Wiring and connections in distribution boards: approved methods according to HD 60364

Connecting, branching, and terminating conductors within an assembly is permitted using four methods: crimping, welding, soldering, and clamps—including screw, bolt, and spring-type connections (in accordance with HD 60364). Simple twisting of conductors is not included in this list; therefore, it is not permitted as a standalone connection method. A flexible stranded conductor must be terminated with a ferrule (such as an H07V-K type) before being inserted into a screw terminal; when placing two conductors into a single circuit breaker terminal, a twin ferrule should be used. The tightening torque must be taken from the specific manufacturer's datasheet: for modular circuit breaker terminals, this is typically 2–2.5 Nm, though specific values are not dictated by general standards.

Copper and aluminium must not be connected directly. In humid environments, a pair of dissimilar metals acts as a galvanic cell, leading to the corrosion of the aluminium. Requirements for such connections are addressed by standard practices regarding transition parts and protective coatings of contact surfaces—in practice, this means using terminals with separate clamps, conductive grease, or Bi-metallic (Al-Cu) ferrules. Conductor identification follows HD 308 S2: N is light blue, PE is green-yellow, and phase conductors are brown, black, or grey.

Why contact connections fail before the cable

In low-voltage assemblies, overheating and fires typically originate from poor contact connections rather than cable damage. Within a construction site distribution board (portable site distribution board to EN 61439-4) or a mechanical switchboard, there are numerous connection points: every circuit breaker, RCD, and terminal block represents a point of contact with a transition contact resistance (R_contact).

The Joule heating law applies here: the thermal power at a connection point is proportional to R_contact. As resistance increases, heating increases; this heating accelerates metal oxidation, and the resulting oxide film further increases the resistance. This creates a self-reinforcing cycle that ends with the melting of the device housing and the formation of an electric arc.

Permitted connection methods

The methods for connecting current-carrying conductors are defined by the HD 60364 series, specifically HD 60364-5-52 regarding the selection and installation of electrical equipment and wiring. The requirements state that the connection, branching, and termination of conductors and cables must be performed using crimping, welding, soldering, or clamping (screw, bolt, spring, etc.) in accordance with the manufacturer's instructions and applicable technical standards.

Simple mechanical twisting is not included in this list. It fails to provide stable contact pressure and weakens due to thermal deformation of the metal: heating during current flow and cooling after disconnection.

Phase conductors in several modular devices are distributed using a busbar rather than makeshift jumpers made from wire scraps: a busbar provides a consistent cross-section across all taps and reduces the number of contact points by half. Spring-type and push-in terminals are also types of clamps (as defined in section 2.1.21) and are permitted alongside screw terminals. The spring maintains the contact pressure automatically, so periodic retightening is not required—unlike with screw connections.

Termination of flexible stranded conductors using ferrule terminals

The internal wiring of portable construction site distribution assemblies is typically assembled using flexible stranded wire, such as H07V-K. Inserting such a conductor directly into a circuit breaker terminal is not permitted: the screw edge crushes and cuts the fine strands, reducing the effective cross-section. Subsequently, the strands "spread" under pressure, causing the contact to weaken.

Therefore, the conductor must be terminated with an insulated pin ferrule, or an H07V-K-2 ferrule if two wires are inserted into a single terminal. The procedure is as follows:

1. Strip the insulation to a length equal to the metal sleeve of the ferrule using a wire stripper. Do not use a utility knife, as there is a high risk of nicking the strands.
2. Slide the ferrule onto the conductor. All strands must enter the metal sleeve, and the wire insulation must sit against the plastic flange.
3. Crimp using a crimping tool with a square, hexagonal, or trapezoidal profile. Standard pliers are unsuitable as they do not provide uniform radial pressure.

Conductor type and termination method	Termination requirement	Installation tool	Node parameter to be verified
Flexible stranded conductor (H07V-K, H05RR-F) for screw terminals	Mandatory crimping with ferrule terminals (e.g. twin-wire ferrules)	Stripper (stripping), crimper (crimping)	No protruding strands, proper sleeve seating depth
Solid conductor (H07V-U, NYM-J) for screw terminals	No termination required; ring shape may be formed for bolt terminals	Round-nose pliers (for forming rings)	Stripped length relative to terminal depth
Connecting two conductors in a single circuit breaker terminal	Twin ferrule (dual-wire) of the appropriate cross-section	Specialised crimper die	Uniform compression of both cores within a single sleeve
Connection to PE and N earth and neutral busbars	Crimping with ring or fork terminals	Crimping tool for insulated terminals	Nut torque tightness, presence of spring washers

Tightening torque for screw terminals and re-tightening

The manufacturer of modular equipment specifies the tightening torque for screw terminals in the product datasheet — MCBs, RCDs, and contactors each have specific values that also depend on the conductor cross-section. For modular circuit breaker terminals, values of 2–2.5 Nm are commonly encountered. There is no single standardised value for this; the definitive source is the device datasheet.

An under-tightened screw results in high contact resistance, causing the terminal to overheat and burn out over time. An over-tightened screw can strip the threads in the housing, bend the pressure plate, or crack the thermoplastic body of the device. Therefore, a calibrated torque screwdriver should be used rather than relying on "feel".

Copper under the screw undergoes slight deformation over time — the mechanism is explained below. Because of this, manufacturers recommend re-tightening all screw connections approximately 24 hours after assembly, before the final commissioning of the installation. There is no regulatory requirement for re-tightening; it is a manufacturer's recommendation and a professional installation practice. Refer to the device datasheet for specific timing and torque values.

Colour and alphanumeric circuit marking

Wire colour coding within the distribution board follows the HD 308 S2 standard for conductor identification:

• Neutral conductor (N) — blue;
• Protective earth conductor (PE) — green-and-yellow combination;
• Line conductors (L1, L2, L3) — preferably brown, black, and grey; red, white, or violet are permitted, but blue, green, and yellow must not be used for line conductors.

At the connection points, wire ends should be fitted with identification rings or sleeves bearing alphanumeric indices according to the board's as-built single-line diagram. These allow the purpose of each line to be identified immediately during fault finding.

What happens at the contact point

Connection reliability is determined at the interface of two metallic surfaces. Even polished copper or aluminium is microscopically rough; therefore, true electrical contact does not occur across the entire surface area of the terminal, but rather at microscopic contact spots—known as Holm's contact spots—where metal asperities are deformed under the pressure of the screw. The remaining space is occupied by air, which acts as an insulator. The constriction of current lines into these spots results in contact resistance, $R_{ct}$.

The contact resistance for a single circular spot with radius $a$ is described by Holm's formula: $R_c = \rho / (2 \times a)$, where $\rho$ is the electrical resistivity of the metal. This is a matter of physics rather than regulation: to reduce $R_c$, the radius of the contact spots must be increased, which requires higher tightening torque.

However, copper and especially aluminium undergo "creep" under constant mechanical stress. Over operational thermal cycles, the conductor "flows" away from under the screw, causing the tightening force to drop. As the contact spot area shrinks, the contact resistance increases. Oxygen enters the gap, and on copper, a layer of $\text{CuO}$ oxide forms—a semiconductor with high resistance. This causes the connection to heat up, eventually melting the insulation and the device housing. This exact mechanism is what torque control and periodic re-tightening are designed to compensate for.

A separate issue is the direct contact of dissimilar metals, such as a copper conductor and an aluminium busbar. In the presence of atmospheric moisture containing dissolved salts and carbon dioxide, a galvanic cell is formed in which the aluminium acts as the anode. This leads to intense electrochemical corrosion, causing the aluminium to disintegrate into a white aluminium oxide powder; in humid environments, this process can occur within just a few months of operation. Requirements for connecting dissimilar metals involve the use of transition components or protective coatings on contact surfaces. In practice, this is achieved using terminals with separate clamps for each conductor, conductive joint compound (such as quartz-vaseline paste), or Bi-metallic Al-Cu lugs.