Protektibong Multiple Earthing (PME) – TN-C-S – (MEN) ug PNB
Ano ang Protective Multiple Earth (PME)?
Ang Protective Multiple Earth (PME) usa ka paraan sa pag-secure nga grounding diin ang earth continuity conductor (ground wire) sa consumer's premises gitugyan sa tanang lokal nga earthing system ug ang neutral conductor sa power supply. Ania usab isip TN-C-S o Multiple Earther Neutral (MEN), kini nga sistema nag-siguro nga kon ang neutral wire mubungkag, ang fault currents mahimo pang mobalik sa seguro sa source pinaagi sa earth connection, minimizahan ang risk sa electric shock ug uban pang hazards.
Sa PME earthing system (illustrated below), ang supply neutral nagserbisyo og duha ka papel: itumong protective earthing ug gihatag ang neutral conductor. Sa wala pa, ang neutral conductor giearth sa daghang puntos sa supply side. Ang sumusunod nga bahin sa article niini nagdiskubre sa mga implicasyon sa usa ka open-circuit PEN conductor (usa ka break sa source neutral), kasama ang mga protective measures ug potensyal nga risks naa sa PME.
Ano ang TN-C-S PME?
Ang TN-C-S PME (Protective Multiple Earthing) usa ka partikular nga configuration sa electrical distribution system diin ang external supply source direkta giearth sa daghang puntos ("T" = Terre, French for "earth" or "ground"). Sa consumer's installation side, ang conductive parts sa equipment gitugyan pinaagi sa circuit protective cables (CPC) sa tanang supply neutral (N) ug sa grounding system.
Ang "C-S" designation nagpakita nga ang neutral (N) ug protective earth (PE) conductors kombinado (C) sa supply source's network ug separado (S) sa consumer's installation.
Key Components of TN-C-S PME
T: Terre ("earth/ground") — Ang sistema adunay direktang independent ground connection separate from the supply conductors.
N: Neutral — Ang return conductor para sa current sa electrical circuit.
C: Combined — Sa upstream supply network (e.g., from the transformer to the consumer’s main panel), ang neutral (N) ug protective earth (PE) conductors merged into a single conductor called the PEN (Protective
Earth Neutral) conductor.
S: Separate — Sa consumer’s main panel or distribution point, ang PEN conductor splits into two independent conductors:
Neutral (N): Carries the return current.
Protective Earth (PE): Connects to equipment frames and ensures safety during faults.
Paano ang TN-C-S PME Nagtrabaho
Upstream (Supply Side):
Ang neutral ug protective earth combined as a PEN conductor, grounded at the source (e.g., transformer) and possibly at intermediate points (multiple earthing).
Downstream (Consumer Side):
Sa consumer’s main panel, ang PEN conductor split into a separate neutral (N) and protective earth (PE).
Ang PE conductor connects to all exposed conductive parts of equipment (e.g., metal casings) to safely divert fault currents to ground.
Ang neutral (N) remains isolated from ground within the consumer’s installation (except for a single bonding point at the main panel to maintain potential stability).
Safety Benefits
Fault Protection: Kon may phase-to-metal fault, ang current flows through the PE conductor to ground, tripping the circuit breaker or fuse quickly.
Neutral Breakage Safety: Kon ang neutral conductor breaks upstream, ang PEN/PE connection ensures that exposed metal parts remain at ground potential, reducing the risk of electric shock.
Flexibility: Combines the simplicity of a combined neutral-earth system (TN-C) in the supply network with the safety of a separated system (TN-S) in the consumer’s premises, making it suitable for both urban grids and building installations.
Kini nga configuration balansehan ang cost efficiency sa supply network ug enhanced safety sa end-user environments, widely used in residential, commercial, and industrial settings.
Ano ang PNB?
PNB, short for Protective Neutral Bonding, usa ka grounding method similar to the PME (Protective Multiple Earthing) system, pero may key difference: ang Neutral-to-Earth (TN) connection established on the consumer side (e.g., at the premises' main panel) rather than at the power supply or distribution transformer.
Sa TN-C-S system, PNB (Protective Neutral Bonding) refers to the configuration where the PEN (Protective Earth Neutral) or CNE (Combined Neutral Earth) conductors of individual consumers are connected to the power source (e.g., transformer) at only one point. Kini nga single bonding point ensures that the neutral and protective earth functions are combined upstream (from the transformer to the consumer’s main panel) and separated within the consumer’s installation (TN-C-S structure).
Key Considerations for PNB
Earth Distance Requirement:The recommended distance between the grounding electrode and the consumer’s main panel (where the neutral-earth bond occurs) is less than 40 meters (≈130 ft.). To minimize voltage risks in the event of a neutral breakage, this distance should ideally be as short as possible, preferably adjacent to the main panel’s earth link bar.
Safety Mechanism:By bonding the neutral to earth at the consumer’s premises, PNB helps stabilize the neutral’s potential and provides a backup path for fault currents if the upstream neutral conductor fails. This reduces the risk of exposed metal parts becoming live and causing electric shocks.
Difference from PME
While both PNB and PME involve neutral-earth bonding, PME typically involves multiple grounding points on the supply side (e.g., at the transformer and along the distribution network), whereas PNB focuses on a single bonding point at the consumer’s location within a TN-C-S framework.
PNB is designed to balance safety and simplicity in smaller-scale installations, ensuring compliance with electrical codes while minimizing the impact of neutral conductor faults in end-user environments.
Asa ug Asa ang PME Earthing System Gamiton?
Under the ESQCR (Electricity Safety, Quality and Continuity Regulations), consumers are prohibited from installing PEN conductors in HV/LV installations; this responsibility lies with the independent distribution network operator (DNO). This is because PME systems involve complex grounding configurations that require professional management to ensure safety and compliance.
Key Benefits of PME
The primary advantage of PME is its ability to mitigate risks during a broken neutral wire (open-circuit PEN conductor). If the neutral fails, the fault current can return to the supply source via the parallel earth path (created by multiple earthing points). This low-resistance pathway triggers protective devices (e.g., fuses, circuit breakers) to trip, as the high current due to low resistance melts the fuse or activates the breaker. Consequently, exposed metal parts remain at near-ground potential, eliminating the risk of electric shock from a broken neutral. Without PME, a neutral break would leave no return path, energizing the neutral wire and posing a severe shock hazard.
Applications of PME
Power supply companies and distributors often employ PME in rural or challenging terrains (e.g., mountainous areas) where:
Individual low-resistance earthing for each building is costly or impractical.
Obtaining suitable earth loop resistance from the transformer to consumer terminals is difficult.
However, using PME requires written approval from relevant authorities due to its technical requirements and potential risks.
Conductor and Bonding Sizing for PME/PNB
For PME earthing, conductor sizing must adhere to BS 7671:2018+A2:2022 regulations:
Potential Risks of PME Earthing
While PME enhances safety, it introduces specific hazards:
Raised Neutral Potential
If the neutral conductor breaks (common in rural overhead lines), all protective metalwork (e.g., equipment casings) bonded to the neutral can become energized. For example:
Silent Faults
Unlike obvious faults, a broken neutral with PME may not trigger immediate protective action. The system can remain energized until someone touches the metalwork, leading to unexpected shocks.
Mitigation Requirements:
Multiple Earthing: Neutral must be earthed at multiple points in the system.
Low Earth Resistance: Each earth electrode’s resistance must not exceed 10 ohms.
Individual Earth Rods: Recommended for each installation to minimize shared fault currents.
Authority Approval: Formal approval is mandatory to ensure proper design and maintenance.
Conclusion
PME is a critical but regulated earthing method, ideal for areas with challenging earthing conditions. Its effectiveness relies on strict compliance with bonding, sizing, and maintenance standards to avoid risks like raised neutral potentials. Always consult qualified engineers and obtain regulatory approval when implementing PME systems.
