Surge arrester MOSA is used for protection of low voltage insulated and non-insulated electrical power networks, distribution transformer’s low side of busbars, electrical motors, individual house connection to electrical grid, and for similar application. Main part of MOSA is non-linear resistor (varistor). Under normal network condition it behaves as insulator. When occurring and passing of overvoltage, MOSA becomes conductive and leads a great part of electromagnetic wave to the earth. Surge arresters MOSA are constructed according to IEC 61643-1 and IEC 61643-12.

Surge arrester type MOSA is devided into two chambers. Varistor with the contact plates and a support screw are located in the upper chamber. The upper chamber is filled in with insulated mass and separated with a partition plate from the lower part of surge arrester. Upper part of a connecting lock is linked to the centre of the partition plate. This kind of construction prevents water and moisture ingress and improves reliability and life-span of MOSA. The other part of connecting lock with a spring and signal plate attached on is located in the lower chamber of MOSA. There is also a copper braid that connects the lock with the earthing screw on the arrester’s body. Signal plate (yellow fluorescent color) has the function to serve as signalization of proper work or malfunction of MOSA. Around the connecting lock the stainless spiral spring is fixed and connected to the signal plate. The signal plate releases itself when overvoltage occurs and it hangs on the spring so we can easily see that the arrester is damaged. That way the location of unfunctional MOSA is very easily detectable without further searching. In this type of surge arrester the signal plate will vibrate vertically regardless weather condition. On the outer side of MOSA body is a plastic teethed screw for tightening earth cable to MOSA. There is a visible mark of earthing on MOSA surface, close to the screw. The insulating mass in upper chamber of MOSA protects varistor from water and moisture ingress and has self-extinguishing properties which is very important in the case of fire. The material of the body has also self-extinguishing properties and it’s UV stabilized. It’s further properties are: resistance to mechanical impacts, resistance to weather influence, toughness, resistance against mould, rottenness, rodents, insects, grease, hydraulic fluids and materials that exclusively consist of carbon and water. Temperature range (allowed) of environment is between -40oC to 85oC.

The varistor is located between two contact plates made of copper. On the upper plate there is a contact screw M8 for connecting onto the line wire. The mechanism (connecting lock) that serves for separating the signal plate is soldered with an ecological alloy to the lower contact plate. Connection toward the earth is realized with copper braid and earthing screw. When the alloy inside the connecting lock starts to melt, signal plate separates itself from the arrester body and hangs on the spring. That way, the damaging of MOSA is clearly visible.