Pro-MoDr

Surge Suppression - MOVs, Modes and Wires

MOVs (Metal usually zinc, Oxide, Varistors or variable resistors) are the basic, impulse voltage (lightning) limiting devices in all modern day lightning arresters, transient voltage surge suppressors (TVSS) and surge protection devices (SPD). Operation of a MOV is very similar to operation of a pressure-relief valve. This is illustrated in the drawing below. Voltage across the MOV is like the tank-to-atmospheric pressure across a valve. Current through the MOV is like flow out of the valve discharge. At low

pressure or voltage there is no flow or current. Neither device does anything. However, at a point X the valve starts to open or the MOV starts to conduct. Pressure or voltage never exceeds a Limit value that is well below the maximum allowable working pressure of the vessel or the puncture voltage of the electrical insulation.

Modes define how many MOVs are used in a TVSS. Three and four mode TVSS are shown below. For three-phase electric power devices can be connected either delta or Y.

A four mode TVSS has a fourth MOV connected from the Y neutral point to ground, G. A, B and C represent the three phase connections.

A six mode TVSS combines the delta and Y three mode TVSS as depicted below.

Wires determine how the TVSS is connected into the power system. The three-mode delta TVSS is just 3-wire because it has no G ground terminal. As a result it can not protect the thinnest and weakest insulation, between any phase and ground. All Pro-MoDr TVSS units are 4-wire as displayed by the three-mode Y, four mode and six mode.