Issues with voltage drop and melting wires

Submitted: Wednesday, Apr 17, 2013 at 03:13
ThreadID: 101742 Views:7488 Replies:7 FollowUps:48
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Voltage drop:
Our compressor is mounted way down the back of the car, in a void beside the drawers and controlled with a switch in the center console. On our recent Simpson trip the compressor became unreliable and only worked intermittently. Sometimes it worked and if it didn't then you flicked the switch until it started. It had just been relocated before trip and the cable to the fridge was used.

Why did it play up? Voltage drop was the answer. Okay for the fridges but too small for the fridges PLUS the compressor.

Don't just add to an existing circuit. Add up ALL the load and use a new run if needed.

Solution:
When we got home a new heavier cable was run just for the compressor all the way from the battery, through the switch on the console and Compressor works a treat now. The new cabling also included a brand new 30 amp per circuit fuse box for all the 4WD accessories instead of a rather suspicious second hand one that the auto sparky had used. It has spoofy bright red leds to indicate a blown fuse. And is labelled!!!!

Melting cabling:
Yesterday on this forum I came across a bloke with a 15 amp switch in a 35 amp compressor circuit. The switch had melted and it's no wonder why. But the fuse was okay. Luckily no fire.

A few weeks back there was a fire in a mates car on a group trip. It was a professionally installed additional battery in the ute tray with the cabling run through a hole drilled in the tray body work. Yes, a good grommet was used. The body of a ute flexes independently of the cabin and thus over time the edge of the hole wore through the large grommet and then the insulation of the big thick positive battery cable putting the positive directly to earth. Bloody ouch. Fires everywhere and even a brake line to the master cylinder was melted. And all professionally installed.

Solution:
Be careful selecting the correct cable and where you choose to run the cable. Go up a size to be safe. Allow for flexing and protection against sharp edges, shorting and rats/mice eating the insulation.

Phil
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Reply By: Member - Rosco from way back - Wednesday, Apr 17, 2013 at 08:19

Wednesday, Apr 17, 2013 at 08:19
G'day Phil

I would suggest the professional installation on your mates car wasn't up to scratch. If it had been fused close to the battery the dead short would not have occurred.

Another point, and possibly pedantic but ... the term voltage drop is used here regularly, however this is a bit of a misnomer as it's not voltage drop at all, rather it's current (amperage) drop due to resistance of the wire. If you were to check with a multi you'd find you still have 12.4V or similar at the end of the wire.

A good basis of comparison is a long garden hose. Connect it to a tap and you will find the pressure (read voltage or potential (to do work) at the end of the hose is the same as at the tap). However, due to resistance (friction) you will only get a small dribble of water out of the end when compared to the unrestricted flow at the tap.

Think of electricity along the same lines .. a thin wire is the same as a thin hose where resistance = friction and voltage = pressure.

So .. for wiring and Ohm's Law V=iR. V is constant so i=V/R ... the availabe current (i) = voltage divided by resistance. The greater the resistance (thin wire) the lesser the current (i). So your fridge won't get enough amps to work at the end of a long thin wire.

Get my drift ... ;o))
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Follow Up By: Tim - Wednesday, Apr 17, 2013 at 09:08

Wednesday, Apr 17, 2013 at 09:08
That's a very interesting theory, I can't say I have ever heard that before.
Tim
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Follow Up By: Member - PJR (NSW) - Wednesday, Apr 17, 2013 at 09:28

Wednesday, Apr 17, 2013 at 09:28
Hi Rosco

You are correct on both counts but there is a practical reason for the choice of both terms, "voltage drop" and "professional". Maybe I am talking to the converted here but for anyone else . . .

I believe that, when the mate used the word "professional", he was saying that it was done by a shop and not DIY. I used it in that vein also so that readers may understand that, just because it was done by a "qualified" autoelectrician shop, that they can NOT be 100% sure that all is fine. It may be - but try to check if you can. In this case all would have been good for years to come if the car wasn't used on the high country flex testing tracks which meant heaps of body movement. Unfortunately the business didn't take that into consideration.

I agree about use of the term "voltage drop" for a public forum. Who knows what skills, knowledge or IQ (questionable with some - joking guys) the audience has. You are right, but for the benefit of the majority I believe that using the term "voltage drop" explains to the reader why they have a loss of "grunt" or a dull light or a compressor that won't turn. They lost "voltage" and to overcome the problem they must use wire that has less "voltage", power or current loss.

This was a pet hate of mine with the younger ranks when giving lecturers. For example in a public forum such as this one and in ageneral non specific thread, I would not try to explain about depletion layers when asked how a computer, valve (whats that?) or a transistor works. Suffice to say it is like a tap for analogue and a switch for digital. Turn the tap on and the music goes through to the speaker etc etc. Turn the switch on and the computer will talk tio the modem. However in a forum run by RMIT (my alma mater) on the atomic of semiconductors then I would be kicked off for using the term "voltage drop".

"Voltage drop" suits this forum situation. It isn't an electronics forum.

It is an issue when a lot of well meaning people try to explain something technical and to us it is high tech gobbledegookk. Your last paragraph was fine but to some it was gobbledegook. Although not very deep, I bet a bundle of readers just skipped it.

Have a good one

Phil
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Follow Up By: Member - Rosco from way back - Wednesday, Apr 17, 2013 at 09:36

Wednesday, Apr 17, 2013 at 09:36
Mate ..
I can't argue with your logic on this one :o)
It's simply the term used is to me akin to an invisible prickle.
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Follow Up By: anglepole - Wednesday, Apr 17, 2013 at 09:42

Wednesday, Apr 17, 2013 at 09:42
Rosco,
Your bit about " If you were to check with a multi you'd find you still have 12.4V or similar at the end of the wire". If you measure the voltage with no load you will have your 12.4V, however if you measure the voltage at the load with the load connected you will find the voltage is less. The difference between the voltage with no load and load is the voltage drop.

Hope this helps

Cheers
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Follow Up By: Member - Rosco from way back - Wednesday, Apr 17, 2013 at 09:51

Wednesday, Apr 17, 2013 at 09:51
Which, I would propose, is due to the added resistance within the load.
Hence i=V/(R of wire + R of load)
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Follow Up By: Member - PJR (NSW) - Wednesday, Apr 17, 2013 at 09:57

Wednesday, Apr 17, 2013 at 09:57
Rosco

Anglepole's post is exactly what I was getting at. That's how a lot understand the issue. Yeah I know a wire doesn't lose volts. They don't just slip out the side and drop off into the dirt. But to a lot they do, and being electricity you cannot see them. But who cares. Problem explained good enough for a lot of people to understand and deal with.

Scenario: Multimeter on DC volts at the load end with the load turned off. Say the meter reads 13v. Turn the device/load on and the meter now reads 12.4V. Look a loss or lower reading. I have lost some volts somewhere in the wire. To the non electronics person this is a "voltage loss". He can see it on the meter display. The scientific reason why they disappeared does not matter. They fell out the side and I must get with with less loss.

To agree or not agree is good. It inspires thinking and nurtures knowledge.

But I am not sure of what you were getting at in your second sentence. Want to try again?

Phil
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Follow Up By: Member - PJR (NSW) - Wednesday, Apr 17, 2013 at 10:12

Wednesday, Apr 17, 2013 at 10:12
Rosco

Invisible prickle sticking in the wire and slowing or resisting the current.

Is that what you meant by invisible prickle? I like it!!!

Phil
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Follow Up By: Member - Rosco from way back - Wednesday, Apr 17, 2013 at 10:24

Wednesday, Apr 17, 2013 at 10:24
Mate

I'm probably getting out of my depth here ... electrickery is not my major forte. Whilst from your contribution, the same may not necessarily be said about your good self.

Obviously Ohm's Law may/does indicate nett voltage losses after something has done work. As you know, voltage or potential difference means it has the potential (to do work).
Hence there must be some losses in voltage after something has actually done work, as a result of losses (heat due to resistance, eddy currents etc)

My original contribution was to try to explain the reasoning, as I see it, behind why thinner wires are unable to do the same work as thicker ones. All taken in the context of what is available to do the work, as against what is available after the work is done.

Does that make sense ??
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Follow Up By: Member - PJR (NSW) - Wednesday, Apr 17, 2013 at 11:02

Wednesday, Apr 17, 2013 at 11:02
The easiest way to describe how a thicker wire has less loss, is to get out of the electronics world and use a plumbing model. No need to go near Ohms law.

Two pipes (don't use a hose), one 6 inches across and one 1/2 inch across, both feeding a water wheel. Pose the question, which one makes the water wheel move faster. Of course the answer is the widest one. Why. It is a limiting factor of the just the plain old physical size of the pipe.

Same with the battery and wiring. The same as the thicjker pipe a thicker wire will carry more current to the load and the light will be brighter, the compressor will work better and your fridge will stay cold easier. And as a bonus a thicker wire is less liable to melt and cause a fire.

You can see why they used the word current in electricity. It is a flow of electrons (water) in a wire (pipe).

Why does it get hot. Go to Google and ask because it can be confusing. Stay away from the word current and ohms law. Practically speaking who gives a rats. They just want it to work.

Keep it simple. As Albert Einstein said “A little knowledge is a dangerous thing. So is a lot.”.

The skill is finding the suitable approach and level.

Phil
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Follow Up By: Member - Rosco from way back - Wednesday, Apr 17, 2013 at 11:13

Wednesday, Apr 17, 2013 at 11:13
Exactly cobber.

It seems to me we are on slightly differing paths, heading to the same destination.
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Follow Up By: Allan B (Member, SunCoast) - Wednesday, Apr 17, 2013 at 12:42

Wednesday, Apr 17, 2013 at 12:42
Rosco,
Your explanations are somewhat confusing.



Take a look at this diagram:

V1 indicates the voltage available at the battery, say 12.4 volts.
V2 indicates the voltage appearing across the cable resistance due to Ohms Law and the current flow, E=IR, say 2.4 volts.
V3 = the voltage available at the load which will be V1 - V2 = V3, 10.0 volts. You will not have "12.4 volts or similar at the end of the wire".
So we started with 12.4 volts and finished up having only 10.0 volts available for use at the load, so indeed, 2.4 volts has been "lost" to us for useable purposes. Its energy appears as heat in the cable and is of no benefit so is "lost".


The current that will flow through the load is a function of the Load Resistance and the voltage available at the load, V3. Current is not "lost", it simply has not been achieved to the value that would have been so if there were less cable resistance.


The term "Volt Drop" is commonly used in electrical engineering to indicate the voltage which is "dropped" across any circuit resistance due to current flow. However it is not used to express the voltage existing at the load terminals.


The issue is not that "your fridge will not get enough amps to work" it is that there will be insufficient voltage available at the fridge terminals to provide the energy for it to properly operate and this in turn will cause less current to flow. The available voltage is the driving force and the flowing current is simply the outcome.

Cheers
Allan

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Follow Up By: Member - PJR (NSW) - Wednesday, Apr 17, 2013 at 13:24

Wednesday, Apr 17, 2013 at 13:24
Technically you are correct.

Your very first post was emphasing the resistive characteristics of wire. That's fine and anyone suitably knowledgeable would understand it.

But would a baker comprehend it. We have three sons, two of which are bakers. Mind you they are all quite well off by now. Good on them. But would knowing about the resistance in a wire help then. Not a chance. But understanding that the light being dull is because the wire is not thick enough. Yep that would help. So I propose a better all round approach is to lose the technical stuff and leave it in the classroom or lecture theater and go practical. That's what a greater majority of the 4WD public will understand. Hence the pipes and if brighter lights are needed then get a bigger pipe for the current to get through. Bigger wire.

You quote Einstein in your signature, thus you should know what he said about how to pass on knowledge. Pick the right level for the audience. As he said “A little knowledge is a dangerous thing. So is a lot.”. Minimise the real technical stuff by replacing it with the pipes. And you had a go yourself.

We are way off what this was all about and I hope that someone gets some knowledge out of it.

I am not going to get into a technical discussion in an open forum about voltage drops, resistance and latent resistance or heat generated by resistance. Sorry mate, but not on an open public non scientific nor technical 4WD touring forum such as this one.

I think that by now we are boring everyone so it's time for a good cuppa and some muffins that I have in the oven. See I can bake! Well heat up a packet mix but don't tell the boys that.

Phil
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Follow Up By: Member - Rosco from way back - Wednesday, Apr 17, 2013 at 13:34

Wednesday, Apr 17, 2013 at 13:34
Allan old mate, you're probably correct and equally it's conceivable I'm talking total gibberish. I have before and no doubt will again before I eventually depart the mortal coil :o)

What got me thinking along these lines was when I first encountered the fridge problem with the wiring in a previous vehicle.

Suffered the old blinking light on the fridge indicating low "voltage".
Checked with a multimeter at the end of the wire and found I had 12.4V.

Eventually replaced it with a heavier wire and still had 12.4V, but this time the fridge worked. So my feeble mind decided that as the voltage was constant (before load application), and I knew the resistance was constant ... (given length of given dia wire = given resistance). If this is the case then I decided, rightly or wrongly that the variable was available current. Higher resistance of thinner wire resulted in lower available current.

This idea was reinforced in my mind by my analogy to a pipe/hose where the pressure is constant as is the friction which leaves the flow as the variable. Increase the pressure (voltage) and/or decrease the friction (resistance) to vary the flow (current).

This all seems to work quite well for a fixed set of circumstances with no load, however it would appear my theory goes out the window as soon as you make it after you introduce a load.

Maybe I should pull my head in, but one thing's for sure ... it makes for an interesting discussion. Particularly when I encounter people who obviously know what they're talking about.
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Follow Up By: Member - PJR (NSW) - Wednesday, Apr 17, 2013 at 13:43

Wednesday, Apr 17, 2013 at 13:43
Tricky when we get technical and try to stay practical isn't it.

Phil

This muffin is great. A bit hot. Eat your hearts out!!!!
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Follow Up By: Member - Rosco from way back - Wednesday, Apr 17, 2013 at 13:53

Wednesday, Apr 17, 2013 at 13:53
Yep.

My head is starting to hurt AND I've got no muffin!!!
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Follow Up By: Member - PJR (NSW) - Wednesday, Apr 17, 2013 at 15:32

Wednesday, Apr 17, 2013 at 15:32
I would love to post this on a thread of it's own on this forum but I think that it would be deleted. Shame. We need good happy posts.

So make the most while it is still visible.

Now that you mention cooking this is a true story and way out of the zone.

One day when I got home from work I came across what looked like the aftermath of a cyclone had gone through our kitchen. I mentioned earlier that two sons are bakers. They are also qualified and excellent pastry chefs. (bragg - bragg - bragg)

Well flower, pastry, cream and icing was everywhere. But - here's the good part - when I opened the fridge I was over the moon. You see I have this little addiction. It's not a bad addiction. It's an addiction to great big fresh chocolate eclairs with lots of fresh puffy whipped creme and chocolate topping. It's got to be real melted chocolate on top. NOT just plain old chocolate flavoured icing.

Well can you picture this. All the normal stuff in the fridge is shoved down towards the bottom and the top half FULL of foot long eclairs with the creme just oozing out the end. Each one about a foot long. And the creme was fresh. Not the stuff that comes out of a spray can. There were around twenty of the pure delights. Heaven! I have died and gone to heaven.

It seems that one son had a lesson on french pastry that day and for an assignment he had to make just one. ONE!!! He made a bloody fridge full. BOY am I pleased that he took my advice and added pastry chef to his studies. You should see our sweets table at Christmas family gatherings. And a student mate and the other son helped. I think they got a little carried away with the decorations though.

And it took me a month to clean the damned kitchen.

Eat your heart out guys and girls. I did.

Cheers and I hope that I am still a member of the forum tomorrow.

Phil
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Follow Up By: Allan B (Member, SunCoast) - Wednesday, Apr 17, 2013 at 16:25

Wednesday, Apr 17, 2013 at 16:25
Rosco, You were close Old Son. Just a little confused with some terms and the order of events
Certainly the current (Amps) will suffer with increasing cable resistance but that is because of the voltage drop reducing the available voltage at the load and hence less current is "pushed through" the fridge. But the current is not "lost", it simply was not created in the first place due to the "lost" voltage being dropped across the cable.


Regarding your "Suffered the old blinking light on the fridge indicating low "voltage". Checked with a multimeter at the end of the wire and found I had 12.4V"
The reason that you had a good 12.4V at the end of the wire was because you unplugged it from the fridge to take the measurement and so interrupted the current flow and accordingly had no voltage drop in the cable. As soon as the fridge is connected and begins to draw current the volt drop returns and robs the fridge. If you made the measurement with the fridge plugged-in then it was whilst the thermostat was off and stopping the current with the same result.
Cheers
Allan

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Follow Up By: oldtrack123 - Friday, Apr 19, 2013 at 11:35

Friday, Apr 19, 2013 at 11:35
HI
re Quote "Another point, and possibly pedantic but ... the term voltage drop is used here regularly, however this is a bit of a misnomer as it's not voltage drop at all, rather it's current (amperage) drop due to resistance of the wire. If you were to check with a multi you'd find you still have 12.4V or similar at the end of the wire"[end quote]

Perhaps you should try explaining THAT theory to qualified electricians
You have it A*SE about
The cable rsistance & the current combine give a voltage drop
The smaller the cable or the higher the current the GREATER the VOLTAGE DROP [VD]
The voltage AT THE END of the cable when under load will ALWAYS be less then the voltage at the source end
The resulant lower voltage may /usually means the LOAD draws less current
VD is the widely recognised term for this effect!!!!

PeterQ
Electrician

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Follow Up By: oldtrack123 - Friday, Apr 19, 2013 at 11:42

Friday, Apr 19, 2013 at 11:42
HI Roscoe
Re Quote"I'm probably getting out of my depth here ... electrickery is not my major forte. Rocoe "[end quote ]

THAT is very obvious!!!
Sometime a little knowledgable can lead to midunderstandings
or worse.

PeterQ
Electrician
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Follow Up By: oldtrack123 - Friday, Apr 19, 2013 at 11:48

Friday, Apr 19, 2013 at 11:48
Hi
"Member - Rosco from way back posted:
Which, I would propose, is due to the added resistance within the load.
Hence i=V/(R of wire + R of load"[end quote]


Even THAT formula is not being used correctly!!

It requires several calculations To find I[amps]
Best do a lot more studying

PeterQ
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Follow Up By: Member - PJR (NSW) - Friday, Apr 19, 2013 at 12:28

Friday, Apr 19, 2013 at 12:28
Peter

In essence everyone, you included, is correct. It is actually a combination of all three that makes the wheel go slower or the light duller when the wire is not big enough. The wire is just the cause.

That's because nature, Ohms law if you like, must remain in balance. Some like to approach the explanation from a current point of view and some from a resistance standpoint and last, but not least, some from a loss of voltage. All three are correct. It is the ohms law trilogy. Change any one entity either up or down and one or both of the other two will change automatically to keep the formula in balance.

Not having a go at you mate but I feel that you made an error. To find current when you know the voltage and the resistance it is one single calculation. Not several.

There are three ways of representing Ohms law, namely;
E = I x R, the traditional way of writing it.
I = E div R, and
R = E div I.

Just as Rosco said "i=V/(R of wire + R of load".

Are you sure that what you just typed was what you meant mate. Maybe a typo?

Phil
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Follow Up By: oldtrack123 - Friday, Apr 19, 2013 at 12:55

Friday, Apr 19, 2013 at 12:55
HI Phil
No mistake, but should have added
That is only true with purely resistive loads, cannot be applied to motors[compressor fridges , or AC inductive loads ,etc ],where the motor load & back emf comes into the picture.[= equivalent resistance/impedance, not actual measured resistance]


PeterQ
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Follow Up By: Member - PJR (NSW) - Friday, Apr 19, 2013 at 16:36

Friday, Apr 19, 2013 at 16:36
I have to go out but I still wonder.

This statement is wrong. "It requires several calculations To find I[amps]". Your words Peter. One calculation for this formula, I = E/R.

We are not talking about AC. Not one person even mentioned it therefore it is irrelevant and disruptive to the discussion. Just plain old 12V DC. It does not help mate. Sorry to be bluint but I must rush.

Cheers I am off to the Aus verses NZ footy.

Phil
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Follow Up By: Member - PJR (NSW) - Friday, Apr 19, 2013 at 23:20

Friday, Apr 19, 2013 at 23:20
Just got home from a great night. You little ripper.

Phil
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Reply By: Ross M - Wednesday, Apr 17, 2013 at 11:43

Wednesday, Apr 17, 2013 at 11:43
G'day PJR Phil
I have seen or been in situations where the word "professional" is used to demand respect of someone ability and that it should never be questioned.
It has caused me to never want to be "professional".

I also find from reading various forums where electricity is used that many people who went to school never seemed to learn anything about electricity.
The good old Tech school situation prepared people for, or alerted them to many situations. They then developed their understanding from a sound grounding in basics.
Things I learned then prepared me and began a developing life long learning process.

I find many people now are having problems with items they either do themselves or have fitted by "professionals" who really don't know either.
Experience might stop an Auto sparky from putting a grommet where it will fail. Basic periodic checking of wiring will pickup the developing failure if looked at.

Many can grasp the idea of connecting something to a battery with a piece of cable but few actually start thinking about the issue in any depth so as to avoid future problems with the design or modification or realize the consequences of what they are doing.

It all looks so simple but each field of endeavour has it's own intricacies which must be under stood to at least some degree.

Therefore many problems seem to be caused by the people themselves.

PS No one should tie 12v cables near brake lines.Easily a foreseeable safety issue.
But if you don't know about the danger or can't perceive it, then there isn't a problem with a "professional" installation is there?

Ross M
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Follow Up By: Member - PJR (NSW) - Wednesday, Apr 17, 2013 at 12:52

Wednesday, Apr 17, 2013 at 12:52
Too true. And the use of titles like professional, engineer, consultant and even the title Electronics Technician (eventually senior tech for the ACT) that I studied so hard for (don't go too far Phil) and achieved are so belittled by today's disrespect for the hard work gone into earning a tertiarry "title". They get them at Woolies or Coles for just sitting in on a seminar (???) for a week or two. There is also a scary tendency not to worry about studying such basics because Google will give the answer. Yeah!!! Right!! Up in the high country or in the desert where there isn't any internet. Not.

Why should they study. Google will remember it for them. All they need is to remember how to use Google. My mind boggles at the lack of knowledge when I watch a quiz show. But what is more frightening is that they won't know what to look for and even if they find what caused the fire how to get the car going again. No google in the desert remember. Scary.

So we try to keep stuff to a low enough level so that everyone may get something out of our "stories". Not too deep but deep enough.

Here's one for the books:
That metal brake line that melted ran from the master cylinder to the back of the car and was possible earthed near the hole through the tub where the positive terminal was shorting. Effectively making an earth connection for the tub and thus the positive terminal in the back. Well that can be the only explanation because there wasn't a cable near the brake lines that we could see.

Scary hey!!!

That last point was interesting. How are they going to learn about the dangers if they don't study, read or listen. Worrying.

Thanks for dropping in.

Do you realise that this thread is one of just a few non complaining or whinging threads that I have been on for ages. I got so sick of reading threads with heaps of whinges and complaints that I started this one.

Phil
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Follow Up By: Nomadic Navara - Thursday, Apr 18, 2013 at 17:03

Thursday, Apr 18, 2013 at 17:03
Oh how our language is being buggerised. When I went to school a professional job meant that it was done by someone who was paid for doing it, as distinct from one done by amateur. These days the word has become a euphemism for quality. Oh how I dislike euphemisms, in this case you don't know if they are talking about a job done by a tradesman or a quality job.
PeterD
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Reply By: Bazooka - Wednesday, Apr 17, 2013 at 20:29

Wednesday, Apr 17, 2013 at 20:29
So after the preliminaries, has (a) the voltage drop team or (b) the current drop/resistance team taken the points? My science hat suggests the latter but my brain's in neutral and doesn't want to do the work required to confirm.
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Follow Up By: Allan B (Member, SunCoast) - Wednesday, Apr 17, 2013 at 20:48

Wednesday, Apr 17, 2013 at 20:48
Hey Bazooka, In my diagram above, 'V2' measures the voltage-drop and 'A' measures the circuit current.
Show me where you could measure the "Current Drop"?
You could win a jar of pickled Ohms!
Cheers
Allan

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Follow Up By: PeteS - Wednesday, Apr 17, 2013 at 21:11

Wednesday, Apr 17, 2013 at 21:11
Oh dear,
And I though an Ohm was where a Volt lives!

........ sorry couldn't help myself.

PeteS
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Follow Up By: Member - PJR (NSW) - Wednesday, Apr 17, 2013 at 21:42

Wednesday, Apr 17, 2013 at 21:42
Just up the road from Ampie Potential who is the current girlfriend of Bruce Watts!!!

Cut it out. Now you have me doing it.

Phil
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Follow Up By: Member - PJR (NSW) - Wednesday, Apr 17, 2013 at 21:50

Wednesday, Apr 17, 2013 at 21:50
Bazooka

It really doesn't matter which one won. As long as the light is bright enough and the fridge keep the grog cold. As long as the message gets through. Good old Albert Einstein was right. The right one is the one with the correct approach for the audience at hand. Not too much and not too little.

When my wife started studying electronics for her ham licence, she asked me and I used ohms law but no mathematics. The sons understood the pipe scenario easiest.

Catchya

Phil
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Reply By: phil300 - Wednesday, Apr 17, 2013 at 20:35

Wednesday, Apr 17, 2013 at 20:35
Solution:
Be careful selecting the correct cable and where you choose to run the cable. Go up a size to be safe. Allow for flexing and protection against sharp edges, shorting and rats/mice eating the insulation.


And put a fuse at both ends were there are two source's of power..

cheers phil.

AnswerID: 509228

Reply By: Flighty ( WA ) - Wednesday, Apr 17, 2013 at 23:05

Wednesday, Apr 17, 2013 at 23:05
Phil.
Have been reading this post with some interest, and will admit to not being very smart in electrics at all, but willing to listen, read and learn.
May I say to you and all other posters thank you for the information in this post.
It lets the average person like myself know that it's not all beer & skittles when it comes to this type of thing.
While admitting to having a basic understanding of what is required for any electrical works when I need it fitted to my vehicle, I do realise the need for an extended knowledge which is from training and experience in this field.
This I can see from this post !!!
I can only hope that it continues this way.
Am beginning to get a bit sad with the continual battering some people are getting from a simple question.
Once again thanks for your post on this subject, and thanks also due to all other contributors for your excellent input.
Cheers & Regards
Flighty

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Follow Up By: Member - PJR (NSW) - Thursday, Apr 18, 2013 at 10:06

Thursday, Apr 18, 2013 at 10:06
Thanks Flighty.

It's good to know we are not just blowing off steam.

Have a good day mate.

I hope you get a bundle of "Thanks" clicked.

Phil.
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Follow Up By: Honky - Thursday, Apr 18, 2013 at 10:23

Thursday, Apr 18, 2013 at 10:23
I also love these posts as I have learned a lot about 12 volt and have wired up my own 12 volt home solar system and a on vehicle solar system with DCDC and alternator charging.
It is a far better than just reading a text book.
Whilst it is online I compare these discussion to doing a "computer course" with no input to "being in a class with all the debate" that would not come up.
I am quite happy to be proven wrong as it means I have learned something I did not know it may keep me out of trouble.

Please continue and keep up the good work

Honky
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Follow Up By: Member - PJR (NSW) - Thursday, Apr 18, 2013 at 10:35

Thursday, Apr 18, 2013 at 10:35
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Reply By: The Bantam - Thursday, Apr 18, 2013 at 16:15

Thursday, Apr 18, 2013 at 16:15
Sorry folks....but the term "Voltage Drop" is the correct one and it accuratly describes exactly what is going on.

This issue is very much poorly understood and as people have testified, very often underestimated.

All wire has resistance.
When current flows in that wire the voltage will differ from one end to the other...ALWAYS...may be a little may be a lot.
There is a "voltage Drop" over the length of the wire......every wire carrying current.

Voltage drop is the correct term, it is used consistently in all the text books and in all the standards and specifications.

Voltage drop can be accuratelly calculated and and easily measured.

However all to often we do not know accurately what is happening.....in the case where the voltage measured is the same before and after heavier cabling has been installed It is clear to me that the measurement was not taken when the full load was not present....IE the fridge motor was not running.

NOW
Back to what we need to know.
In low( extra low to be trechnically correct) Voltage DC situations we have only 12 or sometimes 24 volts...if one or two volts go missing....we have a problem.

On 240V, like in our homes if a volt or two goes missing.....its no biggie...we can loose 12 whole volts and most appliances will still run just fine.



The problem is compounded, because DC equipment draws a hell of a lot of current ( that causes voltage drop) and often suffers badly from reduced voltage.

All too often people underestimate voltage drop and its effects and install cable that is far too small.

In fact in a lot of situations the "correct" or an "adequate" cable may look rediculously large.

In 12 volt situations we should always be using cable heavier than it's required rated current unless we are going very short distances....eg...15 amp cable is only any good for 15 amps if its going a couple of feet..because of voltage drop......

The further you go and the higher the current.....the size of the cable required to minimise voltage drop gets rapidly...very rapidly larger.......easily to the point that it can look rediculoulsy large for the accessory connected.


This is why all the heavy current draw devices fitted standard to a car are very close to the battery.

If we are to run high current demand items in the rear of the vehicle....the cables will need to be large....they may seem rediculoulsy large.

Even many of the "professionals" don't grasp how heavy cables need to be.
Almost every car comming out of the factories is wired in cable as light as it possibly could be and still work.......I hear full ticket 240v sparkies horified when they find out what is normal in how much current is put thru quite small automotive wiring

cheers
AnswerID: 509264

Follow Up By: Nomadic Navara - Thursday, Apr 18, 2013 at 17:43

Thursday, Apr 18, 2013 at 17:43
I will go along with Bantam, It looks like Rosco measured his voltage with no current flowing. As others have pointed out, no current no voltage drop.

When describing technical subjects you must use terms that are correct in all situations. You should not cherry pick terms to use with your own interpretation of them. You will get a voltage drop along a wire but it will have the same current along it until you get to a junction.

When you have a resistive load you will get less current flowing in the circuit if you use thinner wire after doing your measurements with thicker wire. However Rosco's theory of current drop falls down when you are supplying active devices like DC-DC chargers and inverters. These devices, within reason, draw the same input power no matter what the input voltage. If you replace a cable with a thinner one to one of these devices, the voltage will drop but the current will rise to maintain the same power input. Bang goes the current drop theory.
PeterD
Retired radio and electronics technician

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Follow Up By: The Bantam - Thursday, Apr 18, 2013 at 18:10

Thursday, Apr 18, 2013 at 18:10
I think it was Mr Gustav Kirchoff, that discribed this whole relationship building on Mr Georg Ohm's work, both working with the battery that Francios- Marie Aurouet ( aka Voltare) was fiddling with nearly 100 years earlier.

Seriously all this was well discussed and established over 100 years ago.

cheers
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Follow Up By: Allan B (Member, SunCoast) - Thursday, Apr 18, 2013 at 18:18

Thursday, Apr 18, 2013 at 18:18
Well I don't think some of the younger fella's here were around at that time so could be excused of ignorance.
Cheers
Allan

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Reply By: Member - PJR (NSW) - Friday, Apr 19, 2013 at 02:00

Friday, Apr 19, 2013 at 02:00
All well and good. Voltage verses current reminds me of the chicken and egg conundrum. Some like the "voltage drop" approach and others prefer a "limited current" scenario. All fine as long as we get the right wiring done properly.

But the thread is stuck on the "loss" issue. What about the fire that we had! That is life threatening and I would think, much more important.

Questions:
What, if any, fire extinguisher would you carry?
What type and what size?
And if so how big and where would you store it?
Have you ever shown any passengers or even shown the family how to use it?
What could the layman look for that could cause an electrical fire or short?
How many can very quickly put their hand on some gloves and a toolbox? (to disconnect the boiling battery etc)

What have I missed? Feel free to add to the points above.

I (naturally being humble) have some answers but I would like to hear what others say. Maybe even what people have found done "professionally".

This will be telling a lot to suck eggs. But we all need a refresher some times. And maybe learn from our mates. I am open to new ideas.

I have emails and thanks from both new and old 4WDers. So this thread is doing what I wanted. Discussion and education. No darn whinging or complaining. Excellent.

Phil

PS: Guess who can't sleep!!

AnswerID: 509297

Follow Up By: Allan B (Member, SunCoast) - Friday, Apr 19, 2013 at 10:01

Friday, Apr 19, 2013 at 10:01
Phil, Yes adequate fire extinguishers are vital. Just look at the burnt-out wrecks on some tracks! It has been established that a single small extinguisher will not be adequate to deal with a typical fire in a vehicle. I carry three. A 1Kg alongside each seat easily accessible from outside and a 1.5Kg mounted on the rear door of the Troopy. Those travelling through spinifex would do well to also carry a 5 litre water bottle/spray pump to deal with underbody ignitions.


Phil, In regard to quick disconnection of batteries in an emergency, I have sometimes wondered if those "screw-down" isolators mounted on the battery post would be a good idea. My reliance is on the near-mounted circuit breakers.

Incidentally, getting back to your original post, in that fire in your mate's vehicle, was that battery cable appropriately fused at both ends? Probably was not considering the severe outcome of a short to earth.
Cheers
Allan

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Follow Up By: Member - PJR (NSW) - Friday, Apr 19, 2013 at 11:27

Friday, Apr 19, 2013 at 11:27
No fusing or breakers for the battery in the rear. I cannot recall if there was a redarc in the system.

Your point about the small extinguishers is very pertinent to the fire. The 1Kg extinguishers were too small and they all ran out. The trip leader had a very handy 2KG powder and did a better job that the others. Please note that we were back down the track so this is all a bit second hand from a report at our club meeting.

Location was another matter. Some were buried down under all that junk that you carry in the back. Same place as some of the toolboxes were.

Like you ours is in within reach of the driver and passenger. On the floor at the front of the passengers seat. We have a 2KG also. And for the grassy trips I bung a 5 litre pump up sprayer with a good wand and bit of stiff wire on the floor behind the drivers seat.

These battery isolator kill switches and breakers as you suggest need more thought before I will use them. I was warned once not to use them by an auto electrician. He won't even install them. Something about getting the slightest bit of contamination or dirt in them can screw the "balance" for the alternator and put heaps of amp where it is not supposed to go. Too long ago for me to recall why and have not investigated it. In the meantime all the heavy current battery cabling has been tidied up, protected and run safely and neatly and inspected at services.

I have scars from being burnt by fools not doing the job properly or fooling around. I charged a young corporal once for being funny!! We used a lead pencil to quickly diagnose where power was being lost in a radio transmitter. War doesn't wait for the niceties of OH&S. We would hold it over each stage's wires until it got a spark. That meant the signal was looking for a way out and the circuitry after there was faulty. It was "kind of safe" as the pencil outer wood casing was a bad conductor. You never touched either end of the pencil though or ZAP. Well this clown stuck a pin in the side of the pencil and cut off the end of the pin. Follow me here: I am holding the pencil by the sides. first mixer - no spark = all good. Finals pre amp - no spark = all good. PA - - no spark = all good. Output tranny - crap ZAP!!! I was thrown across the room.

There goes Phil again. Apologies.

Back to the topic: And some people wonder why their cars are over GVM. But that's the price of being self sufficient as much as you can.

Catchya

Phil
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FollowupID: 787133

Follow Up By: oldtrack123 - Friday, Apr 19, 2013 at 11:58

Friday, Apr 19, 2013 at 11:58
HI Roscoe
RE Quote"Eventually replaced it with a heavier wire and still had 12.4V, but this time the fridge worked. So my feeble mind decided that as the voltage was constant (before load application), and I knew the resistance was constant ... (given length of given dia wire = given resistance). If this is the case then I decided, rightly or wrongly that the variable was available current. Higher resistance of thinner wire resulted in lower available current" [end quote}

There in lies the problem
You obviosly did not test CORRECTLY
To see the voltage drop in the cable & the true voltage AT the load
THE LOAD MUST BE ON
THE FRIDGE MUST BE OPERATING
THEN you will ALWAYS see some difference in the voltage reading'
It mat be small if the cables are large relative to the load wattage.
Or large if the cables are small for the load wattage

That difference is referred to as the VOLTAGE DROP

PeterQ
electrician
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FollowupID: 787143

Follow Up By: oldtrack123 - Sunday, Apr 21, 2013 at 13:46

Sunday, Apr 21, 2013 at 13:46
HI
RE{quote ]Something about getting the slightest bit of contamination or dirt in them can screw the "balance" for the alternator and put heaps of amp where it is not supposed to go. Too long ago for me to recall why and have not investigated it.[end quote]


Again I would suggest terminology is a bit mixed up!!
& the problem depend WHERE the isolator is connected
Alternator VOLTAGE can rise out of control when there is no load connected to it,
This can damage the alternator itself[ mainly the diodes& possibly the regulator section
The high VOLTAGE can damage equipment which may still be connected to the alternator
BUT if the isolator is only connected in the LOAD circuit Positive from the battery, &at the battery [which is where it should be] the above is not a problem
Any dirty/ poor contact will simply show up as EXCESS VOLTAGE DROP under load!!

Brian /Wanderer
IF you have a close look @some of the screw down isolator
They do have a design problem
They rely on THREAD contact faces area to pick up the current[that area can be quite limited]
Need regular cleaning, I know from personal experience


PeterQ
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FollowupID: 787347

Follow Up By: Member - PJR (NSW) - Sunday, Apr 21, 2013 at 13:59

Sunday, Apr 21, 2013 at 13:59
Outback

Not exactly sure and couldn't recall his explanation but what he said suited me and I also trust him. You see he worked for a bloke called Doug Chivas of Holden Dealer fame, and Doug wouldn't have any idiot working for him. Mate that was back in the early 70's. That's good enough for me and thanks for the added info. Doesn't matter anyway to me because the kill switches are not needed.

Have a good day.

Phil
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FollowupID: 787348

Follow Up By: oldtrack123 - Sunday, Apr 21, 2013 at 21:21

Sunday, Apr 21, 2013 at 21:21
Hi Phil
It hardly matters who he worked for ,he still cannot rewrite the basic laws of electricity or the facts of the effects of having no load connected to a vehicle alternator.['voltage run away]
OR perhaps you misunderstood him???

It is a pretty basic rule "never run a vehicle alternator with out a load/battery connected"


PeterQ
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FollowupID: 787396

Follow Up By: Member - PJR (NSW) - Monday, Apr 22, 2013 at 08:33

Monday, Apr 22, 2013 at 08:33
Mate Can't you read between the lines. I don't care.

Who said anytyhing about no "load". I didn't. So what the hell are you going on and on about. Don't answer that.

Goodbye.

I wish that I could drop this thread. It is being pedantic and boring.

Phil
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FollowupID: 787414

Follow Up By: oldtrack123 - Monday, Apr 22, 2013 at 11:25

Monday, Apr 22, 2013 at 11:25
Hi Phil
Not being pedantic, just trying to make sure some are not mislead by B*^^#it
It is now becoming obvious that you do not understand fully

Your opening posts & your unusual theories indicated that.

Even what your mate, the auto electrician, may have been talking about

In the case of several posts on THIS thread the question of being "on load" or "off load" has been very relevant

Especially the fridge Voltage test & the POSSIBLE PROBLEMS with an isolator which maybe connected so it not just in the LOAD line BUT also IN THE ALTERNATOR TO BATTERY LINE!!!

Peter Q
Electrician
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FollowupID: 787425

Follow Up By: Member - PJR (NSW) - Monday, Apr 22, 2013 at 13:10

Monday, Apr 22, 2013 at 13:10
Mate

You are sooooo wrong. I studied full time for years at RMIT in electronics and communications to achieve my degree. And more advanced studies over the next 40 years in the follow on career that is none of your damned business. I would suggest that you leave that topic alone.

My " unusual theories" as you called them, were NOT THEORIES. there was only one and it was used as an example to help non technical readers understand some basics.

You insult me by not reading and comprehending what I said at the start and just jumping to conclusions. That means fully understanding the real message within my whole post.

Now will you please desist and leave me alone.

Phil
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FollowupID: 787426

Follow Up By: Lyn W3 - Monday, Apr 22, 2013 at 20:02

Monday, Apr 22, 2013 at 20:02
But it's so entertaining reading threads like this!!
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FollowupID: 787446

Follow Up By: Member - PJR (NSW) - Monday, Apr 22, 2013 at 21:12

Monday, Apr 22, 2013 at 21:12
Hi Lyn

I wish these little "johnny come lately" know alls would read the rest of the thread before they go jumping up and down and start accusing people of being ignorant and having strange ideas. I am happy you got something out of this.

Anyway the PMs are entertaining, but unprintable. Nah all good.

Catchya

Phil
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FollowupID: 787457

Follow Up By: oldtrack123 - Tuesday, Apr 23, 2013 at 11:33

Tuesday, Apr 23, 2013 at 11:33
HI
Well ,this "johny come lately "just happens to have spent his whole WORKING life in Electrics.
Covering a very wide range:
Iindustrial , electronics,[Diploma in industrail electronics},switchgear, control systems,Head service technician for a large engineering supply co, [selling Engineering & commercial electrical products of all types]!!!

Now retired!!

Strange with ALL those quals you have, you did not seem to know the problems that a battery isolator could have, IF in the wrong place.

Perhaps you should reread all the posts , YOU would then understand to just which posts my comments applied
The thread did drift of your original subject

PeterQ
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FollowupID: 787507

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