Eaton LZM Circuit Breaker Selection and Engineering Guide

Temperature influence and derating

The environmental temperature has a direct impact on the performance of the thermal magnetic release. All circuit breakers are calibrated at a reference temperature (usually 40 ℃). At higher ambient temperatures, it is necessary to apply a derating factor or temperature compensation factor.

Application Example 1 (Temperature Compensation): An LZM1-A100 circuit breaker (In=100A) operates at an ambient temperature of 60 ℃. The temperature compensation coefficient obtained from the table is 0.86. This means that its actual overload protection action point is equivalent to a circuit breaker set at 100A × 0.86=86A. If the load current is still 100A, it will malfunction.

Application Example 2 (Capacity Reduction): An LZM2-A250 circuit breaker needs to operate for a long time in an environment of 65 ℃. According to the table, its capacity reduction coefficient is 0.85. In order to ensure safe operation, the maximum continuous current allowed to be carried shall not exceed 250A × 0.85=212.5A. When selecting, engineers must adjust the rated current of the circuit breaker upwards based on the highest expected temperature inside the distribution cabinet.

Mechanical Interlock

Preventing two power sources from simultaneously connecting to the same load is a necessary safety function in backup power systems (such as one in use and one backup) or sequential control. LZM provides two mechanical interlocking methods:

Rotating handle interlock: Connect the rotating handle or door linkage handle of two circuit breakers through Bowden cable. The "ON" position of one handle will physically lock the other handle, preventing it from turning to "ON".

Remote operator interlock: used to install two circuit breakers driven by electric operators in adjacent positions. By using a set of mechanical linkages (NZM-XMV series) directly acting on the internal mechanisms of two operators, strict "one-to-one" interlocking is ensured. This solution cannot be used in conjunction with a manual rotating handle and must be clearly selected during system design.

Selective analysis and backup protection

Selectivity/Discretion: The goal is to ensure that the lower level circuit breaker closest to the fault point operates while its upper level circuit breaker remains closed, thereby ensuring continuous power supply to the fault free circuit. There are verified selective coordination tables between LZM series circuit breakers, as well as between LZM and lower level miniature circuit breakers (MCBs, such as FAZ series) or motor protection circuit breakers (PKZ series). Engineers need to refer to the selective limit current (Is) based on the model, rated current, and setting value of the upper and lower circuit breakers. As long as the expected fault current is lower than Is, selectivity can be guaranteed.

Backup Protection: When the expected short-circuit current exceeds the breaking capacity (Icu) of the lower level circuit breaker, a higher breaking capacity LZM circuit breaker can be connected in series upstream as backup protection. At this point, before the upstream LZM action limits energy, the downstream circuit breaker must be able to withstand and break the fault current after the limit value. The backup protection coordination table provided in the technical manual specifies which combinations can be safely used, providing a solution for economically using standard breaking capacity circuit breakers in high short-circuit capacity grid locations.