Busbar hole patterns and what DIN 43673-1 actually specifies

DIN 43673 appears constantly on customer drawings, usually as a bare reference without attached dimensions. Search results for the standard primarily concern its purchase. The document itself is a four-page German-language standard from February 1982.

What the standard covers

DIN 43673-1

, Stromschienen-Bohrungen und -Verschraubungen; Stromschienen mit Rechteck-Querschnitt, published in English translation as "Drilled holes and screw connections for busbars; rectangular cross-section busbars", consists of four pages. It specifies the location and size of the drilled holes and provides base data for the design and construction of the screw connections. It applies to direct current and to alternating current up to 60 Hz.

The scope is tied to two other DIN documents: it applies to the rectangular bars whose continuous current ratings are given in DIN 43670 (aluminium, 1975-12, ten pages) and DIN 43671 (copper, 1975-12, nine pages). The family splits cleanly. DIN 43671 defines the current a given bar section will carry; DIN 43673-1 defines where to put the holes in it. Both are still listed as current by DIN Media, which is unusual for documents of that vintage.

The scope wording specifies that the standard addresses the connection, not only the drilling, and the "base data for the design of the screw connections" is a commonly overlooked component. The 60 Hz ceiling is a defined limit: above it, skin and proximity effects in the joint zone redistribute current around the holes sufficiently that the standard pattern ceases to be the correct one.

DIN 46433 is not a busbar hole standard

This distinction is necessary because the miscitation is pervasive, including in tender documents and in supplier drawings that are copied forward for years.

DIN 46433

is titled Flachdrähte und Flachstangen, gezogen, mit gerundeten Kanten — Maße: "Rectangular wires and rectangular bars, drawn, with radiused edges; dimensions". It is eight pages and is a dimensional standard for drawn rectangular wire and bar: width, thickness, edge radius, tolerances. It contains no information about hole positions, hole diameters or bolted connections. It has been withdrawn and is superseded by DIN EN 13601.

A drawing stating "holes to DIN 46433" is incorrect. The intended citations are DIN 43673-1 for the hole pattern and DIN 43671 for the current rating. If the material specification was intended, that document is EN 13601, the same document that replaced DIN 46433. Verification is required before metal is cut, because a drawing that miscites one standard often miscites the temper as well.

Where the hole pattern meets assembly verification

Hole patterns appear as a fabrication detail. The joint they produce, however, falls within the scope of the assembly standard against which the finished panel is verified.

IEC 61439-1 is explicit. In the temperature-rise test, clause 10.10.2.3.3 requires that "particular attention shall be given to joints in conductors and terminals within the main circuits". Joints are designated measurement points. Clause 10.10.2.3.7 a) requires that where main busbars are tested separately, the tested length is at least 2 m and includes one joint where the bars are extendable, and the short-circuit clause 10.11.5.3.3 fixes that length at (2 ± 0,4) m under the same condition.

The acceptance criterion follows. After the short-circuit test, clause 10.11.5.5 states that "there shall be no loosening of parts used for the connection of conductors". A joint that survives thermally but backs off under electromagnetic force fails the assembly; edge distance and bolt sizing determine that outcome.

Edge distance

The standard fixes hole centres relative to the bar edge. Two independent factors are balanced.

Mechanically, the ligament between hole and bar edge carries bearing load from the bolt during a fault. Peak withstand current drives adjacent bars apart or together at forces that are reacted through the bolt group. Insufficient edge material causes the ligament to yield, the hole to elongate, preload to drop, and the joint to loosen. Outright tear-out is rare in copper of sensible thickness; the primary failure mode is progressive elongation followed by loss of contact pressure, which is more problematic because it remains undetected until the thermal survey.

Electrically, the ligament is a constriction. Current must divert around the hole, so the material between hole and edge carries a higher current density than the bulk of the bar. Placing the hole too close to the edge creates a local hot spot in the most highly stressed part of the joint.

Manufacturing introduces a third constraint. When punching rather than drilling, the die requires surrounding material to react against. Punching with insufficient edge distance in hard-temper copper causes the edge to bulge outward, the ligament to work-harden, and the bar to no longer sit flat against its mate. As shop practice rather than a normative figure, the clear distance from hole edge to bar edge is kept at least equal to the bar thickness, and 1.5 times it for hard temper. The punching process page covers the tooling aspect of this constraint.

Pitch, and current crowding around the holes

The Copper Development Association's Copper for Busbars (Publication 22) provides the clearest published treatment of what holes do to a joint. Joint resistance comprises two parts: the streamline or spreading resistance caused by current diverting through the overlap, and the contact resistance at the interface itself. Bolt holes increase the former, and the CDA estimate depends on the hole diameter and on n, the number of holes measured across the width of the bar.

This single variable resolves a common fabrication argument. Holes should be placed in line along the length of the joint. Staggering them across the width increases resistance because each additional hole across the width is another obstruction in the same current path. Two holes in line along a 60 mm bar produce a better joint than two holes offset diagonally, even though the second arrangement appears to spread the clamping load more evenly.

The overlap length is governed by the same effect. The streamline penalty decreases steeply as the overlap-to-thickness ratio rises to about 2, then improves only slowly out to a ratio of 10. Beyond that, no further gain exists. The CDA conclusion is that the overlap only needs to be long enough to accommodate enough bolts to reach the required contact pressure. Long overlaps waste copper and add weight without improving conductivity.

Two refinements from the same source are simple to implement and rarely applied. Cutting a longitudinal slot through both bars in the overlap reduces contact resistance by 30 to 40 % by evening out the contact pressure in each leg of the joint. Angling the bar ends at less than 45° reduces initial joint resistance by about 15 % and slows the rate at which joint resistance climbs under load cycling by a factor of 1.3 to 1.5.

Hole diameter

Hole diameter involves a trade-off between bolt clearance, tolerance stack-up, and the contact area removed.

Clearance is greater than commonly assumed. CDA's long-standing guidance pairs an M10 bolt with an 11.5 mm hole and an M12 with 14 mm, increasing to 15 mm on wider bars. This is 1.5 to 3 mm of slop, and it is intentional: it absorbs accumulated pitch error across a five- or six-bolt pattern in two bars that were produced on different days, possibly on different machines. Reducing the clearance creates joints that cannot be assembled.

The cost is contact area. Each hole removes copper from the overlap, specifically from the region where clamping pressure is highest. A 14 mm hole in a 100 mm bar removes 14 % of the width from the section at that point. This is why the solution for a marginal joint is more bolts of moderate size rather than fewer large ones. Bolt count provides pressure uniformity; bolt diameter results in section loss.

Hole diameter also interacts with the tooling. Punching a hole smaller than the material thickness is difficult on tooling in copper and degrades hole quality. This is rarely an issue at M10 and above in 5 to 12 mm bar, but it occurs on thin control bar with small fixing holes. The punching force calculator indicates the tonnage required for a given diameter before committing the pattern.

Bolt size, torque and the washer stack

CDA's Table 22 has been the standard reference for decades. The pattern is approximately: 25 to 30 mm bar takes two M8 at 17 N·m; 40 mm takes two M10 at 28 N·m; 50 to 80 mm uses M12 at 45 N·m, with two bolts at 50 mm and four at 80 mm; 100 and 120 mm bar takes five M12; 160 and 200 mm bar takes six to eight M16 at 91 N·m. Sizes in service range from M6 to M20, with four or six bolts being common.

Torque is a proxy measurement, and an imprecise one. Contact pressure cannot be measured in the field, so it is inferred through T = K·F·D, where K is the nut factor. CDA gives K as 0.20 to 0.22 dry, 0.19 to 0.21 with a contact-aid compound, and 0.15 to 0.16 with a molybdenum-disulphide boundary lubricant. This means: the same wrench setting on a lubricated thread produces roughly 30 % more preload than on a dry one. A torque figure from a table and an assembly instruction to grease the threads are incompatible.

The objective is contact pressure across the overlap. CDA's figures are: below about 7 N/mm² is not advisable, above 10 N/mm² is preferred, and above roughly 30 N/mm² there is little further gain. Only around 1 % of the apparent overlap area conducts, the load passing through discrete asperity contacts, which is why surface condition is as critical as bolt count and why bars should be flat and freshly abraded at assembly.

The washer stack performs a specific function. Large, thick washers spread the bolt load away from the hole and widen the region of useful pressure; CDA pairs M10 with a 24 mm washer 2.2 mm thick and M12 with a 28 mm washer 2.7 mm thick. When high-tensile steel bolts are used, differential expansion becomes the problem. Steel expands at about 11.1 × 10⁻⁶/°C against copper's 16.5 × 10⁻⁶/°C, so bolt tension climbs as the joint heats and can be driven past the 95 % of proof stress CDA sets as the ceiling. Disc-spring (Belleville) washers absorb that increase; aluminium bronze CW307G bolts at 16.2 × 10⁻⁶/°C avoid it by nearly matching copper.

A counter-intuitive point exists: CDA does not recommend plating copper-to-copper joint faces unless the environment demands it, because soft plating can flow at elevated temperature and relax the contact pressure established during assembly.

When the drawing says "to DIN 43673" and nothing else

This is the common case, and it is a commercial problem before a technical one.

The bar the pattern belongs to must be identified. The standard indexes hole location and size to the rectangular bar sections rated in DIN 43670 and DIN 43671. If the specified width and thickness is not one of those sections, the reference does not resolve and the customer must provide dimensions.

The intent must be clarified: pattern or joint? Many drawings citing DIN 43673-1 require only the standard bolt pattern for interchangeability with existing switchgear. Some require the screw-connection data as well, which brings bolt grade, washer arrangement and torque into the scope of supply. These are different quotations.

A single physical reference is essential. A sample bar, a marked-up sketch, or a photograph of the mating equipment settles more ambiguity in five minutes than a week of correspondence. The interface holes on the customer's own switchgear are the real specification, regardless of the drawing.

The interpretation returned on the drawing should be stated and signed off. The hole diameter, pitch, and edge distance intended for production must be declared. If the customer holds the standard and the supplier does not, verification against the source is a one-minute check.

The document should be purchased if the pattern recurs. The cost of the four pages is less than a scrapped batch of 200 mm bar. Ordering should be from DIN Media or the relevant national standards body rather than a reseller aggregator.

Getting the pattern onto the machine

Once resolved, the pattern becomes a tooling question. A recurring DIN pattern across several bar widths requires several hole diameters, and the constraint is the number of live tools in the turret. A punch and shear line such as the EMAC-BP-60 handles a fixed tool set well. Where the mix is wide, a processing centre like the IMAC-CENTER 60 with 24 tool stations, extendable to 36, eliminates the changeover issue and holds ±0.05 mm positioning, which maintains pitch stack-up within the allowed clearance.

The rating aspect of the same job is defined by DIN 43671, whose tables are referenced at 35 °C ambient and 65 °C bar temperature with correction factors for other conditions. A joint that is thermally marginal due to bar rating without those corrections will not be salvaged by a good hole pattern. The ampacity calculator applies these corrections rather than omitting them, and the bolted joints article provides further detail on surface preparation than can be accommodated here.