MO Surge Arresters: Distribution & IEC Standards

Section 1

Distribution Systems and the IEC Comparison

The earlier parts covered MCOV and TOV selection, switching-surge energy, protective characteristics, and the lightning discharge current. This closing part applies all of that to two new practical areas: distribution-system arrester selection, and the IEC rating, testing and application philosophy compared with IEEE.

The basic selection rules are the same for every system — but the emphasis changes in distribution networks, and the IEC framework names and classifies arresters differently.

This is Part Five, the final part of the metal oxide surge arrester series.

What this page teaches

why distribution arresters are selected for system-wide applicability;
why the actual feeder voltage — not the nominal — sets the MCOV;
why gapless MO arresters need more careful TOV assessment than old SiC;
how the grounding method controls MCOV across the three distribution types;
why distribution arrester lightning currents can exceed station values;
the IEC rating quantities (\(E_R\), COV, nominal current, line discharge class);
how IEC and IEEE selection compare — protective tests similar, capability tests different;
how to translate between IEEE and IEC catalogue terms.

Section 2

Why Distribution Needs Separate Treatment

In transmission systems a detailed study may be done for each substation or arrester location. In distribution systems that is usually impractical — instead, one arrester MCOV is selected so it can be applied broadly across a similar part of the network.

System-wide applicability

Distribution arrester MCOV is usually selected for system-wide applicability, not for one carefully studied location.

The rules are the same as elsewhere — MCOV above the maximum continuous line-to-ground voltage, TOV below the arrester TOV capability, acceptable energy duty, adequate pressure relief, acceptable environment — but the emphasis shifts to the actual feeder voltage, the grounding method, TOV uncertainty, direct lightning exposure, the arrester discharge current, and practical standardisation across many locations.

Section 3

Distribution MCOV and the Feeder Voltage

The most important rule still holds:

\[ \text{MCOV} \ge V_{\text{LG,max}} \]

Table 1 — Notation for the distribution arrester MCOV selection rule.

Symbol	Meaning
MCOV	Maximum continuous operating voltage of the arrester
\(V_{\text{LG,max}}\)	Maximum line-to-ground rms system voltage

In distribution systems only the customer meter voltage is normally regulated; the feeder voltage itself can vary significantly. The source notes a study where the unregulated feeder voltage reached \(17\%\) above nominal (about \(10\%\) above maximum), averaging about \(7\%\) above nominal (about \(1\%\) above maximum).

Use the real maximum

Do not select distribution arrester MCOV from nominal voltage only — the actual maximum feeder voltage must be considered, because a gapless MO arrester is continuously energised.

Section 4

Why MO Needs Careful TOV Assessment

Older gapped SiC arresters were more forgiving on TOV. Their 60 Hz sparkover was about \(1.2\)–\(1.3\) times the duty-cycle rating — a \(9\ \text{kV}\) duty-cycle SiC arrester could withstand roughly \(10.8\)–\(11.7\ \text{kV}\) for \(10\ \text{s}\) (about \(1.47\)–\(1.60\ \text{pu}\)). By contrast, a \(9\ \text{kV}\) duty-cycle / \(7.65\ \text{kV}\) MCOV metal oxide arrester may have a minimum 10-second TOV capability of only \(1.24\ \text{pu}\) of MCOV.

Table 2 — How gapless metal-oxide and old gapped SiC arresters differ in TOV tolerance.

Arrester Type	Behaviour during 60 Hz overvoltage	Practical Consequence
Old SiC gapped arrester	Sparkover gap gives a high 60 Hz withstand margin	TOV did not need to be known as accurately
Gapless MO arrester	Metal oxide blocks are continuously energised	TOV magnitude and duration must be checked carefully

Tighter TOV margin

MO arresters require more accurate TOV assessment than old SiC gapped arresters — their 10-second TOV margin can be much smaller.

Table 3 — Typical one- and ten-second TOV capability ranges relative to MCOV.

Quantity	Range (Prior Energy)
Actual \(\text{TOV}_{10}\)	\(1.28\)–\(1.46\ \text{MCOV}\)
Actual \(\text{TOV}_{1}\)	\(1.34\)–\(1.54\ \text{MCOV}\)
Minimum \(\text{TOV}_{10}\)	\(1.24\ \text{MCOV}\)
Minimum \(\text{TOV}_{1}\)	\(1.38\ \text{MCOV}\)

Distribution TOVs are not always known accurately, because feeder configurations, grounding resistance, neutral conductor size, distributed loads, fault clearing and ferroresonance all vary — and older practice did not require detailed TOV knowledge.

A non-trivial task

Distribution TOV estimation is not trivial — for four-wire multigrounded systems, ground resistance and neutral conductor size can significantly alter the TOV.

Section 5

Four-Wire Multigrounded Systems

Most North American distribution systems are four-wire multigrounded. TOVs are generally low if the grounding is effective — the earth-fault factor is about \(\text{EFF} \approx 1.25\), provided the grounding resistance is below \(25\ \Omega\) and the neutral conductor is at least \(50\%\) of the phase-conductor size.

\[ \text{EFF} \approx 1.25 \quad\Rightarrow\quad \text{MCOV}_{\min} \approx V_{\text{LG,max}} \]

Continuous voltage usually controls

For a well-grounded four-wire system the arrester TOV capability exceeds the expected fault TOV, so the minimum MCOV is normally the maximum line-to-ground voltage — the continuous-voltage requirement controls, not TOV. This holds only with adequate neutral grounding, neutral size, feeder voltage and grounding resistance; if grounding is poor, it may not.

Section 6

Three-Wire Low-Impedance Grounded Systems

For three-wire low-impedance grounded systems the earth-fault factor is generally \(\text{EFF} \ge 1.4\), and the fault may last up to \(10\ \text{s}\). This makes TOV more severe, and the MCOV may need to be selected above the maximum line-to-ground voltage. For a \(12{,}700\text{Y}/7{,}330\ \text{V}\) system with \(\text{EFF} = 1.6\), \(V_{\text{LG,max}} = 7.33\ \text{kV}\) and minimum \(\text{TOV}_{10} = 1.24\ \text{MCOV}\):

\[ \text{MCOV} = \frac{1.6 \times 7.33}{1.24} = 9.5\ \text{kV} \;\longrightarrow\; \text{MCOV}_{\text{selected}} = 10.2\ \text{kV} \]

The next standard rating above \(9.5\ \text{kV}\) is \(10.2\ \text{kV}\). Here the MCOV is set by the TOV requirement, not by the line-to-ground voltage — the ratio is \(10.2/7.33 = 1.39\). For this category, MCOV ratings may range from about \(1.3\) to \(2.0\) times the maximum line-to-ground voltage.

TOV can control here

In three-wire low-impedance grounded systems the MCOV may be set by TOV, requiring a higher-rated arrester than the line-to-ground voltage alone would suggest.

Section 7

Three-Wire High-Impedance Grounded or Delta Systems

For three-wire high-impedance grounded or delta-connected systems the earth-fault factor is \(\text{EFF} = 1.73\), and the TOV duration can be very long — sometimes \(8\ \text{h}\) or more. Because it is so long, it is treated as practically infinite, so the MCOV must equal the maximum line-to-line voltage:

\[ \text{MCOV} \approx V_{\text{LL,max}} \]

The most severe case

Selecting \(\text{MCOV} \approx V_{\text{LL,max}}\) is much more severe than selecting from line-to-ground voltage — the unfaulted phases sit near full line-to-line voltage for the whole (effectively indefinite) fault.

Section 8

Distribution Grounding Comparison

This is one of the most important distribution lessons — the grounding method drives the MCOV:

Table 4 — How grounding method sets the required MCOV across three distribution types.

Distribution System	Typical EFF	TOV Duration	MCOV Tendency
Four-wire multigrounded	\(\approx 1.25\) if well grounded	Usually low / short	\(\text{MCOV} \approx V_{\text{LG,max}}\)
Three-wire low-impedance grounded	\(\ge 1.4\)	Up to \(10\ \text{s}\)	Higher MCOV may be required
Three-wire high-impedance / delta	\(1.73\)	Very long, possibly hours	\(\text{MCOV} \approx V_{\text{LL,max}}\)

Section 9

Distribution Arrester Currents and the Coordinating Current

Most distribution lines are unshielded, so they are exposed to direct lightning strokes. Distribution arresters therefore often discharge higher lightning currents than station arresters inside a substation. The current depends on ground flash density, stroke location relative to the arrester, shielding by nearby buildings or trees, the number of nearby arresters, and the system layout.

Higher than station currents

Distribution arrester lightning current can be higher than a station arrester current — and it depends on local lightning exposure and surroundings (rural lines may be partly shielded by buildings, trees and nearby objects).

The IEEE Guide gives curves of arrester discharges per year versus arrester current. A current is selected for \(0.1\) discharges/year — one discharge in ten years — called the coordinating current. The arrester discharge voltage at the coordinating current is then used to check the protected equipment insulation strength.

Coordinating current The arrester current selected at \(0.1\) discharges/year (\(1\) discharge in \(10\) years), obtained from expected discharge frequency, ground flash density and exposure. Its discharge voltage sets the insulation-coordination check for the protected equipment.

\[ I_{\text{coord}} \;\longrightarrow\; V_{\text{discharge}} \;\longrightarrow\; \text{equipment insulation strength} \]

Section 10

IEC Arrester Standards and Classification

The applicable IEC documents are IEC 99-4 and IEC 99-5, with frequent reference to IEC 99-1 (in modern naming, the IEC surge-arrester standard family). In IEC, arresters are rated and classified primarily by four quantities:

Table 5 — The four IEC arrester rating quantities compared with IEEE equivalents.

IEC Term	Meaning	IEEE Comparison
COV	Continuous operating voltage	Same as MCOV
Rated voltage \(E_R\)	10-second TOV capability (used in the duty-cycle test)	Not the same as IEEE MCOV
Nominal discharge current	Lightning-current classification	Similar role to the classifying current
Line discharge class	Energy-capability classification	Related to low-current long-duration duty
Pressure-relief class	Fault-current venting capability	Similar purpose to the pressure-relief rating

Section 11

IEC Rated Voltage, COV, Current and Discharge Class

The IEC rated voltage \(E_R\) is defined as the TOV capability at \(10\ \text{s}\), and is also used in the duty-cycle test:

\[ E_R = \text{TOV}_{10} \]

This is an important difference from IEEE, where the duty-cycle rating exists but is not defined directly as the 10-second TOV capability. The IEC continuous operating voltage is the same concept as the IEEE MCOV:

\[ \text{COV}_{\text{IEC}} \equiv \text{MCOV}_{\text{IEEE}} \]

Do not confuse IEC rated voltage with IEEE MCOV

The single most important IEC comparison point: \(\text{COV}_{\text{IEC}} = \text{MCOV}_{\text{IEEE}}\), but \(E_R = \text{TOV}_{10}\). The IEC rated voltage is a 10-second TOV capability — it is not the IEC equivalent of MCOV.

The IEC standard nominal discharge currents are \(1.5\), \(2.5\), \(5\), \(10\) and \(20\ \text{kA}\). The line discharge classes are \(1, 2, 3, 4, 5\) (or none), and are linked to energy capability — not chosen independently of the nominal current:

Table 6 — Which line discharge classes are available at each nominal discharge current.

Nominal Discharge Current	Available Line Discharge Class
\(10\ \text{kA}\)	Class 1, 2 or 3
\(20\ \text{kA}\)	Class 4 or 5
\(1.5\), \(2.5\), \(5\ \text{kA}\)	No line discharge class

The combination of nominal discharge current and line discharge class determines the low-current long-duration test, and therefore the energy capability.

Section 12

IEC versus IEEE Rating Philosophy

IEEE provides standardised MCOV and duty-cycle rating lists. IEC does not tabulate rated voltages and COVs the same way — it provides steps of rated voltage instead. The selection process is still similar; the naming and tabulation differ. The systems an arrester is applied to are generally set by the nominal discharge current:

Table 7 — Typical system applications for each IEC nominal discharge current rating.

Nominal Discharge Current	Approximate Application Tendency
\(20\ \text{kA}\)	Highest voltage / highest duty
\(10\ \text{kA}\)	Transmission and substation
\(5\ \text{kA}\)	Lower-voltage / distribution-type
\(2.5\), \(1.5\ \text{kA}\)	Lower-duty applications

Approximate class equivalence

IEEE station class corresponds approximately to the IEC 10 kA arrester; IEEE intermediate and distribution classes correspond approximately to the IEC 5 kA arrester. These are approximate, not exact, equivalences.

\[ \text{same engineering checks} \;\neq\; \text{same rating labels} \]

IEEE and IEC selection logic is similar — continuous voltage, TOV, energy, lightning current and pressure relief are all checked. The difference is mainly how the arrester is named, classified and tested.

Section 13

IEC Durability Tests

The IEC capability requirements parallel IEEE — high-current short-duration, low-current long-duration, duty-cycle, pressure relief, TOV capability and protective-characteristic tests — but the details differ. The high-current test applies two \(4/10\ \mu\text{s}\) impulses (magnitudes set by nominal current and line discharge class). For \(10\) and \(20\ \text{kA}\) arresters, the low-current long-duration test discharges a charged transmission line into the arrester, performed \(18\) times (\(6\) groups of \(3\), \(1\ \text{min}\) between groups). The duty-cycle test has two forms:

Table 8 — The two IEC duty-cycle test forms and their impulse sequences.

Duty-Cycle Form	Applies to	Sequence
High-current operating duty	\(1.5\), \(2.5\), \(5\ \text{kA}\); \(10\ \text{kA}\) Class 1	20× \(8/20\ \mu\text{s}\) at \(1.2\,\text{COV}\); 2× \(4/10\ \mu\text{s}\); rated voltage \(10\ \text{s}\); COV \(30\ \text{min}\)
Switching impulse operating duty	\(10\ \text{kA}\) Class 2–3; \(20\ \text{kA}\) Class 4–5	20× \(8/20\ \mu\text{s}\) at \(1.2\,\text{COV}\); 2× \(4/10\ \mu\text{s}\), \(100\ \text{kA}\); 2 line discharges; rated voltage \(10\ \text{s}\); COV \(30\ \text{min}\)

Each test is passed if no thermal runaway occurs.

Section 14

IEC Pressure Relief, Pollution and TOV Tests

The pressure-relief test is required only if the arrester has a pressure-relief device; the arrester must vent safely, with components staying within a prescribed circle (as in IEEE). Pressure-relief classes for \(10\ \text{kA}\) arresters include \(10, 20, 40, 50, 63, 80\ \text{kA}\); for \(5\ \text{kA}\) arresters, \(5, 16\ \text{kA}\); all are also tested at \(800\ \text{A}\). At the time of the source, no standard IEC pollution test was established (unlike the IEEE contamination test).

IEC TOV capability curves are built from \(0.1\ \text{s}\) to \(20\ \text{min}\). For lower-duty arresters a \(4/10\ \mu\text{s}\) impulse is applied, then COV for \(30\ \text{min}\), then the curve is established; high-lightning-duty arresters use three \(4/10\ \mu\text{s}\) impulses; \(10\ \text{kA}\) Class 2–3 and \(20\ \text{kA}\) Class 4–5 arresters use two transmission-line discharges to set prior energy first.

Section 15

IEC Protective Characteristics

The IEC protective-characteristic tests are essentially similar to IEEE — steep-front, lightning impulse and switching impulse discharge voltages. The steep-front test applies a \(1\ \mu\text{s}\)-front current at the nominal discharge current; the resulting discharge voltage has a time to crest of about \(0.5\ \mu\text{s}\):

Steep-front = 0.5 μs

The IEC steep-front discharge voltage corresponds to the 0.5 μs discharge voltage — effectively the IEEE front-of-wave protective level (see Part Two).

The lightning impulse discharge voltage uses \(8/20\ \mu\text{s}\) currents at \(0.5\), \(1.0\) and \(2.0\) times the nominal discharge current. The switching impulse discharge voltage uses two current magnitudes with a front \(> 30\ \mu\text{s}\) and a tail \(< 100\ \mu\text{s}\):

Table 9 — Switching-impulse test currents for each IEC arrester classification.

Classification	Switching Impulse Currents
\(20\ \text{kA}\), Class 4 or 5	\(0.5\ \text{kA}\) and \(2.0\ \text{kA}\)
\(10\ \text{kA}\), Class 3	\(0.25\ \text{kA}\) and \(1.0\ \text{kA}\)
\(10\ \text{kA}\), Class 1 or 2	\(0.125\ \text{kA}\) and \(0.5\ \text{kA}\)

Section 16

IEC Rating Selection

The IEC Application Guide selects the COV, rated voltage, nominal discharge current, line discharge class and pressure-relief class — by a logic virtually identical to IEEE, differing mainly in the rating system and terminology:

Table 10 — What system parameter drives each IEC arrester rating choice.

IEC Quantity	Selected by
COV	Maximum continuous system voltage
Rated voltage \(E_R\)	Required 10-second TOV capability
Nominal discharge current	Estimated lightning discharge current
Line discharge class	Required energy discharge capability
Pressure-relief class	Available system fault current

Section 17

Comparing IEEE and IEC

The clearest distinction is that the protective-characteristic tests are essentially similar, while the capability / durability tests are different. An arrester may have similar protective data under both yet be classified differently in duty and capability.

Table 11 — How IEC and IEEE capability and durability tests differ.

Aspect	IEC vs IEEE
Line-discharge energy	IEC tests generally give larger discharge energies, except \(362\ \text{kV}\) 10 kA Class 2 and \(242\ \text{kV}\) 10 kA Class 1 (comparison not direct — test structures differ)
Number of discharges	IEC \(18\) vs IEEE \(20\); after the test IEEE applies MCOV \(30\ \text{min}\), IEC applies rated voltage \(10\ \text{s}\) then COV \(30\ \text{min}\)
Duty-cycle energisation	IEC at \(1.2\,\text{COV}\) (below rated voltage); IEEE at the duty-cycle rating — sequences not identical
TOV capability test	Virtually identical, except higher-class IEC arresters use prior-energy tests; IEEE gives both with- and without-prior-energy curves

Confirm energy from the manufacturer

The IEC guide states the energy capability is at least twice the line-discharge comparison value — but the source notes this is not verifiable from the available data, so energy capability should be confirmed from manufacturer data.

Section 18

A Practical IEEE–IEC Translation

Useful when comparing manufacturer catalogues or standards:

Table 12 — Practical translation between IEEE and IEC catalogue terminology.

Concept	IEEE	IEC
Maximum continuous voltage	MCOV	COV
10-second TOV-based rating	Not the same as duty-cycle rating	Rated voltage \(E_R\)
Arrester class	Station / intermediate / distribution	Nominal current + line discharge class
Lightning current class	Classifying current	Nominal discharge current
Energy classification	Low-current long-duration / energy capability	Line discharge class
Pressure relief	Pressure-relief rating	Pressure-relief class

Section 19

The Distribution / IEC Selection Workflow

Grounding method

Max feeder voltage

Required MCOV / COV

System TOV & duration

Check arrester TOV capability

Coordinating current

Nominal discharge current

Energy / line discharge class

Pressure-relief class

Verify equipment insulation margin

Section 20

Misconceptions, Equations and Final Message

Table 13 — Common distribution and IEC arrester misconceptions with their corrections.

Misconception	Correct Interpretation
“Distribution MCOV can be set from nominal voltage only”	Use the actual feeder maximum voltage and grounding-related TOV
“MO has the same TOV tolerance as old gapped SiC”	Gapless MO may have a lower 10-second TOV margin — check TOV carefully
“Four-wire multigrounded systems always have low TOV”	Only with adequate grounding resistance and neutral conductor size
“IEEE and IEC classes match exactly”	Different rating systems — only approximate equivalences; check duty data
“IEC and IEEE protective characteristics are completely different”	Protective tests are essentially similar; capability tests and terminology differ
“IEC rated voltage equals IEEE MCOV”	IEC COV equals IEEE MCOV; IEC rated voltage is the 10-second TOV capability

Equation Summary

Distribution MCOV rule

\(\displaystyle \text{MCOV} \ge V_{\text{LG,max}}\)

TOV-controlled MCOV

\(\displaystyle \text{MCOV} = \frac{\text{EFF}\cdot V_{\text{LG,max}}}{\text{TOV}_{10}/\text{MCOV}}\)

Worked example

\(\displaystyle \frac{1.6\times 7.33}{1.24} = 9.5\ \text{kV}\)

High-Z / delta systems

\(\displaystyle \text{MCOV} \approx V_{\text{LL,max}}\)

IEC rated voltage

\(\displaystyle E_R = \text{TOV}_{10}\)

IEC–IEEE continuous voltage

\(\displaystyle \text{COV}_{\text{IEC}} = \text{MCOV}_{\text{IEEE}}\)

Coordinating current

\(\displaystyle 0.1\ \text{disch./yr} = 1\ \text{in}\ 10\ \text{yr}\)

Memory map. Distribution arresters are selected for system-wide use → set MCOV from the real maximum feeder voltage → check TOV against grounding (four-wire \(\approx V_{\text{LG,max}}\); three-wire low-Z higher; high-Z / delta \(\approx V_{\text{LL,max}}\)) → estimate the coordinating current from lightning exposure → map to IEC: COV = MCOV, \(E_R = \text{TOV}_{10}\), nominal current, line discharge class, pressure-relief class.

Final engineering message

Distribution arrester application is not just a lower-voltage version of substation practice — the same physical rules apply, but with more uncertainty in feeder voltage, grounding, TOV duration and lightning exposure. For gapless MO arresters, MCOV must be selected using the real maximum voltage and a realistic TOV duty. IEEE and IEC methods are practically similar, but the terminology differs: \(\text{COV}_{\text{IEC}} = \text{MCOV}_{\text{IEEE}}\), while \(E_R\) is linked to the 10-second TOV capability. So when comparing arresters, do not compare names — compare continuous voltage, rated voltage / TOV capability, nominal discharge current, energy class, pressure-relief rating and protective characteristics. Only then are two arresters technically comparable. In short: same engineering checks, different IEEE and IEC rating language.

Distribution Systems and IEC Standards

Distribution Systems and the IEC Comparison

Why Distribution Needs Separate Treatment

Distribution MCOV and the Feeder Voltage

Why MO Needs Careful TOV Assessment

Four-Wire Multigrounded Systems

Three-Wire Low-Impedance Grounded Systems

Three-Wire High-Impedance Grounded or Delta Systems

Distribution Grounding Comparison

Distribution Arrester Currents and the Coordinating Current

IEC Arrester Standards and Classification

IEC Rated Voltage, COV, Current and Discharge Class

IEC versus IEEE Rating Philosophy

IEC Durability Tests

IEC Pressure Relief, Pollution and TOV Tests

IEC Protective Characteristics

IEC Rating Selection

Comparing IEEE and IEC

A Practical IEEE–IEC Translation

The Distribution / IEC Selection Workflow

Misconceptions, Equations and Final Message

Metal Oxide Surge Arresters

Distribution Systems, IEC Standards and IEEE-IEC Comparison

Lightning Impulse Discharge Current