What is nulling?
Nulling — deliberate electrical connection of metal not current carrying parts with the grounded neutral point of the source of the electric power for the purpose of automatic shutdown of the site at fault to frame.
The zero protective conductor — the conductor connecting zanulyaemy parts to the grounded neutral point of the winding of the source of the electric power or its equivalent. It is used for the power supply of the electric unit.
Principle of action of nulling
At fault to frame nulling creates the chain of single-phase KZ that causes operation of the maximum current protection (due to course of the single-phase current of KZ), and the damaged electric unit is disconnected from network. At the same time in the period from the moment of fault to frame before shutdown of the electric unit there is the undervoltage of the body of the damaged electric unit concerning the earth because of redistribution of tension between phase and zero protective conductors and availability of repeated grounding of the zero protective conductor.
Nulling limits time of influence of current for the person and reduces touch tension.
Nulling scope: three-phase four-wire alternating current mains with the grounded neutral up to 1 kV; single-phase two-conductor alternating current mains with the grounded conclusion; three-wire direct current mains with the grounded average point of the source.
As the maximum current protection can be applied: the safety fuses or automatic machines installed for protection against KZ currents, magnetic actuators with the built-in thermal protection; the contactors in combination with the thermal relay who are carrying out protection against the overload; automatic machines with the combined releasers.
For ensuring operability of nulling the conductivity of zero protective conductors has to be chosen such that at fault to frame or on the zero protective conductor there was KZ current exceeding not less than by 3 times rated current of the fusible element of the next safety lock; by 3 times rated current of the non-regulated releaser or the setting of the adjustable releaser of the automatic cut having the characteristic, back dependent on current.
At protection of networks has to provide with the automatic cuts having only the electromagnetic releaser, conductivity of the specified conductors current not below the setting of current of instant operation, increased by the coefficient considering dispersion (according to factory data) and on coefficient of the stock 1,1.
In the absence of factory data for automatic cuts with rated current to 100 A frequency rate of current of KZ concerning the setting it is necessary to accept not less than 1,4, and for automatic cuts with rated current more than 100 And — not less than 1,25.
Total conductivity of the zero protective conductor in all cases has to be not less than 50% of conductivity of the phase conductor. On overhead power transmission lines nulling has to be carried out by the zero working wire laid on the same support as phase wires.
On the ends of air-lines (or branches from them) more than 200 m long and also on inputs from air-lines to electric units which are subject to nulling repeated grounding of the zero working wire have to be executed. At the same time first of all it is necessary to use natural grounding conductors, for example underground parts of support and also the grounding devices executed for protection against the threshold overstrain.Top