TN-S vs TT Earthing: A Real Cost and Safety Comparison for Remote Industrial Sites

The choice between TN-S and TT earthing is one of those decisions that looks simple on paper but has significant cost and safety implications when the site is 20 km from the nearest utility substation, the soil is volcanic rock with resistivity above 1,000 ohm-metres, and the nearest qualified electrician is a 4-hour drive away.

At Batu Hijau, we operated a TN-S system across a mine site spanning several kilometres. The decision was made early in the project and was the right one — but not for the reasons most textbooks cite. The real driver was protection reliability in adverse soil conditions and the certainty of fault clearance that TN-S provides when you cannot guarantee a low earth electrode resistance year-round.

The Fundamental Difference

Both TN-S and TT systems connect exposed conductive parts to earth. The difference is the fault return path.

TN-S: The earth fault current returns to the supply transformer neutral via a dedicated protective earth (PE) conductor that runs alongside the phase conductors. The fault loop is entirely metallic — copper or aluminium all the way back.

TT: The earth fault current returns to the supply transformer neutral via the general mass of earth. The fault loop includes the local earth electrode at the installation AND the supply authority's earth electrode at the transformer.

IEC 60364-1, Clause 312.2 — TN, TT, and IT system definitions

This distinction drives everything else: protection method, touch voltage magnitude, installation cost, and ongoing maintenance burden.

Protection Reliability

TN-S: Deterministic Fault Loop

In a TN-S system, the earth fault loop impedance is calculable and stable:

Zs = Z_source + Z_line + Z_PE

Every element in this loop is a conductor with a known resistance per metre. You can calculate the prospective earth fault current precisely:

If = Uo / Zs

where Uo is the line-to-neutral voltage (230V or 240V for LV systems).

IEC 60364-4-41, Clause 411.4 — Automatic disconnection in TN systems

For a typical TN-S circuit — 32A MCB (Type B), 30 m of 4 mm2 cable with 4 mm2 PE — the earth fault loop impedance is approximately 0.8 ohms, giving a prospective fault current of about 288A. This is well above the 5x In (160A) required for instantaneous magnetic tripping of a Type B MCB within 0.4 seconds.

The key advantage: this loop impedance does not change with seasons, weather, soil moisture, or time. It was 0.8 ohms when installed, and it will be 0.8 ohms in ten years.

TT: Variable Earth Loop

In a TT system, the earth fault loop includes two earth electrodes in series with the mass of earth between them:

Zs = Z_source + Z_line + R_A + R_B

where R_A is the local earth electrode resistance and R_B is the supply transformer earth electrode resistance.

Earth electrode resistance varies with:

• Soil moisture content (seasonal — can double between wet and dry seasons)
• Temperature (frozen soil has very high resistivity)
• Soil type (from 10 ohm-m for saturated clay to >10,000 ohm-m for granite)
• Electrode corrosion over time

For a TT system with R_A = 20 ohms and R_B = 5 ohms, the earth fault loop impedance might be 25.8 ohms (including conductor resistance), giving a prospective fault current of only 8.9A. This is far too low to trip an MCB magnetically.

This is why TT systems depend on RCDs (residual current devices) for earth fault protection. An RCD with a rated residual current of 30 mA will operate at fault currents well below 1A, regardless of the earth electrode resistance. But this introduces a dependency: if the RCD fails (mechanical failure, nuisance tripping disabled by bridging, or simply not tested), the TT system has no backup earth fault protection.

IEC 60364-4-41, Clause 411.5 — Automatic disconnection in TT systems

Touch Voltage Analysis

The ultimate safety metric is touch voltage — the voltage a person is exposed to when they contact a faulted piece of equipment.

TN-S Touch Voltage

During an earth fault in a TN-S system:

U_touch = If x R_PE (local)

where R_PE (local) is the resistance of the PE conductor between the fault point and the main earthing terminal. For a 30 m circuit with 4 mm2 PE conductor (R = 0.138 ohm), and If = 288A:

U_touch = 288 x 0.138 = 39.7V

This touch voltage exists only for the duration of fault clearance — typically less than 0.1 seconds for magnetic MCB tripping. The energy exposure is very low.

TT Touch Voltage

During an earth fault in a TT system:

U_touch = If x R_A

where R_A is the local earth electrode resistance. For If = 8.9A and R_A = 20 ohms:

U_touch = 8.9 x 20 = 178V

This is a lethal touch voltage. However, the RCD should clear the fault within 40 ms at this current level (well above the 30 mA threshold), limiting the duration of exposure. The combination of high voltage but very short duration is within the safety limits defined by the touch voltage/time curves in IEC 60364-4-41, Figure 41.1.

Installation Cost Comparison

For a typical remote industrial site with a 2 km distribution network:

The TN-S system has a higher upfront copper cost but dramatically lower ongoing maintenance. Over 10 years, the TT system costs approximately 3 times more due to the annual testing and RCD replacement burden.

IEC 60364-6, Clause 6.5 — Periodic inspection and testing

The Hidden Costs of TT

The cost table above does not capture the most expensive risk: nuisance tripping. In remote industrial installations, RCDs trip due to:

• Capacitive leakage current from long cable runs and VFDs
• Transient earth leakage from motor starting
• Moisture ingress at junction boxes in tropical/humid environments
• Cumulative leakage from multiple circuits on a single RCD

Each nuisance trip stops production. On a mining site, an hour of lost production can cost more than the entire earthing system installation.

Soil Resistivity: The Deciding Factor

For sites with high soil resistivity (>500 ohm-m), achieving a low earth electrode resistance becomes expensive and unreliable:

At Batu Hijau, the andesitic volcanic rock produced soil resistivity measurements ranging from 800 to 3,000 ohm-m across different parts of the site. Achieving consistent earth electrode resistance below 10 ohms required extensive electrode arrays with chemical soil treatment — an ongoing maintenance commitment and cost that made TT earthing impractical for permanent installations.

When to Choose Each System

Choose TN-S When:

• The site has a dedicated supply transformer (most industrial sites)
• Distribution distances are moderate (up to 3-5 km)
• Soil resistivity is high (>200 ohm-m) or variable
• Continuous production is critical (cannot tolerate nuisance RCD tripping)
• Skilled electrical maintenance staff are not continuously on site
• The installation includes VFDs, motors, or equipment with inherent earth leakage

Choose TT When:

• Supplied from a public utility network where TN-S is not available
• The installation is small and localised (single building)
• Soil resistivity is low and stable (under 100 ohm-m)
• RCD maintenance is feasible and enforced
• Cost of PE conductor for the full distribution is prohibitive
• Local regulations mandate TT (some countries require TT for public networks)

The Decision Framework

The earthing system choice is driven by three questions:

1. Can you guarantee a low, stable earth electrode resistance year-round? If no, TN-S is safer.
2. Can you maintain and test RCDs on every circuit reliably? If no, TN-S is simpler.
3. Can you afford the PE conductor cost for the full distribution network? If yes, TN-S is better value over the installation lifetime.

For most remote industrial sites, the answer to all three questions points to TN-S. The copper cost is real but finite. The ongoing cost and risk of maintaining a TT system in challenging soil conditions is open-ended.