and your theoretical five year old might, hopefully grow up to have some awareness that not everything is explicable the way you seem to want.

Rockets go up. Only a Rocket Scientist can fully understand why.

 

[Herman Trivilino]

the low voltage short caused the resistance to be a lot lower, making the amp draw a lot higher?

The "low" voltage is not relevant. It just means it's lower than some other voltage in another circuit. But there are other circuits with even lower voltage. So it's only relevant when comparing to another circuit, which in this case we aren't doing. So forget about it. Your circuit has some voltage, that's all that we need to know.

Current flows through the circuit, measured in amps. Resistance is defined as the resistance to the flow of current. The less resistance the greater the current. In the case of your short you have very little resistance so you get lots of current, which blows the fuse.

 

[berkeman]

The fuse in the air handler was blown because there was a low voltage short.

The "low" voltage is not relevant. It just means it's lower than some other voltage in another circuit. But there are other circuits with even lower voltage. So it's only relevant when comparing to another circuit, which in this case we aren't doing. So forget about it. Your circuit has some voltage, that's all that we need to know.

@sevensages it is not clear what you mean by "low voltage". Do you mean lower voltage than AC Mains voltage, like for 12Vrms distributed in HVAC systems?

When looking at short circuits and how electrical systems react, yes the source voltage matters but the source impedance also matters. And it matters if there is any sort of short circuit current limiting built into the voltage source.

The short circuit current from a simple voltage source (like the 120Vrms AC Mains or a 12Vrms source derived from the AC mains) depends only on the source impedance. And if there is a short circuit presented to that source, then yes, it will most likely blow the protection fuse in series with that source.

But if the voltage source is a power supply, then there will likely be some sort of short circuit current limiting built into that voltage source, which will limit the maximum current that will flow through an output short circuit.

The two basic short circuit behaviors built into most power supplies are burst current limiting (for switching power supplies) and fold-back current limiting (for linear power supplies). Each will limit the short current values to pretty low levels. You can read more about those here:

https://en.wikipedia.org/wiki/Current_limiting

 

[Rive]

This low voltage short caused the amp draw to go so high that the fuse blew.

Just that the fuse were blown does not means that the current was anything really 'high'. There are fuses for small currents too. They are chosen as to blow at current levels still not catastrophic but already faulty in the given circuit.

There is no absolute definition when can current be considered 'high'. It's just high enough in a context: in this case to blow the relevant fuse.

 

[sophiecentaur]

Just that the fuse were blown does not means that the current was anything really 'high'. There are fuses for small currents too. They are chosen as to blow at current levels still not catastrophic but already faulty in the given circuit.

There is no absolute definition when can current be considered 'high'. It's just high enough in a context: in this case to blow the relevant fuse.

Fuses can be used inside a device to protect internal parts of the circuit as well as in supply leads to avoid passing too much current through the lead and heating / melting / burning it. A car will have many internal fuses (two or more fuse boxes), with fuses feeding all the different circuits and limiting the current to each. The battery can provide many tens of Amps (for the starter) but an entertainment centre may need no more than, say 5A.

This can be important in fault conditions and it's all relative to the fuse rating. In the OP's situation the actual fault current would be lower than if there was a short between the Live and the case. This burst of relatively low surge current could make the fuse take longer to blow and could result in a short but finite time during which the internal electronics could suffer from a brown-out on the way to being switched off and upset programming. The OP may find that interesting because it's not 'just the same' as a high current blow.

 

[Herman Trivilino]

@sevensages it is not clear what you mean by "low voltage". Do you mean lower voltage than AC Mains voltage, like for 12Vrms distributed in HVAC systems?

My guess would be an almost definite "yes". Air conditioning techs deal with "high voltage" circuits connected to the mains that power the compressor and fans; and "low voltage" circuits used for things like relays, switches, and thermostats.

Likely the OP is simply surprised by and wants to understand how a low voltage circuit could draw enough amps to blow a fuse when these circuits won't even give you more than a mild shock, whereas great care must be given when working with the high voltage circuits because they can shock you enough to kill you.

The thing is though, these techs have no doubt seen powerful sparks fly when working with low voltage circuits powered by for example a car battery.

Listening to technicians talk about their understanding of voltage and current in circuit electricity is an interesting and educational experience for me.

 

[Rive]

the actual fault current would be lower than if there was a short between the Live and the case.

Considerably

It is often forgotten, but fuses are not just about a single current rating (and type). They have voltage too, and also a (lot higher than the rated current) breaking capacity (current).

... and so when the 12V automotive fuse catches a short at line voltage, it'll go quite dramatic...

 

[russ_watters]

Fyi, HVAC controls/thermostats typically operate at 24VAC.

 

[sophiecentaur]

Fyi, HVAC controls/thermostats typically operate at 24VAC.

This is a good example of PF firing from the hip and a misunderstanding of the terms from the very start. We all do it and we get some chaotic threads, as a result.
Shame your post was not the first reply in the thread.

 

[sophiecentaur]

Listening to technicians talk about their understanding of voltage and current in circuit electricity is an interesting and educational experience for me.

Been there, done that. Apart from the fact that all our lives are kept going with few real technical problems, I would have said it's a miracle anything works at all. We all have our own internal metaphors for technical processes. Other people's metaphors often seem crazy.

I went to school in the fifties and we were made to use the right terms and use proper maths but there are alternative ways through life. Never worry; AI will sort it all out.