INTRODUCTION TO ELECTRICAL SAFETY
Often times in practical electrical / electronic circuit designs and construction, it’s always paramount that safety measures be observed to avoid an individual sustaining injuries or damaging electronic components, devices, equipment or instruments in a lab. Most of the accidents that occur in electronics circuit design labs can be attributed to individuals not adhering to safety guidelines in handling or operating electrical / electronic circuit devices, components or equipment. This results to costly damages that otherwise would have been avoided if such individuals had knowledge of the necessary safety guidelines required to work in such an environment. The subject of safety is very important in any electrical / electronic field especially when working with power systems; unfortunately, it tends to be the most overlooked. In this section, we will teach you about the electrical safety guidelines you need to follow when working with electrical components, devices and equipment. Our intention is to encourage personal safety consciousness as well as avoiding costly damages or injuries in your course of working with electronic circuits, components and instruments.
GRRRRRRR! WHAT WAS THAT? ELECTRIC SHOCK!
If you have been intrigued by an electronic / electrical device and have wanted to know how it worked by fiddling with such a device just to know what made it tick; or perhaps out of curiosity, you have tried to find out for yourself how certain things in electricity or electronics where possible, chances are that at some point you might have experienced an electrical shock. Depending on the degree of shock, for some, this experience of jolts of pain from electricity can be a total turn-off from ever trying to fiddle with electric devices while for the determined and adventurous, it’s just another bitter lesson learnt, and their curiosity continues to drive them into further discoveries (and bitter lessons?). Power sources capable of high voltage and currents are nothing to be toyed with. When not properly handled and come in contact with a person’s body, it can pass high amounts of current through the victim’s body that can result to severe body injuries such as burns or even a muscle or nervous system breakdown. This hazard is peculiar to both AC (Alternating Current) and DC (Direct current) supply sources; With DC supplies being even more deadlier. As a first step to safety, never touch a conductor passing an ‘unknown’ voltage or current level with your bare hands or other bare body part. ‘Unknown’ voltage or current level implies that there is a certain level of voltage and current that can cause an electric shock to a body while a certain low voltage and current will have very little to no effect. How LOW you ask? you’ll soon find out.
CURRENT FLOW PATH FOR ELECTRIC SHOCK
For one to experience an electric shock, current must flow through the victim’s body. Current flow only occurs when there is a closed circuit path. This is to say, before a person experiences an electric shock, his/ her body must have completed a circuit that allowed current to flow through their bodies. Current flows through the human body because our bodies are conductors; although with some level of resistance. For a person to experience a shock, there must be two contact points through which current enters and leaves the body. A single contact point will never cause an electric shock. it’s possible for birds hanging on wires not to get shocked as a single contact point is made between the bird’s feet and the wire. Unfortunately, for humans standing on the ground, touching a live wire will result in an unpleasant electric shock. This is because contact is made at two point on the person’s body; The GROUND and the Live wire.
The term GROUND refers to an earth link or connection. Most electrical power supply stations have an Earth connection which is a wire that runs through the power supply system and is buried deep in the ground. This is a safety measure observed by power companies as part of power system protection practices. With this ground connection, by standing on the ground, a person has already made a contact point. When a person accidentally touches a live wire, a second contact point is made and the person gets an electric shock.
This is why its advised to wear gloves and shoes/boots when working with live wires. Despite wearing these, one can still get an electric shock. This is because the insulation between a person’s hands, gloves and live wire; or feet, shoe soles and ground varies with the type of material the gloves and shoe soles are made of. Rubber gloves and shoe soles offer the highest insulation but then, sweat from one’s hands and feet as well as moisture and dirt around the shoe soles can compromise the insulation, still resulting to an electric shock when a live wire is accidentally touched. High voltage DC current sources can cause an electric shock if body contact is made between the positive and negative terminals; touching just either the positive or negative terminal won’t cause an electric shock as there isn’t a closed circuit path for current to flow through the body.
VOLTAGE OR CURRENT LEVEL FOR ELECTRIC SHOCK
The main cause of an electric shock is the flow of current through a human body. Without current flow, no electric shock can be experienced. But then, for a specific amount of current to flow, there must be an electromotive force or voltage to push electrons around, hence resulting in current flow. The amount of current flowing through a victim’s body is dependent on the voltage the body is subjected to as well as the resistance of the body at that given time and condition. This is basically Ohm’s law.
The resistance of the human body is not constant but varies from individual to individual and from time to time. The resistance of the human body is influenced by the environment. In a dry environment, body resistance is higher, while in a humid or wet environment, body resistance is effectively lower. When one is sweating, body resistance becomes lower as sweat is rich in salts and other minerals, as such is a good conductor of electricity. The lower the effective resistance of the victim’s body in contact with a power source, the higher the current flowing through the body and the higher the risk of experiencing an electric shock. From research data on effect of electricity on the human body, The minimum amount of current flowing through a human body sufficient to elicit electric shock is 10mA (ten milli-Ampere or 0.01A). Also, any voltage above 30V is enough to cause an electric shock.
- Before working on a circuit or device, always use a good voltmeter to test out voltages on various points of the circuit or device being worked on; In so doing, one can know the voltage he/she will be dealing with and take appropriate safety actions.
- When working with electrical / electronic circuits or power supplies, one should always ensure that his/her hands are dry and is not wearing any metal jewelry as these can cause electric burns to the skin.
- Consider using one hand ( if convenient) while keeping the other hand in your pocket or off any conductive material even concrete walls, during repairs as this will ensure that only one contact point is made with the circuit or device; thus ensuring protection against electric shock.
- Whenever possible, first set a zero energy state by first turning-off power supply to the device or circuit being worked on. One can later restore power to the circuit or device after repairs to test the circuit or device. When a device, circuit or system is put in a state that allows it to be safely worked on, it is said to be in Zero Energy State.
- Always ensure your work table is clear of any conductive object (wires, components, metal tools etc) before placing a circuit board on the table for troubleshooting or repairs. This is to avoid accidental short circuit on the circuit board by the objects on the work table.
- Most multimeters, when set to measure current (Ammeter mode) internally have a shunt low resistance copper wire across the black test lead port and a second red test lead port usually marked ‘A’ for current measurement . This requires that the user must first plug the red test lead into the this port to measure current. If the meter is in the ammeter mode and is accidentally used to measure voltage across a mains supply, it will effectively cause a short on the mains supply which can destroy the multimeter, cause fire or injuries to the user. Check and Cross-check that the multimeter test leads are plugged in correctly into their appropriate ports before use. Always ensure that your Multimeter dial is in the right measurement position before using it on a circuit, device or power supply.
- When working with circuits, devices or power supplies, ensure that your tools have handles made of rubber or other insulating material. Do not use the tool on voltages exceeding the rated voltage insulator can withstand without first ensuring that the circuit, device or power supply is put in zero energy state.
- Most power supply systems make use of capacitors. These capacitors store charge and in some cases retain their charge after power is removed. Before working with such systems, remove power and ensure that the capacitors are discharged first. Do not discharge the capacitor by shunting the leads with a screw driver or another tool; rather, use a hundred ohm (100 Ω). Hold the 100 ohm resistor with long-nose pliers while gently and carefully placing its contacts across the capacitor contacts to discharge the capacitor.