ohms calculation

[eager2learn]

can someone tell me the formula for short circuits in relationship to your meter and the ohm reading you get on it. i heard that according to the meter reading you can figure out an approximate distance to the short or problem

 

[charlie]

Interesting. I seem to remember that from the very distant past but I don't have a clue.

 

[rfwells]

can someone tell me the formula for short circuits in relationship to your meter and the ohm reading you get on it. i heard that according to the meter reading you can figure out an approximate distance to the short or problem

I think there is a NEC table that gives resistance per 1000 feet. Simply take the reading, then calculate the length as a ratio. Better have a good meter.

 

[gar]

090302-1021 EST

eager2learn:

If you assume a zero ohm short at the point of the short and equal diameter wires, then the measured resistance divided by (two times the resistance per foot) of the wire will be the distance. Simple logic. This is because from the point of resistance measurement to the short is 1/2 of the total wire length being measured, and the resistance you measure is the total wire length to the short and back.

There are problems to this method. First is a knowlege that the resistance of the short is very much smaller than the wire resistance. The second is the use of an effective means to measure low resistance. You want to treat the resistance under test as a 4 terminal resistor. Then you want to use an adequate current source and a millivolt or microvolt meter.

.

 

[480 Live]

480 Live

480 Live

Chapter 9 table 8 gives the resistance of the conductors. The resistance is so small that you will not be able to tell where the short is because of all the unknown variable, is it a direct short to the ground wire, shorted to earth, shorted to another hot conductor. If you know the above what is the resistance of the actual short, usually not zero. Since all the values are small and even the where you place meter leads to the conductor may have some resistance. If you want to try it make sure that the total resistance must be dividing in half to find the length of wire.

 

[jumper]

You need a TDR to really do this right.
http://www.ecadusa.com/3140R.htm?gclid=CJCe49XIjpkCFQO5GgodxizzZQ

 

[eager2learn]

so there really isn't a true formula - more like a guess - timation

 

[gar]

090306-2245 EST

eager2learn:

It is not that there is not a formula. The problem is that you lack information on the resistance of the short.

You need to concentrate on fundamentals and understanding circuits rather than just memorizing formulas. A fundamental understanding of circuits and components will allow you analyze most problems correctly. Whereas just plugging numbers into some equation may or may not give you the correct answer if you do not know the fundamentals that the equation is based upon.

Look up four terminal resistors and see if you can understand the reason for their existence. This knowledge could be important to measuring wire resistance.

.

 

[wayne t]

fluke makes a meter to do this for you

 

[eager2learn]

is there an illustration for the four resistors

 

[gar]

090308-2230 EST

eager2learn:

I referred to a 4 terminal resistor, not 4 resistors. A typical resistor has 2 terminals. Thus, a four terminal resistor is a modification of a two terminal resistor.

When measuring or using low resistance resistors the resistance of the connection thru which the current thru the resistor flows my be a large fraction of the resistor's resistance. Thus, to avoid measuring these error voltages when measuring the voltage across the resistor two additional terminals are added to the resistor inboard of the terminals for the current. Only the metering current which is very low flows thru these voltage terminals and thus only very small errors are introduced.

See my web site and photos P19 thru P21 for items related to four terminal measurement
http://www.beta-a2.com/EE-photos.html

.

 

[chiefelec]

can someone tell me the formula for short circuits in relationship to your meter and the ohm reading you get on it. i heard that according to the meter reading you can figure out an approximate distance to the short or problem

I think this will help http://www.marcspages.co.uk/tech/5107.htm

 

[iwire]

Seeing as I only carry a basic meter with me here is how I use it to help find a short.

At the supply end of circuit I take a resistance measurement.

At the far end of the circuit I take a resistance measurement.

Using the info gained from those two tests I get a rough idea which end of the circuit has the problem. This is very helpful when trying to track a ground fault on a fire alarm system.

 

[mikehughes8]

Or you could get a megger or what we call a thumper and just set it on burn mode. Not only will you find your short you could create others in the process. It's lots of fun!