Schemes of protective shutdown of electric units
The big danger is constituted by transition of tension to metal constructive not current carrying parts. The most perfect way of protection against emergence of hazardous voltage on constructive parts of electric equipment is protective shutdown.
Shutdown of electric units at faults to frame in this case is provided with the special devices which are automatically removing stress from installation. As such devices serve the automatic cuts or contactors supplied with the special relay of protective shutdown.
The relay consists of the electromagnetic coil which core in the deenergized state closes the contacts. Contacts of the relay are connected consistently to the feet button in the control circuit of the contactor.
At emergence of tension on clips of the relay coil and course on it current, sufficient in size, the core of the coil is involved and disconnects the contacts in the control circuit owing to what the contactor disconnects the damaged current receiver from network.
Schemes of connection of the relay of protective shutdown can be different. So, the scheme of protective shutdown with the auxiliary grounding conductor in which the relay coil joins on the body of the protected object and on the earth is provided on fig. 1.
The electromagnet is adjusted in such a way that at emergence on the protected object of voltage of 24 — 40 V through the winding of the coil there passes current, the core of the electromagnet is involved under the influence of this relay disconnects the contact and there is the disconnection of the electric motor from network. Ground resistance can be very high (300 — 500 Ohms) that does grounding easily feasible.
One more scheme of protective shutdown is provided on fig. 2. The relay of protective shutdown is attached to the body of the protected object and to the point, the general for the columns connected to network from the selenic vypryamitelny plates connected in the star. The coil can be adjusted so that at course of current via it with a force of 0,01 And there is the retraction of the core and disconnection of contact of the relay with the subsequent disconnection of the object from network by means of the contactor.
Protective shutdown is applied in the following cases:
- in electric units with the isolated neutral to which increased requirements concerning safety, in addition to the device of grounding are imposed (for example, underground works, etc.);
- in electric units with the deadly earthed neutral up to 1000 V instead of connection of the body of the equipment to the grounded neutral if performance of this accession causes difficulties, at the same time the protected installation has to have the grounding device meeting requirements of electric units with the insulating neutral;
- in mobile installations when the device of grounding presents considerable difficulties.