Measuring Mains Voltage

Hello

I have been reading a guide about testing ac voltage and mains voltage. It says that common alternating current is 60Hz (in America anyway, 50Hz in UK)  It then says to measure voltage at this frequency.

My voltmeter isn't very sophisticated; nor is my knowledge of electricity. My voltmeter measures ac and dc current but not frequency

I have read i a forum that multimeters, analogue or otherwise, are designed for measuring mains frequency AC @ 50 or 60hz. Does this sound about right? It does to me. I'm guessing Uk ones are optimized for 50Hz and American ones for 60Hz

In this  tutorial : https://www.youtube.com/watch?v=foGhjGu-FUg (only a minute long)

the guy tests mains' frequency with his 'mulitmeter' and gets 0.04KHz reading , though he does say it is not very accurate at low frequencies. He appears to be touching the probes of the metre to  the screws of a plug that is in a multi plug gang  but the plug itself appears to be completely separate and not wired to an appliance.

1) Is this possible/safe or his he using, for example, a grounding plug. I'm hoping he wouldn't post something so reckless if you do need a grounding plug

2)If it is a normal plug , would he have to modify it in anyway? Again he doesn't say the set up of the plug which I find slightly irresponsible but he knows more of the risks than me. I'm a bit confused as i used to teach basic physics including wiring a plug.  we didn't connect the plug to an appliance; we had short pieces of cable with live/earth/neurtal wires and attached those to the plug and the cable/chord could be seen coming out of the plug. In the video the plug must surely be wired but with the  wires cut
so they are under the cable grip so not showing as how couold he measure the voltage if the plug isnt wired?

3) Regarding the tutorial again, am i correct in saying the mains voltage can be measured even if the socket is switched off. I think I remember the potential difference exists in the socket whether it is on or off. So in theory could he have done his test with his unattached plug in a wall socket? Could he have done it with the plug switched on or off or both?

5) Question 4 may answer question 5: does the multi plug gang in the video need to be attached to a wall socket that is on or off?

Many thanks for our help
Andrea EdwardsAsked:
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JohnBusiness Consultant (Owner)Commented:
I have read i a forum that multimeters, analogue or otherwise, are designed for measuring mains frequency AC @ 50 or 60hz.

Yes. Correct. The powerline frequency in your area will be very stable, so no need to measure. My Fluke Digital Meter does not measure this but measures AC voltage very accurately.

the guy tests mains' frequency with his 'mulitmeter' and gets 0.04KHz reading , though he does say it is not very accurate at low frequencies.  

More or less as noted above. Not required.

If it is a normal plug , would he have to modify it in anyway?

No. A Digital Meter (any Voltmeter) is a high impedance device and you need only touch your probes (carefully) to the correct points. Nothing need be modified.

Regarding the tutorial again, am i correct in saying the mains voltage can be measured even if the socket is switched off.

Only at a point before the socket (since it is off and that is the hot wire that was turned off).

I hope this helps.
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EirmanChief Operations ManagerCommented:
Multimeters are the same in UK/IRL & USA & Europe & Asia & Africa etc,
Voltage is Voltage regardless of frequency.
Getting the right voltage for all equipment is very important.
Getting the frequency correct for some equipment (e.g. A/V) is important.
=============================================================
In the video two holes have been drilled into a standard UK/IRL Plugtop for safe reading of mains voltages
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Regarding the tutorial again, am i correct in saying the mains voltage can be measured even if the socket is switched off
NO - It must be connected and switched on.
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JohnBusiness Consultant (Owner)Commented:
Getting the frequency correct for some equipment (e.g. A/V) is important.   <-- I agree with this, but frequency does not vary (by any significant amount) in North America and (so far as I know) not in Europe either.
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EirmanChief Operations ManagerCommented:
If my mains powered electric clock looses a few seconds due to frequency fluctuations
(usually caused by peaks in demand) It always catches up within a day or two.
Power companies in Ireland/UK keep the average @ 50Hz
http://wwwhome.cs.utwente.nl/~ptdeboer/misc/mains.html
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David Johnson, CD, MVPOwnerCommented:
for everything except devices that use the powerline frequency i.e. cheap clocks the frequency is irrelevant.  A more expensive clock will have its own crystal oscillator and generate its own 1 second pulse. The actual frequency is totally outside of your control  The powerline frequency will vary slightly depending upon the load to the generator (more load frequency goes down, lighter load the frequency goes up.  If you need better frequency stability than what is provided then you have to convert the AC to DC and then use your own oscillator to convert it to AC. i.e. using a crystal oscillator that is in an oven (keeping the crystal at a specific temperature) a gps clock would probably be more accurate.  But using a satellite has its own problems, If the satellite is not geo-stationary then you have (a) doppler effect and (b) distance delay. Not counting the actual delay of the sat's internal clock, the delay going to the transmitter, and the antenna delay.  You have the same problem at your end. Antenna, coax, receiver delays (but this will be a constant) Different satellites even manufactured to a very high standard also will vary slightly from satellite to satellite.

Power companies have very poor short term accuracy in the frequency given but they maintain a  high standard of long term frequency stability. to measure a frequency what you really need is a frequency counter. I used to work with cesium time standards (primary standard) and Rubidium Vapour (secondary standard) and if my clock was off by more than 40 ms it was useless to me. And the short term frequency stability had to be better than 10^-20hz
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JohnBusiness Consultant (Owner)Commented:
I have a very good counter with a crystal oscillator in a temperature controlled oven. It is highly accurate and power line frequencies are very stable where you and I are - fraction of a hertz.
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Andrea EdwardsAuthor Commented:
Hi

Thanks for answering. To confirm with John Hurst (or anyone) I'm not sure what this means

Me: Regarding the tutorial again, am i correct in saying the mains voltage can be measured even if the socket is switched off.

You: Only at a point before the socket (since it is off and that is the hot wire that was turned off).

I think you are agreeing with a subsequent answer to your answer  that the socket must be on to be test the mains voltage. You say (i think) that you can measure before the socket (since) it is off which I think extrapolates to saying where I am testing, the socket must be on. If this is correct I can test mains voltage on any standard plug in any socket that is switched on.

Thank you for answering my questions in a linear fashion. It greatly assists a productive dialogue.
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EirmanChief Operations ManagerCommented:
Andrea: I can test mains voltage on any standard plug in any socket that is switched on.
That is correct.

You can get a variety of socket testers here.
They will show whether the socket it correctly wired and switched on.
If it is switched on, you can test the voltage/frequency with a modified plug top with the drilled holes.
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JohnBusiness Consultant (Owner)Commented:
I wrote  " Only at a point before the socket (since it is off and that is the hot wire that was turned off)."

The socket is connected into the house wiring. You can go to the circuit breaker (say) and measure voltage there or go to another socket known to be on the same circuit and measure the voltage there.
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Andrea EdwardsAuthor Commented:
Thank you for your comments
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JohnBusiness Consultant (Owner)Commented:
You are welcome and I was happy to help
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