maybe dIMM retired to Cornwall?

[D.M.Chapman]

Sounds like a holiday let to avoid...

(pointed out to me by a friend who frequents the forums there)

Darren

[Jonathan Batchelor]

We thought kengreen was a troll, but his persistence is a bit worrying (based on other rubbish he posts). Fortunately we're pretty sure he doesn't actually fly (as per the main theme of the forums!).

JB.

[David WE Roberts]

Just to check that the post copied below is accurate:

***** "Ken, I used to think you knew about electricity and electronics. I am beginning to wonder.

To answer the questions I posed, briefly: inside a switch in the neutral or in the live of a lighting circuit, the highest measurable voltage to be found is 230 volts (assuming you're on 230 volt mains). This is why nobody should work on a switch with the power on.

If the house is wired in accordance with the latest regulations, the highest current available in a switch with a 100 watt bulb connected is a shade below 400mA under either scenario. Touching live and earth will pass 30mA before the RCD trips. Without an RCD, the current will be much higher - certainly in the fatal range in both cases. Few houses, so far, are wired with RCDs in the lighting circuits.

Fatal current is generally reckoned to be 50mA - not across the heart, but across the body. The actual current that will pass will depend on skin resistance; the generally accepted fatal voltage is anything over 50 volts.

So: a person standing on a metal ladder earthed in any way, and touching the "live" wire in a bayonet light socket wired with neutral switching is in grave danger of death.

If the person is not on an earthed surface but the bayonet holder is metal - and therefore earthed in accordance with the regulations - then the shock will be mostly to the hand, but there will be some residual shock to the rest of the body. Still very unpleasant, and potentially fatal to someone with a weak heart (for example). Touching the live pin with one hand while holding the earthed lamp with the other would probably be fatal.

The same person with live switching, in the "off" position, will not be shocked. (If in doubt, the main switch should always be off or the fuse removed.)

It is for this reason that the wiring regulations prohibit neutral switching (and fusing the neutral). It's a pity the kengreen regulations are not the same as the legal ones.

I'm sure you will continue to argue, Ken, but the regulations are not likely to change.

I would strongly recommend that anyone doing wiring in their houses (legally or not) should follow the regulations and switch the live, not the neutral. I feel very strongly about this, which is why I have kept on about it."

******

Just making sure I am adding to my education and not subtracting.

[John Rumm]

Erm, no not really...

Might as well pick it apart:

So far ok.

Complete twaddle. The live wire in the switch will be directly fed from the lighting circuit MCB (typically rated at 6A nominal, however one of those would require a fault current of 30A to move into the "instant" part of its trip curve[1]. Hence the MCB is going to offer no protection from direct contact at all.

[1] pass 30mA before the RCD trips. Without an RCD, the current will be much

This is also twaddle. He has failed to understand that the RCD has no mechanism to limit the current passed. All it can do is limit the duration of the shock, when the shock current exceeds the trip threshold of the device (can be anything from 66% of the rated threshold upwards). The speed of operation should be within two mains cycles (i.e. 40ms) for appreciable shock currents (its allowed by the specs to be longer for ones just over the threshold - but in reality I have never found one that was not 40ms or faster at 30mA or more).

Limiting the shock duration will limit the total energy let through. Its the total energy that counts, so the danger of a shock is closely related to both the magnitude and the duration.

Only part of the story - see above...

As a bold statement that again is not really supportable, but I suspect he may be thinking about limiting touch voltage with EQ bonding etc.

(You can stick you hand on Van de Graaff generator and receive a 100Kv shock and walk away unharmed).

50V and under is generally point at which low impedance (i.e. those that can supply injurious levels of current are considered "touch safe". Its the standard to which equipotential bonding needs to perform when limiting touch voltage for example)

Neutral switching should never be used anyway.

Bayonet connectors do not actually pose as much risk as one might expect since the contact area is very small, and its difficult to maintain contact for any significant duration once shocked.

Possibly but unlikely as a direct result of the shock. Falling off the ladder however is a far more likely way to suffer!

Again, it can deliver the magnitude, but it would be hard to deliver the duration. The bodies natural response will tend to yank the hand away and break contact.

Agreed

Take care even with the fuse removed. Two way switched circuits with borrowed neutrals can leave the neutral of an apparently isolated circuit live and able to bite.

The remainder seems non contentious.

From what brief stuff I have read, the Kengreen posted stuff can be disregarded out of hand as the rantings of a drooling retard. The Keef responses generally are on the right track, but he appears to have misunderstood a few quite key points.

[David WE Roberts]

I thought that the main risk with 240V a/c was that the current was not enough to cause immediate major damage, but the flow of current across the body could interrupt the electrical control of the heart. So lower voltages might not hurt, higher voltagees would just cause the muscles to contract and hurl you away but at 240V you just hang there quivering whilst your heart gets reprogrammed.

Could be urban myth, of course.

Thought also that the risks with 110V were different, but Google is not being my friend this morning.

[Tim Watts]

If you mean site-tools 110V, then those are 55-0-55 (0 is earth) so you are in ELV territory - but it would still zing a bit as you picked rain soaked extension socket out of a muddy puddle whilst standing in the same.

[Andy Burns]

ELV is under 25V RMS AC (or under 60V DC, not so useful for transformers!).

[Tim Watts]

Quite right - I was thinking of DC when I wrote that. I'll get some more coffee...

But - yes, 55V AC is not too bad all the same.

[John Rumm]

Its plenty high enough to cause (fairly) immediate damage.

Generally 240V will in many cases cause you to involuntarily retract you hand etc. The danger of getting "locked onto" something is still there if you can grasp round it though.

110V as in US mains will be similar in theory but more dangerous for other practical reasons usually. 110 as in site transformers etc is designed to be very much safer due to the 55-0-55 nature of the supply.

[Andy Wade]

Wrong, actually. In BS 7671 ELV is defined as "not exceeding 50 V AC or

120 V ripple-free DC, whether between conductors or to earth."

Depends on the circumstances. Electrocution had been known at 24 V, I believe.

[Tim Watts]

It's a bad day. I felt sure 50V AC was the limit, then I foolishly checked Wonkypedia that mentions 25VAC and confused myself into thinking it was

50VDC! Presumaly that was what got the other Andy...

[Andy Burns]

Yes, when the proper standards are only available at a price, people who can't justify the price will turn to whatever sources are free.

It seems the 25VAC/60VDC that wikip refers to are actually the limits for SELV used in a damp environment.

[funkyoldcortina]

I had volts vs amps explained to me at primary school in science lessons as being akin to water in a hosepipe.

The volts (Joules per Coulomb) is like the water pressure in a pipe. The higher it is the harder the energy is pushing those electrons down the wire. In the same way as water stored in a tank at the top of a hill has lots of potential energy, volts is the potential energy of electricity (!) and also called the potential difference.

The amps (Coulombs per second) is like the flow rate of water. The higher it is, the more electricity flows at once. The narrower the pipe (wire) the higher the resistance and the more volts (pressure) needed to overcome the resistance and still maintain a satisfactory current (flow rate).

etc. etc. :)

The combination of volts x amps is the power and like with water, multiplying the

[Andy Burns]

I remembered my library card allows access to a subset of standards online, just double-checked and BS7671 isn't one of them, but it seems that BSi/IET may be allowing online access to it now.

I've asked the library if they anticipate adding 'GBM81' to their subscription package ...

[John Rumm]

Although some free sources are actually worth what you pay for them ;-)

[Andy Burns]

clearly they've squandered the tenner I gave them last year!

do we juke the pagerank of the wiki?

[Andrew Gabriel]

BTW, US changed to 120V some time before we standardised on 240V ;-) (although some states have since reduced to, e.g. 117V, as an energy saving measure).

[Martin Bonner]

My understanding (and I would be delighted to be corrected), is that

110V causes fewer deaths/serious injuries by electrocution (because the voltage is lower), but more by fire (because the current is twice as high, and the heat generated by a slightly dodgy connector is four times as high). Fires caused by electrical faults are a bigger problem *in the UK* than electrocutions, this is not a good trade- off.

However, the problem is not so bad as to justify moving to a different electrical distribution system.

[The Natural Philosopher]

Its a very good one on a building site, where fires are pretty commin anyway.. the plumber whose blowtorch gutted a 1000 year old house..

definitely not. My gut feeling is that 240v around the home is about optimal, and 55-0-55 is about right on a building site!

[John Rumm]

All else being equal that would probably be true. However the US suffers far more electrocutions per capita than we do (by several orders of magnitude). This is probably more a reflection on the quality of their electrical accessories and installation methods, than the voltage however.

What do you propose, 480V per phase and half the current?