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Author Topic: Which mains system is safer?  (Read 9703 times)

Offline Geezer

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Which mains system is safer?
« on: 27/07/2010 19:03:24 »
Most people are probably aware that the mains voltage at wall outlets in the US is only about half (120 V) of what it is in Europe (230 V).

However, almost all homes in the US also receive a 240 V supply that is used to power heavy loads like electric cookers (ranges), water heaters etc.

I understand the US 240 V system is intrinsically much safer than the European 230 V system. Is there any truth to that?


 

Offline SeanB

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« Reply #1 on: 27/07/2010 20:52:09 »
The voltage is not the reason for safety, but the requirement for earth leakage breakers ( GFCI in the USA) that is mandatory in the EU but optional in the US, makes a big difference in the chances that accidental contact will kill you.

The US system, as well as the EU system, are both historical systems that developed from different beginnings. The US system of initially unpolarised ungrounded outlets made it potentially more lethal, as you could be shocked by equipment that was supposedly off, but where the switch is in the cold wire ( nearer ground potential while the rest is hot) and you either touched the contacts of a lamp or had a faulty cord.

About the only advantage in the USA is that incandescent lamps of the same wattage as a 230V unit has a slightly higher light output due to having a thicker filament able to support itself while running hotter.
 

Offline Geezer

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« Reply #2 on: 27/07/2010 20:57:28 »
Wouldn't the fact that the line to neutral voltage never exceeds 120V make a difference?
 

Offline Bored chemist

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« Reply #3 on: 27/07/2010 22:12:54 »
You need to be roughly twice as unlucky to die from a 110 volt shock compared to a 240 volt shock.
The ground fault interrupter can't be the reason for this since.;
1 the two systems were set up before such interrupters were thought of
2 they are not mandatory in the UK- my house hasn't got one (except for the plug in ones that I sometimes use.)

As Geezer points out, the US system is 240V centre tapped so the lines are never more than 120 volts from ground.
(a similar system using 110 volts centre tapped so the wiring is never more than 55 volts from ground is used on building sites etc here)
 

Offline Pumblechook

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« Reply #4 on: 27/07/2010 23:07:27 »
I did read that although phase to neutral/earth is never higher than about 120 V in the US they have more fatalities per capita than Britain. We do have a high standard of elctrical installations with many homes now having RCDs.  I think they allow sockets in bathrooms in the US??  When I was in Paris the main switch - fuse box looked as if it dated from the First World War.
 

Offline Geezer

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« Reply #5 on: 27/07/2010 23:35:53 »
I did read that although phase to neutral/earth is never higher than about 120 V in the US they have more fatalities per capita than Britain.

That may be true, but it's rather unusual to find a 240 V outlet in a bathroom in the US, and the 120 V ones in modern houses do have GFI breakers.
 

Offline tommya300

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« Reply #6 on: 28/07/2010 01:39:11 »
.
I say the systems are relatively safe, it depends on the respect one has when working on the connections of bare terminations.
When comparing 240 volt two phase system, with a 230 volt single phase system.
At this magnitude in voltage at 60 or 50 Hz, they are both lethal.

 Electrocution at such high voltages followed with that high current, your muscles would contract so hard that your grip would take Devinne Intervention to release your grip.

Looking at the wall sockets US vs. Eu, the US 240v system is split to two separate 120v legs, and are both properly,(supposedly), contained in the wall.

The potential voltage in the US, at the wall socket is relatively low, 120 v.
Where as in Eu, the voltage at the wall socket is 230v.

The 120 volts shock will mostly kick you across the room.
Where as at 230 volt will keep you burning in your seat.

It all boils down to how much respect you have for the systems.
Safeguards today are being more and more enforced, all over the place.

"WHY ELECTRONIC AND NOT ELECTROMECHANICAL ELCBS?

South Africa's 40 years of experience in the manufacture and application of sensitive earth leakage protection devices, together with the more recent global experiences covering millions of installed units over several decades have resulted in a better understanding of not only the advantages, but also of the limitations of ELCBs.

In Europe, where electromechanical ELCB's are used almost exclusively, the question of reliability of installed ELCBs has become a subject of major concern and attention. The popular hype regarding perceived reliability of electronic components was not sufficient to prevent CBI-electric: low voltage, some five years ago, from changing their entire ELCB range from electromechanical to electronic technology ELCBs. The improvements in reliability that resulted from this decision are complemented by their freedom from safety performance limitations often found in lower specification electromechanical earth leakage circuit breakers." ---- http://www.heinemannelectric.com.au/ground_fault.php

Now can we compair EMF off the transmission lines. The voltage there being delivered are in tandem with the EMF. This field holds a potential danger with respect to magnitude and time duration of exposure.

.  
« Last Edit: 28/07/2010 01:49:29 by tommya300 »
 

Offline Pumblechook

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« Reply #7 on: 28/07/2010 02:43:37 »
Im am too tired to comprehend what the difference is between an ELCB and an RCD??
 

Offline graham.d

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« Reply #8 on: 28/07/2010 22:00:35 »
The lower the voltage the less the chance of lethal electrocution so that would make the US system inherently safer. However, as has been referred to, electrocution depends on many other factors. Tommya, I don't think there is much difference in how the muscles respond; one of the inadvertent advantages of AC (rather than DC) is that it does not cause the muscles to contract as severely. For people in electronics who are old enough to work on valve (tube) based systems with an HT supply quickly learnt a few tricks such as touching suspect wires/terminals with the back of a finger so contraction from a shock moves it away and always working with one hand and keeping the other behind your back. As recently as about 20 years ago I remember a guy working on a curve tracer testing a high voltage transistor who had by-passed the various annoying safety features (with inserted matchsticks) and failing to follow these rules; he recovered OK but he was nearly killed.
 

Offline Geezer

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« Reply #9 on: 29/07/2010 02:07:17 »
Having zapped myself a few times on both sides of the Atlantic, I must say the experience at 120 V is a lot less unpleasant than it is at 230/240 V. Consequently, I gained a certain appreciation for the system in the US. I sort of assumed I was receiving two phases from a three phase system, but the voltages didn't seem to make sense.

It wasn't until we moved into our present digs that I (finally) figured out how it works. There is a three phase HT transmission line running along the edge of our property. On the pole behind our house, there is a transformer that supplies us and our neighbour, and it's clear that the input to the transformer is only tapped into one of the HT phases.

That's when the penny dropped and I realised it's simply a center tapped transformer with the center tied to neutral. (If we had a "lightbulb" smiley, I'd insert it here.)

I think it's a bit of a pity they didn't/couldn't adopt a similar arrangement for 230/240 V systems in the UK (I don't know much about the rest of Europe) but I'm sure there were probably many reasons that made it impractical.

 
« Last Edit: 29/07/2010 03:39:16 by Geezer »
 

Offline Bored chemist

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« Reply #10 on: 29/07/2010 06:53:07 »
A centre tap grounded system like that in the states needs two pole switches and two fuses to isolate it.
Since you are not meant to touch the wires it shouldn't matter what system you use.
 

Offline Geezer

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« Reply #11 on: 29/07/2010 07:36:29 »
A centre tap grounded system like that in the states needs two pole switches and two fuses to isolate it.
Since you are not meant to touch the wires it shouldn't matter what system you use.

Obviously you are not meant to touch the wires. But the insulation on wires and equipment can, and frequently does fail, for a whole raft of reasons.

So are you saying it's OK if people get electrocuted because it saves a little money in switches and fuses?

EDIT:

Come to think of it, forget the money saved in fuses. I'm willing to bet that most of the cable plugs in the UK have fuses that greatly exceed the current capability of the cables that connect them to the mains (i.e. the vast majority of the plugs contain 13A fuses) so the fuses might make people feel good, but they really are not doing much good at all. Ultimately, they might help to reduce the risk of a fire, but I think there are far better ways to do that. They will not help in the slightest with regard to electrocution.

So, now we are down to the number of poles switched. Outlets in the UK are switched to reduce the risk of electrocution. Outlets in the US are not switched because the risk of electrocution is reduced by virtue of the voltages (to ground) that appear at any outlet.
 
« Last Edit: 29/07/2010 08:52:36 by Geezer »
 

Offline tommya300

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« Reply #12 on: 29/07/2010 16:02:10 »
The lower the voltage the less the chance of lethal electrocution so that would make the US system inherently safer. However, as has been referred to, electrocution depends on many other factors. Tommya, I don't think there is much difference in how the muscles respond; one of the inadvertent advantages of AC (rather than DC) is that it does not cause the muscles to contract as severely. For people in electronics who are old enough to work on valve (tube) based systems with an HT supply quickly learnt a few tricks such as touching suspect wires/terminals with the back of a finger so contraction from a shock moves it away and always working with one hand and keeping the other behind your back. As recently as about 20 years ago I remember a guy working on a curve tracer testing a high voltage transistor who had by-passed the various annoying safety features (with inserted matchsticks) and failing to follow these rules; he recovered OK but he was nearly killed.

Yea the old trick with one hand tied behind their back. The good old 50s and 60s Knuckle test.
This technique was adopted by fire fighter training, when crawling on the floor through a smoked filled run,  one hand forward with a clinched fist. Am I showing my age?
 I was fortunate to live near the neighborhood, Radio, TV & Appliance Repair store.

 Knuckle tests were not recommended by the same people that used this technique in trouble shooting.

 When it came time for changing that Picture Tube (CRT) this test was a different story.
 You would hear from the corner of the store with a chuckle and a far away whisper Discharge the Flyback George before disconnecting that HV Cord.
 The hysteresis is about 25 KVDC on the B&W. If your not careful it will leave a hole in the contact point of the skin of the
 ET or the the kid next store [B)] with a mental Black and blue glow. I actually witnessed a discharge thought the Bose's hand it put a blemish at the finger tip naturally the TV was unplugged.

This may explain it better, my words previously AC DC Muscle description I posted above may need a little face lift.
Here is a direct quote I found on the net that relates to the issue you have pointed out to me.
 
http://www.thenakedscientists.com/forum/index.php?topic=33139.msg317496#msg317496

« Last Edit: 29/07/2010 16:44:03 by tommya300 »
 

Offline daveshorts

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Which mains system is safer?
« Reply #13 on: 29/07/2010 16:25:55 »
Quote
So, now we are down to the number of poles switched. Outlets in the UK are switched to reduce the risk of electrocution. Outlets in the US are not switched because the risk of electrocution is reduced by virtue of the voltages (to ground) that appear at any outlet.

Is it just to reduce the risk of electrocution? It also has advantages in being able to switch things off and know a fault isn't going to produce a fire - reduce transformer leakage etc.

Of course the big advantage of 230V is that you need half as much copper to carry the same amount of power..
 

Offline Bored chemist

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« Reply #14 on: 29/07/2010 19:38:32 »
"So are you saying it's OK if people get electrocuted because it saves a little money in switches and fuses?"
No, I'm saying that since it's usually a combination of ignorance, neglect and bad planning that lead to people getting a shock, the differences between and among the various systems are relatively unimportant.

"Come to think of it, forget the money saved in fuses. I'm willing to bet that most of the cable plugs in the UK have fuses that greatly exceed the current capability of the cables that connect them to the mains "
I'm willing to bet they are not.
Practically all houses are wired with ring mains that use cable capable of handling a lot more than 13 amps (and since they are rings, the real carrying capacity is near twice that of a single cable.)
This sort of stuff

http://www.wickes.co.uk/Twin-and-Earth-Cable/invt/156196

will handle 23 amps and, as you say, most plugs have 13 amp fuses.
 

Offline Geezer

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« Reply #15 on: 30/07/2010 00:24:25 »
Quote
So, now we are down to the number of poles switched. Outlets in the UK are switched to reduce the risk of electrocution. Outlets in the US are not switched because the risk of electrocution is reduced by virtue of the voltages (to ground) that appear at any outlet.

Is it just to reduce the risk of electrocution? It also has advantages in being able to switch things off and know a fault isn't going to produce a fire - reduce transformer leakage etc.

Of course the big advantage of 230V is that you need half as much copper to carry the same amount of power..

Ah yes, but we're comparing two different systems that both supply 230/240 volts, so the copper argument does not apply.

A counter argument re. the switch is that when you unplug something, you can see it really is disconnected. When you open the switch you "think" it's disconnected, but I've seen switches that still leak substantial currents when they are supposed to be open. Also, the switch may not protect equipment in the event of a lightning strike whereas unplugging will.
 

Offline Geezer

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« Reply #16 on: 30/07/2010 00:57:02 »
"So are you saying it's OK if people get electrocuted because it saves a little money in switches and fuses?"
No, I'm saying that since it's usually a combination of ignorance, neglect and bad planning that lead to people getting a shock, the differences between and among the various systems are relatively unimportant.


I thought we agreed earlier that the center tapped system is intrinsically safer when you pointed out that they use such systems on building sites in the UK? Despite the best planning and best of intentions, "stuff happens". Systems fail, so why not allow them to fail in the safest possible manner?


"Come to think of it, forget the money saved in fuses. I'm willing to bet that most of the cable plugs in the UK have fuses that greatly exceed the current capability of the cables that connect them to the mains "
I'm willing to bet they are not.
Practically all houses are wired with ring mains that use cable capable of handling a lot more than 13 amps (and since they are rings, the real carrying capacity is near twice that of a single cable.)
This sort of stuff

http://www.wickes.co.uk/Twin-and-Earth-Cable/invt/156196

will handle 23 amps and, as you say, most plugs have 13 amp fuses.


It's the 13A fuses in the plug tops that worry me. I'm not concerned about the house wiring. It's properly protected at the distribution panel by a fuse or circuit breaker.

The current rating of the fuse in the plug top is intended to prevent the equipment, or its power cord, from drawing too much current. There are many situations where a 13 A fuse is far too large to provide proper protection. For example, a 3 amp fuse might prevent a fire in the equipment or its power cord, whereas a 13 amp fuse will not.

The plugtop fuse is a good idea, but I think it tends to be frequently defeated when people use 13 amp fuses everywhere.


 

Offline tommya300

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« Reply #17 on: 30/07/2010 01:39:20 »
"So are you saying it's OK if people get electrocuted because it saves a little money in switches and fuses?"
No, I'm saying that since it's usually a combination of ignorance, neglect and bad planning that lead to people getting a shock, the differences between and among the various systems are relatively unimportant.

"Come to think of it, forget the money saved in fuses. I'm willing to bet that most of the cable plugs in the UK have fuses that greatly exceed the current capability of the cables that connect them to the mains "
I'm willing to bet they are not.
Practically all houses are wired with ring mains that use cable capable of handling a lot more than 13 amps (and since they are rings, the real carrying capacity is near twice that of a single cable.)
This sort of stuff

http://www.wickes.co.uk/Twin-and-Earth-Cable/invt/156196

will handle 23 amps and, as you say, most plugs have 13 amp fuses.



Relating to your statement
Man made regulation codes must be based on the systems extreem condition to maintain stable delivery within a predetermined range. Line drop is being observed.
Dave was pointing out Ohms law! Setting the power, being a fixed value, and the ratio of (I R) with respect to the cross section of the wire.
Both are on the money as usual
.
 

Offline Pumblechook

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« Reply #18 on: 30/07/2010 01:39:53 »
Yes but when you buy an appliance it comes with the correct fuse in the plug.

 

Offline Geezer

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« Reply #19 on: 30/07/2010 02:16:52 »
Yes but when you buy an appliance it comes with the correct fuse in the plug.



That's very true. The situation must have improved a lot since manufacturers started shipping line cords with moulded on plugs. Is it still possible to buy a "wire-it-yourself" 13 A plug, or have they pretty much disappeared?
 

Offline Variola

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« Reply #20 on: 30/07/2010 09:45:02 »
Yeah you can still buy plugs, to wire yourself. Moulded or otherwise electrical goods all come with their own fuse in the plug when you buy them, if it blows you replace it with one the same.

And after reading this thread I am now completely paranoid about anything electrical in my house!  :o :o

Bloody men.... *grumble *grumble...  :-\
 ;)
 

Offline imatfaal

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« Reply #21 on: 30/07/2010 11:22:48 »
And after reading this thread I am now completely paranoid about anything electrical in my house!  :o :o

Bloody men.... *grumble *grumble...  :-\
 ;)

Yeah, and after reading your thread on the rspb book's surprise inclusions I am itching everywhere!

Bloody women .... ;D
 

Offline Variola

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« Reply #22 on: 30/07/2010 11:26:53 »
And after reading this thread I am now completely paranoid about anything electrical in my house!  :o :o

Bloody men.... *grumble *grumble...  :-\
 ;)

Yeah, and after reading your thread on the rspb book's surprise inclusions I am itching everywhere!

Bloody women .... ;D
[/quote]

 ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D

You are itching your everywhere?? I have never heard it called that before!!!  :D
 

Offline Geezer

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« Reply #23 on: 30/07/2010 18:35:32 »
Yeah you can still buy plugs, to wire yourself. Moulded or otherwise electrical goods all come with their own fuse in the plug when you buy them, if it blows you replace it with one the same.

And after reading this thread I am now completely paranoid about anything electrical in my house!

Well, you are supposed to replace the fuse with one of the same rating, but I wonder how many people make sure it doesn't blow again by sticking a 13 A fuse in instead  ;D (As I recall, when you bought a plug, it always came with a 13 A fuse in it!)

Mind you, if the plugtop fuse popped, and you don't really know why, (like somebody stuck a knife in the toaster for example) it more than likely means there is something seriously wrong with the appliance, so you should either chuck it, or get it serviced by somebody that really knows what they are doing before you use it again.

Uh oh! I think I just made Variola even more paranoid! (Did you turn off the oven before you went to work btw?)
 

Offline SeanB

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« Reply #24 on: 30/07/2010 20:53:58 »
Building supplies are 110VAC, with the centre tap grounded, thus you have a 55V supply if you touch it accidentally. This is classed as non shocking, as generally your skin resistance is so high that most contact is not fatal. The mains voltage is high enough that you get a significant current flow in case of contact, and the voltage is high enough to kill in each system if the current path goes though your heart, as around 9mA is enough to stop the heart if passed though it for a short period.

Earth leakage is the same as RCB, just the name is used differently in different parts of the world. Fuses have been losing favour in consumer units due to the problem of it being too easy to replace them with higher rated units, as circuit breakers are resettable and generally are quite reliable and do not need you to keep spare units on hand. Fuses are still used by the supply companies, as they do have the important thing of failing safe in all circumstances, though these are not user replaceable.

As to saving copper with 230V systems, remember in the USA a dryer needs a special supply, as it cannot be plugged into a normal 115V socket, but needs a 230V socket due to the power draw. In the UK you just plug it right in any outlet.
 

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