230V as grid voltage

RE: the bit about higher voltage being more dangerous because it pushes more current. True if you assume that the breakdown voltage is constant, but it isn't. There are a lot of factors that will determine if a given voltage can establish a current flow through an insulating object. For example, standing on a concrete floor in sneakers in a dry environment and 120Vac probably won't kill you. Try the same stunt in bare feet and you're in for a nasty and possibly fatal shock. Do it with water on the floor and you're morgue meat.

Of course going to 240Vac makes the problem worse. At 480Vac you're pretty much in the hospital at best. That's why US code specifies rubber mats under breaker boxes with 480Vac.

Hello ca1ic0cat:

You're absolutely right. In fact it has been dramatically demonstrated with the advent of repairing high tension lines while they're still hot! A tinfoil suit, a helicopter, and a lightning rod and your business. Just don't get near anything that'll complete the circuit.

PS goofed again forgot to market off topic before I post.

Regarding the 110 Volt and 220 originals, you may wish to look at a previous thread ("Voltage multiples of 11"). Essentially, when the change to AC was started, the early power companies modified the meters using rectifiers, and chose used consistent values for the readings to minimise operator errors.

The basic theory is as follows:
DC values included 100 and 200 Volts. As the rectified value of a sine wave is 0.6366 of its peak, and the RMS value is 0.7071 of its peak, that meant that you need to increase the RMS Voltages to (0.7071/0.6336) x the original DC Voltages, or by a factor of 11%.

This survives pretty-much unchanged in the HV distribution system. For the consumer system, I believe that at some point it became possible to tighten the control tolerances; that allowed transmission systems and user equipments that had been designed for 110 and 220 Volt wide-tolerance operation to be used safely with higher nominal supply Voltages. The higher nominal working Voltages were adopted because they allowed the existing wiring to deliver more power.

Confused? I've barely started.

Fyz

Confuse definitely. As soon as I get a chance I'm going to look at the thread that you suggested. Rectified value has me somewhat confused. If my memory has not totally failed yet I did a demonstration for a class sometime ago. First I measured the outlet voltage with a standard DMM. Next I connected a capacitor to the outlet using diodes to convert to DC, then measured the DC charge on the capacitor and it was in agreement with multiplying the measured AC voltage by the square root of two. Of course this would be expected as that is basically how a standard DMM works. To me this would suggest that the .707 would be the rectified value. I've never tried the experiment with a true RMS meter but I would not expect any difference, as I should have had a virtually uncorrupted sine wave. So I may be missing something here but I can't quite see where the other value comes from.

PS. Having gone out on a limb here as I don't have the diodes to repeat the experiment tonight, I'll pick up some tomorrow just to see if I am wrong, and repeat the experiment using a true RMS meter.

If you have a capacitor, you are peak detecting; this would not have been used because it would have been so sensitive to switching pulses, etc.
However, I should have written "full-wave rectified".

I wouldn't wish to use a meter that relies on peak detection for AC measurements. Even the old Avometers used something approximating full-wave rectification, but with the resistor used for AC modified from the DC value to improve the approximation. Modern electronic meters should give true RMS - though there may be issues due to limited sampling frequency with some waveforms

See also http://en.wikipedia.org/wiki/Mains_electricity

Hello John
That's cheating, however I wish I would've thought of that it would've saved me a lot of typing.

Cheating, maybe, but hell - you spelt it out really clearly - brownie point awarded!

(Edit: I seem to have done the same as you - assuming you gave yourself an "off topic". I pulled myself down 1 vote, but once you've submitted, you need 4 collaborators to help pull it fully OT!)

What Wiki doesn't mention is why 230V is the mains voltage. Of course this is the single phase voltage, while the real power is delivered at 3-phase. Keeping in mind the higher the voltage the better, the limit was the VIR (vulcanised india rubber) insulation used on cables at the time.

Hi BabyGuinness - can you give your comments on my post - Single core or three core cable.

I would like to your expert comments on the following:

1) If there is a single core cable and another 3-core cable both of same size, say 150 sq,mm - Can I use these interchangeably in my power connections as incomers to say a load.

The current rating given for a single core cable of 150 sq.mm from supplier catalog is 389 Amps and for that of 3 core 150 sq.mm is 272 Amps.

{I was told by someone today that the current carried by a three core cable is 272 x3 = 816 Amps. So I am now confused. }

When I sized the cable for a 3ph load say motor, I used kW/(1.732 x 440 x p.f x Eff) to get the Full load current. Then divide this by the current the cable can carry (let us ignore the cable de-rating in this question to keep it simple.)

For example if my FLC is 1500 amps and if I use 3c 150sqmm, then i require (1500 /272 = 5.51) 5.51runs, which is 6 runs. Here I will terminate first core of all the 6 runs in Red phase, second core of all 6 runs in Yellow phase and third core of all 6 runs in Blue phase.

But if I use 1c 150sq.mm, then I will use (1500/389 = 3.85) 3.85 runs or 4 runs. But in this case I will go 6 runs as well as I have three terminals (Red, Yel, Blue) to terminate. Am I doing things correctly?

{I was told by someone today that the current carried by a three core cable is 272 x3 = 816 Amps. So I am now confused. }

Need your help

The reason the current rating per core is lower for the multi-core cable is the proximity of the conductors.

If you had 3 individual cores of the same cross-sectional area (CSA) as the 3-core conductors, and packed them tightly together (as per wrapping them in an outer sheath!) you would have to de-rate them so you ended up with the same rating as the 3-core.

The current rating is generally quoted for the whole cable, fully loaded, but on its own in free air. As soon as you go running 2 or more (single- or multi-cores) together, you have to de-rate.

You don't get owt for nowt.

Base cable rating is for free air installation, and then there are "de-ratings' for installation in conduit, underground, wrapped with other phases, etc. etc.

Hi John,

Thank you. So what is the criteria for going for one 3 core or 3 single core cables apart from cost.

Thank u

This isn't the place for going into all the ins & outs of de-rating.

Refer to our local wiring regulations (you don't say where you're from - the IEE publish the "regs" applicable in the UK and many other countries - tho' I understand this is being merged (harmonized?) with something more Europe-wide ).

Your company should have a copy of whatever regs apply. It contains extensive tables for cable rating and de-rating, depending on cable construction etc., and factors such as those mentioned by Ried in post #21.

The info given by "someone" to you is wrong, it will not be 272 X 3 for 3core cable.

It will be 272 only ( i.e per phase).

Hope this helps.

Good luck.

Thanks - I should have stated explicitly in my #20:

"The current rating is generally quoted for the whole cable, fully loaded, but on its own in free air" - and it is quoted as the current per phase, i.e. per core.

John.

Funny how battery powered hand tools keep upping the voltage. What will be the limiting factor?

I've often wondered about this convention as well; although they are all nominal. In the US, house wiring is normally called 220/110, or 230/115, or 240/120. They're all the same. What is screwy, it that in the US Navy, the power and lighting system is 450/117, the lower voltage used for receptacles and lights. Why 117 you ask........I have no idea. I know you could order transformers in the stock system that were 450/117 or 440/115. Maybe having an ungrounded 3-phase system has something to do with it. Each phase to ground reads about 55-60 volts to the hull, depending on where on the ship you take your measurement.

That is the NATO standard since the 50's for NATO warships..... nothing more nothing less. You need to talk to the NATO designers of yesteryear to find more infos.....

Having 3 phase with no ground (the warship IS ground) and no neutral saves a lot of weight of the extra cables, but needs star/delta transformers to get the 117volts for domestics and lighting!!! They are heavier, but not as heavy in total as a neutral would have been.

This also means that generator phase to phase loading is always equal (except maybe if a fault occurs of course!) and no excessive neutral currents either all on one cable!!!

All major motors for pumps etc. are 3 phase and as such do not need a neutral anyway.....

I got hit once by 440 AV 60Hz, its a real killer, I made sure NEVER to repeat the experience!!! R-Y-B are also the color of the stars I saw!!!

USN ships use delta-delta step down transformers for lighting and receptacles. The only place a wye connection is used is on carriers and amphibs in their avionics spaces for testing. However, back in the 80s when I was redesigning sub-tender galleys to install "Burger King" type equiipment, we had to buy 450/220-110 delta-wye transformers to meet the power requirement.

To add to my comment about the delta-delta transformers, load centers used three single-phase transformers connected delta-delta primary and secondary to form a bank. Single phase transformers are used so that one can be removed due to battle damage and still enable to bank to provide 58% power.

This is a bit like CTA transit system grounding for elevated stations. Trains run 600VDC nominal voltage, with rails being the DC negaive return concuctor. Therefore they (try to ) electrically isolate the support structure from earth ground. Which means that passenger station power is also isolated from earth ground. the station power comes in from the utility through an ungrounded Wye-Wye isolation transformer, and everything is grounded to station structure instead of earth ground. Secondary transformers for 120VAC power are wye/delta similar to what takes place on the "NATO" standard ship power.

This is a very timely post, as I am trying to understand the differences and similarities between US National Electrical Code grounding conventions and the UK IEE earthing conventions. Thanks to the intelligent responders to this post for history and technical background.

If I remember what I was taught in Physics many years ago, the AC voltage that we read on a meter is the "equivalent" voltage in DC that would allow the same amount of "work" to be done.

No more no less.

In this case, the same amount of "work" could be of a purely resistive heater in say boiling a fixed amount of water from a fixed cold temperature.....

A DC voltage of say 120Volts should take exactly the same time as say an AC voltage of 120 volts as read on a meter.

As someone else already pointed out, the "peak" voltage of the AC if measured on say an Oscilloscope, would be significantly higher than the meter reading......

True of modern meters; intermediate meters (e.g. "AVOs") made corrections that gave the correct RMS value for AC provided that the signal was sinusoidal. But at the time that the initial values were created, linesmens' meters were as simple as possible; I'm not certain whether this was mainly for reasons of reliability, or to avoid the risk of taking readings with incorrect settings. (Even I'm not old enough to have experienced this directly).