DC circuit breakers are personal safety precautions in low voltage electrical appliances and low voltage switchgear control equipment.

The significance of indirect contact protection of dc mcb live components is to prevent the occurrence of excessive contact voltage on the housing of terminal electrical appliances or terminal electrical equipment from hurting people. At the same time, several effective methods are proposed to avoid the occurrence of excessive contact voltage.

Generally speaking, the case of a terminal appliance or an electrical equipment is not charged under normal conditions, but if the insulation of the charged conductor is damaged, the case will be charged, and the person touching the charged case will be shocked.

Both IEC 60364-4-41 and GB/T16895.9-2000 currently specify excessively high contact voltages is (1) AC voltages greater than 50V (RMS); (2) DC voltages greater than 120v.

The above voltage is defined as the earth voltage in IEC60364-4-41 and GB/T16895.9-2000. For a three-phase three-wire ungrounded power supply system, the above voltage is defined as a voltage that appears on other conductors when one phase is grounded.

For a complete set of low voltage switchgear, in order to prevent indirect contact, first of all, the corresponding protective measures should be matched to the power system.

Protection insulation, and protection measures and protection insulation must be compatible with the grounding mode of the system; at the same time, it is also required that all electrical equipment should have the ability to automatically cut off the power supply in case of leakage, to prevent the existence and expansion of accidents.

Among all kinds of leakage protection devices, residual current action protector RCD is the best one. It is not only suitable for TT and TN grounding systems, but also suitable for some IT systems.

Low-voltage switchgear must have well-equipped protective circuits, and all non-current-carrying circuit metal structural components such as cabinet structure, cabinet door and mechanism, drawer, pull-out components must be grounded, and the grounded circuit must be continuous.

In IEC 61439-1 and GB72511-2013, the continuous channel of protective conductor is called the continuity of grounding protective conductor.

Personal safety protection and equipment protection in low-voltage distribution network include two aspects: isolation protection and contact protection.

Matters needing attention in selection of dc mini circuit breaker
1. the capacity of the dc mcb type selected should generally be greater than the capacity of the controlled equipment. For equipment with special control requirements, special dc mini circuit breaker models (such as speed and pressure requirements) should be selected.
2. Choose the appropriate type of dc mcb according to the type of control object (motor control, machine tool control, etc.), control requirements and use environment.
3. Determine the technical specifications according to the technical requirements of the controlled object, such as the rated voltage, rated power, multiple of motor starting current, load properties, operating frequency, operating system, etc.
4. Understand the main technical performance (technical requirements) of the dc mini circuit breaker type, such as usage, classification, rated voltage, rated control power, on-off capacity, allowable operating frequency, operating system, service life, process requirements, etc.
5. Should understand the normal working conditions of electrical appliances, such as ambient air temperature and relative humidity, altitude, allowable installation angle, impact vibration resistance, harmful gases, conductive dust, rain and snow invasion, indoor or outdoor work.