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# Electric Safety

by: Feb 28,2024 316 Views 0 Comments Posted in Engineering Technical

Summary：       We all have a friend or colleague who thinks they are immune to the risk of electric shock. How often did someone tell you something in the line of: “I’ve been shocked so many times ..it’s ok, I am used to it…”.. Little do they know. In this article I will tell you exactly what I mean and I will explain why electricity is can be dangerous and most important, why people survive or can get killed by it.

Introduction

We all have a friend or colleague who thinks they are immune to the risk of electric shock. How often did someone tell you something in the line of: “I’ve been shocked so many times ..it’s ok, I am used to it…”.. Little do they know. I don’t believe there is an immunity to electricity as it all depends on circumstances like  current, voltage, resistance, humidity, skin conductance, duration, timing capacitance and yes some luck or bad luck..

In this article I will tell you exactly what I mean and I will explain why electricity is can be dangerous and most important, why people survive or can get killed by it.

The concept of a electric flow.

It’s important to understand that, voltage, resistance and current are closely related. Working with the concept of electrocution on humans, it’s equally important to know that a voltage of current had a entry point and a point of exit. Yes, sometimes this can be multiple points. However, for understanding of the concept, let’s stick to one of each. Let’s assume that a person grabs the ground pole of a car battery with his left hand while he grabs the positive pole with his right hand. In this case, the human body forms a resistance that is directly connected to both poles of the battery and according to Ohms law, a current will flow through the body that is equal to the voltage divided by the resistance. The higher the voltage or the lower the resistance, the higher the current. Most of the time the voltage is fixed, like in this example with the car battery.

Impedance of human tissue

The resistance of the human body is a whole different story. Not only does it depend on the surface of contact but it also depends on the type of tissue and the amount of sweat it produces. Also, resistance in this case is not the really the correct word to use. It’s better to take about impedance because from science experiments we know that frequency plays an important part in this matter.

The human body has two impedance components. One is of skin, the other of the tissue under the skin. Skin has a larger, more variable impedance than body tissue.

Skin Impedance: 2000 to 4200 Ohm

Body Tissue Impedance: 500 to 750 Ohm

The impedance characteristics of the human body are nonlinear. Impedance drops as voltage increases, with a consequent nonlinear increase in current.

AC vs DC

There is a difference on how we experience electricity when we speak in terms of AC or DC

When we look at the effect of both, you can say that being electrocuted with an Direct Current (DC) will most likely result in more severe burns compared to AC. While with Alternating Current (AC) the threshold of “feeling the flow” is much lower compared to DC.

The effect of frequency

Talking in terms of AC, frequency plays an important role. The human skin ( and hearth) is most sensitive for frequencies around 50 to 60Hz. Unfortunately, this is also the net frequency in most

houses.

It’s also known that high frequencies will not penetrate to the inner tissue due to the scientific phenomena called skin effect. An example of a typical skin effect is the use of some tesla coils where people can literally touch the sparks it produces without instant dead occurring. ( warning, this is not an invitation to play around with tesla coils because some tesla coils can kill you indeed )

So the frequency of an Alternating Current has a big impact on humans in terms of electrocution and I already explained that our net frequency of 50 of 60Hz is the most dangerous frequency. So lets dive into that a bit more.

Duration and timing

Let me introduce the term of “let go current” which is the current level where we lose control of our muscles. The muscles will start to contract until the current is removed. If, at that point, we are holding a wire, letting go of the wire is not possible. Someone has to help you by removing the power ( unplugging the source) of has to remove you from the source without being electrocuted themselves. For humans, this threshold is 10mA only. If the current increases, at 20mA chest muscle contractions can cause suffocation and at 30mA it can induce ventricular fibrillation to your heart.

This brings me back to my opening statement….that friend of yours who thinks they are immune to electrocution. As mentioned, it all depends on circumstances like voltage, current and all the other things I already mentioned but, and this is important, what most people don’t know is the effect of timing.

The moment that electrocution occurs might be the most important factor of all. Assuming that voltage and current and all those are enough to kill you, the moment of electrocution and your heartbeat are closely intertwined. Looking at electrocardiogram, there is a part of it that we call the T wave.

If electrocution occurs during this T wave, were the heart is most vulnerable, cardiac arrest of ventricular fibrillation can occur.  It is also good to remember that sometimes an electric shock looks like ‘just a shock that you survived’ in the beginning, giving the impression that all is fine, but due to direct damage to the hearth can still kill you moment or even days later.

Our Electric Net

As I mentioned earlier, electrocution can only occur in closed circuits; we need an entry point and an pint of exit and the electric circuit most be a closed loop. So what about the power we get from our outlets in our homes? We still get an electric shock if we touch the live wire only. How is that possible? Don’t forget the circuit is not a closed loop in this case, or is it? Actually it is due to the way our electric net is constructed.

At the power plant the neutral wire is directly ‘planted’ in the earth. (Depending on the type of infrastructure their might be more places where the neutral is connected to earth). As a result of this, the whole mother earth is now a conductor for the Neutral. Because we are standing on the earth while touching a live wire from the outlet, a current will flow from the live wire trough our body into the earth. Along the way, it will have to come isolated materials like our rubber shoes. The better this isolation, the lower this current will be.

Another thing to consider is the total surface of our body that forms one site of a capacitor while the other site is the earth. Together with the air and the room humidity, this will create a so called parasitic capacitor. Like any capacitor, this can hold and transfer an electric charge.

Turning things around to the safe side of our electric life.

So, in order to prevent electrocuting ourselves while working on electric devices, there is a rather simple solution. Beside the use of proper earth leakage detectors and circuit breakers, I highly recommend that you use an isolation transformer. An isolation transformer is a special transformer. It transforms the incoming voltage to a similar output voltage. So your 230V ( of whatever you have) is transformed to 230V while creating a new net. The output of the transformer has no hard defined neutral of live wire as none of the output wires is connected to the Earth. So this time we have a hard interrupting in the net circuit coming from the power plant and you no longer get electrocuted while touching only 1 wire.

Although the transformer also has an inductive and parasitic coupling between the input and output that will created a leakage current, this current is limited to approximately 50 uA. A current of 50uA is too low to do any damage to the human body and makes it a safer solution for or workspace.

If you want to purchase an isolation transformer you should make sure it can handle the output power you require. The more the better is not always the way to go. With greater power comes a greater rush in current and your circuit breakers might start to complain if this rush-in current is too high.