Schneider C60H-DC Protector Practical Manual

The manual defines three typical fault modes (A, B, C) corresponding to different short-circuit current sizes and protection electrode requirements.

Fault type Fault description Short circuit current size Protection requirements

Fault A grounding fault (such as positive pole to ground short circuit) is equal to the power supply voltage divided by the impedance of the grounding circuit. In the negative grounding system, the fault current is Isc_max at U. The pole of the protective grounding electrode (usually the positive pole) must have a breaking capacity of ≥ Isc_max.

Fault B: Short circuit between poles (direct short circuit between positive and negative poles) Isc_max at U (full voltage). Both poles must be able to disconnect Isc_max at U.

Fault C dual pole simultaneous grounding requires the use of insulation monitoring instrument (PIM) alarm and manual clearing for ungrounded systems. For grounding systems, it is equivalent to fault A or B. See specific wiring analysis.

Design points:

In the negative grounding system, the short-circuit current of the positive pole to ground short circuit (fault A) is equal to the total short-circuit current of the system, and the positive pole protector must be able to disconnect this current. However, a negative pole to ground short circuit will not generate a large current (because the negative pole is directly grounded), so the negative pole usually does not need protection.

In the center point grounding system, the short-circuit current of any pole to ground short circuit is U/2, and the breaking capacity of the selected protector needs to be ≥ U/2 of the short-circuit current.

In an ungrounded system, the first grounding fault will not generate a large current (alarmed by the insulation monitoring instrument), but the second grounding (of different poles) will cause a short circuit between the poles, and the fault current is a full voltage short circuit. Therefore, ungrounded systems usually require protective devices to be installed at both poles, and the breaking capacity needs to match the full voltage short-circuit current.

Buckling Curve and Coordination Design

6.1 Time current characteristics of C-curve

The trip curve of C60H-DC (Figure 3) shows the longest trip time corresponding to different multiples of rated current in cold state:

1.05 times In: 1 hour without detachment.

1.3 times In: Release within 1 hour (hot release zone).

2-5 times In: In the hot release area, the release time varies from tens of seconds to a few seconds, exhibiting inverse time characteristics.

7-10 times In: Electromagnetic trip zone (instantaneous action), with a trip time of less than 0.1 seconds, usually 20-50ms.

In engineering practice, in order to avoid the starting surge current of the load (for example, the starting current of a DC motor can reach 6-8 times the rated current), the C-curve allows for instantaneous action of 7-10 times, providing sufficient margin. But if the duration of the surge current is long (more than 0.1 seconds), it may accidentally trigger the heating trip. At this time, it should be considered to adjust the rated current of the protector or choose a higher current level.

6.2 Selective Collaboration

In complex DC distribution systems, in order to achieve selectivity (i.e. only cutting off the faulty branch without affecting the upstream power supply), it is necessary to match the tripping characteristics of upstream and downstream protectors. Due to the fact that C60H-DC only has a C-curve, if the same model is used upstream and downstream, it may trip simultaneously when the fault current is large. The suggested cooperation method is:

When using C60H-DC with a higher rated current upstream (such as 40A upstream and 10A downstream), and the fault current is expected to not exceed 10kA, it usually has a certain degree of selectivity.

Alternatively, a DC circuit breaker with short delay function can be used upstream (such as a molded case circuit breaker for C60H-DC).

Typical Applications and Case Analysis

7.1 24V PLC control cabinet DC distribution

Scenario: A 24V DC control cabinet with a total load of approximately 15A, including PLC, relays, sensors, indicator lights, etc. Use C60H-DC 2P 16A as the main switch for the incoming line, and use 1P 2A/6A/10A to protect each circuit in the downstream branch.

Attention: It is customary to ground the negative pole in a 24V system. At this point, a 1P protector can be connected in series with the positive pole and the negative pole directly grounded. But for safety isolation (while disconnecting the positive and negative poles), it is still recommended to use a 2P main switch. The branch can use 1P to protect the positive pole.

Selection: MGN61511 (16A 2P)+MGN61501 (1A 1P)+MGN61502 (2A 1P), etc.

7.2 Photovoltaic Array DC Side Protection

Scenario: A group of photovoltaic strings with an open circuit voltage of approximately 300V DC and a maximum operating current of 12A, requiring the installation of a DC protector inside the combiner box. Due to voltage exceeding 250V, 2P C60H-DC 500V series must be used. It is recommended to choose 1.25 times the working current for the rated current, which is 16A.