PHOENIX CONTACT | Topic of the Month

	PEN - Splitting with PEN termination blocks				PE/N bridgeable input termination blocks
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When we are dealing with the installation of PEN conductors according to standards, many questions arise. There are uncertainties in particular where the division into grounding and neutral conductors is concerned. In this case, prefabricated termination blocks can help. In TN-C systems,the PEN conductor has a double function. It functions primarily as the grounding conductor, and secondly as the neutral conductor.

Fig. 1. Division into neutral conductor and grounding conductor

Rules
If the PEN conductor is permanently installed, its cross section must be at least 10 mm² CU or 16 mm² AL depending on the phase conductor and in accordance with VDE0100. Apart from a few special cases in the applications for power supply companies, the conductor may not be installed flexibly. The entire length of the PEN conductor must be green/yellow like the grounding conductor, and in addition it must be marked in blue at the connection point. All terminal points of the PEN conductor must be made with great care in order to prevent undesirable interruptions. Furthermore, the conductor must not be fused.

Division into neutral conductor and grounding conductor
The point at which the PEN conductor is split up into the neutral conductor and grounding conductor is of particular importance. According to the standard, the incoming PEN conductor must be led onto the grounding conductor rail, which is then connected with the neutral conductor busbar (fig. 1). In order to meet all these requirements, PEN termination blocks were especially developed for this purpose. Apart from having color coding, they guarantee permanently good contacting by means of protected screw terminal block connections.

Design of the PEN termination blocks
Strictly speaking, these blocks consist of a green/yellow grounding conductor terminal block with a metal PE foot and a blue modular terminal block, isolated toward the mounting rail (fig. 2). Both are interconnected with a powerful bridge; via this bridge, the power of the neutral conductor coming from the system is led to the PEN conductor. An ingenious and simple solution for the PEN division which everyone can clearly recognize. The question remains as to which mounting rail should be used. The VDE regulations prohibit the use of steel mounting rails if the PEN current is directed entirely or partly along the steel mounting rail. A close look at the point of division reveals when this is the case.

Fig. 2. Design of the PEN termination blocks

Fig. 3. Mounting rails

Conductor connection
On the input side, the green/yellow PEN conductor is first connected to a green/yellow grounding conductor terminal block which connects it directly with the mounting rail via its metal grounding conductor foot. On the output side, the grounding conductor is connected.
Now, a third terminal point is required for the neutral conductor; as already mentioned, the standard requires additional blue identification coding. Multi-wire clamping - i.e., two conductors under the same terminal point - cannot be desired, even less so at a sensitive point like this one. Therefore, a second modular terminal block which must be electrically connected with the grounding conductor terminal block is required. This could, of course, also be a grounding conductor terminal block.

	Connection of the terminal block Both terminal blocks, however, would then be electrically interconnected by means of the mounting rail, and a copper rail would have to be used. In order to still be able to use the less expensive steel mounting rail, the two modular terminal blocks must not be interconnected via the mounting rail but instead by means of a bridge. Just as is the case with the PEN termination blocks. Since the blue terminal block has no metal foot, it is not possible for a partial current to flow along the mounting rail. This means that the VDE requirements are met and no copper mounting rail is necessary.
	Mounting rails If a prefabricated PEN termination block is used, the question of the mounting rail construction becomes irrelevant. Should, however, the rare case occur, that the PEN conductor is to be subdistributed by means of several termination blocks, part of the current will flow along the mounting rail, which may then, of course, not be made of steel (fig. 3). In practice, however, this design will hardly be found. Therefore, a copper mounting rail must only be used if two modular terminal blocks with a directly contacting metal foot are used for splitting up the PEN conductor. The technically and economically more viable solution is thus the use of prefabricated PEN termination blocks. They do not require expensive copper mounting rails and they correspond exactly to the requirements of the standard as concerns their color and technical design.

Relevant standards PEN

VDE 0100
VDE 0611 Part 3: 1996 -06
IEC 947-7-2: 1995
EN 60 947-7-2: 1995
EN 60 947-7-2

Mounting rails/grounding conductors busbars Extract from IEC 947-7-2: 1995/EN 60 947-7-2: 1995/DIN EN 60 947-7-2/VDE 0611 Part 3: 1996-06
PHOENIX CONTACT type	Rail profile	Material	Short circuit strength [mm^2]*	Max. short circuit current[kA]	Max. permitted therm. nominal current with PEN function [A]
NS 15 UNPERFORATED NS 15 PERFORATED NS 15-AL PERFORATED	Top hat rail, acc. to EN 50 045 - 15 x 5 Top hat rail, acc. to EN 50 045 - 15 x 5 Top hat rail, dimensions acc. to EN 50 045 - 15 x 5	Steel Steel Aluminum	10 10 16	1.2 1.2 1.92	* * * * 76
NS 32 UNPERFORATED NS 32 PERFORATED NS 32-CU/35 QMM UNPERFORATED	G-rail, acc. to EN 50 035 - G 32 G-rail, acc. to EN 50 035 - G 32 G-rail, dimensions acc. to EN 50 035 - G 32	Steel Steel Copper	35 35 120	4.2 4.2 14.4	* * * * 269
NS 32-CU/120 QMM UNPERFORATED NS 32-AL UNPERFORATED	G-rail, similar to EN 50 035 - G 32 G-rail, similar to EN 50 035 - G 32	Copper Aluminum	150 70	18.0 8.4	309 192
NS 35/7.5 UNPERFORATED NS 35/7,5 PERFORATED NS 35/7,5-CU UNPERFORATED	Top hat rail, acc. to EN 50 022 - 35 x 7,5 Top hat rail, acc. to EN 50 022 - 35 x 7.5 Top hat rail, dimensions acc. to EN 50 022 - 35 x 7.5	Steel Steel Copper	16 16 50	1,92 1,92 6,0	* * * * 150
NS 35/15-2,3 UNPERFORATED NS 35/15 UNPERFORATED NS 35/15 PERFORATED NS 35/15-CU UNPERFORATED NS 35/15-AL UNPERFORATED	Top hat rail, acc. to EN 50 022 - 35 x 15 Top hat rail, similar to EN 50 022 - 35 x 15 Top hat rail, similar to EN 50 022 - 35 x 15 Top hat rail, similar to EN 50 022 - 35 x 15 Top hat rail, similar to EN 50 022 - 35 x 15	Steel Steel Steel Copper Aluminum	50 35 35 95 70	6.0 4.2 4.2 11.4 8.4	* * * * * * 232 192
* Cross sections calculated acc. to IEC 439-1: 1992 / DIN EN 60 439-1: 1994 / VDE 0660 Part 500: 1994-04. * * Grounding conductor busbars made of steel are not permitted for the PEN function.