A horn gap arrester is one of the simplest forms of surge protection. Two metal horns separated by an air gap divert lightning and switching surges to earth before they can damage a transformer or line. This guide explains the working principle step by step.
What Is a Horn Gap Arrester?
A horn gap arrester consists of two curved metal rods, called horns, mounted on insulators. One horn connects to the live line and the other to earth through a resistor. The gap between them is set so it does not break down at normal voltage.
Horn Gap Arrester Working Principle
- Normal operation: Line voltage is below the breakdown level of the air gap, so no current flows to earth.
- Surge arrives: A lightning or switching surge raises the voltage above the breakdown level of the gap.
- Arc forms: The air at the narrowest point at the bottom of the horns breaks down and an arc forms.
- Surge to earth: The arc carries the surge current through the resistor to earth, protecting the equipment.
- Arc rises and extinguishes: Heat and magnetic forces push the arc up along the diverging horns. As it lengthens, it stretches and goes out.
Diagram Explained in Words
Picture two horns shaped like a letter V opening upwards. The gap is smallest at the bottom, where the arc starts. The line side horn sits above the earth side horn. A series resistance limits the follow current, and a choke coil on the line side stops steep surge fronts from reaching the transformer.
Main Parts
- Two horn electrodes
- Support insulators
- Series resistor
- Choke coil
- Earth connection
Advantages and Limitations
Advantages
- Simple, low-cost construction
- Easy to inspect and adjust
- Works on overhead lines and small transformers
Limitations
- Gap setting can change with weather, dust and bird contact
- Arc may not extinguish quickly, causing a line outage
- Not suitable for high voltage substations, where metal oxide arresters are used
Where It Is Used
Horn gap arresters protect distribution transformers, overhead lines and some older substations, usually up to 33 kV.
Why Surge Protection Is Needed
Overhead lines are exposed to lightning, and switching operations inside the network also create short, sharp voltage rises. These surges rise to many times the normal voltage within microseconds. Transformer windings and line insulators are rated for a limited impulse level, so a surge that exceeds it can puncture insulation and cause a failure that takes weeks to repair. A surge protective device keeps the surge below the withstand level by giving it an easy path to earth.
Construction of a Horn Gap Arrester in Detail
The horns
Two galvanised steel or brass rods are bent into a horn shape and mounted opposite each other on a pair of porcelain insulators. The gap between the horns is smallest at the bottom and widens towards the top. The gap setting depends on the system voltage, and it is fixed so that the gap sparks over at roughly 20 to 30 percent above the normal peak voltage, while staying stable during normal operation.
The series resistance
A resistor is placed in the earth path. Its job is to limit the follow current, which is the power-frequency current that tries to flow after the surge has ionised the gap. Without it, the arc may continue and trip the line.
The choke coil
A small inductor is placed between the arrester and the protected equipment. It slows the steep front of a surge so the gap has time to spark over, and so the transformer insulation does not see a sharp voltage step.
What Happens During a Surge, Phase by Phase
- Normal state. Voltage across the gap is below the sparkover voltage. No current flows and the device is invisible to the network.
- Surge enters the line. The voltage rises quickly toward the sparkover level.
- Spark over. At the narrowest point the air breaks down. A conducting arc channel forms between the horns.
- Discharge. The surge current flows through the arc and the resistor to earth. The voltage on the line collapses to a safe value.
- Follow current. The system voltage keeps the arc alive for a few cycles.
- Arc travels and extinguishes. Heat makes the hot air rise, and magnetic forces push the arc up the widening horns. The arc stretches until it can no longer be sustained and it goes out. The gap recovers its insulation and the system returns to normal.
Where a Horn Gap Arrester Is Installed
- On the line side of a distribution transformer, close to the bushing.
- On overhead lines at points where insulation strength is weak, such as cable-to-line junctions.
- At older substations where the surge protection has not been upgraded.
The arrester should be installed as close as possible to the equipment it protects, with the shortest possible earth lead. Long leads add inductance and raise the voltage seen by the equipment during the surge.
Horn Gap vs Modern Metal Oxide Arrester
| Feature | Horn Gap Arrester | Metal Oxide Arrester |
|---|---|---|
| Principle | Air gap sparkover | Non-linear zinc oxide blocks |
| Response speed | Slower, depends on the gap | Very fast, almost instantaneous |
| Follow current | Present, arc must be extinguished | Negligible, no gap |
| Line outage after operation | Possible | Rare |
| Maintenance | Gap setting needs periodic checks | Low maintenance |
| Cost | Low | Higher |
| Typical use | Low voltage distribution, older networks | All voltage levels, new installations |
Advantages Worth Noting
- Easy to understand, build and repair.
- The gap is visible, so a technician can see the setting and signs of arcing.
- It can be made locally with standard materials.
Limitations You Should Plan For
- The sparkover voltage changes with humidity, pollution and altitude, so protection may vary.
- Birds, branches or wire pieces can bridge the gap and cause a fault.
- Every operation can trip the feeder if the arc does not extinguish quickly.
- Repeated arcing erodes the horn tips and changes the gap, so inspection and adjustment are needed.
Inspection and Maintenance Checklist
- Check the gap with a gauge and compare it with the recommended value.
- Look at the horn tips for pitting and melting, and dress or replace worn tips.
- Clean the porcelain insulators and check for cracks.
- Test the earth connection and measure the earth resistance. Aim for a low value as per your utility practice.
- Look for loose bolts, rust and damaged galvanising.
- Remove bird nests and nearby vegetation.
Choosing Between a Horn Gap Arrester and Other Options
If the network is old, the budget is tight and the system voltage is low, a well-maintained horn gap arrester can still give useful protection. For new work, higher voltage levels and sensitive equipment, a metal oxide surge arrester is the better choice. Many utilities also combine a horn gap fuse with an arrester at the transformer, so that overload and surge are both handled. SPKN India supplies horn gap fuses, lightning arresters and other substation protection equipment. Read our related guide on the 7 key parts of a horn gap lightning arrester and the explanation of lightning arrester types.
Does the Horn Gap Setting Matter?
Yes. If the gap is too small, the arrester may spark over on normal switching surges and trip the feeder again and again. If it is too wide, a dangerous surge may reach the transformer before the gap breaks down. Gap settings are normally given by the manufacturer or the utility standard for each voltage class, and they should be re-checked after any operation. In dusty or coastal areas, the effective sparkover voltage may drop, so inspection should be more frequent.
Role of Earthing
An arrester is only as good as its earth. A high resistance earth lets the voltage at the arrester rise during discharge, and the protected equipment sees that rise too. Use a dedicated earth electrode where possible, connect the arrester earth to the transformer tank and neutral as per local practice, and measure the earth resistance each year before the monsoon.
Practical Tips From Field Experience
- Fit bird guards on the horns of arresters mounted on poles in rural feeders.
- Take a photograph of the gap during commissioning so later inspectors can compare it.
- Keep a spare set of horns and insulators in the store for quick replacement after a storm.
- After a major lightning season, inspect all arresters on the feeder, even if no trip was recorded.
Safety Notes for Technicians
Treat every arrester and its earth lead as live until the line is isolated, tested and earthed. Never touch the horns on an energised line, even in dry weather. During inspection, use an insulated ladder or a bucket truck, wear the specified protective equipment and follow your permit to work procedure. After a lightning storm, wait for the all clear from the control room before approaching the structure, because a second strike can follow the first within minutes.
Frequently Asked Questions
Why are the horns curved?
The shape lets the arc rise and stretch until it is extinguished.
Is a horn gap arrester the same as a lightning arrester?
It is one type of lightning arrester, based on an air gap rather than metal oxide blocks.
What replaces horn gap arresters today?
Metal oxide (gapless) surge arresters, which respond faster and need less maintenance.
Need Arresters or Substation Protection Equipment?
SPKN India supplies horn gap fuses, lightning arresters, isolators and other substation equipment from Faridabad. Send us your voltage and quantity for a quote.
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