Neutral and Earth: Why They Stay Separate (TT and TN-S)

Part 6 of 11 in Cobler's Building Electrical Fundamentals course. New here? See the course page.

Part 5 sized the live conductors for heat and voltage drop. Two more conductors run beside them: neutral and protective earth. What does each one do, where may they meet, and why does an extra link between them cause problems?

In the TT and TN-S distribution circuits described here, neutral and protective earth have separate connections. They do different jobs, and an added link between them can create a dangerous current path. Leave the board cover in place; conductor colours alone cannot establish how a board is wired.

This lesson builds on Earthing and RCDs, which introduced TT and TN-S earthing. Here we look at what each one means for the neutral and protective earth conductors inside a building, where neutral earth bonding belongs, and why an extra link between them causes problems.

This lesson uses examples from Peninsular Malaysia and the requirements of Suruhanjaya Tenaga (ST), the Energy Commission. It compares TT and TN-S earthing. Other arrangements exist, and Sabah and Sarawak have separate regulatory requirements.

Neutral and protective earth do different jobs

Neutral carries normal operating current in circuits that use it, such as a single-phase socket circuit. Some three-phase equipment works without a neutral. Neutral must still be treated as a live conductor and made safe before work.

The protective conductor connects equipment metalwork to the earthing system so that protection can operate during a fault. It is not intended to carry the ordinary load return current, although equipment filters can send leakage current through it during normal operation.

Keeping neutral and protective earth separate in these distribution circuits prevents the protective path from becoming a parallel route for normal load current. Its safety also depends on sound connections and the correct conductor size.

Which arrangement applies

The ST code describes TT as generally used and recommends TN-S for a consumer-owned transformer substation. Section 2.4.3 also specifies TN-S for source-connected earthing above 1 megavolt-ampere (MVA), and TT for separate-electrode earthing up to and including 1 MVA. The designer must check both sections against the installation's supply and load (ST Non-Domestic Electrical Installation Safety Code, sections 2.4.3 and 2.10.1).

In a TT installation, neutral and protective earth stay separate

• Exposed equipment metal connects through protective conductors to the installation's earth electrode system, such as rods or a buried grid.
• The supply source has its own earthing connection, normally at the transformer's neutral point.
• There is no intentional neutral-to-protective-earth link in the consumer's TT wiring, including its main board and downstream circuits.

In TT, an earth fault returns to the source through the protective conductor, the installation's electrode system, the ground and the source's earthing system. The resistance of this loop can make the current too low for a fuse or circuit breaker to disconnect quickly. An RCD normally provides fault disconnection, but the earth path and the RCD must both be checked. A corroded or disconnected earth electrode leaves exposed metal with no reliable path to earth.

In TN-S, protective earth has a separate path to the source

• In the single-transformer arrangement described here, neutral and protective earth meet at the designed source earthing connection. That source is also connected to an earth electrode system.
• Neutral and protective earth remain separate along the distribution circuits. Do not add a link at a main intake or sub-board.

In TN-S, an earth fault has a metallic return path to the source through the protective conductor. A fuse or circuit breaker can disconnect quickly only if enough fault current flows to operate it in the required time. The designer checks the whole fault loop and the protective device. RCD requirements still apply to the relevant circuits.

Have the earthing and protection reviewed when adding a generator, solar inverter, battery system or another supply. Changing the source or its switching can change the fault path.

A note on TN-C-S. TN-C-S combines the neutral and protective-earth functions along part of the supply route. It is not the arrangement to assume for a TNB supply. The ST code discourages it and specifies a voltage leaking indicator where it is used. Do not copy a UK PME or US main-panel connection into a Malaysian board; have the designer confirm the permitted arrangement.

Neutral earth bonding in the wrong place: why an extra link causes problems

• Load current can take an unintended path. An unwanted neutral-earth link can divert current through protective conductors, bonded metalwork or the ground. The path and current depend on the wiring and load.
• Touch voltages can become dangerous. Current in those paths can create voltage differences between metalwork and other surfaces a person can touch.
• An RCD may trip because of the wiring fault. A neutral-earth link downstream of an RCD can divert return current outside its sensor. If the imbalance reaches the operating level, it trips. A link does not always cause a trip, so an RCD staying on does not prove the wiring is correct. Have repeated trips investigated; never bypass the device.

Do not add neutral-earth links to a TT installation or downstream of the source connection in TN-S. Sites with generators or multiple supplies need a design that accounts for each operating arrangement.

How a competent person checks the installation

Ask an ST-registered electrical competent person whose certificate covers the installation and the work to arrange these checks. Different tasks require different authorisations. Occupants should not open boards, take these measurements or alter connections.

Wiring inspections and repairs normally require safe isolation and proof that the circuit is dead. Any measurement that needs the supply on requires a controlled procedure by authorised personnel. Never disconnect a protective conductor to see whether a trip stops.

What to ask for:

• Protective-conductor current. A suitable leakage-current clamp can measure current in the protective conductor. The competent person assesses the reading against the connected equipment and checks whether it comes from normal leakage, a fault or an unwanted connection.
• Residual current in a feeder. Measuring all phase conductors and the neutral, if present, together, with protective earth excluded, shows current returning outside that group. Further tests are needed to find its cause. A low reading alone does not prove the wiring is sound.
• Inspection of connections. With the circuit safely isolated, the competent person checks conductor identification, the drawings and any link between neutral and protective earth.
• Earthing and protection tests. The inspection checks protective-conductor and bonding continuity, the relevant electrode resistance or fault-loop performance, and the operation of fitted protective devices, including RCD trip tests. The required checks depend on the installation.

Records and intervals. Keep the current single-line diagram, protection settings and dated test reports. Regulation 110 of the Electricity Regulations 1994 requires competent-person checks and tests of non-domestic installations at least every five years. It requires checks, tests and calibration of protective relays and devices at least every two years, or as directed by ST. Routine checks and any shorter maintenance intervals still apply, and a defect should never wait for the next scheduled test.

What comes next

With earthing in place, the next protection layer handles a different danger: brief high voltages from lightning and switching, which surge protection sends to earth. The next part, Lightning and Surge Protection, asks: why do storms damage electronics and leave heavy equipment alone, and what does a building use to protect itself?

Check your understanding

1. In a TT installation, is neutral linked to protective earth in the consumer's wiring? No. Equipment metalwork connects to the installation's electrode system. The supply source has its own earthing connection.
2. What problem does an extra neutral-earth link on a sub-board cause? An unwanted link can divert normal load current into protective conductors or other paths and create dangerous touch voltages. It may make an RCD trip because of the wiring fault, but it does not always do so.

Recap: Neutral carries operating current in circuits that use it. Protective earth provides a safety path and may also carry equipment leakage. TT has no neutral-to-protective-earth link in the consumer's wiring. In the TN-S arrangement shown here, they meet at the designed source connection and remain separate downstream. Ask an appropriately authorised competent person to investigate unwanted links or repeated trips.

This is Part 6 of 11 in Cobler's Building Electrical Fundamentals course. Previous: Cable Size and Voltage Drop: Why Distance Matters. Next: Lightning and Surge Protection: Why the Router Dies.

Electrical monitoring does not replace earthing and protective-device tests. Cobler builds CobiNeural, a platform that shows a facility team its building's energy, water and indoor air data as live numbers across the whole site. To see how your building performs, talk to us.