RELAYS 101

Ohm's Law

Z = E / I;  I = E / Z; E = IZ. Also while not part of Ohm's Law calculating Impedance is important. Z = R + jX (and the related) Z = √((R*R)+(X*X)).
So, one can pre-calculate the impedance of a power line as it is being constructed (using known values of Resistance and Reactance), then protection can be accomplished by measuring the amount of "Z" to a fault with the quantities of Voltage and Current as measured within the substation through PT's and CT's.
This is the alpha and omega, the start and finish of protective relay training. Everyone has learned Ohm's Law but few fully realize just how important the law is to Distance Relaying. In many cases only the Protection Engineer sees Ohm's Law as important, while many protective relay technicians forget about it. Many times the relay tech is trying to learn a process of testing relays, without realizing that the distance relay is all about Ohm's Law and that knowing this helps in the understanding of testing protective relays.
There are specific step-by-step learning modules near the end of these presentations. Believe it or not the complex training modules are still based upon Ohm's Law. Before we get to those final distance calculations we learn other techniques that will help complete your understanding of how things work and how we can calculate equivalent test values. 
The reason why Ohm's Law is so important is because the Impedance changes for a number of factors but a change in distance (in miles) of the protected line is proportional to the change in Impedance of the protected line. Therefore, one can monitor Voltage and current and determine MILES!. 
On this page:
Top Section: Enter any two of the three values and the third will be calculated.
Bottom section: Enter your horizontal (R-resistance) and vertical (X-reactance) coordinates and convert them to polar quantities. Or vice versa, enter your polar quantities and convert them to rectangular quantities.