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#141014 - 05/29/04 08:06 AM Debunking myths!
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Hi all,

there has been a thread in alt.engineering.electrical and other groups the last week which has annoyed me. I gave me the idea to debunk a bunch of myths regarding other wiring in other countries.

(We'll leave out the ring/radial this time)

For starters we could start with these ideas. Please comment and add your own questions!

110-120V vs. 220-240V

Pro 120V

"1. The risk of electrocution is significantly lower at 110-120V than at 220-240V."

True.

"2. The voltage is the most important factor when determining the risks with electricity"

No true? Other factors, such as workmanship, protective devices and quality of devices are more important?

Pro 240V

"1. Fire risk is far more important than shock risk. More people die from electrical fires than from electrocution."

True or not?

"2. Currents are lower for the same power, leading to less cable heating and reduced fire risk."

True, but is it significant?

"3. As currents are lower, protective device ratings may be lower."

Only true in theory. In practice, protective devices from 10A to 20A are used on both systems.

"4. Arcing risk. As the voltage is higher, the prospective fault current is higher, making the protective device more likely to trip during a fault."

True?

"5. Users of 110-120V systems believe them to be safer so don't take precautions
against shock, unlike users of 220V-240V systems."

No opinion. Your input?

"6. The lower insulation requirements lead to nasty cheap low quality wiring
accessories"

Not true. The voltages are too low to influence the insulation requirements. In fact, the insulation of the devices are tested with thousands not hundreds of volts.

-------------
I've got a whole bunch of these not related to voltage level but I'll take those some other day.

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#141015 - 05/30/04 02:33 AM Re: Debunking myths!
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
 Quote:
"1. The risk of electrocution is significantly lower at 110-120V than at 220-240V."

Agreed, although those who are unfortunate enough to be electrocuted by 120V end up just as dead as those killed by 240V.

 Quote:
"2. The voltage is the most important factor when determining the risks with electricity"


I think the voltage level involved clearly is one of the factors involved, but I would hesitate to say it's the most important. Once somebody has found himself across the line with current passing through his body, then obviously the lower voltage level will result in a less severe shock, but surely the idea is to reduce the risk of getting connected across the line in the first place?

 Quote:
"1. Fire risk is far more important than shock risk. More people die from electrical fires than from electrocution."


Considering the fairly low electrocution figures in most Western countries, I wouldn't be surprised if it's true. That said, however, how often have there been discussions in ECN over whether a fire blamed on electrical causes was really electrical in nature? So are the electricity-related fire statistics reliable enough to even make a valid comparison?

 Quote:
"2. Currents are lower for the same power, leading to less cable heating and reduced fire risk."

True, but is it significant?

No argument with the first part of the statement obviously, but surely the degree of cable heating is down to I^2*R, and is thus dependent upon the size of the cable?

So long as the cable size is correctly chosen for the current concerned, I don't see this as a significant problem.

 Quote:
"3. As currents are lower, protective device ratings may be lower."


Well, in Britain there's the lower ratings of plug fuses for the protection of appliance cords, but if we're considering only the fixed wiring of the building, then that's discounted anyway.

So long as the OCPD is rated correctly for the cables, is this significant anyway?

 Quote:
"4. Arcing risk. As the voltage is higher, the prospective fault current is higher, making the protective device more likely to trip during a fault."


Maybe true if everything else is equal, but in practice the prospective fault current depends very much on the sorce impedance of the supply transformer, the feeder cables, and so on. Looking at some of the statistics around ECN, I'd say that on average the American supplyis probably capable of supplying a greater short-circuit current than many European supplies.

 Quote:
"5. Users of 110-120V systems believe them to be safer so don't take precautions
against shock, unlike users of 220V-240V systems."


I think this may be down to what people have grown-up with. The average North American might take 120V as "pretty harmless," but because 240V is used for the heavy-dity stuff like ranges and dryers will treat it with a little more respect.

The average Brit/European is used to having 220-240V everywhere, and I've seen plenty of people treat it with very little respect. Yet those same folk would probably be very wary of dealing with 380-415V 3-phase.

 Quote:
"6. The lower insulation requirements lead to nasty cheap low quality wiring
accessories"

Not true. The voltages are too low to influence the insulation requirements.

I'm inclined to agree there. Although devices such as NEMA 5-15 receptacles may be rated at 125V, the construction and insulation materials are pretty much the same as a 6-15 device rated at 250V max.

Besides, I've seen some very cheap-and-nasty 240V devices as well.

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#141016 - 05/30/04 09:49 PM Re: Debunking myths!
djk Offline
Member

Registered: 12/26/02
Posts: 1269
Loc: Ireland
Personally, I think it's a myth that 120V is safer than 230V. The reality is that if you manage to directly connect yourself to either system you are at risk of death!

-----

I agree with paul, American's tend to view 220-240V as being for heavy appliences like stoves and tumble dryers only and thus treat it with some caution. Although I don't think most europeans would mess with a 30A cooker connector either!

-----

The statistics that I have looked through relating to deaths due to electric shock show that in Ireland and the UK it's not very likely that you'll be shocked / killed by domestic wiring. In fact, it's one of the least likely ways to be killed. Most of the very few electrical accidents that happen in Ireland seem to be caused by people coming into contact with distribution and transmission voltages. (>10kV)

-----

Correct grounding of class I appliences and the use of adequate RCD / GFCI protection would seem much more important than the line voltage when it comes to shocks.

-----

When it comes to fires it comes down to regulation and system design. If a system's installed properly and protected correctly against overcurrents it shouldn't catch fire. Regardless of the voltage.

A badly designed 220V system can overload just as spectacularly as a badly designed 110V one!

-----

On the US vs European systems.

Having experienced US electrical systems over the last few days I can say that I feel a LOT safer using an Irish / British or schuko 230V plug than a US 110V equivilant. Plugs visibly spark, it seems like it's quite possible to touch live pins on many plugs.

I also don't like the extensive use of Zipcord on many appliences including things like microwaves, irons and washing machines!! These cables just look fragile compared with a normal European 3-core double insulated cable.

Northern European fittings, cable and fixtures just seem much more sturdy and safer. (even the old ones!)

----

Also, I notice that far fewer US appliences appear to have a ground connection. I've also seen a few cases where the ground pin was bypassed / removed !

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#141017 - 05/31/04 06:37 AM Re: Debunking myths!
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
When considering the overall systems and wiring methods used in North America vs. Europe (rather than just 120 vs. 240V with everything else being equal) I think in both cases it comes down to the quality of the installation, it's proper planning and use of the correct materials.

I know some people from this side of the pond who look at American wiring with apparent horror, often citing things such as no shutters, unswitched outlets, non-sleeved plug pins, and so on.

I think the average American would be equally shocked (no pun intended! ) at some of the methods used here, e.g. the casual way in which service entrance conductors are strung through roof spaces with no conduit, the cramped space in our boxes, etc.

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#141018 - 05/31/04 06:49 AM Re: Debunking myths!
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Hey, great responeses! I'll put it together as soon as possible.

Here are a few more in no particular order:

"1. Wirenuts and Wagos offer a worse connection than connections where the wire is held by a screw."

A single wire held down by a screw is virtually impossible to get loose, but a multiple wires held down by a screw are no better secured than they are by a wirenut. Both types requires the installer to tighten them properly, making careless install a great hazard. Both types work fine with both stranded and solid wires.

Wagos are different in this respect: Either the wire sits there or it doesn't. The wire can be pulled out with substantial force. Only really suitable for solid wires.

"2. Single wires in conduit is superior to cables like NM-B, T&E or NYM."

Only in two cases: If you drive a nail into it, the wires will often move aside. The same thing is supposed to happen with a cable, but it is less likely to happen. Should you drill into the conduit, the wires can be replaced without tearing up the wall. With cables mounted in the right place it is unlikely to happen. Replacing wires inside flexible conduit can be very difficult or impossible.

"3. American circuit breakers don't have instantaneous operation mode, only thermal."

Wrong. The basic design is very similar, although the trip curve is different.

"4. Earth fault protection is not widely used in North America / Europe"

It is widely used around the world, but the protective devices are located in different places: In Europe, Aus/NZ and other places it is typically located in the panel and serves as protection for multiple circuits. In North America, the device is typically located in the socket outlet (receptacle).

"5. Socket outlets in bathrooms introduce a serious danger of electrocution"

No one doubts that the risk of electrocution is far greater in a wet area than in a dry one. For this reason, wiring in wet areas is surrounded by strict regulations. This includes RCD/GFCI protection on both sides of the Atlantic and down under. Without numbers, it is not possible to tell that socket in bathrooms would introduce a siginificant risk of electrocution.

"6. Lack of fuses in the plugs mean that thin appliance cords are only protected by a circuit breaker in the consumer unit."

True. For the circuit breaker to offer sufficient safety two things are required:

a) That the breaker and the expected fault current are matched, in order to ensure an instant trip in case of a fault

b) That the risk of overload on extension cords has been minimised by the use of cords with sufficient current carrying capacity or cord with overcurrent protection.

"9. Neutrals is often used as protective earth in country X."

In practice, it is not very common. Germany did once allowed socket outlets to be connected like this and so did a few other European countries. The US allowed this for certain circuits, but no longer does. In some cases subpanels are fed with this system. These practices are known to have caused various problems.

It remains common practice for utilities to use this systems when connecting buildings. The problems with this appears very limited. (See below)

"8. Bonded neutral and protective earth (TN-C-S) is commonly used by utilities in country X without precautions against neutral failure."

Precautions agains neutral failure include the neutral connected to earth electrodes at regular intervals and/or at the point where neutral and earth are split to form a TN-S system. This appears to be the case world wide. Secondly, it is possible to use a device to monitor the voltage, thereby eliminating the risk. Although simple in theory, it appears this is not used in practice anywhere. The use of overhead lines makes the risk of failure more likely, as the wires and the connections are exposed to the weather in a very different way from underground lines.

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#141019 - 06/05/04 03:10 PM Re: Debunking myths!
:andy: Offline
Member

Registered: 10/18/03
Posts: 272
Loc: Germany
not only the americans may be patriots

 Quote:
Having experienced US electrical systems over the last few days I can say that I feel a LOT safer using an Irish / British or schuko 230V plug than a US 110V equivilant. Plugs visibly spark, it seems like it's quite possible to touch live pins on many plugs.

I also don't like the extensive use of Zipcord on many appliences including things like microwaves, irons and washing machines!! These cables just look fragile compared with a normal European 3-core double insulated cable.


so i agree with that. schuko & H05VV-F 3G1,5 rule

[This message has been edited by :andy: (edited 06-05-2004).]

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#141020 - 06/06/04 04:05 AM Re: Debunking myths!
Texas_Ranger Offline
Member

Registered: 12/17/01
Posts: 2343
Loc: Vienna, Austria
Hehe, yeah, it's easier to rip a Schuko socket w/ box out of the wall trying to pull out a plug than having a Schuko plug fall out of the receptacle...

Still, even though they're more or less illegal by now I like the old round ungrounded sockets. They simply look good.

Maybe I'm just a bit backwards... recently I did some retro-wiring using asphalted cardboard conduit, metal boxes, cloth-covered wires and some cable with black-grey-red color coding. Big room, 5 Schuko sockets 2 old-fashioned ones next to the door and 3 modern ones in hidden places), 6 ungrounded ones (2 of them triples on each side of the window), toggle and rotary switches, some switches and sockets with real glass face plates. Pictures follow as the film is developed.
You get _really_ black sticky hands trying to make joints by twisting the wires and taping them with old cloth tape! Now I know why choc blocks were invented...

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#141021 - 06/06/04 01:30 PM Re: Debunking myths!
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
 Quote:
"1. Wirenuts and Wagos offer a worse connection than connections where the wire is held by a screw."

So long as the right size fitting is used for the wires in question, and those wires are properly stripped and secured, I think they both offer a perfectly secure connection.

 Quote:
"2. Single wires in conduit is superior to cables like NM-B, T&E or NYM."
A conduit system does seem to be more solid at first, but again it comes down to choosing the right materials and installing them properly. PVC conduit still won't stop someone drilling into cables. Different wiring systems for different circumstances.

 Quote:
"3. American circuit breakers don't have instantaneous operation mode, only thermal."

Plain wrong.

 Quote:
"4. Earth fault protection is not widely used in North America / Europe"

I think it's become widely used just about everywhere in the developed world. As you say, the protection levels and locations of the protective devices vary from place to place.

 Quote:
"5. Socket outlets in bathrooms introduce a serious danger of electrocution"

Only if people start to do stupid things with appliances in bathrooms, in my opinion.

 Quote:
"6. Lack of fuses in the plugs mean that thin appliance cords are only protected by a circuit breaker in the consumer unit."

Clearly a quote from a Brit! A counter-argument is that so many people here fit a 13A fuse "because it's a 13A plug" that there's really little difference in practice between a cord on a BS1363 13A-fused plug and one on a 15 or 16A circuit.

 Quote:
"9. Neutrals is often used as protective earth in country X."

"8. Bonded neutral and protective earth (TN-C-S) is commonly used by utilities in country X without precautions against neutral failure."

As far as the U.K. is concerned, the utilities have always insisted on tighter requirements where PME (TN-C-S) is used.

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#141022 - 06/28/04 03:10 PM Re: Debunking myths!
teach Offline
Member

Registered: 06/28/04
Posts: 15
Loc: UK
BS 7671 quotes the threshold from extra low voltage to low voltage as 50V ac and 120v dc
so either would electrocute

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#141023 - 06/29/04 03:12 AM Re: Debunking myths!
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
I'm sure we've all come across those laymen who ask a question along the lines of "What's the highest voltage you can touch and still be safe?" We all know that there's no easy answer to that one.

On the definition of extra-low voltage, let's not forget that limits have risen over the years.

If we go back to the 14th edition of the IEE Regs. we find that extra-low voltage is defined as:
 Quote:
Normally not exceeding 50 volts between conductors, and not exceeding 30 volts a.c. or 50 volts d.c. between any conductor and earth.


So some supplies now considered to be extra-low voltage would have been classed as low voltage prior to 1981.

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