In the TN-CS system, if the detection circuit is placed between the PEN conductor and the live (L) wire in the TN-C section, leakage current on the PE line of the TN-S section may not be detected, leading to a failure of the circuit breaker to trip. This happens because the fault current does not pass through the zero-sequence current transformer, which is essential for detecting imbalances.
In a TN-S system, if the neutral (N) line is mistakenly connected to a switch, a break in the N line combined with a leakage on the live (L) line can cause the residual current device (RCD) to fail to operate. The detection circuit will not sense the leakage current because the neutral line is interrupted, resulting in an apparent malfunction.
In the TN-CS system, if the N line and the PE line are incorrectly connected together, any leakage current will bypass the RCD's detection mechanism, causing it to refuse to trip. This improper connection creates a parallel path for the leakage current, making it undetectable by the protection device.
Another common issue is when the sensitivity of the leakage circuit breaker is set too low. If the actual leakage current does not reach the threshold, the device will not activate, even in the presence of a fault. This can lead to serious safety risks if the leakage is not properly addressed.
Malfunctions in leakage circuit breakers often occur due to several key reasons. First, in TN-CS systems, if the PE and N lines are reversed during installation, the device may not function correctly, leading to false trips or failures. Second, in three-phase four-wire systems, using a three-pole RCD instead of a four-pole one can cause issues, as the neutral line may carry current that is not accounted for in the detection process.
Additionally, high-power electrical appliances located near the RCD can generate electromagnetic interference when turned on or off, potentially triggering false trips. Another factor is low insulation resistance between phase and neutral lines, which allows some current to leak to ground, creating an imbalance that the RCD may misinterpret as a fault.
If the PE line of an equipment’s casing is accidentally connected to the working neutral line, it can also result in unwanted tripping. Furthermore, if the three-phase power lines are not passed through the zero-sequence current transformer in the same direction, the magnetic fields may cancel each other out, leading to incorrect operation.
Lastly, if the sensitivity of the RCD is set too high, it may trip unnecessarily even under normal conditions, causing inconvenience and reducing user confidence in the protection system. These factors highlight the importance of proper installation, correct selection of devices, and regular maintenance to ensure reliable operation of leakage circuit breakers.
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