7. The estimated test modes of electrical systems

7.3. Overcurrent protection

Protection devices must disable any overload current flowing through the conductors, before this current might cause increased conductor temperatures dangerous for the isolation of the compounds, clamps or the medium surrounding the conductors.

GOST R 50571.5-94 "Electrical installations of buildings. Part 4. Security requirements. Overcurrent protection" requires the following protective devices.

433.2. Consistency conductors and protective devices.

The performance of any safety device that protects the cable from overload, must meet the following conditions:

I a = n = z;

I 2= <1,45 I z,

where I in - operating current of the circuit; I z - allowable continuous current of the cable; I n - nominal protection device current (for the protection device with adjustable characteristics of the rated current I n is the current selected setting); I 2 - current, ensuring reliable operation of the protection device, practically an I 2 shall be equal to:

current response for a given response time for circuit breakers;

Melting current fuse for a given response time fuse.

Note. In certain cases, in order to prevent unintended operation of protection should take into account the current peak load. In the case of the cyclic load values I n and I z are selected based on the values of I and I z in thermally equivalent to a constant load.

Compliance with the parameters RCD conditions of the plant on the rated making and breaking capacity of the residual current is determined by the design by calculating the current value of the ground fault in the electrical installation.

Checking the RCD most important parameters - conditional short-circuit current calculated by the appropriate calculation of short circuit current in the electrical installation.

In all cases, the use RCD its parameters must comply with the maximum load currents and short-circuit currents.

The circuit-switching the RCD should provide consistent protection device (circuit breaker, fuse) that protects against overcurrent.

Rated load current RCD should be equal to or one rank higher than the nominal current of the protective device serial.