Which voltages are safe for work? With many electrical incidents, current is the critical factor that determines safety, with Ohm’s Law (V=IR) deciding the outcome. Individuals have been electrocuted at voltages of 110 volts AC and 42 volts DC, while currents nearing 100mA through the heart can cause ventricular fibrillation, which can result in death.
According to OSHA, about 350 electrical workplace deaths occur annually. While this is a small number of individuals relative to the electrical workforce at large, I personally do not know an engineer who has not experienced their own electrical incident at some point in their career.
Electricians and electrical engineers are typically well versed on the “Dos and Don’ts” of electrical systems, but mechanical engineers, maintenance personnel, managers and salesmen are also highly likely to come into contact with electrical systems and may not have had as extensive training. Any time I come into contact with an electrical system, I consciously and subconsciously evaluate the system by using my mental checklist below. These are tips, not necessarily a comprehensive checklist for every situation. If you are unsure of an electrical environment, always seek a knowledgeable person to assist you.
Do not make assumptions. Mistakes happen, and systems can be wired incorrectly or mislabeled improperly. External factors such as bad line power, bad grounds or other electrical loads can also reflect back to the current system.
Work unpowered. While this is not always possible or practical, it is the best way to mitigate risk. Things to watch out for include:
- Power storage devices such as capacitors, drives, power supplies and batteries. These can hold a dangerous charge even after main power has been removed.
- Illegal bypass of safety devices. Often, service persons will bypass the power circuit to troubleshoot a panel while open. This may mean overcoming the disconnect or shorting-out shutoff devices.
- Lighting and logic circuits may not always be designed to shut off with the rest of the system.
Do not work alone. When servicing a machine, most plants require an electrician who knows the system or someone who can respond to the electrical incident to be present.
Assess your environment. Moisture is dangerous when it comes into contact with electrical systems, and spills or condensation are immediate red flags. Other things to observe include panel location, dangling or messy wires and the location of safety and shut-off devices.
Wear proper safety equipment. Safety equipment can range from full electrical suiting to properly insulated tools. Always use recommended safety equipment and check for holes in electrical suits or gloves. Likewise, be sure to use insulated tools without chips or scratches in insulation. Additionally, use surge protectors or electrical tools with proper safety shut-offs. It is also crucial to have a reliable multimeter. Keep in mind that a $10 unit battery checker may not have the proper fusing to protect you from high-current systems.
Use proper machine equipment. When building or servicing a panel, adhere to the following equipment recommendations:
- Always use recommended gauge wire and proper sized fuses or breakers, not just what is commonly stocked.
- Do not install excess wires in a single terminal block connection.
- Use certified components and check ratings. Improperly tested or underrated components can be key failure points.
- Set proper device parameters, such as limiting a servo drive’s output current to not exceed a motor’s rated current.
Leave your workspace safe. If you are leaving your workspace for any amount of time, make sure it is safe for another person to come service the machine. A LOTO (lock-out-tag-out) may be required in many plants. You should also properly terminate wires and remove bypass of safety devices.
If you are working near, on or selling electrical components, please take the time to familiarize yourself with your specific systems, taking the time to always read manuals. Never assume that any system is safe.
For more information on shop safety, please take a look at Tim Tiebert’s recent article on Maintaining a Safe Shop Environment.
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