What happens if the air gap is too small?

If the gap is too small, the air can become ionized. When this happens, the air changes from a good insulator, to a good conductor. This is what allows an electrically charged thunderstorm cloud to suddenly release large amounts of current in a lightning strike.

web.mst.edu - Avoiding Elec. Shock - MST.edu

Do you need permission to replace a front door?

The Quick Answer: Planning permission is NOT needed for houses to replace the windows and doors on condition that the new items are 'like for like...

Last updated Dec 7, 2020

Can rats climb on plastic?

If you've ever wondered how rats find themselves in odd places way up high, the answer is that rats are excellent climbers. They can climb any...

Last updated May 29, 2020

Promotion

Safe Distances From Exposed Conductors (DC to 60 Hz)

Avoiding electric shock seems reasonably simple - never touch energized electrical conductors. Some conductors, such as those in the cord of a typical appliance, are completely encased in a highly reliable insulation. If the insulation is undamaged, a user can safely handle the cord. But in other applications, such as overhead power lines, the energized conductor is not coated with insulation. The only insulation is the air gap separating conductor from the user. While large air gaps are reliable insulators, small air gaps are not. It is up to each person to maintain a safe distance from these exposed conductors. A small air gap is hazardous for a variety of reasons If the gap is too small, the air can become ionized. When this happens, the air changes from a good insulator, to a good conductor. This is what allows an electrically charged thunderstorm cloud to suddenly release large amounts of current in a lightning strike. The conversion from good insulator, to good conductor, can occur suddenly and without warning. As people move about, they may not carefully track the location of every part of their body, every tool they may be using, and every fold of fabric of their clothes. They may stumble, slip, or have other unexpected body movements. These unanticipated movements may bring unexpectedly close to, or even cause them to touch, a nearby conductor. Overhead electrical power lines should not be considered stationary. They can be blown to the side by the wind, and will sag varying amounts as they warm up and cool down. Even if a worker is stationary, the movements of the line may cause a dangerous situation. The distance between the general public and energized conductors must always be large enough to insure people do not touch the conductors, the air does not become ionized, and common body movements do not cause a problem. The calculation of minimum safe distances is as much a political question, as it is a physics problem. One needs to take into account human behavior, such as the likelihood of various actions, the cost of injuries, and the cost of preventing those injuries. One of the commonly used standards in this area, is published by the National Fire Protection Association, and is known as NFPA 70E - Handbook on Electrical Safety in the Workplace. It describes safe distances for electric shock protection. The Handbook is primarily intended for systems that use direct current, or the alternating current frequencies of 50 hertz and 60 hertz. The NFPA 70E handbook recognizes two types of people: qualified persons and unqualified persons. A qualified person has received specialized training on the safe construction, maintenance and operation of electrical systems - far more training than this guide provides. Anyone who has not received this specialized training is considered an unqualified person. If you are the least bit uncertain if you are an NFPA 70E qualified person, then you should assume you are an unqualified person. The NFPA 70E handbook rules periodically change. The 2021 edition of the handbook describes two regions around exposed electrical conductors, related to electric shock risk. These regions are known as the Restricted Space, and the Limited Space. The lines that separate the regions are known as the Restricted Approach Boundary, and the Limited Approach Boundary. There is one other important safety boundary, called the Arc Flash Boundary, which is not discussed here. An unqualified person may move about freely in the unrestricted space, without fear of electric shock. However, an unqualified person should normally not cross the Limited Approach Boundary, and should never cross the Restricted Approach Boundary. In addition to not crossing the Limited Approach Boundary themselves, an unqualified person should not touch any object (other than the ground) which crosses the boundary. This is particularly important when they are using long tools, such as ladders, pole saws, etc. It is also important to watch temporary structures, such as scaffolding, to make sure it does not cross the limited approach boundary

Can squirrels chew through PVC?

Squirrels can easily rip off asphalt shingles or chew holes through fascia boards and even PVC pipes. Basically, anything that's not metal they can...

Last updated Aug 1, 2020

Why do builders use cheap windows?

The terms “builder grade windows” or “contractor windows” refer to stripped-down no-frills lower-quality windows. Builders and land developers...

Last updated Apr 10, 2020

Can I build my own shed?

Yes, with the right plans you can build your dream shed.

Learn More »

Promotion

A qualified person should avoid crossing the limited approach boundary if possible, but are welcome to cross if if necessary. A qualified person can also cross the restricted approach boundary, but only if it is necessary, and they have all of the appropriate personal protective equipment. The PPE may include gloves, goggles, boots, hearing protection, and other objects not shown in the illustration below. An unqualified person may cross the limited approach boundary, if they are under the direct and continuous supervision of a qualified person. The qualified person must first educate the unqualified person as to acceptable behavior and actions, and must continuously monitor their activities until they leave the limited space. An unqualified person may never cross the restricted approach boundary, even if they have PPE and supervision. The NFPA 70E handbook has tables that describe where the limited approach and restricted approach boundaries must be located. The distances are measured from the boundary, to the nearest exposed energized conductor. The distances are different for the Limited Approach and Restricted Approach boundaries. The tables are also different for direct current (DC) systems and alternating current (AC) systems. Note the AC entries assume a frequency of 50 Hz or 60 Hz, and do not apply to higher frequencies. The AC voltages are measured from phase-to-phase, and are the root mean square (RMS) values. There are sometimes different boundaries for "movable" and "stationary" conductors. Overhead power lines are considered movable. The cables have enough slack in them, that they can sway in the wind. They also can sag as they warm. Most electrical equipment such as transformers, ridge pipes, and permanent indoor wiring is considered stationary. The information given below is derived from tables 130.4(E)(a) and 130.4(E)(b) in the 2021 edition of NFPA 70E. Voltage Difference (DC) or RMS Voltage Between Phases (AC) AC (50 Hz or 60 Hz) DC Limited Approach Restricted Approach Limited Approach Restricted Approach Movable Stationary All Movable Stationary All Below 50 V Not Specified Not Specified Not Specified Not Specified Not Specified Not Specified 50 V - 150 V 3.0 m 1.0 m Avoid Contact 3.0 m 1.0 m Avoid Contact 150 V - 300 V 3.0 m 1.0 m 0.3 m 3.0 m 1.0 m Avoid Contact 301 V-750 V 3.0 m 1.0 m 0.3 m 3.0 m 1.0 m 0.3 m 751 V-1 kV 3.0 m 1.5 m 0.7 m 3.0 m 1.0 m 0.3 m 1.1 kV - 5.0 kV 3.0 m 1.5 m 0.7 m 3.0 m 1.5 m 0.5 m 5.1 kV - 15.0 kV 3.0 m 1.5 m 0.7 m 3.0 m 1.5 m 0.7 m 15.1 kV - 36.0 kV 3.0 m 1.8 m 0.8 m 3.0 m 2.5 m 0.8 m 36.1 kV - 45.0 kV 3.0 m 2.5 m 0.8 m 3.0 m 2.5 m 0.8 m 45.1 kV - 46.0 kV 3.0 m 2.5 m 0.8 m 3.0 m 2.5 m 1.0 m 46.1 kV - 72.5 kV 3.0 m 2.5 m 1.0 m 3.0 m 2.5 m 1.0 m 72.6 kV - 75.0 kV 3.3 m 2.5 m 1.0 m 3.0 m 2.5 m 1.0 m 75.1 kV - 121 kV 3.3 m 2.5 m 1.0 m 3.3 m 3.0 m 1.2 m 121.1 kV - 145 kV 3.4 m 3.0 m 1.2 m 3.3 m 3.0 m 1.2 m 145.1 kV - 150 kV 3.6 m 3.6 m 1.3 m 3.3 m 3.0 m 1.2 m 150.1 kV - 169 kV 3.6 m 3.6 m 1.3 m 3.6 m 3.6 m 1.6 m 169.1 kV - 242 kV 4.0 m 4.0 m 1.7 m 3.6 m 3.6 m 1.6 m 242.1 kV - 250 kV 4.7 m 4.7 m 2.8 m 3.6 m 3.6 m 1.6 m 250.1 kV - 362 kV 4.7 m 4.7 m 2.8 m 6.0 m 6.0 m 3.5 m 362.1 kV - 500 kV 5.8 m 5.8 m 3.6 m 6.0 m 6.0 m 3.5 m 500.1 kV - 550 kV 5.8 m 5.8 m 3.6 m 8.0 m 8.0 m 5.0 m 550.1 kV - 800 kV 7.2 m 7.2 m 4.9 m 8.0 m 8.0 m 5.0 m Above 800 kV Not Specified Not Specified Not Specified Not Specified Not Specified Not Specified Voltage Difference (DC) or RMS Voltage Between Phases (AC) AC (50 Hz or 60 Hz) DC Limited Approach Restricted Approach Limited Approach Restricted Approach Movable Stationary All Movable Stationary All Below 50 V Not Specified Not Specified Not Specified Not Specified Not Specified Not Specified 50 V - 150 V 10′ 0″ 3′ 6″ Avoid Contact 10′ 0″ 3′ 6″ Avoid Contact 150 V - 300 V 10′ 0″ 3′ 6″ 1′ 0″ 10′ 0″ 3′ 6″ Avoid Contact 151 V-300 V 10′ 0″ 3′ 6″ 1′ 0″ 10′ 0″ 3′ 6″ Avoid Contact m 301 V-750 V 10′ 0″ 3′ 6″ 1′ 0″ 10′ 0″ 3′ 6″ 1′ 0″ 751 V-1 kV 10′ 0″ 5′ 0″ 2′ 2″ 10′ 0″ 3′ 6″ 1′ 0″ 1.1 kV - 5.0 kV 10′ 0″ 5′ 0″ 2′ 2″ 10′ 0″ 5′ 0″ 1′ 5″ 5.1 kV - 15.0 kV 10′ 0″ 5′ 0″ 2′ 2″ 10′ 0″ 5′ 0″ 2′ 2″ 15.1 kV - 36.0 kV 10′ 0″ 6′ 0″ 2′ 9″ 10′ 0″ 8′ 4″ 2′ 9″ 36.1 kV - 45.0 kV 10′ 0″ 8′ 0″ 2′ 9″ 10′ 0″ 8′ 0″ 2′ 9″ 45.1 kV - 46.0 kV 10′ 0″ 8′ 0″ 2′ 9″ 10′ 0″ 8′ 0″ 3′ 6″ 46.1 kV - 72.5 kV 10′ 0″ 8′ 0″ 3′ 6″ 10′ 0″ 8′ 0″ 3′ 6″ 72.6 kV - 75.0 kV 10′ 8″ 8′ 0″ 3′ 6″ 9′ 9″ 8′ 0″ 3′ 6″ 75.1 kV - 121 kV 10′ 8″ 8′ 0″ 3′ 6″ 10′ 8″ 10′ 0″ 3′ 10″ 121.1 kV - 145 kV 11′ 0″ 10′ 0″ 3′ 10″ 10′ 8″ 10′ 0″ 3′ 10″ 145.1 kV - 150 kV 11′ 8″ 11′ 8″ 4′ 3″ 10′ 8″ 10′ 0″ 3′ 10″ 150.1 kV - 169 kV 11′ 8″ 11′ 8″ 4′ 3″ 11′ 8″ 11′ 8″ 5′ 3″ 169.1 kV - 242 kV 13′ 2″ 13′ 0″ 5′ 8″ 11′ 8″ 11′ 8″ 5′ 3″ 242.1 kV - 250 kV 15′ 4″ 15′ 4″ 9′ 2″ 11′ 8″ 11′ 8″ 5′ 3″ 250.1 kV - 362 kV 15′ 4″ 15′ 4″ 9′ 2″ 20′ 0″ 20′ 0″ 11′ 6″ 362.1 kV - 500 kV 19′ 0″ 19′ 0″ 11′ 8″ 20′ 0″ 20′ 0″ 11′ 6″ 500.1 kV - 550 kV 19′ 0″ 19′ 0″ 11′ 8″ 26′ 0″ 26′ 0″ 16′ 5″ 550.1 kV - 800 kV 23′ 9″ 23′ 9″ 15′ 11″ 26′ 0″ 26′ 0″ 16′ 5″ Above 800 kV Not Specified Not Specified Not Specified Not Specified Not Specified Not Specified

Are pavers better than concrete?

Paving stones are more durable than concrete and can withstand more pressure per square inch. Whether you use cement pavers or want stone driveway...

Last updated Mar 19, 2018

Can I remove a car parked on my property?

You can pursue a civil case for trespassing and if the civil courts rule in your favour, the vehicle would be removed from your drive. A solicitor...

Last updated Nov 30, 2020

Promotion

If you live in one of the few countries that do not use the metric system, here’s the same information in feet′ and inches″.

web.mst.edu - Avoiding Elec. Shock - MST.edu

Is cardboard good for compost?

Yes, all cardboard will start to break down because it is biodegradable. Once you start soaking cardboard, it will release the carbon and be a...

Last updated Apr 21, 2019

How thick should a concrete path be?

Your concrete slab for a path doesn't have to be more than 7.5 cm thick. It also doesn't need a gravel base or anything like that. It is advisable...

Last updated Dec 26, 2018

Promotion

How far can you span a 2x6 floor joist?

According to the International Residential Code 2018 (IRC), the maximum length a 2x6 can span as a floor joist is 12'-6”, as a ceiling joist is...

Last updated Feb 4, 2018

Can I build my own shed?

Yes, with the right plans you can build your dream shed.

Learn More »

Promotion

What kind of math is needed for carpentry?

Carpenters use arithmetic, algebra, geometry, calculus and statistics to measure materials, add up volumes and complete other project-planning...

Last updated Mar 20, 2020