Thread: Wet Sanding

Yes Standardization of Electrical systems , Plug designs and wiring specifications, on an international platform just does not exist, same as the Railways in Australia with 3 or 4 different gauges .
The original designers as you say pulled the magic volts & frequency out of the bag ( for want of a better phrase)

And we now have to work with what was decided all those years ago.

I have a BOSCH PEX 400 sander 125mm, and it is very good , with a vac attached and using Zirconia Discs I sanded the fairing compound on the hull of mu last boat in less than 2 hours, and used about 5 discs.

Jeff

Australia, and nearly everywhere apart from the US, Japan, Taiwan and parts of South America, use 230VAC (near enough) because of the lower costs of transmission. Higher volts means lower current to provide the same grunt since power (watts) = Volts x Amps. With lower current we need smaller cross sections on transmission lines and so we save a bucket stringing the nation together.

It was Edison who decided on the lower voltage of 100 volts because his lamps would last longer and therefore take the market share from gas. If he'd listened to Tesla (who's ideas he stole) he would have taken AC from the beginning. But he decided to push DC rather than AC because generation was simpler. Talk about juice making you grab a wire, try a DC belt if you like but remember... electrical cooking is a slow and painful process!!

Now I don't know what this lecturer was about, b.o.a.t., (man that's a punctuation nightmare) but it's current that kills - not voltage. Remember the old Van DeGrath static generators? Thousand of volts but sod all current so we just had fun zapping each other at school without killing anyone.

But back to the original point, I've wet sanded with cheapo Ozito sanders and it works very nicely. The only problem I had was with the dust caking into and destroying the bearings. Mind you, I have every circuit in the workshop protected by earth leakage, or RCD, breakers.

Originally Posted by PAR

I once took a shot of 240 VAC on the tip of my nose.

I gotta know... How, why, ??

Originally Posted by Darce

Now I don't know what this lecturer was about, b.o.a.t., (man that's a punctuation nightmare) but it's current that kills - not voltage. Remember the old Van DeGrath static generators? Thousand of volts but sod all current so we just had fun zapping each other at school without killing anyone.

Hi Darce

Punctuation nightmare indeed - that' why i paragrahd it.ff
V= IR
or if you prefer
I=V/R

240VRMS @ 50Hz / Resistance of averagely electrolyted huming bean = lethal current pulses.

Change the voltage, and you change the current.
Change the frequency & you change the current pulse duration.

Both increase likelihood of survival.

Since neither of htese options is likely to happen in yours or my life-time...

Stick an RCD in the circuit, and ***provided your electrocution is to earth,
rather than between active and neutral*** your survival is almost
guaranteed. (if it's between active & neutral... well a bloke in my brigade is
a commercial flower farmer.. I can get you a good price for the wreaths &
bouquets.. )

The point is, what is 'safe' in the USA is **potentially** ruddy dangerous
here in AU. Or UK. Or anywhere else using 240V 50hz AC. I'm cool with
using an angle grinder in the rain to cut drains across my concrete driveway.
I know the risks & take the appropriate precautions. Likewise wet sanding a
dinghy bottom with a finishing sander.

Your static generators didn't kill because they couldn't sustain the voltage
(and therefore current) for more than a few (dozen?) microseconds. Not a
long enough sustained current to do more than tickle - insufficient power.
P=VI and all that.

Apocryphal story perhaps... DC may be making a comeback on long haul
transmission lines. Capacitance being the big power loss with AC....
For example the Bass Strait connector is DC.
Also the vast majority of submarine communications cables use high voltage
DC to power the optic fibre signal regenerators

DC at those voltages is seriously Bad Karma

cheers
Alan J

Originally Posted by PAR

The USA used to cycle at 50 hz too, but this change was implemented long ago, because of the shock hazard.

I don't know where this comes from but the electrical impedance (effective resistance) of the human body is at a minimum between 30 and 80Hz (Engineering handbook - National Association of Broadcasters - Google Books) so there is little difference between 50 and 60 Hz in terms of electric shock.

Like most countries the US had frequencies between 20 and 400 Hz (see Utility frequency - Wikipedia, the free encyclopedia) and settled on 60 Hz but it had nothing to do with electrical safety. There was a big argument between Edison and others about DC versus AC but not about 50 versus 60Hz.

Saying it's the current that kills is correct but it is a bit more complicated than this;

The key factors in electrocution for both 50 and 60 Hz are
1) The electrical impedance of the actual electrocution path on/in the human body.
2) The applied voltage - increasing the voltage also decreases 1)
3) The conditions under which electrocution takes place. eg If the electrical device cannot deliver more than 1 microamp then no more than this will flow and no one dies.
4) 1) 2 and 3) then determine the final current and it is this that does the physiological damage.

It is very difficult to state what is safe and what is not. Voltages as low as 50 V AC, and currents as low as 10 microamps have been sufficient to kill in some cases.

I have been fortunate to survive a 6kV jolt to the tip of my left ring finger. Luckily my left hand was resting on the metal chassis and a track of skin between the finger tip and the point touching the chassis turned yellow/green, then brown and eventually sloughed off. The shock was followed by a slight smell of burnt meat.

There was a MYTH BUSTERS show about 2 months ago , on electrocution,. they found that a current of 110volts US Standard at 70milliamps was sufficient to cause death in their tests.

Personally , I have a major aversion to sustaining or getting any electrical shocks..

voltage does not kill in it self it is the frequency / amperes that cause the drama

Jeff

About a half an amp across the heart will kill you.

The Edison and AC advocates debate was long before the 60 Hz cycle was established. Edison favored DC, because of his investment in it and related equipment (generators), but this required huge transmission voltages because of drop. The AC debate, partly fostered by a previous Edison employ (Tesla) solved the transmission difficulties and most utilities adopted it as a simple business decision.

If household voltage is cycled at 60 Hz you will let go of a live line if you grab it. If it's not cycled at this rate (the same rate your muscles fire) you will not let go of a a live wire and much more harm can result. I've hired helpers, who's sole job was to knock me or some other electrician off a hot leg if we happened to screw up. Their only tool was a 2x4 which was placed between the worker and the live panel they where working on. In the event of a hit, they where to push the worker off the load, before they started smoking. And yes, it's sometimes necessary to work a live panel, where one misplaced wrench can mean you're cooking.

The old post and tube systems in this country where 50 Hz, and caused lots of deaths and hard hits. These systems where replaced and the rate increased to 60, though commercial applications may use different rates, household service is always 60Hz for this reason. Once this change was made, household electrocutions dropped by 90% in the USA.

Again, using electric tools for wet sanding can be safe, at least in this country, but you do need to be careful, particularly when it comes to the amount of water you use. Most applications only need a slight trickle of water over the work to be effective. A slight dribble is all you need. Anything more then this and you run the risk of a hard hit, which will at least make a bad hair day for you.

You know, this conversation reminds me of a poster in the workshop...

1. Beware the lightning that lurketh in a capacitor undischarged lest it cause thee to bounce upon thy buttocks in a most unseamanlike manner
2. Cause thou the breaker that supplieth large quantities of juice to be opened and thusly labelled prior to labour, that thy days may be long in this earthly vale of tears
3. Prove to thyself that all circuits that radiateth and upon which thou worketh are grounded and thusly labelled lest they lift thee to radio frequency potential and causeth thee to make a like radiation too
4. Tarry not amongst those fools that engage in intentional shocks for yea, they are non believers and are not long for this world
5. Ensure thou useth the proper method when thou taketh the measure of high voltage circuitry so that thou doth not incinerate thee and thyne test meter for verily, though thou hath no Naval Stores Number and may thereby be written off, the test meter doth hath one and much woe be bringethed upon the Account Holder
6. Take care thou tamperest not with interlocks and safety devices, for this incurreth the wrath of thine chief and bringeth the fury of thine Commander about thy head and shoulders
7. Work not on energised equipment for if thou so doest thy shipmates will surely be buying beers for thy widow and consoling her in ways generally not acceptable to thee
8. Verily, verily I say unto thee, never service equipment alone for electrical cooking is oft a slow and painfull process and thou may sizzle in thine own fat upon a hot circuit for hours on end before thy maker sees fit to end thy misery and drag thee into his fold
9. Trifle not with radioactive tubes and substances lest thou commence to shine in the dark like a glow worm and thy wife be frustrated nightly and have no more use for you apart from thine own wages
10. Looketh not in the face of the fibre optics for, verily, if thou doth see the light, thou mayest ne’er see else again

somewhere about the place I have one like it for fuel.
Might see if I can find it.