Dynamo Voltage (possible) Hazard

Posted by: ConRAD

Dynamo Voltage (possible) Hazard - 10/28/18 07:12 PM

Originally Posted By: "ConRAD"
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.

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 !!

Posted by: derSammy

Re: Dynamo Voltage (possible) Hazard - 10/28/18 10:03 PM

Hi ConRAD,

I'm not sure, if your assumptions apply. A bike dynamo is a constant current source, with a current of about 500mA.
You are right, unloaded the voltage can be extremely high, I remember something like ~1V per km/h? But the voltage drops significantly with the presence of a load. To the best of my knowledge it is almost impossible to get more than 10W from a bike dynamo. Are these circumstances suficient to get a significant hazzard for the human body?

Moreover I want to point out two aspects:
(1) The danger to harm your fingers or even to do a front spin with the bike seems to be much more likely than electrical issues, when you perform ajustments like you descriped.
(2) When you cycle faster than -let's say 20 km/h- it is a realy good idea to keep both hands at the bars. People doing dynamo adjustments at 50km/h apply for the Darwin Award. Not because of electrical issues, but because of general safety rules.

And, last but not least, I highly recommend the use of a dynamo hub, which avoids a lot of the drawbacks of the old-fashioned bike dynamos.
Posted by: ConRAD

Re: Dynamo Voltage (possible) Hazard - 10/29/18 09:25 AM

Hi derSammy, thanks for your reply.
Yes I agree with you, at 50 km/h is definitely much better to keep both the hands on the handlebar !!
Here below nevertheless some of my points:
A bike dynamo is a constant current source till the load is a prevalent resistive load and its value is negligible compared to the internal reactance of the dynamo; 500-600 mA is the maximum output you can get from a dynamo, even at 100 km/h on a dead short-circuit load !!
Yes I confirm: no-load voltage is something like ~1V per km/h.
A 6V-3W hub dynamo is capable of delivering 6W at 20 km/h, 10 W at 50 km/h.
Output voltage of a dynamo of course is likely to drop with load but the typical 1500 Ohm of a human body will not affect substantially this value at all.
... 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 frown plot below shows that 100 mA for 1 sec might be lethal.

Posted by: derSammy

Re: Dynamo Voltage (possible) Hazard - 10/29/18 09:26 PM

I don't know where the diagram is from and I need to admit, that I did not think about the deadly hazzard limits of electricity yet.
But I nowhere found an indication, wheather the values hold for DC or also AC (and what is more dangereous)?

However, 1s seems to be quite long in my perspective. 54km/h is 20m/s or about 10 roations of the wheel per second. In other words the current will have changed direction about 130 times in 1s.
Posted by: ConRAD

Re: Dynamo Voltage (possible) Hazard - 10/30/18 09:15 AM

The above diagrams are based on IEC electrical safety related standards and in my case more specifically on publications of Prof. V.Carrescia teaching “Electrical Safety” at the Polytechnic of Turin, Italy.
Considering the very low currents involved I don't think there are big differences between DC and AC, except for body resistances/impedances varying according to specific frequency.
However here below some actual measurable numbers:

50 km/h is 13.9 m/s (50000m/3600s)
13.9 m/s is approx 6.3 rps (rotations per second based on a 28” wheel)
6.3 rps is approx 82 Hz based on a 26-poles hub dynamo

Posted by: derSammy

Re: Dynamo Voltage (possible) Hazard - 10/30/18 09:44 AM

Your calculation is correct, 54/3,6=15, not 20 as I claimed. And I probably mixed up the number of magnets and the number of poles.
Posted by: kangaroo

Re: Dynamo Voltage (possible) Hazard - 10/23/19 11:50 AM

i dont know whats the point. You cant have the Current and the Voltage so high that its getting dangerous. You can maybe reach one but not both.

50VAC or 120VDC