Terminal connections compared — spring terminal versus screw terminal, and the accessibility requirement for junction boxes

The ferrules for flexible conductors guide and the cable lugs & crimp connections guide cover how a conductor end itself is prepared. This article covers a slightly different question: how two or more conductors are joined together inside a junction box, and what requirements apply to that connection itself.

Spring terminal (lever/cage-clamp, Wago-type)

A spring terminal clamps the conductor with a constant spring force, not a tightening torque applied by the installer. That gives a number of practical properties:

Self-adjusting: the clamping force automatically adapts to the actual conductor cross-section and remains constant even under vibration or temperature cycling — there is no tightening torque that can weaken over time.
Tool-less installation for lever-type designs: the conductor is inserted and the lever closed, without a screwdriver.
Suitable for solid and (with a ferrule) flexible conductors, depending on the specific clamp type.
Reusable: with most types the conductor can be removed without damaging the terminal and reinserted.

Screw terminal (choc block / kroonsteen)

A screw terminal clamps the conductor by a screw that compresses it either directly or via a pressure plate. This principle is simple and widespread, but has a known failure mechanism:

Torque-dependent: too loose immediately gives an excessively high contact resistance; too tight can damage the conductor.
Cold flow of copper: copper gradually deforms ("creeps") under sustained mechanical pressure, so that a correctly torqued screw terminal can still loosen over time — particularly after repeated thermal cycling from heating and cooling under load — without anything visibly changing on the outside.

Note: this loosening through cold flow is exactly the mechanism behind one of the most common findings in thermographic inspection (see the thermographic inspection guide) and in contact resistance testing (see the contact resistance testing with a micro-ohmmeter guide): a connection that was correct at commissioning can, years later, still develop an elevated contact resistance and heat generation.

Comparison

Aspect	Spring terminal	Screw terminal
Clamping force	Constant, spring-driven	Dependent on tightening torque
Prone to cold-flow loosening	No	Yes
Installation	Often tool-less (lever type)	Requires a screwdriver
Re-check needed (torque)	No	Yes, periodically recommended
Reusability	Generally good	Limited (thread/plate wears)

Accessibility requirement for the junction box

Regardless of the chosen terminal type, an additional requirement applies: every connection must remain accessible for inspection, testing and maintenance (§526.3), unless it concerns a specifically certified maintenance-free connection system that is permitted to be permanently enclosed and made inaccessible (for example cast into concrete or behind a solid wall finish).

For an ordinary junction box with spring terminals or screw terminals, this means in practice: the box itself must remain reachable, for example via a removable cover or an inspection hatch — not fully plastered over or hidden behind a fixed ceiling finish without an access point.

Practical relevance

When choosing a terminal type for a specific application, the expected load, the accessibility for future maintenance and the conductor type (solid or flexible) all play a role. During a periodic NEN 3140 inspection, a screw-terminal connection is a logical place to check the tightening torque or, where that is not practical, to carry out a contact resistance measurement or thermographic scan — precisely because this terminal type is prone to gradually loosening without a visible cause.

Common mistakes

Enclosing a junction box making it fully inaccessible (for example cast into concrete) without using a connection system certified for maintenance-free use.
Never re-checking screw terminals for tightening torque during periodic maintenance — cold flow is a gradual process that only becomes visible through heat generation or elevated contact resistance.
Using a spring terminal beyond the specified cross-section/current rating of that particular type.
Inserting a solid conductor into a terminal specifically designed for flexible conductors (with a ferrule), or vice versa, without checking the manufacturer's specification.