Mains frequency

[Mit]

Why is the current frequency in the UK 50Hz when it's 60Hz in most other countries? Isn't 50 Hz more dangerous?

[Tim Baxter]

I have no idea why we use 50Hz but it is no more dangerous than 60Hz.  All it means is that the voltage changes from 0V to +240V and back to 0V, 50 times per second instead of 60 times per second.

[kempie]

Actually only about 20% of countries use 60Hz (including the US) which by no means can be called 'most'.

http://www.school-for-champions.com/science/ac_world_volt_freq.htm

[kempie]

Tim Baxter -  240 V (actually officially standardised at 230 V +10% −6%) has a peak value of ~339 V, so in 1 cycle the voltage change is:

 0 V → +339 V → 0 V → -339 V → 0 V

[Tim Baxter]

Kempie, I don't understand where you're coming from.  Using your figures the voltage varies from +339 to -339 giving a voltage swing of 678V.  How can this be?

Additionally, there is no -ve value swing in our supply it is from +240v to zero and back.  If this weren't the case then both supply wires would swing both +ve and -ve alternately.  As you know, the cables are termed live and neutral therefore only one has the voltage swing.  Surely you're not suggesting that 1 supply cable has 678V at peak?

[rojash]

As kempie says, the peak value is 339 V, but the swing is:

0 _ 339 _ 0 (as Tim Baxter notes there is no -ve swing)

Giving an RMS voltage around 230 - 240 V (I can't be bothered to calculate it exactly)

[kempie]

There is a -ve swing.

Peak to peak voltage of a nominal 230 V ac supply = ~679 V

http://en.wikipedia.org/wiki/Alternating_current

[Tim Baxter]

You are stubborn kempie.  The URL doesn't obviously reveal where you're at.  Ok so you're using peak (as you stated) value which converts to rms of 240V.  But there is still no swing -ve relative to 0V.  The voltage is a sinusoid above the zero line, ok if you put a capacitor in series you would get a swing about zero of 240V (rms), ie. + and - 120V but it doesn't explain where you're coming from.  Anybody else help here?

[BillyBB]

Tim and Rojash, what you are describing is a DC supply that turns on and off. An AC supply alternates between positive and negative supply feed.

[kempie]

Tim Baxter - read the section 'Mathematics of AC voltages' in my previous link and this explains the principle perfectly. Otherwise try this link

A sinusoidal wave of min. 0 V and max. +339 V as you describe would have the effect of a fluctuating direct current.

As BillyBB stated the hint to ac is in the name; the current alternates in its direction of flow. A flow in one direction gives a positive voltage and likewise a flow in the opposite direction gives a negative voltage.

[Tim Baxter]

It's all very well to quote from an encyclopaedia about the characteristics of a.c. but in the real world one of our supply lines is tied to earth.  Because of this one of that line cannot have any p.d. ('cos it's shorted to earth) w.r.t. earth, therefore no voltage variation.

If you look at overhead domestic power lines you will see 4 of them, 3 individual phases @ 240V each (w.r.t. earth) and one earth/neutral.  Each of these 3 phases is fed to individual houses in strict rotation giving each house a supply of 240V (max w.r.t. earth).

Are you sure that you're not confusing the UK with the US system which does have a 2 phase domestic supply which will give a negative going voltage?

[Space]

kempie is right to be stubborn.
Alternative current works just like the way they taught you at school. Not in another way.

The reason why it's called 240V is because 240 is the averaged value of the voltage squared. And that is consistent with the fact that it is a sin wave oscillating between +339 and -339 .

[kempie]

I dont know why you mention the multi-phase nature of transmission lines but, to reiterate, the single phase domestic ac supply is sinusoidal and as such has a +ve and a -ve component which may well be transformed, rectified & smoothed into dc for use by the appliance.

Back to School 

[Tim Baxter]

I (little ol' stubborn me) am now understanding where you're at with all this.  My apologies to you all, you're quite right.  Incidentally, why did we convert from a.c. to d.c. supply?

[Eddiecurrent]

It simply relayes to the rotational speed of the alternator that the power stations use. In the early days of electrical supplies there were many different voltages (both dc and ac) and with regards to frequency there were many different frequencies. In order to standardise the power systems in this country they decided that 50Hz should be standard. This frequency relates to 3000 rpm in those countries that use 60Hz the rotational speed is 3600 rpm. It make no difference to the danger (in fact ac is slightly safer as it passes through a 0 volt point that can enable people to let go, dc doesn't allow this).