§526 — Electrical connections: why a loose terminal is the most common cause of electrical fire

The guide on micro-ohmmeter contact resistance testing and the guide on thermographic inspection both cover how a deteriorating connection is detected. This article covers the source of the problem: the requirements that §526 places on every electrical connection at the moment of installation, so that this deterioration ideally never occurs.

Why this chapter exists

Every electrical connection — a terminal in a distribution board, a cable lug on a busbar system, a lug connection in a switchboard — is a potential weak link. A poorly executed connection ends up with elevated contact resistance; that resistance causes local heating when current flows (I²R loss), and that heating in turn accelerates further deterioration of the connection itself (oxidation, relaxation of spring pressure, expansion and contraction of the material) — a self- reinforcing process that can end in a glowing connection or an arc. §526 therefore sets concrete requirements for how a connection must be executed.

The core requirements of §526

Accessibility for inspection: a connection must in principle remain accessible for inspection, testing and maintenance. §526 allows a limited set of exceptions: connections designed to remain buried, connections filled or encapsulated with resin compound, connections made by welding, brazing or crimping, and connections that are part of equipment complying with its own product standard (for example a factory-made motor terminal connection). Outside those exceptions, a connection may not be buried inaccessibly.
The correct tightening torque: a terminal connection must be tightened according to the torque specification of the manufacturer of the terminal or switchgear — "hand-tight" is not a torque specification. Too loose gives excessive contact resistance; too tight can damage the conductor or deform the terminal, which likewise leads to a deteriorated connection. A calibrated torque wrench or screwdriver is, for critical connections (large cross-sections, busbar systems), the appropriate way to demonstrate this, not an estimated hand force.
Combining conductor materials: combining a copper and an aluminium conductor in the same terminal requires a terminal explicitly approved for that material combination (see the guide on aluminium-copper connections for the underlying galvanic corrosion issue) — a standard copper-on- copper terminal is not automatically suitable for this.
Combining conductor cross-sections and types: a terminal not designed to accept two conductors of different cross-section, or a solid and a fine-stranded conductor together, may not be used for that purpose either — even if both conductors happen to fit "loosely".

Why this leads to fire risk

A deteriorating connection is rarely an acute short circuit; it is typically a creeping process in which contact resistance gradually increases, causing rising local temperature, without the regular overcurrent protection (which reacts to current magnitude, not to local temperature) doing anything about it — the current through the circuit stays within normal limits after all. That makes a loose terminal connection one of the few failure mechanisms that ordinary overcurrent protection does not catch, and why periodic thermographic inspection (see the guide above) and contact-resistance measurement at commissioning form a separate check alongside the regular NEN 1010 testing.

Practical relevance

At commissioning and periodic inspection, a spot check of the tightening torque of critical connections (main busbars, main fuse connection, large motor terminals) belongs among the tasks, alongside a visual check for discoloration or melt marks on plastic terminal blocks — an early sign of a connection that is already starting to heat up.

Common mistakes

Tightening a terminal "by feel" instead of to the manufacturer's specification — both too loose and too tight increase the chance of a deteriorating connection.
Combining copper and aluminium conductors in a standard terminal not explicitly approved for that material combination — this accelerates galvanic corrosion at the interface.
Burying a connection inaccessibly without need (for example cast into concrete or behind a fixed panel) when it does not fall under one of the explicit §526 exceptions.
Assuming that regular overcurrent protection catches a deteriorating terminal connection — the current through the circuit stays within normal limits, so a circuit breaker or fuse does not react to this.