Tjømemuffe, Olastubben or grounding sleeve. It has many names, but do you know what it is used for - and why?
A Tjømemuffe is a plastic drainage pipe that is lined with copper on the inside. This copper coating is connected to a copper wire, which comes out through a nipple in the drain pipe. The drain socket must be placed lower on the drain pipe than the lowest drain/drain grate in the building.
Tjømemuffe is used in IT networks and TT networks, to equalize the ground potential in an electrical installation, where voltage rises to ground can occur. A conductive coating forms on the inside of plastic drainage pipes, which causes them to conduct electricity.
Regulations on electrical low-voltage installations require that where potential differences can arise between drainage pipes made of insulating materials and other conductive parts, there must be a satisfactory electrically conductive connection to the inside of the pipe, or to the drain. This means that installations with IT networks must have Tjømemuffe, while it is not normally required in TN networks (FEL appendix 1 Earthing of pipe systems).
In the event of an earth fault in your own or someone else's electrical system, the fault is connected to earth. Between the drain pipe and the earth electrode there is an unknown earth resistance. In unfortunate circumstances, the earth fault current can travel towards the drain pipe, and follow the conductive coating up the drain pipe. When you take a shower, there will be a voltage on the drain, and when you turn on the water, the body becomes part of the earth fault current circuit. The return route goes via the water pipe down to the main ground rail, and down into the soil again.
Illustration 1: IT network without Tjømemuffe. Ill.: Trainor AS
In an IT network with 230 V, the voltage is reduced to 132 V when it goes to earth. However, in the event of an earth fault, one or more in the transformer circuit, the voltage can be much higher. This voltage will be distributed across the resistors that are in series in the circuit where the earth current flows. In such cases, the voltage across the body can reach well over 50 V. Current flow of this magnitude can be life-threatening.
In the case of installations with Tjømemuffe, there will not be a voltage rise to earth since the drain pipe and the water pipe will have the same potential and there will be no potential differences.
In facilities with an IT network where there should have been a Tjømemuffe but this is missing, there are several methods to solve the equalization of the drain grate.
Illustration 2: IT network with Tjømemuffe. Ill.: Trainor AS