Probably everyone who reads this article has heard about such a device as an "RCD", and perhaps even used it in everyday life. But in fact, we do not use an RCD, but an UDT (differential current device).
In this article I will talk about how this device actually functions in ideal electrical circuits in which there are no leakage currents. On the nuances of UDT operation in real electrical circuits of electrical installations of buildings, in which leakage currents always flow, which can cause false positives of UDT, I will write a separate article (within the framework of the "Zen short article" format - this is not possible, since the article will large).
Now, for general development, useful information about the "basic" principle of the UDT.
It should be added that I∆n is set by the manufacturer of the device and is usually indicated on its case, for example, I∆n = 0.03 A for household UDT.
Let us consider an example of the functioning of a two-pole UDT used in single-phase electrical circuits.
The figure below illustrates the operation of the DT UDT under normal conditions and under conditions of damage in the electrical circuit:
Under normal conditions
Electric currents flow in the phase and neutral conductors of the UDT main circuit, in such a way that:
Note: vertical bars, eg | I1 | - means the absolute value of the electric current I1.
The currents I1 and I2 are directed in opposite directions, which means that if we add them vectorially, we get that the vector sum (differential current) of the indicated electric currents is (en) zero:
I.e:
As a result of this, the absolute value of the electric current flowing in the secondary winding of the differential transformer will also be zero:
Under these conditions, the RTD, which is connected to the secondary winding of the diesel fuel, cannot operate.
Therefore, the first practical conclusion:
Under normal conditions of the electrical circuit, the UDT does not work and, therefore, does not disconnect the external electrical circuits connected to it.
Under damage conditions
Then we get that:
That is, in fact, in this situation, the differential current will be equal in absolute value to the earth fault current.
Further, we have that:
From this we get that:
As a result, we get the second important conclusion:
As a conclusion
I tried to explain the principle of the UDT in the simplest possible way, but it did not work out as I wanted, because in this article it was necessary to adhere to the terminology as strictly as possible.
Now we know how UDT works and that UDT should not be triggered by leakage currents, because it is not intended for this. On the contrary, the RCD detects and disconnects earth fault currents (see. (See Damage Conditions section of the article).
P.S.
I highlighted some paragraphs with pictures, because the Zen editor does not allow creating superscripts or subscripts.
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