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Author Topic: What's the highest 'low-voltage' considered 'safe'?  (Read 17579 times)

Offline peppercorn

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If installing and wiring up electrical equipment for a friend (or a customer), what is the highest voltage allowed prior to regulation being required (in the UK specifically) ?

I believe the law is still a little different on the homeowner doing certain work these days, but I'm talking about any installation outside this category.

What if the system is outdoors? Like garden lighting?
Armoured cable, for instance, isn't a requirement for low-voltage work, is it? But what constitutes low voltage?
 


 

Offline RD

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Re: What's the highest 'low-voltage' considered 'safe'?
« Reply #1 on: 22/09/2012 12:09:43 »
A car battery charger is only 12 Volts, but is apparently sufficient to electrocute a cat ... http://news.bbc.co.uk/1/hi/england/cumbria/3049297.stm

The current, rather than the voltage, is the killer ...
Quote
A common misconception is that larger voltages are more dangerous than smaller ones. However, this is not quite true. The danger to living things comes not from the potential difference, but rather the current flowing between two points.
http://hypertextbook.com/facts/2000/JackHsu.shtml
 

Offline syhprum

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Re: What's the highest 'low-voltage' considered 'safe'?
« Reply #2 on: 22/09/2012 13:47:04 »
The transformers that outside workers use produce 48 volts ground free so that is what is considered safe in the UK
 

Offline peppercorn

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Re: What's the highest 'low-voltage' considered 'safe'?
« Reply #3 on: 22/09/2012 14:36:38 »
An electrician friend of mine says the regs allow up to 25V AC or 60VDC without needing certification.

The old 'bell' wire (to power a EM ringer) on phones is 50v still, if I'm not mistaken - ?


RD, that is surprising on current being the killer. I thought there was a minimum voltage needed to overcome the  skin's resistance (unless your laying chest down in a pool of water I suppose)  ....
 

Offline JP

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Re: What's the highest 'low-voltage' considered 'safe'?
« Reply #4 on: 22/09/2012 16:37:14 »
RD, that is surprising on current being the killer. I thought there was a minimum voltage needed to overcome the  skin's resistance (unless your laying chest down in a pool of water I suppose)  ....

Current is literally what kills you, but voltage tells you how much current your power source can potentially put through a given resistance.  Voltage = Current x Resistance (Ohm's law), so for a fixed resistance, voltage and current are proportional.

You can do the calculations using the numbers on Wikipedia: http://en.wikipedia.org/wiki/Electric_shock

The human body's resistance (from hand to hand) for low voltages is listed to go as low as ~1000 Ohms.  A lethal alternating current (via fibrillation) is stated to be as low as 60x10-3 Amperes.  Therefore V=60 volts is roughly the maximum safe voltage for AC according to their numbers if the current passes in one hand and out the other.  So 25 V AC seems like it allows a pretty safe margin.
 

Offline Bored chemist

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Re: What's the highest 'low-voltage' considered 'safe'?
« Reply #5 on: 22/09/2012 17:21:46 »
Saying "it's not the voltage that kills you but the current" is like saying that it's not the height that you fall from that kills you, but how fast you hit the ground.
One is determined by the other.
 

Offline evan_au

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Re: What's the highest 'low-voltage' considered 'safe'?
« Reply #6 on: 10/10/2012 10:26:22 »
Certain areas are recognised as being especially dangerous, and should not be wired up by amateurs:
  • In hospital and medical equipment, people may have electrodes embedded in the body, bypassing the skin resistance. This is even more of a risk if the electrodes are positioned in or near the heart; normally only a small fraction of the current passing from hand to hand passes through the heart, but with a nearby electrode, most of the current may pass through the heart.
  • In mines where explosive gas mixtures may occur (eg coal mines), even a small spark can trigger an explosion. All wiring must be enclosed in hermetic enclosures, or be designed so that the maximum energy delivered by a circuit is insufficient to trigger an explosion.

It is an unfortunate accident that the AC frequency used in commercial power grids (50-60Hz) is in the range that can strongly trigger muscles. Aircraft systems often work at higher frequencies like 400Hz, which is less lethal, because our nerves can't retrigger that quickly.
 

Offline SeanB

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Re: What's the highest 'low-voltage' considered 'safe'?
« Reply #7 on: 11/10/2012 19:51:58 »
Below 50V is considered safe for incidental contact ( touching by accident) in many instances, but medical devices are only safe by having no current flow allowed, as you can be connected to them with a very conductive means, so no current flow is allowed, but the voltage is 110V.

In most cases other than wet areas you are reasonably safe with under 50V.
 

Offline wolfekeeper

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Re: What's the highest 'low-voltage' considered 'safe'?
« Reply #8 on: 11/10/2012 22:24:08 »
Saying "it's not the voltage that kills you but the current" is like saying that it's not the height that you fall from that kills you, but how fast you hit the ground.
One is determined by the other.
No, if you stick the electrodes through the skin, you can be killed with very low voltages across the chest; blood is a very good conductor.

It really is the current that kills.
 

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Re: What's the highest 'low-voltage' considered 'safe'?
« Reply #8 on: 11/10/2012 22:24:08 »

 

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