Application of the article
In any electrical installation, and has certain current conductor to earth through the protected area.The current is commonly known as leakage current.Is the most common form of leakage current flows through the conductor insulation around, and around the home or office to protect electronic equipment filter.So what's the problem?In has the leakage protector to protect circuit, leakage flow, intermittent any unnecessary trip.In extreme cases, can cause can reach the conductor parts of voltage increases.
The reason of leakage power
Insulation has both resistance and capacitance, conduction current through the two components.In view of the insulation performance for high resistance, leakage current is actually quite small.However, if the insulation aging or damage, low resistance, large current may circulation.In addition, a long conductor capacitance is bigger, can cause larger leakage current.So the leakage protector manufacturers recommended maximum limit the length of the one-way feeder for 76.2 m.
At the same time, for voltage surge and other fault protection of electrical equipment also increase leakage current, such as filter.Usually these filter in the input capacitance, which further increased the total wiring system capacitance, and leakage current.
Reduce the leakage current
So, if to eliminate or minimize leakage current?Quantitative leakage current, and then identify the source of trouble.One way is to make use of the leakage current clamp table.These pliers table and used to measure the load current clamp type table is very similar, but when measuring current under five ma's performance is much better.Most of the clamp table just can't detect such a small current.
Conductor in the clamp table jaw clips, clamp table reads the current value depends on the intensity of magnetic field around a conductor.
In order to accurately measure the current, the need to make sure that jaw occlusal surface clean, no crack or damaged.Avoid twisting clamp table jaw, otherwise it will cause measurement error.
The power supply
load
Clamp table to detect the magnetic field around the conductor, such as single core cable, armored cable, pipes, etc., or phase line and neutral line of the single-phase circuit, or a three-phase circuit (such as leakage protector or residual current device) all of the charged conductor.
In the test circuit group charged conductor, load current of the magnetic field generated cancel each other out.Any unbalance current leakage from the conductor to the earth or other parts of the current.In order to measure the current, the leakage current clamp table should be capable of detecting current is less than 0.1 mA.
For example, in the measure ac 240 V circuit, all loads are disconnected, may cause 0.02 mA (20 mu A) leakage current.Said the value of insulation resistance is:
240 V/(20 x 10-6) = 12 M Ω.Ohm's law (R = V/I)
If in the circuit of power insulation test, the result may be 50 M Ω or higher range.This is because the dc voltage insulation tester is used for testing, without considering the effect of capacity.Insulation resistance value is the actual value under normal working conditions exist.
If the measured load of office equipment (PC, monitor, copier, etc.) of the same circuit, because of these devices on the input filter capacitor, the result may be significantly different.When the circuit has many sets of equipment, the effect will be accumulated together.That is to say, the leakage current is higher, could reach milliamperes.If the electric leakage protector in the protection circuit to increase the new equipment, may trigger the leakage protector.And because the amount of leakage current ways of working together with the equipments, so the leakage protector may be random trip.This kind of intermittent fault is very difficult to diagnose.
Clamp type table that can detect and measure the measured conductor through a wide range of ac current or change.When existing telecommunications equipment, clamp type table indicating the leakage current value may be far higher than 60 Hz insulation impedance results.This is because the telecom equipment often use filter, produce function grounding current, and harmonic.If you use through a narrow band-pass filter to filter out other frequency current clamp type table, can only measure the characteristics of leakage current under 60 Hz.
Measurement of the floor drain current
Connection with load (switch), measured leakage current leakage current of load equipment.If the load when the leakage current is low enough to accept, so the line even lower leakage current.If you need a separate line leakage current, then disconnect (switch off) load.
Phase line
Neutral wire
load
Figure 1
Single-phase circuit test:Grip the phase line and neutral conductor.The measured values for all the current flowing to the earth.(see figure 1)
load
Figure 2
Three-phase circuit for testing:In all three phase conductor.If there is a neutral line, should be clamped with the phase line.The measured values for all the current flowing to the earth.(see figure 2)
load
Exposed conductive parts
Figure 3
Phase line
Neutral wire
The grounding conductor
load
By accident grounded circuit leakage current
Figure 4.
Phase line
Neutral wire
panel
Leakage current IE
Figure 5
Through the grounding conductor leakage current measurement
For measuring flow to the total leakage current of special grounding connection, using clip-on table clamp grounding conductor.(see figure 3)
By accident grounded circuit leakage current measurement
Group line/neutral grounding clamp together, can identify the unbalance current, show the socket or panel leakage current, by accident grounded circuit (such as the panel installed on the concrete foundation) leakage.If there are other electrical joint connection (for example) is connected to the pipe, also can produce a similar imbalance.(see figure 4)
To track the leakage current source
This a series of measurements to identify the total leakage flow and the source of trouble.First of all to the measurement of main lines connected to the switchboard.Then perform item 2, 3, 4, and 5 measurements, identify the bearing have larger leakage current circuit (see figure 5).(see figure 5)
conclusion
Leakage current is one of the important instructions on the conductor insulation effectiveness.When connected with electrical equipment with a filter circuit to duck can appear higher leakage current, voltage will lead to interfere with the normal work of the equipment.Take advantage of low leakage current clamp table, methodically performed the measurement, can locate leakage fault source.If necessary, can redistribute installed around the load, accordingly make it more balanced.
