Electricity supply to homes

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mwatson
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Joined: Wed Oct 22, 2008 4:53 pm

Electricity supply to homes

Postby mwatson » Wed Oct 22, 2008 5:05 pm

I was wondering if someone could explain the safety, insulation or other reasons for electricity being supplied to our homes at 240V. If a higher voltage was used (say 500 V), wouldn't that result in a lower, safer current for household users?

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joe
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Location: Sydney

Postby joe » Thu Oct 23, 2008 9:39 am

I was wondering if someone could explain the safety, insulation or other reasons for electricity being supplied to our homes at 240V. If a higher voltage was used (say 500 V), wouldn't that result in a lower, safer current for household users?


Loosely speaking, it is the potential difference ('voltage') applied from one point to another on the human body that determines the damage. Assuming reasonably good, sustained electrical contact, 240V or 500V applied between two hands, or hand to feet, is likely to be fatal. 12V is not -- you wouldn't even feel it.

We don't use 12V because we'd require very thick wiring and lots of lost energy. If your house is using say 5000 W of power at some time (lots of stuff turned on), it requires 20A at 240V, but 400A at 12V. To carry 400A without huge losses in the wiring would require very fat cables, even for short distances. Have a look at the wires connecting the battery of a car to the starter motor: these carry something like 100A, but only a short distance.

So, what kills you?
The larger the voltage applied, the larger the current that flows inside you. The larger the current flowing through you, the larger the voltage developed across vital bits, such as the SA node or pacemaker cells of your heart.

If you apply a smaller voltage, then no matter what current the power supply is capable of producing, only a small current will flow through you.
Provided that a source is capable of providing tens of milliamps or so, the higher the voltage, the more likely it is to kill you.

Be careful!

Joe

mwatson
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Joined: Wed Oct 22, 2008 4:53 pm

Voltage

Postby mwatson » Thu Oct 23, 2008 5:52 pm

Thanks Joe!

Am I therefore correct in saying that choosing the best voltage to step down our power supply to is a balancing act between safety considerations and some practical considerations like cost. That is, a higher voltage (like 300V) might allow for cheaper, thinner wiring but is more likely to cause a potentially fatal current to flow through a person.

Does that also mean that in Australia (where we use 240 V) that appliances like hair dryers would need more insulation than in the US where they use 120V?

Thanks again.

mw

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joe
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Postby joe » Thu Oct 23, 2008 9:29 pm

That is, a higher voltage (like 300V) might allow for cheaper, thinner wiring but is more likely to cause a potentially fatal current to flow through a person.


Yes. Electric power is often delivered by 220,000 V lines, but that would be seriously dangerous domestically. Switch design would be a nightmare.

Does that also mean that in Australia (where we use 240 V) that appliances like hair dryers would need more insulation than in the US where they use 120V?


At the margin, where one makes only poor electrical contact, one can imagine situations in which 110 V would not be fatal but 240 V would. But both are so dangerous that the way to operate is never to touch them. I don't know whether any US makers make appliances with an insulation that wouldn't meet European standards.

However, you'll notice that, in Australia, appliances are more often earthed than in the US.

Joe


Joe


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