Having some parents pressuring you into engineering? Wondering what is electrical engineering or how the grid actually works? Eyeing that job at Hydro Quebec? Look no further!
In the following weeks i will give you a solid crash course into electrical systems. While it will not turn you into a full fledged professional engineer, it will give you an idea of what lies ahead should you choose this career path. For seasoned professional engineers, you might learn something new as i hope to learn from you as well.
To make this more palletable, i will also add charts and infographics in the weeks to come, as an image is indeed worth a thousand words.
Today i will go over electrical conductors and insulators. By the end of this post you will know:
- Which materials are used as electrical conductors and which are used as electrical insulators.
- What is electrical resistance and its relation with distance
- What is an electric arc and why clearance distances are important
- What is the dielectric strength of electric cables
1- What is an electrical insulator
Here is a definition taken from Wikipedia:
An electrical insulator is a material whose internal electric charges do not flow freely; very little electric current will flow through it under the influence of an electric field.
Therefore, an electrical insulating material stops electricity and keeps you safe from getting electrical shocks. Think about the rubber or plastic your laptop power cord is made of.
Here is a few materials ranked in terms of their insulating properties from highest to lowest:
- Glass or ceramic
- Plastic – Rubber
Main insulating materials used in the electrical industry are air, rubber, plastic, glass & ceramic. The choice for insulating material are based on factors such as:
- Cost of material
- Easy to manufacture
- Easy to install
- Mechanical resistance
- Heat resistance
As you can see, choosing an insulating material is tricky. To standardize the choices and make it easier for electrical engineers is the National Electrical Manufacturers Association or NEMA. It establishes standards to which the electrical industry must comply to bear its seal. It is also a quality standard of having a NEMA approved product installed. Most engineering firms technical specifications required installed products to be NEMA approved.
Here is link to their description of electrical insulating materials.
Also, if you are looking for manufacturing specifications of insulating materials, i recommend that you look at Hubbell products:
2- What is an electrical conductor
Again, here is the definition from Wikipedia:
In physics and electrical engineering, an electrical conductor is an object or type of material that allows the flow of an electrical current in one or more directions.
Some materials are better electrical conductors than others. Here is a short list of conductive materials, from lowest to highest:
It can be seen that most of the good conductors of electricity are metals. The more conductive a material is, the less heat is generated as electricity pass through it. While power generation and overhead lines as less concerned by this phenomenon because heat is dissipated through the air, conductivity is a major factor in electronics. That is why silver is used in computer chips. Gold is sometimes used in electronics, but rarely due to its price.
Back to electrical utilities, brass, copper and aluminium are the most widely used materials due to their relative abundance on earth and low cost.
Also, if you are looking for manufacturing specifications of electrical conductors and cables, i recommend that you look at Nexans and General Cable:
3- Resistivity and conductivity
While copper and aluminium are used as electrical conductors because they easily allow the flow of electricity, they do offer some resistance. This resistance may be very low for short distances, but increase greatly as you get further and further away.
Also bear in mind that as the cross sectional area of a conductor increases, the resistance goes down. That is, the larger the conductor is, the easier it is for electricity to flow through it. The drawback is that a larger conductor is more expensive.
4- Resistance and voltage drop in electrical conductors
As the length of the conductor increases, the resistance also increases. This is major factor when designing a factory and its electrical equipment. For example, if you are 200 ft from an electrical source, voltage may drop by 5%, maybe even 8%. This will impede the normal operation of your equipment.
That is why electrical engineers conduct what is called a voltage drop study, where the electrical equipment will be checked whether they are too far from an electrical power source and should be relocated. Nowadays, voltage drop calculations are done on power modeling software such as CYME or SKM.
More on voltage drop studies in another blog post. More on CYME and SKM in another blog post!
5- Air and electric arcs
Air is a good insulator, but has its limits. If we approach an uninsulated high voltage conductor, like an overhead transmission line, the electricity wants to leave the live conductor and find a path back to its source. In Quebec, it would probably be the hydroelectric dam that produces this electricity.
If we get close enough, the air will break down and turn from an insulator into a conductor. An electric arc will form through the air, like lightning.
This is why during thunderstorms, one should never take refuge beneath the highest tree, as lightning will seek its quickest path to the ground through that tree. Similarly during a thunderstorm, one should never walk in an open field, as lightning will seek its quickest path to the ground through you!
6- Arcing distance
The higher the voltage applied to the conductor, the farther the electricity will jump through the air. The distance that the electricity can jump is called the arcing distance.
The arcing distances depends on the weather. When it is raining or humid, the electricity can use the moisture to break down the air more easily.
The electrical system operators know the worst case arcing distances and use this information to set safe minimum distances from live equipments, which increase as the system voltage increases.
7- Arcing distance and electrical clearances
The electric clearance is a safety distance that further protects linemen, operators and electricians when working near live conductors.
Clearance distances are much greater than arc distance to protect electricians and take into consideration the worst case scenario with rain.
This distance takes into account risks of missteps, for example tripping over and falling towards the live conductors. If properly implemented, an electrician will still be safe from an electric arc.
The best practice is to deenergize conductors and equipments while an electrician is working close by. Because this is not always possible or because the job is to diagnose faulty electrical equipments, an electrician may have to work close to live conductors.
8- Arc flash dangers
The dangers of an electric arc flash are not only in terms of electricity passing through the human body, but also to surrounding equipments. An operator exposed to or close to an arc may be thrown back by the explosive force, turned deaf by the noise, blind by the flash of light, experience 2nd and 3rd degree burns. He or she may even suffer from internal damage due to concussive forces from the explosion.
On this picture, the electrician is protected by a Level 4 Personal Protective Equipment. This is the highest level of protection there is. More on the damages from electric arc flash in another post!
The danger of the electric arc flash is well known and now mandatory per Canadian Standard CSA Z462 norm, while in the United States, it is the NFPA70E norm.
More on the CSA Z462 in another blog post! More on the NFPA70E in yet another post!
9 – Dielectrical strength
The ability of an insulator to stop an electric arc passing through it depends on the material dielectric strength and material thickness. It is the ability to withstand high current and voltage. It is of great importance to the condition of any live electrical cable.
As an electrical cable ages, its dielectrical strength diminishes, making it more likely to experience an arc. At the microscopic level, we can start seeing physical damage through the insulation layer of a cable.
Since this damage looks like little trees, this phenomenon got the name of electrical treeing. As the cable ages, these trees grow. Eventually, they will be long enough that electricity will flow from the copper core, through the trees, through the insulation and out of the cable.
At that point, an electric fault has occurred and the cable must be deenergized and removed from service. The cable insulation is said to have failed. Electrical treeing is especially problematic for cables in humid underground applications, because of the moisture that may appear inside the cables. More on how electrical treeing is mitigated in another post!
10- Insulator tracking
If we have a potential voltage difference across an insulator, it will limit the current flowing through it due to its high resistivity. As the voltage increases, the electricity can’t pass through it, so it will try and go round the outside of the insulator. This is called insulator tracking.
This tends to be an issue when the system voltage gets to a high level (+36kV), when the insulator is located outside and subjected to rain and pollution that sticks to the outside of the insulator, creating a lower resistance path for the electricity.
One way to reduce the effect of tracking is to increase the distance around the outside of the insulator. By increasing the linear distance that the electricity needs to flow from one side to the next, we make it more difficult for tracking to occur. That is why high voltage insulators have that shape. If we want to use the insulator for a higher voltage, we can simply stack the insulators in series.
With this blog post, you have learned conductive and insulating materials used in power cables and that resistance increases as distance increases. You have also learned what are electric arcs and why power cables eventually fail because of the electrical treeing phenomenon.
Next post will be about circuit breakers, why and how we use them. Stay tuned!