Which is more scary: volts or amps?

An Eccentric Anomaly: Ed Davies's Blog

It both amuses and scares me how people are frightened of high voltages but often seem amazingly relaxed about low-voltage high-current situations.

High voltages are relatively easy to deal with. Wrap them up so people can't contact them with bodily parts and put an over-current protection device (fuse or circuit breaker) in the way so they're not likely to start a fire. For extra points, put an RCD in the circuit to defeat those more intent than normal on a Darwin award and even an AFCI to stop relatively small sparks causing fires.

High current circuits, particularly DC ones, are intrinsically harder to make safe even at low voltages as any poor connection, with higher than nominal resistance, can dissipate significant amounts of power as heat and so, maybe, cause a fire. Over-current protection doesn't help as normal operating currents can cause such problems.

However, it is still essential to protect cables from overcurrent with a suitable fuse or breaker if a power source connected to the cable is capable of sourcing more current than the cable can safely carry. A typical example would be a large lead-acid or LiFePO₄ battery which can produce enormous currents if suitably provoked.

The immediate trigger for this rant is a thread on the Navitron forum where a few worthies have been getting particularly snotty about a potentially [sorry, accidental pun retained with malice] lethal mains-voltage level setup which is actually reasonably safe if handled correctly (modulo a few qualms about proper isolation from the supplier's lines depending on the earthing arrangements). What I find ironic is that these same worthies deemed it appropriate to dismiss concerns about unfused battery-to-wind-turbine connections.

The problem with wind turbines is that you really need to keep them on load all of the time (or, at least, all of the time when there's any noticeable amount of wind) otherwise they tend to overspeed then undergo a rapid unscheduled disassembly. This argues against putting a fuse between the turbine and the battery as fuse failure would be a possible cause of unloaded operation.

On the other hand, a fault anywhere along the cable or, more likely, a fault in the tower or turbine causing a short circuit could easily result in a current out of the battery well in excess of the carrying capability of the cable resulting in a fire.

For a simple system it's a choice: risk losing the turbine (possibly flinging blade parts around the countryside) if the fuse fails open-circuit or risk losing your house and possibly life if there's a short circuit anywhere on the cable up the rattling tower or in the whirling turbine. I know which I'd chose to worry about more.

Of course, the ideal solution is to put the batteries outside the house in an outbuilding you don't mind too much losing. That's probably not very practical for various reasons, not least that batteries don't work so well if they're very cold. Also, the battery bank is not cheap and you don't want to lose that either to fire or thieving scrotes.

I'm not sure if I will want or need a turbine. If I do I think the arrangement I'll go for is to, of course, fuse the connection to the battery (separately from the fuses/breakers for the PV panels and loads) then have a box outside the house (e.g., at the base of the turbine tower or a metre or two away from the point the cable enters the house) containing a dump load with an arrangement to switch it in if the turbine appears to be running off-load (e.g., the voltage is a bit higher than the maximum the battery should ever get to).

More generally, the point is that even low-voltage devices which can source significant current need, in order to be operated safely, some careful thought — more than for higher voltages which are used regularly so problems are better understood and solutions well known.