The operations often involving electric shock and how to prevent it from occurring
There have been four fatal occupational accidents during this June and July, three of which were caused by the electric shock hazards. Obviously it shows that the electric shock incident easily occurs when someone is working and sweating a lot in the summer time.
The rubber-soled shoes provide a certain degree of insulation to help protect someone from conducting current through their feet. When a bare hand contacts a low-voltage, the electric current flows through the body and there is no path for the current to exit. Dry rubber-soled shoes can resist about 1,000 volts. The electric current is forced to flow out via the original entering point, and it forms an equivalent electric potential. The person is not aware of the potential shock, which results in more hazards. If any other parts of the body, such as the neck, chest, knees or the other hand, contact a good conductor, like metal, the electric current will have an exit path to flow out. The electric current flowing from high potential to low potential, the ground considered a zero electric potential, forms a circuit in which the human body will feel the shock.
From these set example cases of electric shock in occupational accidents, Taipei City Labor Inspection Office is summarizing three operations where the electric shock frequently occur, and offers prevention methods for your reference.
1. Electric shock caused by not turning off the power when the light bulb is replaced
Replacing a light bulb with a bare hand when the power source is not shut off, and touching the copper at the bottom of the socket with a finger while another part of the body contacts a metal conductor, results in a shock.
Ways of preventing electric shock:
Make sure that the power source is turned off before replacing a light bulb. Once you have the light bulb replaced and are ready to test the appliance, turn the power switch back on. Taking an extra step helps to ensure safety. Many people believe that it’s impossible to be lethally electrocuted by 110 volts, which isn’t enough to be deadly. As a result, it can be easily overlooked. Actually there have been many electrocutions caused by 110 volts.
The severity of injury from electric shock depends on the amount of current passing through the body. The research shows that the current instantly passing through the heart greater than 100 milliamps (mA) or the current constantly passing through the heart greater than 50mA will cause ventricle fibrillation. Consequently, the heart beats irregularly from 300 to 600 beats per minute, and the cardiac systolic and diastolic function is lost. If a defibrillator is not used, young people will die after about four hours and the elderly have about an hour.
The dry human body skin resistance is about a few hundred thousand ohms (Ω). And if the skin is wet, that resistance is lowered to a few hundred ohms. For example, the sweating or damp body, supposedly the wet skin resistance is 800 Ω, contacts a circuit with current leakage as high as a 110 volt, no matter how the electric shock occurs instantly or constantly it may cause fatal ventricle fibrillation. The current flowing through the body is (A) = voltage resistance = 110/800 = 0.14A = 140mA. Never assume that 110 volts can only make you feel numb. In fact it is likely to cause death.
2. Electric shock caused by not testing the devices to make sure the device is energized, as well as the occurrence of current leakage, when working on air conditioner, cooling tower and pumping motor
On the air conditioner, cooling tower and pumping motor, the insulating materials are installed to isolate the internal current and its other internal metal components for preventing the metal parts from holding a current when current contacts with the metal. The devices installed outdoors where they’re exposed to the weather meaning the winds, sun and rain falls; and moreover they’re not in good maintenance by the technicians. This causes the internal insulating materials to gradually decay. And after a time if the circuit contacts the internal metal, often there will be a short circuit and the frame of the device will be energized.
Electric shock from a circuit is formed, when the technicians touch the energized device and the other part of his body contacts with a metal conductor. For example, he’s sweating a lot in summer, his pants are soaked in sweat, or he kneels on the metal plate when repairing an air conditioner.
Ways of preventing electric shock:
Always shut off the power and double check it prior to repairing the maintaining any electrical devices. If working on a powered circuit, under load, you have to wear an insulated glove to prevent shock occurring when contacting any part of the energized device. Why is it necessary to detect electric current with a meter probe after the power is off? That’s because some power switches of the equipment are not installed next to the equipment but are installed in the power distributing box far away. And sometimes inside that box there are many sets of no-fuse breakers to control many devices. A possible electric shock occurs by not testing the power before working on it. And in a condition that we don’t know which breaker gives power to which equipment because of poor labeling the circuit when turned on ends up powering the wrong device.
3. Electric shock caused by working on replacing a billboard advertisement and cutting the electrical lines without checking for power in advance
Make sure to cut the original circuit before you remove the old billboard advertisement. The old style billboard uses 220 volts, two 110 volt lines. One line is directly connected to the billboard and the other is connected through the Time relay, which controls the billboard turned on at dusk. During the day the billboard is not on and there is a 110 vote energized line. Cutting directly into the electrical line will easily cause shock.
Ways of preventing electric shocks:
When working on replacing a billboard, firstly shut off the power; secondly, use a metered probe to test for power and make sure no current is present; and then work on it after making sure both circuit and billboard are unpowered. The purpose of double checking the power is not only to avoid cutting the wrong circuit, but also to test for current leakage from other nearby billboards. An electric shock accident occurred while working on replacing the billboard advertisement in Taipei City in July. In that incident although the enterprises who was in charge of the billboard had cut the power of the billboard, the worker was still accidently electrocuted. After investigation, it showed that the accident was caused by a broken line of a nearby billboard resulting in a bare line with power, contacting the billboard being worked on. The tragedy could have been avoided if the power box had been tested.
The competent authority of the billboard affairs is the Taipei City Constructive Management Office. According to the Management Autonomous regulation of Advertising Products implemented since July 22, 2016, the installments of the advertisement for a product are supposed to pass the approval of the competent authority in advance. As to the censorship of applying for the approval to the advertisement of products, the competent authority can entrust the Advertisement Engineering Association of Taipei City or other related professional organization to engage in censorship.
To apply for the permission of installing an advertisement of a product, the safety certification of electrical appliances for the advertisement of a product should be attached. The power supply on the billboards and lighting fixtures should be installed, signed and verified by a technician or an electrician with a class A certificate.
According to Article 59 of the Regulations Governing Indoor Wiring Installation and Article 14 of the Regulation Governing Building Technologies, on the billboard installment, a circuit breaker should be installed for preventing a person from being shocked because the circuit breaker will have the power shut off instantly when the billboard has current leakage. Additionally the metal frame of the billboard’s light and its steel support poles should be grounded.
For saving money, some enterprises don’t hire a licensed electrician to install a billboard advertisement. If they fail to apply to the authority nor install the circuit breaker on the billboard, and as a result of the negligence, the technician working for the cable company or the maintenance person who is dead because of the current leakage caused by the billboard, the enterprises could be criminally responsible for it. Herein we are urging our enterprises, who need to install a billboard, to contact the Advertisement Engineering Association of Taipei City for the procedures or to ask for a recommendation of professional construction contractors from them. The billboard circuit uses the safer double solenoid valve control, two circuits being turned on and shut off simultaneously, and a circuit breaker is installed to help prevent the technician working for the cable company or the maintenance person from being electrocuted.
Finally, remember to shut the power off first and then test for the presence of a current, and make sure to cover the energized part with insulations.