Tightening torque of electrical connections — torque wrenches, creep and the Belleville washer

The guide on §526 connections lists the correct tightening torque as one of the basic requirements for a terminal connection, and the guide on aluminium-to-copper connections briefly notes that aluminium "creeps" under clamping pressure. This article goes deeper into both points: the mechanics behind correct tightening torque, why both under- and over-tightening lead to the same problem, and the role of the Belleville spring washer in maintaining clamping force over the long term.

Why "hand-tight" is not a tightening torque

A bolted or clamped connection in an electrical circuit must deliver a specific clamping force, specified by the manufacturer of the terminal or switchgear — expressed as a tightening torque in Nm (or lb-in/lb-ft in American documentation, for example tables per UL 486A-486B). That clamping force determines the contact area and thereby the contact resistance of the connection. Tightening "by hand feel" does not produce a reproducible clamping force, let alone the correct one, and is not a substitute for an actual value specified in Nm.

Two sides of the same failure mode

Under-tightening: insufficient clamping force leaves too small an effective contact area between conductor and terminal, giving a higher contact resistance. Under operating current this leads to local heat generation (P = I²R) at exactly that contact point, which can further degrade the connection — often gradually, over months.
Over-tightening: excessive torque can deform the conductor itself (particularly a fine-strand or multi-wire conductor), plastically deform the terminal so that clamping force actually decreases once the torque is released, or, on a shear-bolt terminal, cause the head to snap off prematurely before the intended clamping force is reached.

Both extremes ultimately result in the same measurable failure mode — increased contact resistance — which is why periodic contact resistance measurement is a worthwhile check, regardless of whether under- or over-tightening was the original cause.

Why aluminium needs an additional measure: creep (cold flow)

Aluminium exhibits creep (cold flow) under sustained mechanical clamping pressure more strongly than copper: the metal gradually deforms, even at room temperature and well below its yield strength, so that clamping force decreases over months to years — even without anything else changing on the connection itself. On an ordinary, rigid bolt-and-nut connection on aluminium, this leads to a progressively looser connection, with the same contact-resistance/heat-generation spiral described above for under-tightening, but arising well after the original, correctly executed installation.

The Belleville washer as a solution

A Belleville washer (a conical, dish-shaped spring washer) placed between the bolt/nut and the terminal compensates for this creep behaviour: as the aluminium under the terminal gradually deforms over time, the washer — which itself has not yet been fully "flattened" — partially maintains the clamping force, instead of the clamping force dropping off directly and completely as it would with a rigid flat washer. Manufacturers of terminals and connection hardware for aluminium typically specify the point to which the washer must be compressed (for example "until the crown of the washer is no longer discernible") rather than only a Nm value — because it is precisely that degree of compression that determines the lasting clamping force.

Note: a Belleville washer is a supplement to, not a substitute for, the correct terminal assessment for aluminium (suitable terminal geometry, possibly contact paste — see the guide on aluminium-to-copper connections). Not every terminal is designed for use with a spring washer; follow the manufacturer's specification.

Torque wrenches: type and calibration

For critical connections (main busbars, high-current service connections, motor terminals) a torque wrench — not an ordinary open-end or ring spanner — is required to actually achieve the specified tightening torque:

Click-type torque wrenches give an audible/tactile click once the set torque is reached; they require periodic calibration (typically annually, or more often with intensive use) because the internal spring can weaken over time, which can result in an actually lower tightening torque without this being visible.
Torque screwdrivers are used for smaller terminals (circuit breakers, smaller switchgear terminals) where a full-size torque wrench does not fit.

An uncalibrated torque wrench gives a false sense of security: the user believes the correct torque has been applied, while the actual clamping force can deviate without this being apparent in any way until a later contact resistance measurement or thermographic inspection reveals it.

Practical relevance

When installing or periodically checking critical electrical connections (main busbars, high-current service connections, motor and generator terminals), the applied tightening torque should be documented (for example with a torque wrench with logging capability, or noted manually), so that if a later contact resistance measurement shows a deviation, it can be verified whether the original torque was correctly applied, rather than having to guess.

Common mistakes

Tightening "by hand feel" or using an ordinary spanner instead of achieving the manufacturer-specified tightening torque with a calibrated torque wrench.
Continuing to use an uncalibrated torque wrench for years without periodic calibration checks — the internal spring can weaken without any visible signal.
Rigidly tightening an aluminium connection without a Belleville washer or comparable compensation where the manufacturer specifies it — the connection then loosens over time due to creep, without this being visible at the time of installation.
Over-tightening "to be safe" on the assumption that more clamping force is always better — this can damage the conductor or the terminal itself and actually reduce the clamping force.