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harry.m1byt
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Water & electrics and fuse boxes
« on: Dec 29th, 2007, 11:34am » |
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Just a word or two of explanation on the problems when these get together. Keep in mind that the higher the voltage, the thicker the insulation or spacing needs to be to work effectively...
Perfectly clean pure water has a surprisingly high resistance/ low conductivity, but is much worse than dry air as an insulator. Air itself is not as good an insulator as most of the solid materials commonly used as insulation, such as plastics etc. To deliberately improve the conductivity of water (lower its resistance) it is usual to add common salt.
One project I worked on long ago at a power station was a motor speed controller involving a water bath to act as a resistance for the motor. Imagine a tank 30' x 15' x 15' high, with electrodes lowered into the salted water to control the power input to the motor.
The cinematic scenes of air dryers dropped in an occupied bath as a means to murder the occupant surrounded by lots of sparks are very unlikely.
During floods, it is quite common to see mains lights under several feet of water, yet continueing to work normally. Only after long submersion does the water start to break the insulation down, to start to become conductive are there real problems.
When moisture gets into the fuse boxes, it is unlikely to cause any immediate problems. Only when it has been in the fuse box for a considerable period will it start to have an effect. What happens is the copper tracks start to oxidise due to the moisture, making the water conductive shorting out the tracks and also causing poor connections in the box - hence the resulting weird effects on the cars electrical operation.
Last year I attended a bank where the basement was flooded by 8' of water. Meter plus 3 phase distribution boards all deeply submerged and the supplies plus bank all working perfectly normally.
I have several times dropped my mobile phones in deep water and once even in mild acid. All I do is pull the battery and give the phones a good soak and flush in clean water, followed by a good dry out. I also regularly clean up electronics by the use of water after making sure there is nothing capable of absorbing/ storing/ trapping the moisture long term.
So the point is to catch the fuse box in good time before damage occurs and get it properly dried out. Because of the mult-layered structure - The only way to do that is to remove them, strip them down, clean them and thoroughly dry them out.
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big_neil
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Re: Water & electrics and fuse boxes
« Reply #1 on: Dec 29th, 2007, 2:26pm » |
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you are dead right on the mains aspect harry, i worked for 35 years on underground mains cables and many times i was called out to flooded properties where the mains meters and incoming supplies were under water in cellars. to lift a floorboard and shine a torch below and see the meters still going round under three feet of water never failed to amaze me. it was a bit dodgy though when the consumer had a payment meter and try as i could i couldn,t persuade people to put there hand in the water and put another 50p in the meter when it ran out! (i know it,s a slight digression from the original thread but quite interesting all the same)
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harry.m1byt
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Re: Water & electrics and fuse boxes
« Reply #2 on: Dec 29th, 2007, 9:26pm » |
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Hi Neil -
No problem, I just thought it worth pointing out how the electricity/water mix actually works out in practical sense.
I was quite amazed to learn that one of the methods used to track down underground mains cable faults, was to simply ignore them and let them develop to the point of service interruption or until the pavement showed obvious signs of the fault beneath it.
The bank I referred to had a large underground dissused safe vault in its basement, all of it centrally heated with a large radiator down there. All of the thousands of gallons of water were being kept at around 40 degrees by the central heating making the bank above like a sauna. Of course none of the staff wondered as to why the atmosphere had suddenly turned so tropical in the bank. I walked in on one of my regular check arounds, noticed the humidity, but it didn't quite register properly, started making my way down the stairs, still not fully awake and suddenly felt my feet getting very, very wet 
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big_neil
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Re: Water & electrics and fuse boxes
« Reply #3 on: Dec 30th, 2007, 9:30pm » |
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many a night i,d walk the route of a cable after a fuse had blown in the sub , my mate used to ram a 400 amp fuse in and i,d listen for the bang under the flags to know where to dig, sometimes on frosty nights the flags used to thaw out where the fault was. i was called out one day to a council house with a no supply, when i got on site there was water everywhere and the council plumber said there was a leak, as he couldn,t find the stopthingy he,d dug a hole in the garden and cut the lead water pipe and bent it back on itself but he hadn,t stopped the leak, as i was talking to him the water started bubbling in the hole that he,d dug (yes you know what,s coming next don,t you?) i baled the hole out with a bucket and low and behold he had cut thro the lead electric cable (old cables were plain lead with no armours on them) with a junior hacksaw right up to his armpit in water with his bare hands , then folded the cable end, i.e. the live end back on itself to try and cut off the water as he so thought. when i showed him what he,d done he fainted right there in front of me!. to this day i will never know how he didn,t get a shock and was killed. as we all know electricity always takes the shortest route and the fault current would probably have gone back to the sub thro the lead sheath, although it didn,t blow the sub fuse, quite an amazing story i think.
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mr._floppy
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Re: Water & electrics and fuse boxes
« Reply #4 on: Dec 31st, 2007, 12:41am » |
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So, are you saying when James Bond tosses BlowJob ( or is it OddFelt ? ) into the bath and throws a plugged in electric heater into the water for good measure and says " Frying Tonight " it is unlikely to kill him ?
One for Stephen Fry's QI I think. 
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big_neil
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Re: Water & electrics and fuse boxes
« Reply #5 on: Dec 31st, 2007, 11:48am » |
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right on mr floppy,,,,,on the other hand ,,,, a consumer of ours went to the shops one day and left the kettle on sitting on the draining board, this was before auto switch off kettles, as the kettle boiled dry it had one of them thermal plug ejects at the flex point and the plug ejected into the sink full of water, as harry and i both say, water can sometimes not be too good a conductor and the supply didn,t cut off, this lady came home from the shops , washed her hands in the sink and was electrocuted. as i said i,ve worked with live electricity for 35yrs and always treat it with the utmost caution, probably why i can write this today,,,,take care ,,,,neil
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taliban aka Cheekyboy2
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Re: Water & electrics and fuse boxes
« Reply #6 on: Dec 31st, 2007, 11:52am » |
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i think that is likely too kill him........
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harry.m1byt
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Re: Water & electrics and fuse boxes
« Reply #7 on: Dec 31st, 2007, 12:32pm » |
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on Dec 31st, 2007, 12:41am, mr._floppy wrote:So, are you saying when James Bond tosses BlowJob ( or is it OddFelt ? ) into the bath and throws a plugged in electric heater into the water for good measure and says " Frying Tonight " it is unlikely to kill him ?
One for Stephen Fry's QI I think.
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Basically, yes - but obviously not something to be experimented with.
To suffer injury or death the current has to have a path through your body. The worst possible scenario is live in one hand, earth, neutral or another phase in the other hand. That path would be through your heart which is normally controlled by electrical impulses. Those impulses would be overcome by the flow from the external source (the mains) and you could possibly die as a result. Hence it is good practise when working live to wear insulated shoes, keep one hand safely in your pocket and stand clear of anything conductive. You then cannot get a shock when your working hand touches something live, apart from across that one working hand.
If you were to stand clear of all earths and poke your finger tip into a live lighting lampholder, then the worst you can get is a tingle as the current flows over the skin of your finger tip [2].
I don't have any figures, but would suggest most injuries from electrical shock are physical injuries resulting from the muscle contraction from the shock, rather than direct damage from the flow of current through the body and contact burns. In other words you get the shock and the involuntary muscle contraction throws you off the step ladder you are standing on and possible burns on the contact area of the skin.
The highest risk to consumers/users, is one of contact to live, whilst having contact with earth - which is why RCD's have become so popular. These things notice that their is a leakage to earth (through you) and cut the supply off.
Getting back to the fire in the bath scenario....
The risk depends entirely on the voltage gradiant [1]produced in the water, which decreases the further away you are from the live conductor. The nearer you are to the live conductor in the fire, the higher the risk of electrocution. If the live point is nearer to a metal side of the bath, then that it is the path most of the current will take.
The suddenly boiling water amid impressive flashes is also nothing more than cinematic licence. Assume the fire is fitted with a 13amp plug and is 3Kw -the same as a large kettle. How long does it take the kettle to boil a pint of water? Here we are discussing a bath full of many gallons of water, it would take an hour or two to produce boiling water in the bath.
Now for the flahes...
If you were to stick the bare live and neutral into a bath of water, the best you would get is a steady fizzle as the H2O is converted to H and O [2]. You can often hear the fizzle on high voltage overhead lines in damp conditions as the insulators leak due to moisture.
[1] Voltage gradient is what makes electric showers resonably safe to use. Old and corroded metal cased element which eventually splits open exposing the live to the flow of water. The water around that point will become live and you are in that flow of water - yet by the time the water has got to you the voltage gradient has fallen to almost nil.
[2] How do I know? Well as a youngster I was very inquisitive of all things, I experimented.
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big_neil
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Re: Water & electrics and fuse boxes
« Reply #8 on: Jan 5th, 2008, 1:01pm » |
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it actually takes well less than half an amp to kill if taken across the chest as harry says.So really all domestic lighting circuits , which are 5 or 6 amps can and are fatal. The reason why i could work on underground cables live without any disruption to the consumer was i always had to have two means of insulation , i.e. rubber gloves, a mat, or proper live jointing wellies, any one of the two were allowed under the safety regs, as you couldn,t feel connections properly with the gloves on we used to use the other two but it wasn,t fun in a dirty wet hole nearly up to the knees in water ,we often got the odd tickle. i t was quite safe as we would only be working on one conductor at a time.. and yes harry you only get a shock if it has a path to go , the worst job was working at consumers meter positions on fault conditions coming upon the unexpected, as most were in pokey cupboards under the stairs, when there was a bang you couldn,t see owt for ages except blue rings in front of your eyes, bit like a weld flash, as the sub fuse was sometimes 400 amps these bangs kept on until it blew open circuit. to think i used to have black hair when i started!
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TiberiuS
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Re: Water & electrics and fuse boxes
« Reply #9 on: Jan 5th, 2008, 5:16pm » |
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Guys, not intending to take Harry's thread off topic, I've got a suggestion about the problems people get with the looms on their cars, gremlins pointing to duff sensors and the like. I was getting some weird gremlins a while back, I pulled every connector on the engine loom while I was doing some other work, doused every connector in switch cleaner and plugged/unplugged it a dozen times to clean the contacts, then flooded each plug with dielectric grease and put it all back together. No gremlins since then , no flashing 'CHECK ENG' lamps, everything works just like it should.
Most of the problems I've had are through the connectors losing contact, I think a lot of people replace sensors and things without even trying the cheaper fixes first. This has nothing to do with the problem with the wire insulation breaking down on some early Scorpios, but still worth checking if you're getitng problems.
Regards, Bruce.
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Bruce - '07 Jaguar XKR coupe, '95 Jaguar XJ Sport 4.0, '82 Ford Capri 2.0 Ghia, '15 Honda Civic (sloooooww..)
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harry.m1byt
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Re: Water & electrics and fuse boxes
« Reply #10 on: Jan 6th, 2008, 12:59pm » |
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on Jan 5th, 2008, 1:01pm, big_neil wrote:| it actually takes well less than half an amp to kill if taken across the chest as harry says.So really all domestic lighting circuits , which are 5 or 6 amps can and are fatal. |
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Depending on the individual and the circumstances, 10 to 60mA is all that it takes at 240v.
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harry.m1byt
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Re: Water & electrics and fuse boxes
« Reply #11 on: Jan 6th, 2008, 3:04pm » |
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on Jan 5th, 2008, 5:16pm, TiberiuS wrote:| Most of the problems I've had are through the connectors losing contact, I think a lot of people replace sensors and things without even trying the cheaper fixes first. |
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I'll second that, especially true on such low voltage as the 12v on cars, with such low currents involved. Of all the electrical problems I've dealt on my cars in the last 10 years, all but one has proved to be due to exposed connections and moisture ingress oxidising contacts causing them to develop high resistance or even the copper wire itself to corrode away.
It does not take much moisture in an almost sealed connection to start attacking the connections and causing problems.
I have so far had on the Scorp - cruise control, ABS and temperature gauge faults - all traceable to moisture causing oxidation at their connection points. All solved by simply cleaning them up.
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