Il problema dell'alimentazione elettrica per chi usa la bicicletta, sia nell'uso quotidiano che per viaggi di più giorni, sta diventando sempre più impegnativo. Serve sempre più energia elettrica non solo per illuminare la strada nelle buie ore della notte ma anche per alimentare il sempre crescente numero di dispositivi elettronici a cui non sappiamo più rinunciare. Questo sito vuole essere un primo passo per provare ad orientarsi nella non sempre facile scelta degli strumenti necessari per gestire in modo possibilmente autonomo le proprie necessità.

The electric power supply problem for people who like to cycle either in everyday use and along trips of several days, it's becoming increasingly challenging. More and more electricity is indeed required not only to illuminate the road in the dark hours of the night but also to feed the ever growing number of electronic devices that we can't live without. This site aims to be just a first step in the attempt to orientate the user in the not always easy commitment to choose the most appropriate tools.

Thursday, 14 November 2019

Dynamo Voltage Hazard

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 !!

Here below an example of how, e.g. SHIMANO, implemented a surge protection in its switch SW-NX10; in this case ZD1/ZD2 zener protection is there not for "regulating" but for avoiding to have dangerous voltages on the "live wire" whilst the S1 switch is open, i.e. when the lights are off. In such a case indeed, in the event to have a voltage surge this would be routed through the bridge instead of through the cyclist in the always possible circumstance to have an accidental contact with the live parts of the circuit.