Do I need an earth wire??

I see a lot of people have read your post but no replies. I don't know much about electricity either, but am I right in assuming you are going to charge the batteries via a 12v to 240v inverter?
There appear to be differing opinions but one is that in the event of an accident, and someone comes to your aid, they could be at serious risk with 240v. flowing through the car. Suggest you talk to a auto electrician.

KenD
bris

Hi Lyndon,
The charger will be properly earthed via its 240 volt cable from the mains, just be sure that you use an extension cord which includes an earth conductor. A standard 3-pin extension will have this.
The charger could also be double-insulated and thus require no earth.
The 12 volt side of the charger does not require earthing.

Reading your post I assume that you will be charging on board batteries from the mains supply, in which case I would not run a dedicated earth to the vehicle as the charger output will be grounded by the negative connection to the battery.

Chris

Thanks for all the replys. Looks like it with be ok, no flames yet anyway :-)
I noticed that the inverter in the car has been earthed to the car, hence the question. The wire I'm using has not got hot which seems to tell me it is handling the current flow ok. Now a new post for the battery and charging Guru's

Lyndon

If in fact you are running it off an inverter doesnt that run off the batteries????????

Are we all missing something here or is it just being explained really badly.

If it is in fact running off the batteries via the inverter I would think the looses in the circuit would negate the charging effect.

No such thing as a free lunch or perpetual motion or making more electricity than you are using ?????????????

Im sure an expert will correct me but this has been hashed over previously and the answer was as I have replied.

Thats what I thought but he goes on to say later that his inverter is earthed to the car body so if he isnt charging the batteries with it the same cautions would still apply.

I had one and got rid of it for safety reasons.

Only takes one drink and bye bye.

Cheers

Hmmmm... I realise that this thread last had activity 20 or so hours ago, however I feel the need to provide some "expert" input here. No, I'm not an electrician, or an engineer. Rather, I'm a doctor, who feels quite confident talking about mechanisms of electrical injury. I have seen and treated victims of electric shock, and have a decent understanding of the physics and pathology behind electrocution/electrical injury.

I will not offer an opinion on the matter at hand, which is beyond my area of expertise. However, I will correct some misconceptions about electrical injury, and summarise the topic in a piece I hope will be read by anyone who surfs onto this thread as a result of a future search, or link from another thread.

I mean no disrespect to any contributor on this Forum. You have my admiration for writing, and we all add our input in good faith. Please understand that I am merely combining mostly what others have written into a summary of correct information.

There are three main mechanisms behind electrical injury:

1) thermal effect causing burns, damage to skin/muscle/organs etc caused by the resistance of body tissues, which depends on the length of time a subject is exposed, the voltage, and the location where the current is applied

2) secondary injury caused by muscular contraction/involvement of the nervous system in which involuntary movement places a person in harms way, ie: if a hand were to be propelled into a moving radiator fan, or if tetany caused the palm to grip a live wire

3) cardiac arrest caused by the induction of lethal arrhythmias (ventricular fibrillation) where the only definitive treatment is DC defibrillation - another form of electric shock (delivered by a defibrillator, usually involving 200J of energy)

There are other more subtle mechanisms: increased permeability of cells, diaphragm paralysis, central nervous system dysfunction. These are probably beyond the scope of this discussion for the time being.

There are five main factors that determine the extent of electrical injury:

-Type of current (direct [DC] or alternating [AC])
-Voltage and amperage (measures of current strength)
-Duration of exposure (longer exposure increases injury severity)
-Body resistance
-Pathway of current (which determines the specific tissue damaged)

Alternating current (especially at low frequency, such as the 50-60 Hz supply common in household circuits and power inverters) is 3-4 times more dangerous than DC current, for several reasons, which I will elaborate on later. In fact, 240V at 50-60 Hz, is one of the most dangerous combinations you could select for a power system! (The pathophysiology was not as well understood when Thomas Eddison and George Westinghouse were fighting their "War of Currents").

Voltage and amperage are debated in this thread. The old saying that "current kills", or as one member put it, "It's the voltage that matters, not the current" circle around a larger issue: Ohm's Law.

V=IR states that current is proportional to voltage. You can rearrange that to state that I=V/R. Hence, the larger the resistance at a given voltage, the smaller the current.

The variable is the resistance of the body. It varies depending on the location that the potential is placed across, the amount of callous on one's hands, the presence of metallic jewelry, sweat, water, their fat/water content, and most importantly, the presence of shortcuts to the heart, such as a pacemaker lead.)

The effect of electricity on the body depends on the current flowing, which as we have just discussed (by Ohm's law, and the resistance of the body) is a direct function of voltage and resistance. You cannot "force" 400 A at 12 volts, unless you can control the resistance (ie: make the resistance of the body 0.03 ohms - the resistance of a foot submerged in water is about 100 to 300 ohms, which would cause a current of 100 mA - this is enough to cause severe pain, or difficulty breathing! But it's a long way off 400A.) You could kill with a 1.5 V battery if the resistance to the heart were sufficiently small, but this would require nothing short of direct electrodes (the native potential of a cardiac muscle cell in its polarised state is -70 mV, and it depolarises to +110 mV, so you can see how small the voltages are that we're playing with here).

At a usual resistance of dry skin, approximately 40k ohms, you can see that the current drawn from a 12V source would be 0.0003 A - or 0.3 mA - not enough to be felt by the average male in good health. The current CANNOT be larger than this, unless the resistance is lowered by moisture (which might reduce the resistance to 4K ohms, where you would receive a slight sensation at the hand), or by another means of lowering resistance as described above.

Alternating current is different again - at 0.3 mA (ie: 12V 60Hz AC through dry skin) you will have a slight perception in the finger in contact with the wire. (This is only an example - I'm sure most of us don't routinely deal with AC this low). Generally, the effect seen is three to four times greater with AC - ie: it takes three times less current at 50-60 hz to bring about a particular effect than with DC current. Strangely enough, the effect of higher frequency AC current (ie: 10 kHz) is diminished. It's actually between 5 and 10 times SAFER than DC current.

Pathway of current refers to which parts of the body are in the circuit. The most dangerous pathways cross the heart: left to right hand being the number one. Right hand to right foot is the safest of the limb paths, however this does not account for diaphragmatic paralysis or thermal effect.

Alternating current is more likely to induce ventricular fibrillation (VF) than DC current, although DC can certainly do so (you just have to be less lucky, and you need more current). It takes 500 mA at 3 seconds of exposure to induce VF in a healthy adult with DC, but only 100 mA at AC. (In fact, in some conditions, ie: across the chest, VF can be induced with 17 mA of DC current). To draw 500 mA with 12 V DC requires a resistance less than 288 ohms, which could occur if your hands were immersed in solutions of salt water (a contrived situation, I know). The point is, 12 V can kill. With 240 V 60 Hz AC, the minimum resistance to induce VF after 3 seconds of exposure is 2.4k Ohms - this can occur in a finger/thumb grip of a wire with dry hands.

If a person is unlucky enough to be wearing a ring, or gripping a pipe their resistance may fall to 1000 ohms. In these situations, the DC voltage required to generate 17 mA (the lowest current that can cause VF in ideal conditions) is 17 V. Under less than ideal conditions, 17 V can still induce tetany, which can cause secondary injuries.

Keep in mind, this is for a lethal cardiac arrhythmia - at EVEN LOWER currents, thermal injury, muscular contraction, and diaphragmatic paralysis CAN occur, and these can be lethal.

AC is much more likely to induce fibrillation because it disrupts the electrical pathways of the heart, and continues to do so, creating "circuses" of turbulent current patterns. This is due to the cyclical nature of the current - it "pushes and pulls", so to speak. DC generally depolarises the heart completely, which will cause arrest whilst the current is applied, but the heart may begin beating normally once the current is removed (unless specific conditions are met, which is why 3 seconds can induce VF at 500 mA). In fact, this is how defibrillators work - a DC current is applied across the heart, which causes total depolarisation of the heart muscle, and its intrinsic "pacemaker" (usually) takes over. The term "jump start" isn't correct - it is more akin to a "reboot". Defibrillators can apply a biphasic current of between 15 and 40 A across a chest with a resistance of 50-200 ohms (reduced through the use of the gel conducting pads). The machines are clever - they measure the resistance across the chest (actually the impedance, but enough on that), and adjust the peak current and duration of the shock accordingly.

Thermal injury has been "touched" on, but suffice to say that it is a function of current as well. One contributor talked about static electricity - this is actually due to many thousands of volts, but is with relatively few electrons, and therefore brief, also altered by the large resistance of air (the physics behind it is even more complicated - Q=it comes into play). The currents in a lightning strike are extremely large, as is the voltage. The exposure is very brief. Their injury and lethality often comes from thermal injury - looking at lightning strike victims is not for the weak-stomached! Surviving a direct hit from a lightning bolt is EXTREMELY rare. I also reiterate - you CANNOT alter the current - it is a function of voltage and resistance.

I hope this has cleared up some misconceptions about the role of electricity in injury. To summarise, ANY voltage, AC or DC, can kill under the right conditions. AC (certainly in household voltages) is EXTREMELY dangerous, and much moreso than DC. Current is a function of voltage and resistance. You cannot independently control how much current is drawn, if you have a known voltage and resistance (which is a function of the pathway that the current is flowing, based on multiple factors). Some pathways are more dangerous than others.

Overall, ELECTRICITY IS DANGEROUS and poorly understood. This includes the output of a 240V AC power inverter, which is JUST as dangerous as domestic supply.

With the deepest of respect and noble intentions,

Charles

_______________________________
Charles Jenkinson
Perth, Western Australia

Gracie "The Grey Ghost"
1991 Toyota Landcruiser GXL (HDJ80R)
4.2L 1HD-T Turbo Diesel
358,864 km and counting!

Appliances are earthed for one reason - to reduce the chance of an electric shock. If there is a fault and 240volts is connected to the case, a fuse will blow.

If the Charger has an Earth Terminal, you connect this to the bodywork of the car and if the Extension Lead has good earth conductor, then you will have similar protection in case of a fault e.g. the Extension Lead gets pinched in the door and only the Active conductor contacts the bodywork. The fuse will blow, even if there is no Safety Switch (RCD).

I don't see any reason to assume that either the Positive or the Negative output of the Charger will be connected to Mains Earth.

Safety Switches and Double Insulation are other approaches which provide protection.