In accordance with the current Rules for the technical operation of electrical installations in devices up to 1000 V, in order to control the sensitivity of the protection to short-circuits, it is necessary to measure the resistance of the "phase-zero" loop. For this, devices with various connection schemes, accuracy and field of application are used.
It is necessary
- - measuring instruments;
- - project documentation.
Instructions
Step 1
Review the design documentation and previous measurements and system tests in advance.
Step 2
Select and prepare instruments for monitoring or measuring parameters of grounding devices. It can be an M-417 resistance meter, an EKO-200 voltage meter, an EKZ-01 device. High measurement accuracy is ensured by the combined use of an ammeter and a voltmeter.
Step 3
Determine the control electrical receivers for testing. Measurements should be carried out on the most distant devices, allocating at least 10% of their total number.
Step 4
Use the formula Z = Z1 + Z2 / 3 for calculations, where
Z - resistance of the "phase-zero" loop;
Z1 is the total resistance of the loop wires;
Z2 is the resistance of the supply transformer.
For copper and aluminum conductors, the loop resistance should be about 0.6 ohm / km.
Step 5
Knowing the resistance of the "phase-zero" loop (Z), determine the current of a single-phase earth fault: I = U / Z. If it follows from the calculations that the frequency of the circuit is more than 30% higher than the permissible parameters of the protection operation established by the rules for electrical installations, carry out additional measurements of short-circuit current with the EKZ-01 device.
Step 6
When using the ammeter-voltmeter method, disconnect the electrical equipment to be tested from the mains. Measure with AC through a step-down transformer. To measure, you will need to perform an artificial short circuit of one of the phase wires to the body of the electrical receiver, guided by the connection diagram (see Fig. 1).
Step 7
Check the measuring current I after the voltage U is applied to the electrical receiver. It must be at least 15-20 A. Determine the loop resistance value using the formula Z = U / I. Add the resulting Z value arithmetically to the calculated resistance value of one of the phases of the working transformer. Formalize the measurement results in the form of executive documentation.
