Cables, Wires, Conductors, is what is used to transport electrical power from one point to another. Some overhead transmission lines transport electricity hundreds of miles and some circuits within your home transport electricity just a few feet from the panel. The electric conductor is highly conductive metal such as copper or Aluminum covered by a highly resistive material such as plastic or rubber. The result is a safe, reliable and efficient channel that controls the flow of electrical energy from a source to it's load.
The Conductor
The conductive Path is designed based on the load that will be required. Just like any other force in nature, the stronger the link the more force or work can be produced. Conductors are often compared to fluid dynamics because many of the same laws apply, the bigger the pipe, the higher the pressure the more fluid that can be transported. The same is true with electricity. The bigger the conductor, the higher the voltage, the more electrical energy can be transported.
Cables and wires are sized according to the cross section area of the copper within the conductor, the greater the cross section area, the greater the amount of current that can be conducted through the cable. The National Electrical Code in section 310 identifies recommended and often required sizes of electrical conductors for the amount of current or load that will be expected of them. a 20A load will require a much smaller wire than a 200A Load. This doesn't mean that a wire such as a #12 AWG rated for 20A could not conduct 200A of electrical power like a #3/0 AWG would, it just means that if it trys to, it will most likely get very hot in the process and end up igniting into flames any flamable material laying against it. This is because the resistance of a #12 wire is much greater than that of a #3/0 wire. The #3/0 Wire can conduct 20A, but it is a waste of copper if it is never used for more than that, but a #12 will never be used for a 200A load because it would eventually fail due to excessive heat.
The Insulation
Back to our comparison of fluid dynamics, Insulation is what contains the pressure of the line. A thin wall garden hose might hold 10 pounds per square inch, but it would blow up if 200 pounds per square inch of force was put on it. In Electrical Energy, Voltage represents the amount of pressure on the line. It is separates the potential difference between the conductor and the return path it is trying to get to. A 600V rated cable will withstand 480V for many years, but apply 4,160V to it, it won't last but a few seconds.
Under Construction
Before we talk about testing cables, we must first expand the understanding of how electrical conductors work. Let's start with the conductor, What happens when the conductor increases with resistance? More heat is generated by converting electrical energy to thermal energy thus causing the less energy to be transfered to the load and cause thermal damage or fire to nearby combustable materials. Using stranded copper and aluminum sized for the amount of current that the load requires is essential for continued service. It is impractical to test this feature, becuase of the very low risk of failure and the very high cost to measure this attribute. Focus should be placed at connection points to ensure proper termination of cables. Thermal Inspections, regular cleaning and torquing of bolted connections and measurements using a digital low Resistance Ohm (DLRO) Meter are ways to have confidence that the conductive path will continue to serve energy to it's connected load.
Now let's consider the insulation. It's purpose is to resist the flow of electrons. Electricity requires an established path from a source to a load and back to the source in order to produce the desired results. Normally the source of electricity is at point A and the load is at point B with the shortest connection both directions being in a straight line. If you have both the source and return paths traveling adjacent to each other, they need a level of insulation between them to eliminate short circuits from happening. The flow of electrical energy is measured in voltage or pressure, and current or flow. The coductor must be sized for the flow, the insulation must be sized for the pressure.
There are other aspects of conductos that must be understood such as grounding, and shielding, but for now we will go to the next step of determinging if the insulation level is sufficient for safe, reliable and efficient operation.
Consider a 120V circuit. If resisted by a 2,000,000,000 Ohm resister it will produce a current of .06 micro Amps. or 7.2 micro watts which produces no noticable results. This is a sign of good insulation. Now lets consider the same 120V circuit resisted by a 2,000 Ohm insulation. This will conduct .06 A current and waste 7.2 watts or about the same as a small light bulb. This will still work as a conductive path, but now we are producing some heat within the insulation. Now if we have a short circuit of 2 Ohms it will produce a 60A current and waste 7200 Watts. Considering that a 20A circuit breaker will time out and trip it's circuit within a minute under these circumstances, this will not work reliably and the heat energy would be great so it could be a safety hazard as well.