Dynamo Voltage (possible) Hazard

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ConRAD
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Dynamo Voltage (possible) Hazard

Postby ConRAD » 27 Oct 2018, 7:09am

When I was young I remember that I often used to adjust with the right hand the sidewall dynamo connection-wire sometimes accidentally coming loose while cycling. Making sure to secure at the same time with the left hand the handlebar on a not very well insulated grip I remember also that some perceptible and quite annoying leakage current was indeed crossing my body.

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Well, IEC-International Electrotechnical Commission states that for voltages around 50V human body on a hand-to-hand pattern has an impedance of about 1500 Ohm or even less, that one significantly depending on actual body mass, skin conditions, contact area, applied voltage, frequency, etc.
Now the point: at 50 km/h at no-load conditions, or on a 1500 Ohm load with no significant difference at all, out of a dynamo without a built-in voltage protection you may have something like 50V that applied to your body might induce in turn something like 30 mA. Something definitely unlikely to happen, I agree, but still possible … especially for grounded hub dynamos and spoilt head lamps with exposed/not well protected live parts.
From the point of view of a possible exposure hazard IEC worked out the below Current-vs-Time plot identifying four zones:

Zone 1: represents the limit for current perception estimated to be 0.5 mA

Zone 2: represents the danger threshold generally recognized still to have no dangerous physiological effects

Zone 3: it’s a sort of an “alert” area coming just before any possible atrial fibrillation, mainly characterized by yet reversible physiological effects such as muscular contraction (tetanization), difficult respiration and cardiac disturbances. As you can see 30 mA for one sec may already have some annoying effect.

Zone 4:is characterized by permanent effects, such as fibrillation, depending, beyond current and time, also on specific health conditions. Contact times as low as 10 ms may be lethal but fortunately these current values seem to be much higher than those ones actually supplied by a dynamo at 50 km/h !!

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PDQ Mobile
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Re: Dynamo Voltage (possible) Hazard

Postby PDQ Mobile » 27 Oct 2018, 10:04am

So how come an electric fence shock of about 7500 volts! directly across the chest area to a fully grounded hand does not result in any long lasting damage?

Not pleasant I agree, but not likely to be lethal unless a pacemaker or similar is involved.

My Dad always said " it's Amps that kill".

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ConRAD
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Re: Dynamo Voltage (possible) Hazard

Postby ConRAD » 27 Oct 2018, 10:30am

PDQ Mobile wrote:.... so how come an electric fence shock of about 7500 volts! directly across the chest area to a fully grounded hand does not result in any long lasting damage? ...

... because an electric fence energizer converts mains or battery power into a high voltage pulse about once every second around 150 microseconds long ... just enough for cows to keep clear !!
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axel_knutt
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Re: Dynamo Voltage (possible) Hazard

Postby axel_knutt » 27 Oct 2018, 11:36am

PDQ Mobile wrote:So how come an electric fence shock of about 7500 volts! directly across the chest area to a fully grounded hand does not result in any long lasting damage?

Because it's not 7500V. The source voltage (EMF) might be 7500V, but the loaded voltage (PD) presented to the body depends on both the load and source impedances:

PD = EMF*Zl/(Zl+Zs)

If the load impedance is low and the source impedance high then the voltage across the skin will be a fraction of the 7500V EMF. If the source impedance is very much higher than the load (and it is for an electric fence) the total impedance (and hence the current) will be almost independent of the load impedance, which is what you need if you want to ensure the same (painful but safe) experience for anyone touching the fence.

My Dad always said " it's Amps that kill".

Voltage and current are not independently variable, the ratio of one to the other is the body impedance. The risk is determined by the current flow, but most electric sources posing shock risk are constant voltage sources (eg. mains). Since the body impedance can vary wildly according to circumstances, this presents a problem in defining what voltage will be hazardous, as I alluded to above. If you want to administer a 'safe' shock, the way to do it is by ensuring a defined current irrespective of body impedance, ie. a constant current source. This is what has been done by making both the EMF and the source impedance very high.
“I'm not upset that you lied to me, I'm upset that from now on I can't believe you.”
― Friedrich Nietzsche

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ConRAD
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Re: Dynamo Voltage (possible) Hazard

Postby ConRAD » 27 Oct 2018, 12:54pm

axel_knutt wrote:... if you want to administer a 'safe' shock, the way to do it is by ensuring a defined current irrespective of body impedance ...

TRUE !!! ... nevertheless don't forget that body impedance can vary a lot depending on applied voltage.
Body resistance measured with a low voltage battery powered multimeter is a sort of a nonsense!

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PDQ Mobile
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Re: Dynamo Voltage (possible) Hazard

Postby PDQ Mobile » 27 Oct 2018, 1:20pm

Thanks for the explanations guys.
I looked up impedance! Wondering if it was the same as resistance?
I am not much wiser.
https://en.m.wikipedia.org/wiki/Electrical_impedance

What I can say is, is that I must be of little impedance!! :D
I have experienced hefty leccy fence shocks across the chest area, one hand on moist ground and close to the energizer and its earth.
While the wallop leaves one rather, um, stunned, it doesn't feel like mortality knocking!
Though the short duration is perhaps the life saver?

50 volts from a dynamo is a barely perceptible tickle.

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ConRAD
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Re: Dynamo Voltage (possible) Hazard

Postby ConRAD » 27 Oct 2018, 1:29pm

PDQ Mobile wrote:... 50 volts from a dynamo is a barely perceptible tickle ...

... what ??? .... just a barely perceptible tickle ?? ps: the dynamo was running at 10 kph, sorry for the language !!

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ConRAD
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Re: Dynamo Voltage (possible) Hazard

Postby ConRAD » 29 Oct 2018, 7:55am

PDQ Mobile wrote:... my Dad always said " it's Amps that kill"...

... sometimes just a few mA can kill and that's why worldwide households electrical protection, at least against "indirect contacts", has been generally specified to be put in place through the installation of a good grounding system in combination with a 30 mA RCD-Residual Current Device.
The IEC :( plot below shows that 100 mA for 1 sec might be lethal

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