MV metal-oxide surge arresters (IEC 60099-4) — continuous operating voltage, residual voltage and protective margin

The SPD guide covers surge protective devices inside a low-voltage building installation. This article covers the medium-voltage equivalent: the metal-oxide surge arrester (MOSA), standardised in IEC 60099-4 ("Surge arresters — Part 4: Metal-oxide surge arresters without gaps for a.c. systems"), used to protect transformers, switchgear and cable terminations against lightning and switching overvoltages on MV networks.

Gapless metal-oxide, not the old spark-gap arrester

Older arresters combined a silicon-carbide (SiC) resistor block with a series spark gap: the gap had to break down before the resistor could conduct, which introduced a response delay and ageing spark-gap contacts as an extra failure mode. A modern MOSA has no series gap — the zinc-oxide (ZnO) varistor blocks conduct a highly nonlinear current directly as soon as the voltage across them rises, and return to a near-insulating state as soon as the overvoltage subsides. This gapless, fully solid-state behaviour is what makes response time effectively instantaneous and eliminates gap-related ageing.

Continuous operating voltage (Uc) versus rated voltage (Ur)

Two voltage ratings define an arrester, and confusing them leads to incorrect selection:

Continuous operating voltage (Uc) — the RMS voltage the arrester can withstand indefinitely under normal system conditions. This must be selected at or above the maximum continuous phase-to-earth voltage the arrester will actually see, including normal system voltage variation.
Rated voltage (Ur) — the RMS power-frequency voltage the arrester is designed to withstand for a defined short duration (typically 10 seconds) during a temporary overvoltage (TOV) test. Ur is always higher than Uc and is the value used for classification and ordering, but it is not the voltage the arrester is expected to carry continuously.

Residual voltage — the actual protection level

The parameter that determines how well the arrester protects the downstream equipment is the residual voltage (Ures): the voltage that appears across the arrester's terminals while it discharges a specified surge current (commonly the 8/20 µs waveform at the rated nominal discharge current In, e.g. 10 kA for a typical distribution-class MV arrester). A lower residual voltage at a given discharge current means a lower voltage stress reaches the protected equipment — but a lower residual voltage generally also means less margin for other design constraints, so arrester selection is always a coordinated choice, not simply "pick the lowest Ures available."

Line discharge class — energy absorption capability

IEC 60099-4 classifies arresters into line discharge classes 1 to 5, which describe the arrester's capability to absorb the energy of a switching-surge discharge without damage — a higher class indicates a higher energy-absorption capability, relevant mainly for arresters applied on longer transmission lines or at locations exposed to significant switching-surge energy, rather than for a typical short MV distribution feeder.

Pressure relief — surviving a failed arrester

If a MOSA fails internally (for example due to long-term moisture ingress degrading the ZnO blocks), it can become a near short-circuit path for power-frequency fault current until the upstream protection clears the fault. IEC 60099-4 defines a pressure relief class (a rated short-circuit current the housing must withstand without violent, fragmenting failure) so that a failed arrester vents safely rather than exploding — an important safety property given that MV arresters are often mounted close to personnel-accessible switchgear.

Placement: lead length matters

Protection is only as good as the connection between the arrester and the equipment it protects. A steep-front lightning impulse travelling through the connecting leads causes an additional inductive voltage drop (proportional to the rate of current rise and the lead inductance) on top of the arrester's own residual voltage — so a long or looped connecting lead effectively raises the voltage the protected equipment actually experiences, even with an arrester rated for a low Ures. Arresters should therefore be mounted as close as practically possible to the equipment being protected, with the shortest, straightest possible leads.

Periodic monitoring: leakage current

A MOSA degrades gradually rather than failing instantly: moisture ingress or repeated surge duty increases the small resistive leakage current that flows continuously at normal operating voltage, long before the arrester approaches a dangerous failure. A surge counter with leakage current meter installed in the arrester's earth connection tracks this trend over time — a rising resistive leakage current, monitored periodically, is an early warning of arrester ageing that a purely visual inspection cannot detect.

Common mistakes

Selecting Uc based on nominal system voltage without margin — the continuous operating voltage must cover the actual maximum phase-to-earth voltage the arrester will see, including normal system voltage variation, not just the nominal value.
Choosing the arrester with the lowest available residual voltage without checking insulation coordination — protection level is one input among several; it must be coordinated with the protected equipment's impulse withstand and the rest of the installation.
Mounting the arrester with long or looped connecting leads — the inductive voltage drop in the leads themselves adds directly to the residual voltage, undermining an otherwise correctly selected arrester.
Treating leakage-current monitoring as optional — a MOSA that is ageing internally shows no external sign until failure; periodic leakage-current trending is the only practical way to catch degradation early.