Discover why ultra-immunized RCDs are designed to avoid nuisance trips

Devices without ultra-immunized systems can trip due to a leakage that is 50% above their rating, as required by the regulation on RCDs, which can cause a trip with a leakage current that is much lower than the trip limit.

The CBS residual current protection units have an ultra-immunized system to better guarantee continuity of supply in the installation.

Trip margin set to 85% of the leakage value

Devices without ultra-immunized systems can trip due to a leakage that is 50% above their rating, as required by the regulation on RCDs, which can cause a trip with a leakage current that is much lower than the trip limit.

Suppose we are protecting a channel or a shunt line and we install a differential relay with a sensitivity of 300 mA. If we don't use a relay with an ultra-immunized system, it is possible for the combined earth leakage current to be below 300 mA and for the device to trip as low as 150 mA, a threshold that is well below 300 mA, the value at which a current starts posing a danger to people.

Therefore, there is a real leak that is not dangerous to people or to the installation that is causing the RCD to trip, causing service outages and raising the indirect costs of the installation.

To prevent this effect, we suggest installing ultra-immunized RCDs that are guaranteed not to trip below 85% of the circuit breaker's rating. In other words, continuing with the example above, these devices will only trip with currents above 250 mA, without ever exceeding the 300 mA required by the protection system, and bringing the trip value as close as possible to the actual rating of the RCD.

Harmonic filtering (high-frequency leaks)

Ultra-immunized RCDs offer a better frequency response because they filter out all leaks whose frequency exceeds 50/60 Hz. High-frequency currents can affect the measurement of toroidal transformers, causing erroneous readings and nuisance trips.

As a result, by "making them invisible" to harmonic currents, we guarantee supply continuity by triggering only with actual leakage values at 50 or 60 Hz.

Ultra-immunized RCDs are not affected by transient disturbances that may appear in electrical installations, such as those associated with environmental factors, like lightning strikes, or those created when connecting or disconnecting loads - whether inductive or capacitive - to/from the installation.

It is common for the distribution grid to be affected by external phenomena such as lightning strikes, which cause fleeting or transient surges that dissipate very quickly. These disturbances can affect RCDs, which are not designed to deal with this kind of signal and can thus cause a nuisance trip.

Because of this, an ultra-immunized system ensures continuity of supply by not affecting the installed RCDs when a transient of up to 3 kA and lasting 8/20 μs occurs.

Easier to interpret

The CBS-40A/400A residual current protection relay unit makes it even easier to interpret its readings than its predecessor, the CBS-4.

Its backlit screen changes colour depending on the device's status to identify the status of each of the 4 lines being measured at a glance, showing values in red when any of the 4 available relays trips due to leakage accumulation; in yellow when the leakage level exceeds the pre-alarm limit; in blue when in programming mode, and in white when it is monitoring with no alarms.

The pre-alarm level is fully adjustable and can be set to a value from 20% to 100% of the adjusted trip point sensitivity, so that preventive maintenance can be carried out during a scheduled outage before the trip point is reached.