Other Electrical Safety Thread

Thehack

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Foreword: I am an electronic technician (supervisor level) that deals with equipment repair and electrical safety. I am writing this thread as an informative post in respect to the SFF community on electrical safety. I am not an electrical engineer. I bridge the gap between the engineer (designers, manufacturers) and users (breakers of things). If there are mistakes, or misinformation, please discuss below so that I can make necessary changes to this post.

*If you have ever been zapped while using simple consumer products in your home, PLEASE get a certified electrician to do a home circuit safety check. Your home circuit may be outdated or defective!*


Introduction

The purpose of this thread is to provide insight on electrical safety, the mechanism that protects us, and why it is important. As SFF community pushes the envelope of what is possible inside such a small form factor, we often take unorthodox approaches. Many of our mods and builders are truly pushing the envelope of SFF. As we continue to innovate, it becomes more important to pay attention to electrical safety to ensure injury and damage is minimized.

1. Why is electrical safety important?

This is simple. It protects us from injury and protects the equipment we use. There are many mechanism that engineers have developed to ensure we don't Darwin ourselves from the gene pool (arguably, they might have slowed down evolution a bit). It only takes about 200mA (less than 3 watts at 12V) to stop our heart. It is especially important to ensure electrical safety around children and elders, who are more prone to injury and lack awareness when handling equipment.

2. Watch this video:

3. Information Injection:

A) Electricity travels from high potential to low potential and/or vice versa, depending on how you view electrical theory and what kind of stuff you're doing. It requires a complete circuit.

B) You can become the complete circuit. If your feet is touching the ground, the ground has neutral/low potential, and the 120 VAC from the socket will travel through you to the ground

C) A load device is anything that is designed to use electricity. The load devices determines how much power is used, not the source of power.

D) It only takes 200mA to stop the heart. This is why technicians do "one hand" troubleshooting. Using two hands makes it possible for current to travel from one hand to the other, hitting your heart.

C) Mains power is considered the source for powering your equipment, from the wall for the purpose of this guide.

4. There are a few ways we implement electrical safety:


A) Ground Fault Interrupter. GFI "are designed to protect from electrical shock by interrupting a household circuit when there is a difference in the currents in the "hot" and neutral wires." They are also known as GFCI (ground fault circuit interrupter) are commonly used in places where it there is a high chance of shorting the outlet. This includes outdoor, kitchen, bathrooms, and basements. Know that if you try to hurt yourself by throwing a toaster in the bathtub, an electrical engineer out there tried to stop you.

B) Isolation. Isolation is the physical separation of a circuit from mains power. This means should you short or damage the equipment, it is not physically connected to the wall outlet, thus preventing you from pulling the entire breaker's worth of electricity. It is especially important to have isolated circuits in critical areas like hospital and medical care.

C) Insulation. Insulation is simple, it prevents you from getting shock by covering the circuits with an insulated layer.

D) Fuse/current limiters. Fuses or over current protection circuits prevents the device from pulling too much current. They are specifically programmed or specced to the circuit and will trip if it exceeds what the device is capable of supporting. They may be circuit controlled, or just a simple one time fuse.

E) Grounding. Grounding allows an alternate path to low potential. In the case of equipment failure, where are able to touch high potential, the device is more likely to take the grounding path than you as the path. However, if the circuit is NOT connected to ground, either through design or failure, YOU may be the path to ground regardless.

5. As users how can you implement some of these electrical safety mechanisms?

A) GFCI. If you are working in an area that is prone to shorts, ensure your wall outlet is has a GFCI built in, or you implement a GFCI portable adapter. You should include a GFCI in kitchens, bathrooms, basements, and outdoors if your house doesn't already have them in place.

B) Isolation. If you are doing some crazy experiments use an isolated transformer box. This ensures that whatever you're working on, is completely isolated from the wall and any other devices that may be connected to the wall.

C) Insulation. Simple: keep it out of reach of prying hands. If you are working on a project, make sure your work bench is cleared, and consider using an insulated (+ ESD if working with sensitive components) mat.

D) Fuse/Current limiters. Most PSUs include this as additional protection. If you are doing a scratch build or crazy experiments, use a fused C14 power entry with the a fuse rated for the power requirements you're using.

E) Any device that is handled, without an insulation layer, that mains power reach needs to be grounded. If your device is covered with plastic panels, and the exposed screws are NOT connected to any circuit or chassis, then you can forego grounding. However, most of our projects are open and a child can easily stick a utensil in our components so best to go with a grounded chassis.
6. Best practices (to come whenever I feel like it)
A) Remove metallic objects when doing repairs or modifications. This include watches, rings, bracelets, etc.

B) Disconnect the power source when making modifications/adjustments. No, turning off that switch in the back does not count. The AC source is still getting to the chassis, albeit "off."

C) One hand troubleshooting on live circuit. This prevents currents from going across your heart.

D) Use insulated working mats, have a buddy when you're dealing with high voltage. If you are getting electrocuted, have him KICK you off the circuit.​

7. Electrical Safety Specifications. The NFPA outlines the following electrical safety specifications for consumer use products:
A) Chassis to Ground Resistance: 0.500 Ohms. How to test: Using a multimeter, set to resistance reading. Short the leads and note the measurement. This is your baseline resistance. Take the ground cord and measure from the ground prong to exposed metal on the chassis, usually a part of the PSU. The reading should be lower than .5 ohm above the baseline measurement you had taken earlier.

B) Chassis leakage current: 500 uA for consumer products, 300 uA for critical areas product. How to measure: pretty much impossible to measure at this resolution without an electrical safety analyzer. It is the amount of leakage from a touch point on the chassis while the ground is disconnected, and connected. Generally, with ground connected, leakage current should read 0.

C) Ground Leakage current: 500uA for consumer products, 300uA for critical areas product. Similar to chassis leakage current, this is measures the amount of leakage through the ground prong.

D) Other electrical safety standards are excluded since this post concerns only consumer products.​
 
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AleksandarK

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May 14, 2017
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I like this thread very much!

Electrical safety is something we all need to look after, but many people underestimate it. Really like that you summed it up here:thumb:
 
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Phuncz

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This thread deserves a warm hug ! Thanks for you sharing your time and knowledge to educate us.