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A technical interpretation of experimental research into miniature circuit breaker terminal temperature rise and the influence of terminal frame geometry.
Terminal temperature rise is an important performance parameter in miniature circuit breakers (MCBs). Excessive heating at the terminal can affect product reliability and, under certain conditions, lead to terminal burning.
In actual applications, terminal damage is not necessarily caused by electrical current alone. Conductor type and cross-sectional area, tightening torque, installation conditions and terminal frame geometry can all influence the thermal behavior of the connection.
The research paper Experimental Study on Terminal Block Temperature Rise of Miniature Circuit Breaker, by Huang Jia, Ren Changbin and Zhang Yanyang, investigates these factors through comparative experiments.
The study first evaluates terminal temperature rise under different connection parameters. It then examines the influence of conductor insulation entering the terminal frame and compares two terminal frame designs: a ribbed structure and a compound-groove structure.
This article follows the original research sequence, presents the three original data tables in English and retains the two original terminal frame illustrations to explain the study's findings.
The researchers selected single-pole MCBs rated at 100 A based on actual customer application conditions. The samples were tested in a controlled laboratory environment.
The main test conditions were as follows:
Rated current of the MCB: 100 A
Reference ambient temperature: 25°C, maintained within ±1°C
Test samples: 36 MCB samples for the connection-parameter comparison
Test duration: 50 hours per sample under the reported cyclic energization procedure
Daily energization period: 09:00–17:00
Temperature measurement: A multichannel temperature measurement instrument
Tightening torque control: A torque screwdriver
Sample arrangement: Individual placement, with no other circuit breakers or heat sources within 20 cm on either side
The researchers recorded three temperature readings during the final minute before power was switched off each day and averaged them to obtain the daily terminal temperature-rise value.
For the conductor-material tests, aluminum conductors were tested without copper-to-aluminum transition lugs. The thermal protection trip elements were removed from the MCBs before testing to prevent thermal tripping from interrupting the experiment.
The study subsequently conducted a separate comparison between two terminal frame designs using the same MCB model and a specified tightening torque.
These experimental conditions are important when interpreting the results: the findings describe the tested samples and configurations, rather than universal performance limits for every MCB terminal.
The first set of experiments examined four variables:
Tightening torque
Conductor type
Conductor cross-sectional area
Electrical current
Table 1 reproduces the connection-parameter results reported in the original paper.
Tightening torque (N·m) | Conductor type | Cross-sectional area (mm²) | Current (A) | Reported result |
|---|---|---|---|---|
2.3 | Copper | 10 | 100 | 135.2 K |
2.3 | Copper | 10 | 75 | 89.5 K |
2.3 | Copper | 10 | 63 | 78.3 K |
2.3 | Copper | 6 | 100 | Terminal burning after 50 h |
2.3 | Copper | 6 | 75 | 100.4 K |
2.3 | Copper | 6 | 63 | 85.3 K |
2.3 | Aluminum | 16 | 100 | Terminal burning after 0.1 h |
2.3 | Aluminum | 16 | 75 | Terminal burning after 0.3 h |
2.3 | Aluminum | 16 | 63 | Terminal burning after 0.5 h |
2.3 | Aluminum | 10 | 100 | Terminal burning after 0.04 h |
2.3 | Aluminum | 10 | 75 | Terminal burning after 0.05 h |
2.3 | Aluminum | 10 | 63 | Terminal burning after 3 h |
1.2 | Copper | 10 | 100 | 206.2 K |
1.2 | Copper | 10 | 75 | 114.8 K |
1.2 | Copper | 10 | 63 | 99.7 K |
1.2 | Copper | 6 | 100 | Terminal burning after 11 h |
1.2 | Copper | 6 | 75 | 201.1 K |
1.2 | Copper | 6 | 63 | 185.8 K |
Source: Table 1 of the original paper. The paper records terminal-burning times where the samples burned before the test endpoint. Temperatures at the time of burning were not recorded because they exceeded the thermocouple's measurement range.
For the 10 mm² copper conductor, the reported terminal temperature rise was higher at 1.2 N·m than at 2.3 N·m at all three tested currents.
For example, at 100 A, the recorded value increased from 135.2 K at 2.3 N·m to 206.2 K at 1.2 N·m.
This comparison supports the paper's conclusion that insufficient tightening torque can adversely affect terminal temperature-rise performance.
However, the result should not be interpreted as a recommendation to tighten terminal screws beyond their specified torque. The appropriate torque depends on the particular terminal assembly and manufacturer requirements.
The results for the copper conductors generally show higher temperature rise at higher current under comparable conditions.
The study also found that the smaller 6 mm² copper conductor produced less favorable results than the 10 mm² conductor in several comparisons.
These observations are consistent with the importance of current loading and conductor resistance in terminal heating. The results also show why conductor size and tightening conditions should be considered together rather than evaluated independently.
Under the tested conditions, the aluminum-conductor configurations experienced terminal burning much sooner than the copper-conductor configurations in several comparisons.
This was particularly apparent in the 16 mm² aluminum-conductor tests, where terminal burning was recorded within 0.1–0.5 hours at the three tested current levels.
The authors concluded that aluminum conductors were more prone to terminal burning under the conditions investigated.
This is a result of the specific test configurations. It does not establish that every aluminum-compatible terminal will perform worse than every copper connection; terminal compatibility, conductor preparation and installation requirements remain important.
The researchers also investigated whether conductor insulation pressed into the terminal frame affected temperature rise.
Two MCB samples of the same model and rating were used for the comparison. The test was conducted at 100 A, with a tightening torque of 3.5 N·m and a conductor cross-sectional area of 35 mm². Terminal temperature rise was recorded after four hours of energization.
Current (A) | Tightening torque (N·m) | Conductor cross-section (mm²) | Without insulation trapped (K) | With insulation trapped (K) |
|---|---|---|---|---|
100 | 3.5 | 35 | 51.2 | 76.5 |
100 | 3.5 | 35 | 56.8 | 123.7 |
100 | 3.5 | 35 | 49.8 | 73.2 |
100 | 3.5 | 35 | 47.4 | 70.6 |
Source: Table 2 of the original paper. The four readings are reproduced in their reported order.
All four paired readings show higher terminal temperature rise when conductor insulation was pressed into the terminal frame.
In the second reading, the temperature rise increased from 56.8 K to 123.7 K.
The result illustrates that terminal performance depends not only on the component's design but also on the quality of conductor installation.
When insulation enters the clamping region, it can interfere with the intended conductive interface between the conductor and terminal. This can adversely affect the connection and its thermal behavior.
The practical implication is straightforward: conductor preparation and insertion must follow the terminal manufacturer's instructions. The conductor insulation should not be trapped in a clamping region designed to establish electrical contact with the conductor.
The next part of the study focuses on terminal frame geometry.
The researchers compared two designs:
A ribbed terminal frame with a triangular bottom profile
A compound-groove terminal frame with recessed features along the sides and bottom
Both designs were made from Q195 steel. The comparison used the same MCB model and a tightening torque of 2.3 N·m.
Original Figure 1. Ribbed terminal frame design.
The original paper describes this design as having a triangular bottom profile combined with ribs. The reported rib dimensions are 6.8 mm in length and 0.6 mm in height.
Original Figure 2. Compound-groove terminal frame design.
The compound-groove design uses right-angled triangular grooves. The paper reports a groove depth of 0.25 mm, a rib length of 9.6 mm and a rib height of 0.5 mm.
The two figures are the original illustrations from the paper. They are included to preserve the structural comparison on which the following experiment is based.
The researchers tested both terminal frame designs under the same specified tightening torque and compared their terminal temperature-rise behavior.
Current (A) | Conductor type | Cross-sectional area (mm²) | Ribbed design | Compound-groove design |
|---|---|---|---|---|
75 | Copper | 6 | 100.2 K | 97.0 K |
63 | Copper | 6 | 85.4 K | 81.2 K |
75 | Aluminum | 16 | Terminal burning after 0.3 h | Terminal burning after 2.6 h |
63 | Aluminum | 16 | Terminal burning after 0.5 h | Terminal burning after 3.5 h |
Source: Table 3 of the original paper. For the aluminum-conductor tests, the reported values are times to terminal burning, not temperature-rise measurements.
For the 6 mm² copper conductor, the compound-groove design produced lower reported temperature rise in both comparisons.
At 75 A, the recorded temperature rise decreased from 100.2 K to 97.0 K. At 63 A, it decreased from 85.4 K to 81.2 K.
The differences are measurable, although the results alone do not establish a universal performance advantage for every compound-groove design.
The difference between the two designs was more pronounced in the reported aluminum-conductor tests.
At 75 A, the ribbed-design sample experienced terminal burning after 0.3 hours, while the compound-groove sample burned after 2.6 hours.
At 63 A, the corresponding times were 0.5 hours and 3.5 hours.
These results indicate that the compound-groove design performed better in the specific aluminum-conductor configurations tested. Because the temperatures at burning exceeded the thermocouple's measurement range, the study did not report corresponding terminal temperature-rise values for these events.
The authors proposed three mechanisms to explain the results.
The triangular grooves along the sides and bottom of the frame provide space for conductor expansion.
According to the paper, when the conductor heats and expands, it can be pressed into these grooves, increasing the contact area and reducing contact resistance.
This is the proposed explanation for the lower temperature rise observed with the compound-groove design in the comparative tests.
The side ribs work together with the curved bottom profile to guide the conductor toward the center of the terminal during tightening.
The authors suggest that this arrangement improves conductor positioning and connection reliability.
They also discuss how repeated heating and cooling can change the dimensions of the conductor and affect clamping conditions. If the conductor moves and the clamping force decreases, the connection can enter a negative cycle in which reduced clamping promotes further movement and further loss of clamping force.
The compound-groove structure is intended to reduce this risk by helping maintain conductor positioning and mechanical retention.
The curved bottom profile and two ribs work together to resist axial conductor movement.
The paper reports that terminal frames without the relevant rib features failed the 16 mm² stranded-wire movement test and showed poorer performance in the corresponding pull-out tests.
This observation is important because terminal performance involves both electrical contact and mechanical retention. A connection must establish the intended conductive interface while maintaining the conductor's position under the specified mechanical conditions.
The experimental results support four principal conclusions reported by the authors:
Under the tested conditions, lower tightening torque, smaller conductor cross-section and higher current were associated with less favorable terminal temperature-rise performance.
Aluminum-conductor configurations were more prone to terminal burning than copper-conductor configurations in the tests.
Pressing conductor insulation into the terminal frame increased the measured terminal temperature rise.
Terminal frame geometry influenced the results. The compound-groove design produced lower temperature rise with copper conductors and longer times to terminal burning with aluminum conductors under the reported test conditions.
The study demonstrates that terminal temperature rise is not determined by current alone. Conductor properties, installation conditions, tightening torque and terminal frame geometry interact to influence connection performance.
For terminal frame designers and circuit breaker manufacturers, the practical lesson is that structural design should be evaluated through comparative testing under defined electrical, thermal and mechanical conditions.
The reported results apply to the particular MCB assembly and terminal frame designs investigated in the paper. They should not be interpreted as universal performance guarantees or as proof that every grooved terminal frame will outperform every ribbed design.
Huang Jia, Ren Changbin and Zhang Yanyang, “Experimental Study on Terminal Block Temperature Rise of Miniature Circuit Breaker” (小型断路器接线端子温升研究), Electrical Appliances and Energy Efficiency Management Technology, 2021, No. 8, pp. 40–42.
DOI: 10.16628/j.cnki.2095-8188.2021.08.008.
This article is an English technical interpretation of the cited research. The experimental values, design dimensions and observed outcomes are reproduced from the original paper; the proposed mechanisms are attributed to the paper's authors.
