Dear all,
I know that when a live wire touched a natural wire it would become a short circuit. And the mcb would trip.

1) But my question is what happens when 2 phases of a 3 phase power supply (415V) comes into contact. I have read online that for a single phase live wire to come in contact with another single phase live wire, nothing happens as its all the same phase. Right ?

2) Just one more question. In terms of electrons how does a lose connection causes heat and eventually a electrical fire occurs. I have always thought that heat only happens when there is arcing. But for a loose connection there no arcing but there is still heat which eventually will burn whatever is around.

Thank You.

 

The two phases of 3 phase power are equivalent to the live and neutral of a single phase circuit (full power pop, flash and burn with a short circuit).

No need to think about electrons really. The loose connection has high(er) resistance from many factors like smaller cross-section or oxidation. This causes a voltage drop across the loose connection. Voltage * Current = watts = heat and burning.

 

The two phases of 3 phase power are equivalent to the live and neutral of a single phase circuit (full power pop, flash and burn with a short circuit).

No need to think about electrons really. The loose connection has high(er) resistance from many factors like smaller cross-section or oxidation. This causes a voltage drop across the loose connection. Voltage * Current = watts = heat and burning.

Thank you for the reply.

But how does a lose connection cause a voltage drop ?

Looking at V=IR . If my resistance go up, my current will drop right ?

 

Dear all,
I know that when a live wire touched a natural wire it would become a short circuit. And the mcb would trip.

Do you mean a "neutral" wire?

1) But my question is what happens when 2 phases of a 3 phase power supply (415V) comes into contact.

Current will flow between them.

I have read online that for a single phase live wire to come in contact with another single phase live wire, nothing happens as its all the same phase. Right ?

As long as there's no voltage difference between them, I think that's right.

2) Just one more question. In terms of electrons how does a lose connection causes heat and eventually a electrical fire occurs.

Heat is caused because of the resistance of a loose connection.
For example, if you have a connection which looks like 10 milliohms and there's 120 volts feeding 20 amperes through it to a downstream load, that 10 milliohms will drop E = IR = 20A X 0.01μ = 200 millivolts and the connection will dissipate P = IE = 20A X 0.2V = 4 watts, quite a lot.

Let's say though, that instead of 10 milliohms the connection is loose and looks like 100 milliohms. In that case it'll drop 2 volts and it'll dissipate 40 watts, and if it's small enough it could get hot enough to catch something on fire/

I have always thought that heat only happens when there is arcing. But for a loose connection there no arcing but there is still heat which eventually will burn whatever is around.
Thank You.

There can also be arcing in a loose connection, just to make things worse,

 

Do you mean a "neutral" wire?

Current will flow between them.

As long as there's no voltage difference between them, I think that's right.
2) Just one more question. In terms of electrons how does a lose connection causes heat and eventually a electrical fire occurs.

Heat is caused because of the resistance of a loose connection.
For example, if you have a connection which looks like 10 milliohms and there's 120 volts feeding 20 amperes through it to a downstream load, that 10 milliohms will drop E = IR = 20A X 0.01μ = 200 millivolts and the connection will dissipate P = IE = 20A X 0.2V = 4 watts, quite a lot.

Let's say though, that instead of 10 milliohms the connection is loose and looks like 100 milliohms. In that case it'll drop 2 volts and it'll dissipate 40 watts, and if it's small enough it could get hot enough to catch something on fire/



There can also be arcing in a loose connection, just to make things worse,[/QUOTE]



Are you saying that the loose connection will dissipate 4 watts as heat ?

 

1(a): short circuit

1(b): no

2: electrical resistance results in heat. The law of the conservation of energy. Heat is a "lower" form of energy. Who said there is no "arcing", AND who said no arcing is no heat!?!!!? The arcing to heat connection is way out there, in fact examining the levels of types of energy, light is far "above" heat, and Edison's incandescent lamp made people examine those levels and laws much more closely, in order to understand them.

 

Thank you for the reply.

But how does a lose connection cause a voltage drop ?

Looking at V=IR . If my resistance go up, my current will drop right ?

The current drop will be very small if the increase in resistance is small in reference to the entire circuit like in the above post. An increasing fraction of total power in the circuit moves from the load designed to dissipate power safely to the loose connection that usually has only a very limited capacity to safely dissipate power causing dangerous point heating of contacts, wiring and insulating materials.

 

Thank you for the reply.

But how does a lose connection cause a voltage drop ?

Looking at V=IR . If my resistance go up, my current will drop right ?

Only if the voltage across the resistance is fixed.

Consider a 120 Vac that is applied to a 6 Ω load through a connector having 1 mΩ of resistance on both terminals. So the total resistance seen by the source is 6.002 Ω and the current is 19.99 A. The voltage across each connection is 19.99 mV. The heat dissipated in the connector is i²R = (19.99 A)² (2 mΩ) = 799 mW. That's a quite noticeable amount of heat (consider that typical resistors are rated for somewhere between 100 mW and 250 mW), and since most connectors seldom get warm to the touch, shows that that is actually a significant amount of resistance for a connection intended for this much current. But now let's say that corrosion or wear or a loose connection that doesn't press the mating parts together as firmly (hence reducing the effective contact area) raises both of the connections to 100 mΩ. Now the total resistance is 6.2 Ω and the total current is 19.35 A. So, yes, the resistance went up and the current went down. But now look at the connector. The voltage across each of the connections is now 1.935 V and the total power dissipated in the connector is 74.92 W. Small pencil soldering irons, which are fully capable of catching things on fire, go down to the 7.5 W range and here we are talking ten times the amount of heat. If those contacts were to each go to just 1 Ω the total current would drop to 15 A but the power dissipated in the connector would shoot up to 225 W.

 

The phase to neutral voltage vectors are a magnitude of 240 volts, and at 120 electrical degrees apart, referring to the sine wave spacing produced at the generator. A voltmeter measuring between any two phases, phase to phase, will show a reading of :
\(\sqrt{3}*240=415\) volts

 

Heat is caused because of the resistance of a loose connection.
For example, if you have a connection which looks like 10 milliohms and there's 120 volts feeding 20 amperes through it to a downstream load, that 10 milliohms will drop E = IR = 20A X 0.01μ = 200 millivolts and the connection will dissipate P = IE = 20A X 0.2V = 4 watts, quite a lot.

Let's say though, that instead of 10 milliohms the connection is loose and looks like 100 milliohms. In that case it'll drop 2 volts and it'll dissipate 40 watts, and if it's small enough it could get hot enough to catch something on fire. There can also be arcing in a loose connection, just to make things worse,

Are you saying that the loose connection will dissipate 4 watts as heat ?
No. in the loose connection example I cited a resistance of 2 ohms causing a dissipation of 40 watts as heat, while in the non-loose connection 200 milliohm example I cited a 4 watt heat dissipation.

 

2) Just one more question. In terms of electrons how does a lose connection causes heat and eventually a electrical fire occurs. I have always thought that heat only happens when there is arcing. But for a loose connection there no arcing but there is still heat which eventually will burn whatever is around.

"In terms of electrons" doesn't completely explain why the loose connection gets hot. The electrical energy normally travels in the space surrounding the wire/connections when circuit resistance is low and the electrons (that usually don't carry electrical energy in the circuit) form part of a waveguide for energy from source to load. When the electrical resistance increases at a spot in the circuit the electrical energy surrounding the conductors starts to move into the actual conductors from the surrounding space into that spot causing heating as the electrical energy is transformed into thermal energy and is dissipated there instead of flowing to the normal load. We normally engineer electrical power systems to reduce the amount of electrical energy carried by electrons to the lowest amount practical because space is a much better energy moving media for energy fields than matter with free electrons.

 

Dear all,
I know that when a live wire touched a natural wire it would become a short circuit. And the mcb would trip.

1) But my question is what happens when 2 phases of a 3 phase power supply (415V) comes into contact. I have read online that for a single phase live wire to come in contact with another single phase live wire, nothing happens as its all the same phase. Right ?

2) Just one more question. In terms of electrons how does a lose connection causes heat and eventually a electrical fire occurs. I have always thought that heat only happens when there is arcing. But for a loose connection there no arcing but there is still heat which eventually will burn whatever is around.

Thank You.

2 phases touching usually make a bigger bang than phase to common.

Single phase ELCBs usually check that all the current out of the live come back in the neutral instead of going somewhere it shouldn't.

There should be no current in a 3-ph common, so I don't know how that scheme would work.

The delta configuration may not even have a common.

 

Touching 2 phase wires together will result in fireworks of a size dependent on the source current available. I am aware of an incident time some ago where two electricians were killed by arc flash caused by using a meter set to measure current instead of voltage and shorted phase to phase in a power station switchboard.
nsaspook, your apparent explanation of transmission of electricity is rather amusing! Never have I heard that electrons don't constitute the means of current flow! Your explanation seems to suggest we do not need larger cables for increased current since electrons do not carry the current only the magnetic field.

 

If anyone would like to see something interesting check out this link of arcs and sparks. Scroll down to the 480 volt 3-phase Arc Flash Demonstration. Read about it and think about it. The phases were bridged using AWG 28 which is all it took to start things cooking.

An electrical explosion, or "arc flash", occurs when one or more high current arcs are created between energized electrical conductors or between an energized conductor and neutral (ground). Once initiated, the resulting arc(s) can bridge significant distances even though the voltage is relatively low. In the above demonstration, arcs were intentionally initiated by bridging #28 AWG wires across three bus bars in a testing laboratory. When power is applied, the wires immediately explode, forming a conductive plasma which evolve into high-current power arcs between the bus bars. In the above example, three one inch copper bus bars were separated by one inch, and were connected to a 480 volt open circuit source (a large delta-connected distribution transformer). During the 842 millisecond event, the average short circuit current was 17 kiloamperes, and the peak current exceeded 30 kiloamperes. The energy dissipated within a power arc is limited only by the fault current capability of the upstream power source and the duration before protective hardware "clears" (interrupts) the short circuit. In many low voltage (480 - 600 volt) electrical power distribution systems, fault currents can exceed 70,000 amps. The thermal energy liberated within these high-current arcs can be many tens of megawatts - equivalent to several sticks of dynamite. The arc core may reach 35,000 degrees F (four times the surface temperature of the sun!). As the arc "roots" vaporize portions of the copper bus bars, the copper vapor explosively expands to over 60,000 times its solid volume. The incandescent copper vapor rapidly combines with oxygen in the atmosphere, forming dense clouds of cupric oxide, blackening the air and covering nearby objects with black "soot". Globules of molten copper are also violently ejected, showering the immediate vicinity with 2,000+ degree droplets at speeds that can approach 700 miles per hour.

Magnetic forces also propel the arc along the bus, extending it in the process. The high currents also generate huge magnetic forces that can bend thick bus bars or even rip them from their mountings, possibly creating additional shrapnel. Any unprotected individual unlucky enough to be anywhere near this event would be seriously injured or killed. Because of the extreme danger, most countries now require electrical workers to wear protective clothing and headgear whenever working near energized high-energy equipment. Some additional video clips that demonstrate the effects of 480 volt industrial arc flashes and their effects on manikins clothed in regular (unprotective) and protective clothing can be seen on the Westex site. - See more at: http://teslamania.delete.org/frames/longarc.htm#480_volt_arc_flash

Run the .wmv video, pretty neat.

Ron

 

nsaspook, your apparent explanation of transmission of electricity is rather amusing! Never have I heard that electrons don't constitute the means of current flow! Your explanation seems to suggest we do not need larger cables for increased current since electrons do not carry the current only the magnetic field.

My explanation might seem that way to someone who has an electron centered concept of electrical energy transmission.

"We normally engineer electrical power systems to reduce the amount of electrical energy carried by electrons to the lowest amount practical because space is a much better energy moving media for energy fields than matter with free electrons."

That means lower resistance from larger cable or better conductors if needed to reduce the kinetic energy of the electrons to close to zero. Of course electrons as charge carriers are the means of current flow in most connectors but that doesn't mean those charge carriers carry any more than a tiny fraction of the electrical energy in a good conductor. The electromagnetic energy and electron charge carriers form an efficient system of energy transmission through space.

When "electricity" (the charges already inside the wires) is flowing, it is called an Electric Current but "electricity" is not a form of energy.
We have "the quantity of electricity" Coulombs.
We have "the quantity of energy" Joules.

Energy and charge are two different things, so they cannot both be the electricity. It's like saying in the electronic–hydraulic analogy that water is the energy. Electricity enters the loose connection through one wire, and the same amount of electricity leaves through the other wire but energy is dissipated at the resistance instead of flowing past like "electricity". With "Alternating Current" the electricity does not move forward at all yet we still have energy transfer to specific points in the circuit.

 

Here https://www.google.com/url?sa=t&rct...00/pdf&usg=AFQjCNGGz3GPLE5WK6h5Z39sd8webpMHWA is a little blurb about arc faults.

Loose and "stupid" connections contributed to a lot of problems at work.

example: A stupid connection

Using a PCB trace to carry 40 Amps at 20 V, when a lug at the same point is a bette choice, This same 40 A had a 40 kV pulse riding on it.

Using the wrong gauge wire in an expensive dryer. The old dryer was vapor based. The object is to dry a piece of glass, so it microscopically clean. The old way was Freon TF.

Loose connections:

It's really bad when it's the service to a house. A fried had a bad splice at the entrance head. I identified the issue and the power company located the fault.

A 100 kV at 0.1 Amps supply made X-ray analysis very noisy.

In another regulated power supply 13 kV at 1.5 A. It used a tube as the pass element. My predecessor just kept replacing the driver transistor about 1x per year. It took me a little longer, but I went after the root cause. I ended up replacing about three 625K power resistors that was used as part of the HV divider.

It also took took out a 1 Megohm 200 W resistor which was used as a bleeder. I re-checked for any stresses in transistor tube driver and repaired that.

Then I tightened and used locktite all of the connections I could find. The system was good after that.

I think water destroyed the transformer at one point, but the only failure I remember after my repair was the 100 A 3 phase breaker.

Loose connections decrease reliability.