Electrical tests for terminal blocks

The derating curve represents the current carrying capacity of a component as a function of the ambient temperature and neighboring contacts. It is affected by the contact material and the insulating housing. To determine the current-carrying capacity of plug-in terminal blocks, arrangements with a variety of positions are selected, which are electrically connected in series using conductors with the same cross-section. For the practical determination of the derating curves, the current carrying capacity of the plug-in terminal blocks is determined in accordance with DIN EN 60512-5-1. Here, after applying various currents and setting the temperature balance, the maximum temperature increase that occurs on the test objects is measured. When the upper limiting temperature of the insulation material (here, always assumed to be 100°C) is taken into account, these values yield a derating curve dependent on the ambient temperature (the “base curve”). An adjusted capacity curve, the “derating curve”, is generated in accordance with DIN EN 60512-5-2. In accordance with this standard, the permissible load current is 0.8 times the respective base current. The derating factor “...takes into account manufacturing tolerances in the connector contact system. It also takes into account uncertainties in the temperature measurement and the measuring arrangement.” For plug-in terminal blocks from Phoenix Contact, derating curves with 2-, 5-, 10-, and 15-position arrangements are specified.

The rise in temperature of a terminal block due to the Joule effect must be kept to an absolute minimum. The contact resistance must therefore be as low as possible. In this test, the rise in temperature that occurs at room temperature during exposure to a test current is documented.

IEC 60947-7-1/-2
Here, five terminal blocks are horizontally mounted on a rail and connected in series using 1 or 2 m conductor loops with the rated cross-section. The terminal blocks are exposed to a test current that is as high as the current carrying capacity of the rated cross-section. The rise in temperature at the middle terminal block is documented. Assuming a room temperature of about +20°C, a maximum rise in temperature of 45 K (kelvin) is permitted in the terminal block. Additionally, a voltage-drop test must be performed on the terminal block.

UL 1059
The process basically corresponds to the IEC test, only the conductor lengths differ. In the UL 1059, three terminal blocks are horizontally mounted adjacent to one another. The measurement is taken at an ambient temperature of 25°C, whereby a maximum rise in temperature of 30 K (measured as close as possible to the terminal point) is permitted. Due to the high-quality contact materials used in Phoenix Contact terminal blocks, all connection technologies offer lower heating values than required by the specified standards. High-quality copper materials and reliable contact transitions guarantee low contact resistances in the terminal blocks.

Terminal blocks must, in practice, also be capable of resisting short-circuit currents until the relevant safety equipment cuts off the current without sustaining any damage. This can last a few tenths of a second and can occur at several times the nominal current. For testing purposes, a terminal block is mounted on the support and wired to a conductor with the rated cross-section. Protective conductor terminal blocks are subjected in three stages of 1 s each to a current density of 120 A/mm² of the rated cross-section. The requirements are met if, after the test, the individual parts are undamaged and they can still be used. Before and after the test, the terminal block must pass the voltage-drop test. The voltage drop before and after the test must not exceed 3.2 mV per terminal block nor may it exceed 1.5 times the value measured before the test. In the case of a 240-mm² high-current terminal block from Phoenix Contact, a test current of 28,800 A is passed through the terminal block for one second without loss of quality.

Carrying out a dimensional check of air clearances and creepage distances confirms that electrical insulation properties are adequate with respect to the following:

• Design
• Expected contamination
• Expected ambient conditions

The distances are verified by measuring between two adjacent terminal blocks and between the live metal parts and the support, taking into account the shortest distances. This involves considering the isolation of the air as the clearance and the distance along the surface as the creepage distance. The minimum distances are defined in IEC 60947-1.

For the clearance, this means:
It is the shortest path through the air between two electrical potentials. The deciding factor for measuring the minimum clearance is the rated surge voltage, the overvoltage category of the terminal block, and the expected pollution degree. The rated surge voltage is derived from the neutral voltage with respect to the overvoltage category. If not documented otherwise, overvoltage category III is used for the terminal blocks. The category describes equipment in fixed installations and is intended for such cases where there are special requirements for the reliability and availability of the items. The associated clearance is described in Table 2 (extract) of IEC 60664-1. Further specifications here are the generally non-homogeneous field for the application and pollution degree 3 (conductive pollution occurs or see Table 2 – IEC 60664-1: A non-conductive pollution that becomes conductive since condensation is to be expected).

For the creepage distance, this means:
It is the shortest path along the surface of the isolation between two electrical potentials. The RMS value of the DC or AC voltage system (conductor to conductor, conductor to ground, conductor to neutral conductor) is decisive for determining the minimum creepage distance; see Table 3a and 3b of IEC 60664-1. Table 4 of IEC 60664-1 shows the relationship between the RMS value of the voltage, the pollution degree (3), and the insulating material group (I.) of the terminal block.

As of April 2006, the NEC (National Electrical Code) requires the short-circuit current rating of industrial controllers to be specified. These SCCR (short-circuit current rating) values can be calculated with the help of UL 508A. In the USA, the calculation must be summarized on the rating plate of all industrial switchgears, for all main circuits, as well as for the feed-in of the control voltage supply. Standard values for non-specified components are listed in UL 508 A – Table SB 4.1. A standard value of 10 kA is specified for terminal blocks. This SCCR value describes the short-circuit rated current flow of a system or component under specification of a rated voltage. This is the maximum permissible symmetrical fault current that does not lead to significant damage that could impair use or lead to dangerous handling. On the complete system side, this SCCR value is based on the weakest installed components in the associated distributor or feed-in circuit. Terminal blocks in the CLIPLINE complete system are documented as having SCCR values of 100 kA in the UL file XCFR2_ E60425. They help you to create powerful systems with high measured SCCR values.

For circuits in which the installation of higher documented components is not possible, the entire circuit can be rated higher by connecting a correspondingly high-current listed fuse terminal block upstream. The UK 10,3-CC HESI N fuse terminal block allows the SCCR for downstream circuits to be raised to 200 kA.

In every terminal point of a terminal block, one or more conductors are connected – depending on the connection technology. Current transfer is strongly affected by the electrical resistance between the conductor and the current bar. High-quality contacts create a gas-tight connection. This guarantees a long-lasting and reliable connection. This electrical test therefore determines the voltage drop on a terminal block (two terminal points), from which conclusions about the contact resistance and the contact quality can be drawn. The terminal blocks are wired with the rated cross-section. For measuring purposes, a direct test current corresponding to 0.1 times the current carrying capacity of the rated cross-section is applied to the terminal blocks. The voltage drop is picked off at a distance of ≤10 mm from the middle of the terminal point (see diagram). At a room temperature of about 20°C, the voltage drop must not exceed 3.2 mV per terminal block before and after the test, nor may it exceed 1.5 times the value measured at the start of the test. Terminal blocks from Phoenix Contact are up to 60% below the limit values required by standards.

Proof of sufficiently large clearances between two neighboring potentials is provided using the surge voltage test. The test is carried out with the surge voltage five times for all polarities in relation to the rated insulation voltage. The time intervals in this process are at least 1 s. The distance between neighboring terminal blocks or between the terminal block and the rail is examined. There must be no unintentional sparkovers during the test. Rated surge voltages for Phoenix Contact terminal blocks are between 6 and 8 kV in accordance with IEC 60664. The respective height is derived from the nominal voltage. Operationally safe use of the documented operating voltages of the terminal blocks is thereby effectively checked. Category III of overvoltage category 4 is the default setting.