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The function and working principle of zero sequence current
(residual current / leakage current) transformer:
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Two abnormal phenomena occur when the electrical equipment leakage:
First, the balance of the three-phase current is destroyed, there
is zero sequence current;
The two is that when the normal non charged metal casing appears on
the ground voltage (normal, the metal shell and the earth are zero
potential).
The zero sequence current transformer is the leakage protection
device through the zero sequence current transformer detection to
obtain abnormal signals, through the intermediate mechanism to
transfer, so that the implementation of the action, through the
switch device to disconnect the power supply.
The structure of the current transformer is similar to that of a
transformer. It is composed of two coils that are insulated from
each other on the same core. When a coil has a residual current,
the two coil induces an electric current.
The zero sequence current transformer installed in the line, a coil
with the grid lines connected to and in the secondary coil and the
leakage protector tripping device connection.
When the normal operation of the power equipment for electric water
heaters, and line current is equilibrium, transformer current
vector and zero (current is the direction of the vector, such as
according to the direction of the outflow for "+", return direction
"-", back and forth in the transformer current are equal in
magnitude, in the opposite direction, plus or minus offset each
other). Since there is no residual current in a coil, so not
secondary induction coil, switch device for earth leakage
protection device in closed state operation.
When electric leakage electric water heater equipment shell and
human touch in a timely manner, at the fault point shunt, this
leakage current by human - earth grounding, return to the
transformer neutral point (not the current transformer), resulting
in transformer Shen into, out of the current appeared imbalance
(current vector and is not zero), a secondary coil Shen residual
current. Therefore, secondary induction coil, when the current
value reaches the value of the action current of the leakage
protector is defined, automatic switch trip, cut off the power
supply.