Reliability, efficiency and safety are crucial factors for a high quality electrical connection. A stable and permanent connection ensures a continuous flow of current and minimizes potential interference or downtime. In addition, it is important that the connection can resist the loads and environmental conditions in industrial surroundings in order to ensure reliable performance over a long period of time. With regard to terminal blocks, these specific requirements are defined in international norms and standards and the resulting test procedures. They refer, for example, to the current flow, short circuit, voltage drop or heating within the terminal blocks. For sufficient isolation between the potentials, the required clearances and creepage distances are identified or tested.
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The voltage drop test is used to verify the contact quality of the connections. Low contact resistances at the contact points are necessary for the lowest possible and stable voltage drop. Compliance with the maximum permitted voltage drop is verified during the following tests:
Mechanical strength of clamping units
Temperature rise test
Short-time withstand current
Aging test for screwless terminal blocks
For the test, 5 terminal blocks are mounted on a DIN rail and the connection points of the terminal blocks are wired with the rated cross-section. The measuring current is 10% of the rated wire current. The voltage drop is measured over the total distance between the feed-through terminal block or protective earth terminal block and DIN rail.
The verification is considered to be passed if the voltage drop across the feed-through terminal block, before and after the respective test, does not exceed 3.2 mV or 1.5 times the initial measured value. For the PE (ground) terminal block, the voltage drop across the DIN rail must be no higher than 6.4 mV or 1.5 times the initial measured value.
The maximum permissible self-heating of the terminal block under defined conditions is verified with the temperature rise test. Low contact resistances at the contact points are necessary for the lowest possible temperature rise.
For the test, 5 terminal blocks are lined up on a standard-compliant DIN rail. They are wired in series with harmonized PVC insulated wires of the rated cross-section. The terminal blocks are loaded with a defined test current until a constant temperature is reached.
The verification is considered to be passed if the terminal blocks do not heat up by more than 45 K during the test and if the measured voltage drop across the terminal block, before and after the test, does not exceed the required limit values.
The verification of impulse withstand voltage test is used to verify sufficient clearances between two adjacent potentials. The rated voltages derived from the rated voltages of the power supply for the corresponding grid type in conjunction with the overvoltage category are the deciding factors for the dimensioning of the clearances.
For the test, 10 terminal blocks are lined up on a DIN rail conforming to the standard and the connections of the terminal blocks are wired. The voltage is first applied between adjacent terminal blocks and then between the terminal blocks and the DIN rail. The impulse withstand voltage test is performed with a pulse form of 1.2/50 µs with at least 1s break between the pulses. 10 pulses each are applied with alternating polarity.
The verification is considered to be passed if there is neither a flashover nor a breakdown between two adjacent terminal blocks or to the standardized DIN rail.
The verification of the short-time withstand current ensures that the terminal blocks can withstand the short-circuit currents occurring in practice without damage in the event of a fault until the protective device intervenes and switches off the current.
For the test, terminal blocks are mounted on a DIN rail and the connection points of the terminal blocks are wired with the rated cross-section. Protective earth terminal blocks are subjected to this test procedure over 3 runs, feed-through terminal blocks over one run. The terminal block is loaded with a test current of 120 A/mm² for 1 second. The voltage drop over the total distance of the feed-through terminal block or protective earth terminal block and DIN rail is measured before and after the test.
The verification is considered to be passed if there is no damage to the terminal block and further use is guaranteed and the measured voltage drop complies with the specified values.
The insulation test is used to verify the dielectric strength of the insulating material used. The normative test voltage is derived from the rated voltage of the terminal block.
For the test, 5 terminal blocks are lined up on a DIN rail conforming to the standard. The connections of the terminal blocks are wired and the defined test voltage is applied for at least 60 seconds between the adjacent terminal blocks and the DIN rail.
The verification is considered to be passed if there is neither a flashover nor a breakdown between two adjacent terminal blocks or to the standardized DIN rail.
The verification of clearances and creepage distances demonstrates compliance with the normative specifications with regard to the rated voltage and the insulating material which is used. The lengths to be maintained are determined as part of the insulation coordination.
For the test, terminal blocks are lined up on a standard-compliant DIN rail. Proof of clearances and creepage distances must be provided between two adjacent terminal blocks and one terminal block and the mounting support. The minimum requirements of the respective distances are defined in IEC 60947-1.
The clearance is the shortest distance in air between two electrically conductive components. A dimensioning of the clearance is obtained taking into account the following factors:
• Expected overvoltage (rated impulse withstand voltage)
• Applied overvoltage protection precaution
• Environmental conditions
The verification is considered to be passed if the required minimum distances are complied with normatively.
The creepage distance is the shortest distance along the existing insulating body between two electrically conductive components. A dimensioning of the creepage is obtained taking into account the following factors:
• Intended rated voltage
• Insulating materials used (group of insulating materials)
• Environmental conditions ( degree of pollution)
The verification is considered to be passed if the required minimum distances are complied with normatively.
Proof of the SCCR rating (Short Circuit Current Rating) ensures the safety of the switchgear in the case of a short circuit against electric shock, fire and secondary risks. Terminal blocks have a short-circuit rating known as SCCR, which defines the level of short-circuit current that the particular device can safely withstand when protected by a specific overcurrent protective device as identified by SCCR. According to Table SB 4.1 in UL 508A, 10 kA is required as the default value for non-specified components such as terminal blocks.
For the test, 3 terminal blocks are mounted on a DIN rail and the connection points of the terminal blocks are wired with the rated cross-section. While maintaining the rated voltage, a short circuit is generated in the circuit, which is safely interrupted by the upstream fuse element (see UL file with the number E60693). All UL-certified terminal blocks from Weidmüller have an SCCR value (short-circuit rated current) of 10 kA. Different measured or proven SCCR values above 10 kA are documented and can be viewed in the UL file with the number E60693.
The verification is considered to be passed if there is no damage to the terminal blocks and further use is guaranteed. Furthermore, at the end of the test, the measured voltage drop must comply with the specified values.
With the proof of the derating curve, the current carrying capacity is documented as a function of the pole number and the ambient temperature. Since no derating is required in the IEC 60947-7-1/-2/-3 terminal block standard, the pluggable terminal blocks are treated like plug-in connectors with regard to the derating curve and are therefore tested in accordance with IEC 61984.
For the test, pluggable terminal blocks are mounted on a DIN rail and the connection points of the terminal blocks are wired with the rated cross-section. The current-carrying capacity curve, which is dependent on the ambient temperature, the so-called base curve, results from the upper limit temperature of the insulating material. For the material WEMID, this is 130 °C. In order to compensate for measurement inaccuracies or a scattering of the contact resistance in the contact area, IEC 60512-5-2 requires a correction of the load capacity curve by a factor of 0.8 to the base current.
The derating curve is calculated and visualized from the measured values.
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