If we assume the worst case scenario i.e. With pvc plastic sheathed cables we generally assume that the limiting temperature for the cable is 70☌ and any rise in temperature in the conductor should be kept below this figure. A temperature rise is not what we want, it is inevitable though and we must design circuits and select cable sizes to make allowances for the heat rises. This perfect adiabatic scenario is actually the worst case condition for our cables. In the perfect situation described in the preceding paragraph no heat would be lost from the cables into the surrounding air or building materials. More current and the potential is there for more heat production. We know that as current passes through the cables, there will be heat generated by this current flow. This perfect scenario, which we cannot achieve in everyday life, is used as a starting point for calculating the heating effect that electrical current has on electrical conductors. Imagine it as a perfectly sealed heatproof unit. Nothing can leave the box either, any heat generated inside the box cannot seep through the walls of the box and escape into the atmosphere. It is so perfectly insulated that no heat can enter the box, even leaving it in the sun all day will not cause any heat to enter into the inside of the box. Relating to or denoting a process in which heat does not enter or leave the system concerned.Īnother way to look at it is to imagine a perfectly insulated box. We will end with some worked examples of the equation in use. If we look in the Wiring Regulations Book we will find two Formulas to do with the Adiabatic Equation, so let’s start with what adiabatic means and then move on to what the formula is and what we can do with it. We know it is to do with selecting the correct size earth conductor but how do we use it? And why do we even bother? We are often asked what is an adiabatic formula or equation and indeed what does adiabatic actually mean in relation to electrical installation work.

Twin and earth table 7671