A metal-oxide (zinc oxide) surge arrester is a voltage-limiting device connected in parallel with the equipment it protects. Below its reference voltage the metal-oxide varistor blocks nearly all current. Above it, the varistor becomes sharply conductive and diverts the surge energy to earth, then returns to the blocking state as soon as the voltage falls back. That non-linear behaviour means the arrester needs no series spark gap and no maintenance — the reason the older silicon-carbide, gapped arrester has been almost completely replaced.
| Parameter | Meaning |
|---|---|
| Continuous operating voltage (MCOV / Uc) | The highest power-frequency voltage the arrester can withstand continuously. For a 10 kV system with an earthed-neutral arrangement, Uc is typically 8.0–8.4 kV; for an isolated or resonant-earthed system it must be higher. |
| Rated voltage (Ur) | The voltage at which the arrester can absorb a specified energy for 10 s. Typically 12.7 kV for the 10 kV class. |
| Nominal discharge current | 5 kA (distribution) or 10 kA (station / higher-energy). |
| Residual (discharge) voltage | The voltage that appears across the arrester at the nominal discharge current; must be below the withstand level of the protected equipment. |
| Energy capability | Expressed in kJ/kV of rated voltage; determines how many surges the unit can absorb before failure. |
On a 10 kV network the choice of Uc depends on how the system neutral is treated. In an effectively earthed system the healthy-phase voltage rise during a single line-to-earth fault is limited, so a lower Uc can be used and the protection margin is larger. In an isolated or resonant-earthed (Petersen-coil) system the healthy phases rise to line voltage for the duration of the fault, so a higher Uc and a higher Ur must be selected. Choosing the wrong one either reduces the protection margin or causes premature arrester failure.
Composite silicone housings have largely replaced porcelain for outdoor arresters. They are lighter, resistant to shattering, and their hydrophobic surface keeps pollution performance stable in humid and coastal environments. For heavy pollution, select a housing with a longer creepage distance and confirm the clearance to earthed structures after installation.
How do I know an arrester has failed?
A failed arrester usually shows as a short circuit to earth, or its leakage current rises sharply. Where fitted, an action counter and a leakage-current monitor give the clearest indication.
Should I fit arresters on both sides of a transformer?
Wherever a surge can arrive from the line side, fit an arrester at the transformer terminals; if the transformer feeds a cable that can be switched, an additional arrester on the secondary side is good practice.
