How to Select Surge Arresters for 11kV, 24kV and 33kV Overhead Lines

Ask an engineer how they picked the surge arrester for a pole-mounted transformer and the answer is usually the system voltage. An 11 kV line gets an "11 kV arrester." A 33 kV line gets a "33 kV arrester."

That single step skips the two parameters that actually determine whether the arrester survives — continuous operating voltage (Uc) and rated voltage (Ur) — both of which depend on how the network neutral is earthed, not just on the line voltage. Get them too low and the arrester heats up under temporary overvoltage and fails, often taking the feeder out with it. Get them too high and the protective level rises, leaving the transformer exposed to the surge you installed the arrester to stop.

And even when the calculation is right, a two-metre connection lead can quietly cancel a third of the protection.

This guide works through the selection in seven steps for 11 kV, 24 kV and 33 kV overhead distribution, with the full rating tables you need to specify a part number.

What a metal-oxide arrester actually does

A modern gapless arrester is a stack of zinc-oxide varistor blocks with a strongly non-linear voltage/current characteristic. At normal operating voltage it conducts a leakage current in the region of a milliamp and behaves as an insulator. When a lightning or switching surge arrives, its resistance collapses by orders of magnitude, it conducts the surge current to earth, and it clamps the terminal voltage at its residual voltage — then returns to the high-resistance state as the surge passes, without any follow current for the system to interrupt.

Three practical consequences:

1. The arrester is continuously energised. It must tolerate normal operating voltage for 30 years without thermal runaway. That is the Uc requirement.
2. It must survive temporary overvoltage (TOV) during earth faults, which on some networks lasts hours rather than cycles. That is the Ur requirement.
3. It clamps, it does not eliminate. The protected equipment sees the residual voltage plus whatever the connection leads add. That is the protective margin question.

The product standards to quote in a specification are IEC 60099-4 for the arrester itself and IEC 60099-5 for selection and application.

Step 1: Set Uc from the system voltage and the neutral earthing

Continuous operating voltage (Uc) must exceed the highest continuous power-frequency voltage that will appear across the arrester, phase to earth:

Uc ≥ Us ÷ √3

where Us is the highest system voltage — 12 kV for an 11 kV network, 24 kV for a 22 kV network, 36 kV for a 33 kV network. That is the floor. Whether you can sit near it depends on Step 2.

Add margin where the network runs above nominal voltage, where voltage regulation is poor, or where harmonic distortion is significant.

Step 2: Set Ur from temporary overvoltage — this is where neutral earthing decides everything

During a single phase-to-earth fault, the two healthy phases rise. How far, and for how long, is entirely a function of the neutral arrangement — and this is the step most commonly skipped.

On an isolated or resonant-earthed 33 kV network — common in rural distribution across much of Asia, Africa and Latin America — a sustained earth fault puts close to full phase-to-phase voltage across the arresters on the healthy phases, potentially for the rest of the shift. An arrester chosen on the "0.8 × Us" rule for a solidly earthed system will cook.

This is why our range offers several Ur values inside each voltage class rather than one. Confirm the neutral earthing arrangement before selecting a part number — it is the single most valuable line in an RFQ.

Step 3: Choose the discharge class — 5 kA or 10 kA

Nominal discharge current (In) defines the 8/20 µs current at which residual voltage is specified and, together with the long-duration ratings, describes how much energy the arrester can absorb.

The price difference between the two classes is modest against the cost of a failed 315 kVA transformer and a day of crew time. On feeders in high-keraunic regions, or where the arrester also has to handle switching and cable-discharge energy, the 10 kA class is the defensible choice.

Step 4: Check the protective margin

The lightning impulse protective level (Upl) is the residual voltage at nominal discharge current, 8/20 µs. The protected equipment has a lightning impulse withstand level (LIWL/BIL). The margin between them is what you are buying:

Protective margin (%) = (LIWL − Upl) ÷ Upl × 100

A margin of at least 20 % is the usual minimum, and more is preferred because the margin erodes in service (Step 5).

Worked from our published residual voltages against typical transformer insulation levels:

Note the trend: margin shrinks as voltage class rises. At 33 kV there is far less room for error than at 11 kV, which makes the next step disproportionately important on 33 kV lines.

Step 5: Lead length — the field error that undoes the calculation

The transformer does not see Upl. It sees Upl plus the inductive voltage developed along the connection leads on both the line side and the earth side.

Conductor inductance is roughly 1 µH per metre. A 10 kA 8/20 µs surge has a rate of rise in the order of 1.25 kA/µs, so each metre of lead contributes approximately:

ΔV ≈ 1 µH/m × 1.25 kA/µs ≈ 1.25 kV per metre

Four metres of total lead — two metres up to the line connection, two metres down to the earth — adds around 5 kV to what the transformer experiences. On a 33 kV installation with Upl of 108 kV that is a 5 % erosion. On an 11 kV installation with Upl of 36 kV it is closer to 14 %, and careless routing of 3–4 m on each side can eat a quarter of the protection you paid for.

Four rules for the crew:

1. Mount the arrester as close to the transformer bushing as physically possible.
2. Keep both leads short and straight. No loops, no service coils, no generous slack "for later."
3. Bond the arrester earth lead directly to the transformer tank, and take the tank and the arrester to the same earth electrode. The transformer insulation is stressed by the difference between its terminal and its tank — connecting them at the same point removes the lead drop from that difference.
4. Verify earth electrode resistance against the local standard; a low protective level into a poor earth is a partial solution at best.

Step 6: Housing, creepage and pollution

Composite (silicone-housed) arresters dominate modern distribution work for reasons that go beyond price:

Creepage follows the same pollution logic as any other outdoor MV component — roughly 16 mm/kV in light pollution, 20 mm/kV medium, 25 mm/kV heavy and 31 mm/kV very heavy per the IEC 60815 classes. Coastal, desert and heavy-industry routes need the extended-creepage version specified at tender, not discovered at year two. The same reasoning that governs your drop-out fuse cutouts and composite insulators applies here — specify the whole pole to one pollution class, not component by component.

Step 7: Disconnector, position and mounting

Grounding-lead disconnector. If an arrester fails, it fails to earth — and without a disconnector that becomes a permanent earth fault that keeps the feeder out. A disconnector releases the earth lead when the arrester fails, restoring service and leaving a hanging lead that a crew can spot from the ground. On radial rural feeders this single accessory pays for itself the first time it operates. Confirm whether your quoted units include one.

Where to install on the feeder:

• At every distribution transformer, on the HV bushings
• At overhead-to-cable transitions, at both ends of the cable section
• At line terminations and open points
• At riser poles and at the entry to switching equipment
• On both sides of a normally open sectionaliser where the line may be back-fed

Mounting. Confirm bracket type, phase spacing and clearance to the pole and to the drop-out fuse cutout mounted alongside, and match hardware corrosion protection to the rest of the power fittings on the structure.

Full rating reference: 11 kV, 24 kV and 33 kV

All values are published data for our composite zinc-oxide range. Residual voltages are given for 1/4 µs steep front, 8/20 µs lightning and 30/60 µs switching impulses.

11 kV class — 11kV Zinc Oxide Composition Arrester

24 kV class — 24kV Composite Zinc-Oxide Arrester

33 kV class — 33kV Composite Zinc Oxide Arrester

Reading the model code: YH5W-24/72 is a 5 kA metal-oxide arrester with 24 kV rated voltage and 72 kV residual voltage at nominal discharge current. YH10W is the 10 kA class equivalent.

Five mistakes that shorten arrester life

1. Selecting on system voltage alone. Without the neutral earthing arrangement, Ur is a guess.
2. Under-rating Ur on an isolated-neutral network. A fault that persists for hours is a thermal duty, not a transient.
3. Long connection leads. The most expensive arrester on the market cannot outrun four metres of inductance.
4. Separate earths for arrester and transformer tank. This puts the lead drop directly across the transformer insulation.
5. Omitting the disconnector on radial feeders. One failed arrester then equals one feeder outage until a crew locates it.

What to include in your RFQ

1. Nominal and highest system voltage (Us)
2. Neutral earthing arrangement and expected earth-fault clearing time
3. Required Ur and Uc, if already calculated — or send us the above and we will calculate
4. Discharge class: 5 kA or 10 kA
5. Protected equipment LIWL/BIL (transformer, cable, switchgear)
6. Pollution class or site description
7. Housing: composite or porcelain
8. Disconnector required: yes or no
9. Bracket and mounting arrangement
10. Site altitude, ambient temperature range and lightning ground flash density if known
11. Quantities, delivery schedule and destination port

Datasheets, structure drawings and test certificates are available from our Download centre and Certificates pages.

FAQ

What is the difference between Uc and Ur on a surge arrester? Uc is the continuous operating voltage the arrester can withstand indefinitely; Ur is the rated voltage, defined by its ability to survive temporary overvoltage for a specified duration. Uc sets the everyday duty, Ur sets the fault-condition duty.

How do I choose the arrester rated voltage for an 11 kV line? Start from Uc ≥ 12 ÷ √3 ≈ 6.9 kV, then set Ur from the neutral earthing: roughly 0.8 × Us on an effectively earthed system, and at least equal to Us on an isolated or resonant-earthed system.

Should I use a 5 kA or a 10 kA arrester? 5 kA is standard for distribution transformers in moderate lightning conditions. Choose 10 kA for high ground flash density, cable transitions, substation entries and higher-value assets — it carries the higher long-duration and high-current withstand.

How long can the arrester connection leads be? As short as practical. Each metre adds roughly 1.25 kV to the voltage the equipment sees during a 10 kA surge, so keep the line and earth leads combined to a metre or two and bond the earth lead directly to the transformer tank.

Does a surge arrester replace a drop-out fuse? No. They address different faults. The arrester limits overvoltage; the drop-out fuse cutout interrupts overcurrent and provides visible isolation. A pole-mounted transformer normally has both.

Composite or porcelain housing? Composite for most modern distribution work — lighter, safer failure mode, better under pollution and vandalism. Porcelain remains viable where utility practice and inland conditions favour it.

Send us your line data and we will size the arresters

Give us system voltage, neutral earthing, transformer schedule and site conditions, and we will return Ur, Uc, discharge class and part numbers with datasheets — coordinated with the cutouts, insulators and fittings on the same structure.

Delixi Electric supplies composite zinc-oxide arresters across the 11 kV, 24 kV and 33 kV classes in 5 kA and 10 kA versions, alongside drop-out fuse cutouts, composite insulators, overhead conductor, power fittings and distribution transformers — IEC and CUL certified.

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