415V Power

[Andrew C]

Anyone still struggling with the concept would do well to draw the graph of the voltage of the 3 phases. It is quite obvious once you see it that the difference is about 400v. It also shows (if you assume the graph to be current rather than voltage) why the neutral doesn't overload. Harmonics excluded!

[MarkPAman]

Anyone still struggling with the concept would do well to draw the graph of the voltage of the 3 phases.

Here's one I made earlier! BR Wiki

 

;)

[topsy100]

do you know what stop playing with stuff that can kill and get someone in who knows what they are doing and understands the significance of balancing loads etc.

[GridGirl]

do you know what stop playing with stuff that can kill and get someone in who knows what they are doing and understands the significance of balancing loads etc.

 

Er, did you not read the parts where the OP said:

 

 

 

Thanks for the quick reply.

 

I wouldn't dream of butchering a cable to suit.

 

I'm just not sure where the 415V comes in. Its more for my own interest. If there is a pre made break out box anyone can recommend I'll have one.

 

 

Thanks

 

I understand 3 Phase, it was just the 415v but that confused me. I was trying to work out in my head 3 x 240 doesn't go into 415 !!!

 

I'll get a 63a 3 phase distro ordered.

 

Thanks for all your help.

 

Cheers

 

Plenty of good and constructive advice has been given in this thread, and clearly the OP has listened and taken heed of what's been said so your comment really isn't necessary!

[Tomo]

Anyone still struggling with the concept would do well to draw the graph of the voltage of the 3 phases.

Here's one I made earlier! BR Wiki

The scale in that diagram is a bit out - you've drawn it as being -240 to +240.

240V is the RMS voltage, and the peak voltage of a sinusoidal AC supply is actually √2*(RMS Voltage), so the scale should be +/-339VAC.

[MarkPAman]

Yes of course it is.

 

It came from my trying to explain to some students how 240 becomes 410 in a 3 phase situation. I figured if I started by saying 240 is really 339 I'd spend more time explaining RMS than 3 phase.

 

So, maybe the graph is showing the RMS value??? But that's not right either. And shouldn't we be talking about 230V?

[Tomo]

It came from my trying to explain to some students how 240 becomes 410 in a 3 phase situation. I figured if I started by saying 240 is really 339 I'd spend more time explaining RMS than 3 phase.

Yes, you've got a point there. It's a tough job deciding which lies-to-children are appropriate for a given situation.

 

And shouldn't we be talking about 230V?

Yeah, but no, but yeah... 230VAC RMS nominal, except that in most of the UK you'll actually see 240VAC.

You'll only get 230VAC from new substations. (Newbuild venues and new housing estates.)

 

'Harmonising' regulations is such a great idea, isn't it?

[pete10uk]

Could we end this topic now?

 

The question was answered in the first 10 min. Some really good comments and for that I thank you all. The post should help anyone else who has the same question.

 

For info I took delivery of a nice shinny Distro this morning.

 

Cheers

 

http://essentialsupplies.co.uk/acatalog/63a_415_Distro_2.jpg

[dbuckley]

So, maybe the graph is showing the RMS value???

A graph of voltage against time cannot show RMS value "on the graph", it can only show the intantaneous voltage at each moment in time. The highest point the sine wave reaches is the peak voltage.

 

You can't "measure" RMS voltage, only calculate it.(*) So fFrom all of those instantaneous voltages you can calculate the RMS voltage of the entire cycle, and then draw that as a solid line across the graph.

 

Could we end this topic now?

No, spoilsport :)

 

 

(*)Actually, you can measure RMS voltages directly, using a hot wire voltmeter, but thats a bit unusual, and I doubt anyone here (including me) has actually used one of these instruments.

[Andrew C]

(*)Actually, you can measure RMS voltages directly, using a hot wire voltmeter, but thats a bit unusual, and I doubt anyone here (including me) has actually used one of these instruments.

Used, not really, but we do have some!

[PaulDF]

How about using a true RMS voltmeter?

[TomM]

Most true RMS voltmeters are calculating an average, and somewhere in the manual it will tell you what time interval it is working over.

[Seano]

Most true RMS voltmeters are calculating an average

RMS is an average, by definition: its the Root of the Mean Square.

It needs to be an average over a long time relative to the period of the AC you're talking about (ie: significantly longer than 1/50s for 50Hz), otherwise its meaningless.

[dbuckley]

RMS shares with "average" the concept that there are a number of instantaneous readings over a period of time that are mathematically processed to give a single number, but "average" is different processing to "RMS". The average voltage is not the same as the RMS voltage.

 

RMS is a bit of a misnomer: RMS is the square root of the mean of the squares of the values.

[Seano]

RMS shares with "average" the concept that there are a number of instantaneous readings over a period of time that are mathematically processed to give a single number, but "average" is different processing to "RMS".

 

I think you're confusing 'average' and 'arithmetic mean'.

Mathematically, that concept of taking a range of values and doing something with them to get a single 'representative' value is really all the term "average" means - it isn't any one specific process, there are lots of them.

Mean, mode and median are all examples of different kinds of average.

Arithmetic mean and RMS (aka quadratic mean) are both examples of different kinds of mean.