Introduction

We all know someone who believes they are immune to the risk of electric shock. How often have you heard someone say, “I’ve been shocked so many times… it’s fine, I’m used to it”? Little do they realize, there is no immunity to electricity. The impact of an electric shock depends on various factors like current, voltage, resistance, humidity, skin conductivity, duration, timing, capacitance, and yes, sometimes luck or misfortune.

In this article, I will clarify what I mean and explain why electricity can be dangerous, and most importantly, why it can both save and kill.

The Concept of Electric Flow

It’s crucial to understand that voltage, resistance, and current are interconnected. When considering electrocution in humans, it’s essential to recognize that the voltage has both an entry and exit point. While there can be multiple points, let’s focus on one entry and one exit point for simplicity. Suppose a person touches the ground pole of a car battery with their left hand and the positive pole with their right. The human body forms a resistance between both poles, and according to Ohm’s Law, a current will flow through the body that is equal to the voltage divided by the resistance. Higher voltage or lower resistance leads to higher current. In most cases, the voltage is fixed, as in the case of the car battery.

Impedance of Human Tissue

The resistance of the human body is more complex. It depends not only on the contact area but also on the type of tissue and the amount of sweat produced. The term “resistance” isn’t entirely accurate here; “impedance” is more fitting because frequency plays a significant role, as demonstrated by scientific experiments.

The human body has two impedance components: one for the skin and one for the tissue beneath it. Skin has a larger and more variable impedance compared to body tissue.

Skin Impedance: 2000 to 4200 Ohms

Body Tissue Impedance: 500 to 750 Ohms

Impedance in the human body is nonlinear. As voltage increases, impedance decreases, leading to a nonlinear increase in current.

AC vs DC

The way we experience electricity differs between AC (Alternating Current) and DC (Direct Current).

In terms of effects, electrocution from DC is likely to cause more severe burns compared to AC. On the other hand, AC has a lower threshold for “feeling the flow” than DC.

The Effect of Frequency

When dealing with AC, frequency plays a critical role. Human skin (and heart) is most sensitive to frequencies around 50 to 60 Hz. Unfortunately, this is the frequency used by most electrical grids worldwide.

High frequencies do not penetrate deep into the body due to a phenomenon called the skin effect. For example, with certain Tesla coils, people can touch the sparks they produce without being instantly harmed (though caution is crucial, as some Tesla coils can still be deadly).

Thus, the frequency of an AC current greatly affects its impact on humans, with 50 to 60 Hz being the most dangerous. Let’s delve deeper into this.

Duration and Timing

Introducing the concept of “let-go current,” which is the current level where we lose control of our muscles. As current flows through the body, muscles begin to contract until the power is removed. At this point, if you are holding a live wire, letting go is impossible without external assistance. For most people, the “let-go current” threshold is just 10 mA. If the current increases, at 20 mA, chest muscle contractions can lead to suffocation, and at 30 mA, ventricular fibrillation may occur, potentially causing cardiac arrest.

This brings me back to that friend who thinks they’re immune to electrocution. While voltage, current, and other factors are critical, what many fail to realize is the role of timing.

The moment of electrocution is crucial. If it happens during the vulnerable T-wave of the heart’s electrocardiogram, it can lead to life-threatening arrhythmias like ventricular fibrillation.

Even if the shock seems harmless initially, internal heart damage can lead to death moments or even days later.

Our Electric Grid

Electrocution can only occur in a closed circuit. But how can we still receive a shock from a single live wire? This is because our electrical grid is designed to complete the circuit.

At the power plant, the neutral wire is grounded (often multiple times). This effectively makes the Earth a conductor for the neutral. When you touch a live wire while standing on the ground, the current flows from the live wire through your body into the Earth. Insulating materials, such as rubber shoes, can limit the current flow.

Additionally, the human body and the Earth can form a capacitor, with the air and humidity contributing to a parasitic capacitance that can store and transfer an electric charge.

Turning Things Around for Safety

To avoid electrocution while working with electrical devices, a simple solution exists. In addition to using proper earth leakage detectors and circuit breakers, I highly recommend an isolation transformer. This type of transformer converts incoming voltage to an equivalent output voltage while creating an isolated circuit. The output wires of the transformer are not grounded, ensuring a safer environment where touching a single wire will not result in a shock.

While isolation transformers do have inductive and parasitic coupling that can produce a leakage current, it is typically limited to around 50 µA, which is too low to cause harm.

When purchasing an isolation transformer, ensure it can handle the necessary output power. Remember, higher power doesn’t always equate to better performance; too much current can trigger circuit breakers.

If you have any questions about PCBs or PCBA, feel free to contact me at info@wellcircuits.com