The leakage current compensation


Why is it useful to compensate for the leakage current?

 

All modern residential electrical installations integrate a protection device against electric shock. The device is nothing more than a residual current circuit breaker (also said RCD) able to disconnect the system from the power supply in case of ground fault. A simple integrated sensor performs the measurement of the difference phase-neutral current (in other words current flowing to the ground) and, when it reaches a threshold, the switch operates by isolating the plant downstream. In order to have an effective protection for against direct contacts is necessary that the value of the current doesn't reaches dangerous values for people and, in the case of residential installations, it can never exceed the total 30mA. This is the limit imposed by law that we must never exceed for our safety!

No doubt, the RCCB device has become such an important component for electrical safety that none of us would be willing to give it up.

Unfortunately, in certain situations, it happens that intervenes even in the absence of failure or accidental contact, and require a manual reset. If the interventions are too frequent we could reach a comfort level so low as to be led to exclude the whole device. We try to understand better why the circuit breaker can intervene in the absence of failures.

 

The problem: the permanent leakage current or parasite

 

A small leakage current to earth is always present in all electrical installations and is almost never linked to the quality of insulation of conductors but is bonded to the extension of the system itself. More facility is extended and the greater its ground fault. Let's see, now, to understand why. The electrical cables arranged along the entire installation introduce a parasite capacitance that is proportional to their length. The capacity introduced by a meter of electrical cable is modest but, on an average extended installation, leads us to have a total capacity far from negligible. The current through this parasite capacitance contributes to the overall leakage current. In addition to the eddy current described above, normally, there are leakage currents due to built-in filter capacity in all modern electronic devices which, taken individually, we could easily overlook. The total parasite current due to these phenomena is not negligible and is often able to considerably reduce the circuit breaker tripping margin bringing it close to the triggering threshold. At first sight the problem seems unavoidable, but there is good news!

Cancel the capacitive leakage current

 
Since the parasite leakage current is responsive capacitive-type, you can cancel it by introducing a simple phase plug. Neither more nor less than what you get with power factor correction systems for inductive loads. These devices introduce a capacitive current to cancel the inductive component of the load.

In a similar manner, roles reversed, we can cancel the capacitive component by introducing a leakage inductive compensation current. Remember that we are always talking about the small leakage current to earth and not of the load current flowing between phase and neutral, are two different problems.

B-Stop fixes the problem

 
It is a device that generates an inductive compensation current can greatly reduce the permanent capacitive leakage current to earth. This takes us away from the trigger threshold and inevitably it will be reduced unwanted tripping (for no apparent reason) without degrading safety. The residual current device will continue to intervene regularly, on schedule, in the presence of a earth fault or in the event of accidental direct contact.

Let's an example to understand

 
Suppose we have a capacitive leakage current to earth equal to 17mA (very common situation). The RCCB installed has a nominal operating current of 30mA, but we know that can take place from the middle of the rated current as required by the reference standards. We assume an installed specimen effective intervention threshold equal to 20mA: the tripping margin, in the best case, it will be of only 3mA! There is to be taken into account, also, that both the actual tripping threshold that the leakage current may change slightly with variations in environmental conditions such as temperature, humidity and line voltage. The margin of only 3mA does not give many guarantees that the threshold may not be exceeded under certain unfavorable conditions!

If we add B-Stop, configured to supply 11mA, we will have that the leakage current of 17mA will be reduced to 6 mA (17mA - 11mA). The tripping margin will now be equal to 20mA (intervention threshold) - 6 mA (leakage) = 14mA against the initial 3mA!

We have increased the margin of almost 5 times!

The security first of all

 

Be wary of who offers solutions that lead to exclude the RCD or to increase the threshold for intervention over the legal limit, which can never exceed 30 mA in civil electrical installations!

Adding B-Stop can eliminate the problem of unwanted tripping without modification to the existing plant and maintaining the same level of protection.