# Why only one prong in the plugs are not showing response to tester pen?

I have 2 pin outlet for A.C current . Now when I connect a TV plug in that and when I touch one of the prongs with my tester, the tester is showing response. But on the other prong the tester shows no response.

I am using an A.C current(50 amps)(230 volts).

As far as I know,

Both the two prongs will act as a phase for every 1/50th second alternatively.

i.e For the first 1/50 second one hole in the outlet must act as a phase and in the next 1/50 second other outlet hole must act as a phase. And this cycle must continue...

So , If i touch any prongs my tester should show response. But only one is showing response.

If my concept is wrong please ignore my mistake and explain where Ive gone wrong.

• Any electician apprentice learns the why in the first weeks. And in any case : extremly off topic. -1 – Georg Jul 21 '11 at 18:06
• @Georg ok sorry to ask here again, could you answer it. I couldn't find the answer. thats why i am asking. I am eager to know the answer too.and why i am posted here is I thought this question is related to electricity and transitively to physics. – EAGER_STUDENT Jul 21 '11 at 18:14
• 1st withot knowing where You lice, there is no way to answer: UK? USA?, civilized world? :=) Look in Wikipedia for Your countries wiring scheme and what "live" "neutral" and "earth" for the three wires mean. – Georg Jul 21 '11 at 18:28
• @Georg What ever the country may be... The "basic concept" of A.C current transmission is same na.... My question is though 2 ports of the outlet acts as a phase alternatively for every 1/50 th second, why the tester pen responds to only one port. please tell me whether my concept of A.C transmission is correct... well I am an Indian. – EAGER_STUDENT Jul 21 '11 at 19:00
• The systems are not the same! And the assuption You repeat is wrong!. Did You look for the keywords I wrote ? – Georg Jul 21 '11 at 19:59

Does not work like this. In EU and UK (and most of the world), one prong is at nearly zero voltage, other prong is sine wave, with amplitude 340v or so (240v RMS), with equation $v\sin(\omega t)$

US uses 120v for appliances such as TVs, lightbulbs, etc. However, in the US you might also encounter two-phase wiring where one prong is at $\frac v 2\sin(\omega t)$ and another is at $-\frac v 2\sin(\omega t)$ , used for electric stoves that run at 240v. You might also encounter the wiring that uses 2 phases of three phase system, with voltage of about 200v between them but 120v from each to ground. Which happens because the equations are $v \sin(\omega t)$ and $v \sin(\omega t + \frac 2 3 \pi)$ , the phase is offset by third of a circle. Ahh, and my apartment seem to be using full three-phase (with 400v between two phases) for the electric stove. Not quite sure. All in all there's a wide variety of wirings.

edit: ahh and also:

As far as I know,

Both the two prongs will act as a phase for every 1/50th second alternatively.

i.e For the first 1/50 second one hole in the outlet must act as a phase and in the next 1/50 second other outlet hole must act as a phase. And this cycle must continue...

This is always incorrect, everywhere in the world. The voltage of a sine wave source in general is given by $v\sin(\omega t + \alpha)$ where $\frac {\omega} {2\pi}$ is the frequency and $\alpha$ is time offset. The 'phase' aka 'live' is any wire with such variable voltage. The 'neutral' is wire with nearly zero voltage. The role of phase and neutral never alternates. What you are thinking of is a two phase system where one phase is positive and other is negative for 1/100th of a second, then vice versa, then back again (cycle repeats in 1/50th of a second)

The answer is that one prong has near zero (ground) volatge, and the other varies. You get three wires coming off the distribution antenna, a floating ground, and two hots, of different sign. The voltage between the two hots is 240, and between a hot and the float ground 120. Most plug pairs utilize opposing hots, so combining both plugs you have (non code compliant) 240V available. The floating ground may drift a few volts from actual ground, thats what the third post is for, so you shouldn't count on it as grounding for safety purposes.

• Omega, can You imagine there is a world with reasonable wireing/voltages/number of phase outside US? Eager_student is from India! – Georg Jul 21 '11 at 20:01
• yes this answer is not applicable in most other countries and it is kind of dangerous to discuss it without proper caveats because you can kill yourself if you fiddle around without knowing exactly what you're doing :) – BjornW Jul 21 '11 at 20:24