Insulation Resistance of a Cable
WHY CABLES ARE INSULATED? An INTRODUCTION
With the exception of power transmission cables that are on electric poles, almost all the cables that are in use today are insulated. The level or degree of insulation resistance of a cable depends on the purpose for which the cable was designed for. Apart from saving energy from being lost or dissipated to the surrounding, one paramount reason why cables are insulated is to save us from the danger of being electrocuted.
Electricity is very dangerous. The first touch can be the last touch and it never gives even a single chance. A slight touch of a cable carrying an electric current can lead to a fatal accident. Our body conducts electricity partially. When our body comes in contact with a current-carrying conductor, the electric current will tend to flow from the conductor than to our body. Our body being a partial conductor will not be able to conduct away from the electric current. When the current too much than our body can contain, it then kills the person in question.
In order to avoid this kind of accident in our homes, it became necessary that cables be insulated. The insulation prevents current leakage as well as from reaching us thereby preventing us from being electrocuted.
WHAT IS AN INSULATOR?
An Insulator is a material or a substance that does not conduct heat or electricity. Insulators do not conduct heat or electricity because they have no free-moving electrons. Conductors are said to be insulated when they are covered with an insulating material such PVC etc. The process is called insulation. The insulator around the conductor prevents electrical energy and signals from escaping to the surrounding.
EFFECT OF TEMPERATURE ON INSULATED MATERIALS
An increase in temperature increases resistance in conductors while resistance decreases with an increase in temperature in semiconductors as well as insulators. An increase in temperature can make a semiconductor a good conductor, an insulator a semiconductor.
INSULATION RESISTANCE OF A CABLE
A cable conductor is provided with insulation of suitable thickness to avoid the leakage of current. The thickness of any cable depends on the purpose of its design. The path of current leakage in such cable is radial. The resistance or opposition offered by the insulation to the flow of current is also radial throughout its length.
For a single core cable conductor of radius r1, internal sheath radius r2, length l, and insulation material resistivity ρ, the perimeter of the conductor is 2πrl. The thickness of the insulation will be given as dr.
Rins = ρdr/2πrl
When integrated, we will have:
Rins = ρ/2πl[loge r2 /r2 ]
Rins is inversely proportional to 1/l contrary to R = ρl. Where ρ (rho) is a constant known as resistivity.
There are some cables that have more than one insulating layer and more than one core. The main wire being at the center, serve as the main conductor. The other core serves the purpose of grounding and preventing the electromagnetic waves and radiations from escaping from the cabled. It serves as a shield. Cables under this category are the Coaxial cables.
Coaxial cable conducts electrical signal using an inner conductor (the inner or main conductor could be any good conductor but copper is mostly preferred because of its low resistivity, the copper could also be plated) is contained in mostly PVC case. Before the outer PVC case, there are two or more other insulators with either aluminum foil or copper strands between them. The cables are protected from the external environment by the outermost PVC case. While voltage is passed through the inner conductor, the shield or case has little or no voltage passing through it.
The advantage of coaxial design is that electric and magnetic fields are confined to the dielectric with little leakage outside the shield. Due to the level of insulation in the cables which prevents outside electromagnetic fields and radiations from penetrating into it, interference is avoided. Since conductors with large diameters have less resistance, less electromagnetic field will be leaked. The same goes for cables with more insulation. Knowing that weaker signals are easily interrupted by little interference, cables with more layers of insulation are always a good choice for conveying such signals.
FEATURES of an INSULATED CABLE
Having noted that cable insulation resistance is determined by its purpose of design, there are some factors that an engineer would have to consider before designing a cable. Coaxial cables would require more insulation because the cable will not only prevent power leakage, it traps the electromagnetic radiations. The insulation ranging from one layer to two, three, or four. Cables are engineered for different purposes.