Choosing settings for security devices

The settings of the protective devices, as well as the rated fuse currents of the fuses, must be selected from the following conditions:

Condition 1… The rated current of the release or fuse must not be less than the rated current of the electrical consumer.

Condition 2… The protective device shall not trip the electrical receiver during normal operating overloads. To ensure this, you need the following:

a) rated fuse current fuse must be at least:

Aznagore peak / K,

where K is the coefficient.

At a low starting frequency and a short duration of acceleration (up to 5 seconds), K = 2.5. With a high starting frequency and a long acceleration time K = 1.6 — 2.5, for automatic switches, the setting of the thermal release should be checked by the time-current characteristic for the time of operation in the overload zone, and the electromagnetic release by the disturbance conditions from the inrush currents.

Condition 3... The settings of the protective devices must be checked for selectivity in tripping, that is, in such a way that for any violation of the normal mode, only the damaged section is tripped, but the protective devices in the upper links of the network do not work. The check is performed according to the characteristics of the current over time.

At currents exceeding the starting currents, the fuse or circuit breaker must first be tripped and only then magnetic switch (or contactor) for which the condition must be fulfilled:

T advance (automatic) <(t svz x K) / Kzap,

where T pre (auto) is the operating time of the fuse (breaker) according to the characteristic of the current over time, K is the coefficient equal to 1.15 and taking into account the deviation from the own time of the starter; T svz — self-time magnetic starter (or contactor); Kzap - safety factor equal to 1.5.

The security device settings adopted must meet the requirements PUE… When the distance of the receiver from the substation is large, it is necessary to check the operation of the protective device in the case of a single-pole circuit in accordance with PUE.

For thermal relays the rated current of the electrical consumer must be within the operating current of the heating element of the relay.

Selection of interrupting current

In the event of a short circuit, the motor must be switched off immediately. Interruption is done by momentarily de-energizing the relay. Breaking current iss are selected based on the starting current of the electric motor:

iop = Kzap x Azpik = Kzap x Kn x Aznom,

where, Peak — peak (start-up) current of the electric motor; KNS — the multiple of the starting current of the electric motor, Kzap = 1.3

Testing protective devices for short-circuit current stability

The stability of protective devices in the event of a short circuit is indicated in the catalogs and plant information, therefore the check is reduced to comparing these values ​​with the short circuit currents at the installation points of the devices.