(A reply to Peter Ladkin)

It is noteworthy that there are far more international standards relating to electrocution (which has fortunately become rarer since the introduction of RCDs), than to prevention of fire.  Personally, I worry much more about the latter. 

Peter Bernard Ladkin: 
 

But now, everything seems to depend on a reliable electricity supply. 

Yes Peter, we have allowed ourselves to become totally dependent on it.  If you want to really see how far down this road we've gone, and what the consequences of an extended power cut would be, I recommend Marc Elsberg's “Blackout” - ISBN 978-1-784-16189-7. 

(which has fortunately become rarer since the introduction of RCDs), than to prevention of fire.  Personally, I worry much more about the latter. 

I think correctly so - the proliferation of always on electronic ‘thingys’ of variable quality is not helping, and the standards that are meant to underpin CE marks and so forth are largely unenforced at the shallow end.  Against this, there may be a tendency for fires caused by clothes on heaters and pans left on rings to be attributed to ‘electrical fire' when in many cases ‘lack of thought' would be more true.

There may be quite a few un-reported shocks, but electrocutions in the UK are jolly rare, some tens per year in fact- so they end up in the news, unlike fires, which are just as tragic, but more common, and so not as remarkable.

Mike

how about a low power DC standard of, say, 48V with connectors rated at, say 10A?

The nearest we have at present is USB - up to about 2A at 5V...

Christopher Moller: 
 

(A reply to Peter Ladkin)

It is noteworthy that there are far more international standards relating to electrocution (which has fortunately become rarer since the introduction of RCDs), than to prevention of fire.  

That has to do with history, I think. In Germany, the electrotechnical engineering professional society VDE was founded in 1893 and that same year the electrical-safety low voltage standard VDE 0100 was published. 

I don't actually definitively know if electrocution was the main concern then, or electrical fires, or both.

It is a bit different in the UK, since the forerunner of the BSI was founded separately from the STE/STEE/IEE and IIE. 

Christopher Moller: 
 

how about a low power DC standard of, say, 48V with connectors rated at, say 10A?

David, you're 100% aligned with our thinking!  ……

We would be keen to hear your (and anyone else's) views on this!

When I was involved in some standardisation activity in Germany around electric vehicles some 8 or so years ago, the interest in ultra-low voltage was increasing because of LEVs, and now there is a working group (WG). My old pedelec (2010 vintage) is 24V; they have been at 36V now for a while. I seem to remember the standardisation interest was in 72V (or lower). What is the reasoning behind 48V?

Connectors are an often underrated issue. One of the guys who helped design the Mennekes-Mercedes 7-pin connector for EVs, which is now EN, sat on a standarisation committee I was chairing in 2012-4, and had lots of interesting tales! 

What is the reasoning behind 48V?

Most countries distinguish between “low voltage” (LV) and “extra low voltage” (ELV).  LV would typically be 120V or 230V house mains, and is dangerous to touch.  ELV is generally safe to touch.  The line between the two is usually set at about 50V.

So 48V is the highest battery voltage that's safe enough to touch, so requires much lower standards of insulation.

The 50V is safe thing is a convenient round number, not a matter of fact. - really it is shock current that matters, and wet vs dry  hands or shoes vs bare feet, or both, can make the difference between an bit of a tingle from 230V and yet the odd fatality from voltages as low as 28 - though the only deaths at 28V  I am aware of were army personnel already badly  injured, and being wet with lost blood probably contributed to the lower body resistance.

In regulation land it is a black/white limit, in engineering reality - and much like the other one at 1kV, it is a bit more grey.

The other thing against higher voltages at DC vs low frequency AC is that there is no natural extinction of arcs, so switch gear has to be rated for VA product, rather than just peak volts based on gaps and peak amps based on heating metal calculated  separately (the rate of growth of  volume of plasma  at the arc has to be less than the rate of increase of the volume between the contact)   (a demo video) This means that  DC switches must be bigger and more ‘snappy’ than their micro-gap AC counterpart for the same voltage . If you have played with an arc welder you will find that the length of arc you can hold is mostly current dependent, rather than voltage, once you have enough open circuit volts to strike reliably. (and yes you can do acceptable welding  with just 2 car batteries in series, but it is not very good for them and you need to be quick)  

For electrics in more developing places, I'd expect the double fault to danger principle that requires both a fuse and an earth to be widely ignored, and a more pragmatic approach to risk of single insulation. Certainly the South American shower wiring I have seen with twist and tape wiring does not seem to kill the locals, but it makes one of a western education feel quite nervous on first contact.

Mike