Forum Electric: Electrical forum, Electrical engineering, electrician, power plant, energetics,wires

crale · Joined: 30 May 2007 Posts: 2

Can someone explain to me how an IEEE 67 device works (AC Directional Overcurrent Relay)?

My confusion starts and ends with the fact that AC reverses direction 60 times per second so it never really gets anywhere. If that is the case, how do you have a flow of electrons in an AC circuit in any direction since the net flow would be zero?

contrex · Joined: 29 Oct 2006 Posts: 57

you need to do a lot more reading! Your idea of what electrons do in an electric circuit is mistaken. How do you think the power gets from the power station to your house many miles away? Radio waves "reverse direction" many million times a second, but you still hear the DJ on your radio set. Ocean waves "reverse direction" periodically, but they still break on the shore. Sound waves do, but sound still gets to your ear.

yanici · Joined: 15 Jul 2006 Posts: 208

I saw a funny video and if I can find it I'll post the site. The video explains that the power companies generate electric current and gets it back again after it completes the circuit. Then they send that same electric current out over and over again. So, they have already filled the wires with current and haven't really had to make any since 1937. Amazing! :wink:

crale · Joined: 30 May 2007 Posts: 2

I'm not sure how power gets from the power station to my house. That's why I'm asking the question on this forum.

I thought that's what this forum was: "Electric theory discussion." If I have posted to the wrong forum then please point me in the right direction. I was hoping that someone would be able to explain it simply, not ridicule my ignorance or suggest a video of how the power company sells the same electricity.

Thanks for nothing.

yanici · Joined: 15 Jul 2006 Posts: 208

Hey crale. Loosen up a little. I think your shorts are too tight.

contrex · Joined: 29 Oct 2006 Posts: 57

kaaud · Joined: 08 Feb 2007 Posts: 6

Actually that is a good question,I never thought of that,an all theories about current flows are based on DC current,And I think u guys r making these jokes just because u r not sure of the right answer,and to the poster who talks about sound and ocean waves,u r talking about transverse and longitudinal movments of propagations,I know that,but this doesnt relate to AC current,sorry, nice try though

Can anybody help?? this is what the forum for,thx in advance.

WFO · Joined: 21 Jan 2007 Posts: 19

The best way to think of AC vs DC is to consider the AC wave at one moment in time. If you freeze a sine wave at one point, then you can look at the current and voltage relationship to each other and the math (power =volts X amps) works just like the DC (I'm ignoring phase shift here for simplicity).

During one half of the cycle, work is being done. During the second half of the cycle, work is also being done. By their theory, you could never cut through a board with a hand saw because the pull would undo the work of the push.

Push down on a spring. When you release pressure, the spring rebounds. Theoretically, you are back where you started and nothing has been accomplished. But obviously it took work to compress the spring and the rebound could be harnessed to supply an equal amount of work.

I'm not that sure about a 67 relay (I thought a 67 device was a definite time adder for an instantaneous) but I'm sure it is based on referencing the voltage to the current. In one direction, the current would be in phase with voltage. In the other it would be 180 degrees out. I know that zone relays use this phase shift compared to the impedance of the line to calculate the distance to the fault.