Amendment to IEC 60947 - 7-1 - what it means for terminal blocks

Screw connectors or terminals have been around for a very long time. They are very simple in their most basic forms, and they have the significant advantage that they have no compatibility problems with mismatched sizes or shapes. Moreover, the connections are very secure, both physically and electrically, because they firmly contact a large section of wire.

However, this can sometimes also be a disadvantage, because it can take time to tighten or un-tighten a set of connections that could otherwise be simply plugged or unplugged. Also, if the connected wire too thin, the screw terminal can partly cut through the wire. In addition, screw connectors may come loose over time if not done up tightly enough initially. In the UK all screw connectors on fixed mains installations are required to be accessible for servicing for this reason.

Terminals can be arranged in many ways, including a terminal strip or terminal block, with several screws along (typically) two long strips. This creates a bus bar for power distribution.

The modern take on screw terminals are DIN-rail mounted types, such as feed-through and PE terminals, which use clamping yoke or other screw technologies.

Then there are screw-less types, such as those that use spring clamps. These provide fast, durable and practical secure connections that reduce wiring time, minimise troubleshooting and perform well in severe conditions such as vibrations. Such maintenance-free IEC spring-clamp terminal blocks need just a screwdriver for easy installation, and can reduce wiring time by more than 50%.

Whether of the screw type or screwless, modern terminal blocks are available in a wide range of sizes and configurations to suit almost every kind of application, including multi-circuit and isolation blocks. Two-level terminal blocks are available for applications requiring control signals to be distributed through the same block. Many different wire sizes are catered for.

Standards:

There are a number of standards that cover terminal blocks, but one of them is the IEC 60947 series. These are:

• IEC 60947-1 (Low Voltage Switchgear and Controlgear: General Rules)
• IEC 60947-7-1 (Low Voltage Switchgear and Controlgear: Terminal Blocks for Copper Conductors)
• IEC 60947-7-2 (Low Voltage Switchgear and Controlgear: Protective Conductor Terminal Blocks for Copper Conductors)
• IEC 60947-7-3 (Low Voltage Switchgear and Controlgear: Safety Requirements for Fuse Terminal Blocks).

IEC 60947-7-1:

IEC 60947 - 7-1 - 'Low-voltage switchgear and controlgear - Part 7-1: Ancillary equipment - Terminal blocks for Copper Conductors', was published last year, and it specifies requirements for terminal blocks with screw-type or screwless-type clamping units primarily intended for industrial or similar use.

Such terminal blocks are designed to provide electrical and mechanical connection between copper conductors, for use in circuits having a rated voltage not exceeding 1,000 V AC (up to 1,000Hz) or 1,500 VDC, and they are to be fixed to a support.

IEC 60947 - 7-1 also applies to terminal blocks designed to connect circular copper conductors, with or without special preparation, having a cross-section between 0.2mm2 and 300mm2.

This standard can, therefore, be used as a guide for the following:

• Terminal blocks requiring the fixing of special devices to the conductors (eg: quick connect terminations or wrapped connections).
• Terminal blocks providing direct contact to the conductors using edges or points penetrating the insulation - eg: insulation displacement connections (IDC).
• Terminal blocks of a special type - for example, disconnect devices.

Note that, generally, the term 'clamping unit' has been used instead of the word 'terminal'.

The ammendments:

IEC 60947-7-1 has recently been amended - what does this mean? The main technical modifications are as follows:

• Added in - now, this standard may be used as a guide for special types of terminal blocks (such as diodes or varistors or similar component holders)
• The conventional free air thermal current added in 5.2 in the list of information to be stated by the manufacturer
• An update of 7.1.4 that the colour combination green-yellow is not allowed for terminal blocks
• The consequences of failed single tests and failed tests within test sequences as specified in 8.2
• Requirements regarding clearances and creepage distances replaced by reference to Annex H of IEC 60947-1. Annex A has been deleted.
• Requirements for tightening torques for the tests improved and referenced to Table 4 of IEC 60947-1. Annex C has been deleted.
• In 8.3.3.3, there is a changed reference for pull-out force to IEC 60947-1
• The wording of the voltage drop test has been improved in 8.4.4, and the wording of the short-time withstand current test has been improved in 8.4.6
• Annex D - with additional requirements for test disconnect terminal blocks - has been added. The scope has been modified accordingly.

Test procedures:

All screw and screw-less terminal products that relate to IEC 60947-7-1 should comply with the requirements of the following tests:

• Dielectric test
• Short-time current withstand test
• Voltage drop test
• Tension test (pull-out)
• Temperature rise test
• Thermal cycling test.

Such tests determine whether the terminals concerned meet certain requirements, including connection integrity, the distance between two poles within a terminal block (no flashover or breakdown must occur), the strength of the insulating materials, the conductor not pulling out of the terminal unit, the clamping unit breaking while undergoing certain forces, and others.

Note that IEC 60947-7-1 should be read in conjunction with IEC 60947-1:2007 - Low-voltage switchgear and controlgear.

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