Tower footing resistance is the electrical resistance between a transmission tower’s earthing and the general mass of the ground. POWERGRID’s specification sets a limit of 10 ohms for every tower, measured in dry weather after the tower is erected and before the earth wire is strung. This guide explains why the limit exists and how to measure against it, with both an ordinary earth tester and a high-frequency kit. It then ranks the remedies to try when a tower reads high: extra pipe electrodes, deeper electrodes, counterpoise wire and chemical earthing.

What is tower footing resistance, and why must it stay under 10 ohms?

Tower footing resistance is the resistance a tower’s earthing presents to current flowing into the ground. It must stay low because it sets how high the tower’s voltage rises when lightning strikes. When lightning hits a tower or its overhead earth wire, the current runs down the tower into the ground. The voltage that builds up on the tower is roughly that current multiplied by the footing resistance. If the voltage exceeds what the insulator string can withstand, an arc jumps from the tower across the insulators to the phase conductor. The result is called a back-flashover, and it trips the line.

A grounding guide from the Transmission and Distribution Engineering Committee of the US rural electric cooperatives works through the arithmetic for a 69 kV wood-pole line. A 25 kA lightning stroke through 25 ohms puts 625 kV on the structure, above the line’s insulation level of about 420 kV, so a flashover is likely. The same stroke through 10 ohms puts 250 kV on the structure, which the insulation withstands (TDEC Guide for Transmission Line Grounding, August 2025). The guide adds that the full calculation is more involved. Current waves travel along the earth wire to other structures and reflect back, so the multiplication shows the principle rather than a design value.

The limit itself is a design choice. POWERGRID requires every tower to stay at or below 10 ohms (Technical Specification Section IV-C, Revision 7, clause 1.7). Uttar Pradesh Power Transmission Corporation applies the same figure in its tower earthing specification. Eskom, the South African utility, sets a nominal 20 ohms for 66 to 132 kV lines. Where the value cannot be achieved, its standard says a higher back-flashover rate must be expected on the affected sections (Eskom distribution standard for earthing of sub-transmission line towers, clause 4.1.1).

When should tower footing resistance be measured?

Measure each tower in dry weather, after erection and before the earth wire is strung. POWERGRID’s specification (Section IV-C, clause 1.7) and the Uttar Pradesh specification set this timing, and the 10 ohm limit applies to that reading. Two reasons support it.

The first reason is the earth wire. Once strung, the earth wire connects the tower to every other tower on the line. An ordinary earth tester then reads the tower in parallel with all the others and shows a figure that is too low. A study by Korean researchers states the point directly. Towers are connected in parallel through the overhead ground wire, so a tower’s footing resistance is measured after the earth wire is disconnected (Journal of the Korean Institute of Electrical and Electronic Material Engineers, 2010).

The second reason is soil moisture. IS 3043 notes that soil moisture normally ranges from about 10 percent in dry seasons to 35 percent in wet seasons. Below about 20 percent moisture, resistivity rises very abruptly as moisture falls (IS 3043:1987, clause 8.6). A dry-weather reading is therefore close to the worst case for the site. Readings can also drift after commissioning. ITU-T K.107 notes that soil settling through annual weather cycles tends to reduce earth impedance during the first year or two (ITU-T K.107, clause 6.2).

Eskom adds one more condition: the measurement is taken with the foundations, earth straps and any counterpoise all connected.

How do you measure tower footing resistance with the fall-of-potential method?

Pass a test current between the tower’s earthing and a current probe driven into the ground far from the tower. Then read the voltage at a second probe placed between the two. The tester divides that voltage by the current and shows the resistance. Do this before the earth wire is strung. The steps are:

1. Keep the earth wire out of the circuit. Test before stringing, as the specifications require.
2. Lay the leads in a straight line from the centre of the tower, at right angles to the line (Eskom, clause 4.13.2.1).
3. Drive the current probe far from the tower. IEEE Std 81, as summarised in ITU-T K.107, puts it typically at five times the largest dimension of the earthing. EN 50522 asks for at least four times the maximum extension and not less than 40 m. Eskom uses 100 m for ordinary tower earthing.
4. Drive the potential probe on the same line at 62 percent of the current probe distance. For a 100 m current probe that is 62 m. Eskom specifies 60 m for standard tower earthing and 61.8 m for towers with counterpoise, which it calls the 61.8 percent method.
5. Read, then move the potential probe a few metres each way and read again. Two or three consecutive equal readings show that the probe is clear of the influence of both the tower and the current probe. ITU-T K.107 calls this the flat slope method. If the readings keep changing, move both probes further out.
6. Wet the soil around the probes if their own resistance is high, or connect several probes together (Eskom).

Counterpoise towers need longer probe distances. A tower with four 70 m counterpoise wires has an earthing extension of about 70 m. The EN 50522 rule quoted in ITU-T K.107 then puts the current probe at 4 x 70 = 280 m or more. It puts the potential probe at 2.5 x 70 = 175 m or more, which is close to 62 percent of 280 m. A 100 m lead is too short for that layout. These distances are a calculation from the stated rule, not figures quoted from a tower specification.

How do you measure a tower after the earth wire is strung?

Use a high-frequency tester or a method that measures the current leaking into each tower leg. A normal earth tester cannot isolate one tower once the earth wire is in place. ITU-T K.107 describes the high-frequency method. The test frequency is high enough, up to 5 kHz, that the earth wire and the neighbouring towers become a negligible parallel path. The instrument then reads a single tower without lifting the earth wire (clause 7.2.4). Eskom’s standard allows a high-frequency instrument, subject to the utility engineer’s approval, where the electrode cannot be disconnected from the structure. The meter must be calibrated to the surge impedance of the line, because without that calibration the neighbouring towers still affect the result.

Test frequency matters. A study of 154 kV towers in Korea compared methods and found that readings settled only above about 40 kHz. The same study notes that the CIGRE working group recommends either a high-frequency method or a method that measures the current leaking into each of the four tower legs. It also notes that an accuracy of about 20 percent is enough, because weather and soil moisture move the result by about that much. Ask the supplier of any high-frequency kit for its test frequency and calibration basis.

POWERGRID asks for more than a resistance reading on counterpoise towers. The contractor also measures footing impedance with a high-frequency kit, and both values must stay at or below 10 ohms (clause 1.7.3). ITU-T K.107 explains why the two can differ. Earthing is usually measured as resistance because reactance is negligible for small electrodes. The reactive part grows with the size of the earthing and when it is tied to the shield wire, and it must be determined when surge currents are analysed. An instrument maker’s demonstration on a four-legged lattice tower shows the effect. The measured impedance rose from about 3.6 ohms at 41 Hz to over 24 ohms at 5,078 Hz (AEMC tower ground testing bulletin, Winter 2015). A low-frequency reading alone can therefore understate what a lightning surge meets.

How much does soil resistivity change the result?

Footing resistance rises in direct proportion to soil resistivity, so identical earthing that gives 33 ohms in 100 ohm-metre soil gives 131 ohms in 400 ohm-metre soil. IS 3043 gives the resistance of a pipe or rod electrode as R = ρ / (2πL) x ln(4L / d). Here ρ is the soil resistivity in ohm-metres, and L and d are the electrode length and diameter in metres (IS 3043:1987, clause 9.2.2). The standard prints the formula with lengths in centimetres and a factor of 100.

Soil resistivity comes from a Wenner four-electrode survey. OPTCL’s tower specification calls for that method, per IS 3043, with at least eight test directions around the site (OPTCL specification, clause 3.1.2). The table applies the IS 3043 formula to a single 25 mm x 3 m pipe electrode and counts the electrodes needed in parallel to reach 10 ohms.

The table assumes uniform soil, as IS 3043 does. It also spaces each electrode at least its driven depth from the next, the condition under which IS 3043 treats resistance as falling in proportion to the number of electrodes. It ignores the tower foundation’s own contribution. Real soils are layered, so use the table as a first estimate and rely on the measured reading. The 1,500 ohm-metre row marks where the arithmetic stops working, because 50 electrodes per tower is not practical. POWERGRID’s specification moves from 4 x 30 m to 4 x 70 m of counterpoise wire at the same 1,500 ohm-metre figure.

Which remedies bring a tower under 10 ohms, and in what order?

Try parallel pipe electrodes first, then longer electrodes where the ground allows, then counterpoise wire, and use chemical earthing last. The order follows cost and the strength of the evidence for each remedy.

Parallel electrodes. IS 3043 says a number of rods or pipes in parallel give a resistance practically proportional to the reciprocal of the number. This holds so long as each sits outside the resistance area of the others. In practice that means a separation equal to the driven depth, and little is gained beyond twice the depth. The standard also notes that pipe diameter has a minor effect on resistance. A 40 to 60 mm GI pipe electrode with back-fill sits at about 1,500 to 3,500 rupees per 3 m unit in VLI’s earthing rod guide. That is an indicative September 2026 market band, not a quotation. Four electrodes therefore cost 6,000 to 14,000 rupees in material, before pits, clamps and installation.

Longer electrodes. In 100 ohm-metre soil, the formula gives 32.8 ohms for a 3 m electrode and 18.2 ohms for a 6 m electrode, a reduction of 44 percent. Two 3 m electrodes in parallel give 16.4 ohms, lower than the single 6 m electrode. That is why IS 3043 says that, except in special conditions, a number of rods in parallel is preferred to a single long rod. The special condition is soil whose resistivity falls with depth. IS 3043 reports measured cases where resistance dropped sharply once rods reached a clay layer beneath gravel (clause 9.2.2, Figure 12).

Counterpoise wire. A counterpoise is a set of galvanized wires buried radially from the tower legs. POWERGRID’s specification prices it as 120 m of wire per tower below 1,500 ohm-metres and 280 m above (clause 1.7.4). It also allows a shallower layout where hard rock stops trenching to the normal 1 m depth. IS 3043 notes that strip and conductor electrodes have special advantages where high-resistivity soil lies beneath a shallow layer of low-resistivity soil (clause 9.2.3). VLI’s counterpoise earthing page covers the wire.

Chemical earthing. Where normal pipe or counterpoise earthing does not reach 10 ohms, POWERGRID’s specification provides for chemical pipe or counterpoise earthing with a ground enhancement compound. The compound should conform to IEC 62561 Part 7 and must not depend on continuous moisture to stay conductive. Its set-form resistivity must be below 0.12 ohm-metre, backed by test reports for resistivity, leaching, sulfur and corrosion. A 2021 computer simulation covered three 500 kV towers in rocky ground in Peninsular Malaysia, where soil resistivity exceeded 500 ohm-metres. The utility’s enhanced earthing designs cut footing resistance by 54.7 to 66 percent, and adding enhancement compound reduced it by a further 16 to 20 percent (Applied Sciences, 2021). The figures come from a simulation, not field readings.

After each change, repeat the measurement the same way and at similar soil moisture, so the readings can be compared.

What to record for each tower

A reading is only useful later if the conditions behind it are on file. For each tower, record:

• Tower number and type, and the date and time of the test.
• Recent rainfall, so a dry-weather reading can be told from a wet one.
• Soil type and, where measured, the Wenner resistivity.
• The earthing as built: number and size of electrodes, counterpoise length, or chemical earthing.
• The method, instrument and probe distances, including the repeat readings used for the flat slope check.
• The result against 10 ohms, and for counterpoise towers the impedance reading as well.
• Any remedy applied and the reading after it.

Earthing material for a footing resistance fix

VLI supplies the material for each step above: earthing pipe, earth electrodes, counterpoise earthing, copper clad earthing and the wider earthing materials range. The guide to transmission line accessories shows where earthing sits among the other tower fittings. Send the soil resistivity readings and the tower schedule through the contact page, and the team will quote the electrodes or counterpoise wire against them.

Frequently asked questions

What is the tower footing resistance limit in India?

POWERGRID’s specification and the Uttar Pradesh Power Transmission Corporation’s tower earthing specification both set 10 ohms for every tower. The reading is taken in dry weather after erection and before the earth wire is strung. Other utilities set their own figures, so check the tender. Eskom, for example, uses a nominal 20 ohms for 66 to 132 kV lines.

Can you measure footing resistance after the earth wire is strung?

Yes, with a high-frequency tester or a method that measures the current leaking into each tower leg. A normal earth tester cannot, because the earth wire puts the tower in parallel with its neighbours. ITU-T K.107 describes the high-frequency method, and POWERGRID requires a high-frequency impedance reading on counterpoise towers.

Why does the reading change with the season?

Soil moisture changes. IS 3043 notes that moisture normally ranges from about 10 percent in dry seasons to 35 percent in wet seasons. Resistivity rises very abruptly once moisture falls below about 20 percent. New electrodes also settle during the first year or two, which tends to lower the reading, according to ITU-T K.107.

How many earth pipes are needed to reach 10 ohms?

The number depends on soil resistivity. By the IS 3043 formula, a 25 mm x 3 m pipe gives about 33 ohms in 100 ohm-metre soil. Four pipes spaced at least 3 m apart then give about 8 ohms. In 400 ohm-metre soil the same arithmetic calls for 14 pipes, which is why counterpoise wire or chemical earthing is used instead.

What if a tower still reads above 10 ohms after the usual remedies?

POWERGRID’s specification provides for chemical pipe or counterpoise earthing with an enhancement compound at such locations. Eskom’s standard refers the matter to the utility’s engineer and notes that a higher back-flashover rate can be expected on the affected section. Record the final reading and report it to the line owner.

Cover photo: earth conductors clamped to the foot of a lattice tower leg, Badaia mountains, Alava, Spain, by Basotxerri, via Wikimedia Commons, CC BY-SA 4.0, cropped.