System Topology Explanation

Our HSP-A40 Type 2 AC SPD series is compatible with TN‑S, TN-C-S and TT systems. IT systems require specially evaluated SPD solutions. TN-C system is shown in the wiring diagram for reference only. It is generally not used for new building power installations nowadays, mostly only seen in older legacy installations.

（Note: For TT systems, the grid neutral is earthed at the transformer, while equipment uses independent local earth electrodes.）

1. TN-S System (Three-Phase Five-Wire)

Characteristics: 3P+N+PE. Neutral and PE split after transformer earthing point.

Advantages: Stable earthing; no neutral drift shock risk.

Installation: Parallel phase-to-PE & inter-phase wiring.

Application: Hospitals, data centres, precision workshops.

2. TN-C-S System (PEN Combined Earthing)

Characteristics: Shared PEN upstream; N and PE separate after the split point.

Advantages: Lower wiring cost, combines TN-C & TN-S benefits.

Installation: Parallel phase wiring; split PEN at building incoming cabinet.

Application: Factories & buildings with partial power safety upgrades.

3. TT System (Independent Local Earthing)

Characteristics: Grid neutral earthed separately; equipment has independent local earth.

Advantages: Prevents fault potential transfer to equipment enclosures.

Installation: Phase & neutral connect to grid; dedicated separate earth for equipment.

Application: Rural / outdoor supplies with no public shared PE line.

4. IT System (Isolated Neutral)

Characteristics: Source neutral ungrounded or high-impedance earthed.

Advantages: Continues running; single earth fault will not cut power.

Installation: Fit IMD insulation monitor between phase and earth.

Application: Hospital ORs, mines, chemical plants (zero-interruption critical loads).