Direct Answer

What cross-section should the equipotential terminal box in a server room or switch room have, and which wires must it take? The answer: the cross-section of the equipotential busbar must be able to carry the bonded fault/lightning current (matched to the cross-section of the connected earthing main), and it must bond the building metal structure, the PE main, the shielding layers and the entering/exiting pipes on the same busbar. The key of selection is not the appearance, but whether the current-carrying capacity of the cross-section and the number of connection points cover all the metal bodies in this area that need bonding.

At a server room, the equipotential terminal box had a too-small cross-section and only took the PE main, not the tray and the shielding: during a strike, the potential difference between the tray and the cabinets was large, and equipment was damaged. After matching the cross-section to the connected earthing main and bonding the tray/shielding/pipes to the same busbar, the problem disappeared. The key distinction: an equipotential box is not an "ordinary junction box" — the cross-section handles the current-carrying capacity (matched to the earthing main), and the connection points handle potential equalization (covering all the metal bodies in this area); both are indispensable.

Why:

  • Cross-section: carries the bonded fault/lightning current, matched to the earthing main;
  • Connection points: cover the metal bodies to bond (structure/PE/shielding/pipes);
  • Contact: the connections are reliable and low-impedance;
  • Location: the main and local equipotential points are set as needed.

Key insight: "Selection looks at two things: is the cross-section enough to carry, and are the connection points complete — treating it as an ordinary junction box means it is installed for nothing."

At a switch room, a newly added cable tray was not connected to the equipotential busbar; during a strike, the tray potential was unequal to the equipment, arcing damaged it, and connecting the tray to the busbar fixed it. Lesson: newly added metal bodies (trays/pipes/equipment enclosures) must be brought into the equipotential system in time — every missing connection point leaves a potential-difference risk.

1. Matching the Busbar Cross-Section to the Earthing Main

Step one: how the cross-section is set.

The role of the cross-section:

  • Carries the bonded fault/lightning current;
  • The current equalizes and shunts through the busbar;
  • A too-small section → heating/melting;
  • It is the first indicator of selection.

The matching basis:

  • Matched to the cross-section of the connected earthing main;
  • Of the same order as the PE main/down-conductor;
  • Reference the standard minimum-section requirements;
  • Stricter for large-current scenarios.

Material and corrosion protection:

  • Mainly copper (good conductivity);
  • Tinned/galvanized for corrosion;
  • Stronger protection in humid environments;
  • The material affects contact and life.

Key insight: "The cross-section is matched to the earthing main to carry the bonded current — too small means heating and melting; look at the carrying capacity first, not the appearance."

2. The Types of Metal Bodies That Must Be Bonded

Step two: what the busbar must take.

Building metal structures:

  • Steel frames/column bars;
  • Slab reinforcement/exterior metal;
  • The structure is a natural equalization net;
  • It must connect to the busbar.

The PE main:

  • The protective earthing main;
  • The PE of each circuit;
  • Bonded at one point on the busbar;
  • Avoid multi-point earthing loops.

Shielding layers:

  • Cable shielding/trays;
  • The lightning shielding system (connecting to LPZ);
  • Shielding metal must be equipotential;
  • Prevents induced overvoltage.

Entering/exiting pipes:

  • Water/gas pipes/trays;
  • Metal pipes entering the area;
  • Bonded at the entrance;
  • Prevents "potential coming in through the pipes".

Key insight: "The structure, PE, shielding and entering/exiting pipes — four types — must meet on the same busbar; missing one leaves a potential-difference channel."

3. The Contact Reliability of the Connections

Step three: is it connected firmly.

Connection methods:

  • Bolted (removable, easy to maintain);
  • Welded (permanent, low resistance);
  • Crimped terminals/lugs;
  • Choose the method by the part.

Contact requirements:

  • Clean, unoxidized contact surfaces;
  • Enough tightening torque;
  • Low contact resistance;
  • Regular continuity checks.

Common problems:

  • Oxidized contacts → rising resistance;
  • Loose bolts → virtual connection;
  • Unremoved paint/oxide layers;
  • Unreliable connections → failed equalization.

Key insight: "The connections must be reliably contacted and low-impedance — oxidation/looseness/virtual connections make the equalization nominal; test the continuity regularly."

4. Where to Set the Main and Local Equipotential Points

Step four: where to install and how many.

Main equipotential bonding (MEB):

  • Set at the building entry;
  • Summarizes all the building metal;
  • At the incoming switchgear;
  • The whole building's potential reference.

Local equipotential bonding (LEB):

  • Server rooms/switch rooms/bathrooms, etc.;
  • Locally bonds the metal in the area;
  • Key for sensitive areas;
  • Connected to the main.

Setting points:

  • Set at the area entrance;
  • Reliably connected to the earthing system;
  • Independent local busbars in local areas;
  • Clear main-local hierarchy.

Key insight: "The main handles the whole-building reference, the local handles sensitive areas — set them by area; do not mix the hierarchy."

5. The Selection Check: Cross-Section, Connection Points, Material and Corrosion

Step five: how to check in practice.

The checklist:

  • ① The cross-section matches the earthing main, with enough carrying capacity;
  • ② The connection points cover all the metal bodies in this area;
  • ③ Good conductivity and adequate corrosion protection;
  • ④ Reliable connections, testable continuity;
  • ⑤ A clear main/local hierarchy.

Common pitfalls:

  • Selecting by the "junction box" mindset;
  • Too small a section and incomplete points;
  • Missing newly added metal bodies;
  • Unprocessed contact surfaces and virtual connections.

Practice verification:

  • Verify the connections by continuity testing;
  • Monitor online with the FR Earth-Resistance Monitor;
  • Recheck the equalization after strikes;
  • Bring new metal bodies into the system in time.

Key insight: "Select with a checklist: enough section, complete points, good material, firm connections, clear hierarchy — only when all five pass is the selection right."

6. Product Implementation: Equalization and Earthing as One

WeiWuLian products manage the equipotential system:

  • FR Earth-Resistance Monitor: online earthing and equalization state;
  • Continuity recheck: continuity tests of the connection points;
  • Strike-event correlation: rechecking the equalization after strikes;
  • Data loop: digitizing the section/points/state.

The design principle in one sentence: "Section by the main, complete connection points, reliable contact, clear hierarchy — manage the equipotential system with data."

7. Common Misjudgments

  1. "Treat it as an ordinary junction box": ignoring the equalization role. Result: small section, incomplete points. Correct: match the main and cover the metal bodies.
  2. "Any section works": ignoring the carrying capacity. Result: heating and melting. Correct: match the main's section.
  3. "Missing the tray is fine": ignoring the potential difference. Result: arcing on a strike. Correct: bring new metal bodies in.
  4. "Connected and done": ignoring contact reliability. Result: virtual connection, failed equalization. Correct: test the continuity.
  5. "Mix the main and local": ignoring the hierarchy. Result: unclear reference. Correct: a clear main/local structure.

Scope and Limitations

This article explains the section matching, connection-point coverage, contact reliability, main/local equipotential settings and a selection checklist for equipotential terminal boxes and earthing busbars, applicable to selecting and understanding equipotential systems. The exact sections and layouts follow GB 50057-2010, GB 50343-2012 and their revisions; this article explains only the selection logic and judgment basis. All cases are anonymized (a server room, a switch room) and do not refer to any specific customer.