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General
1.5.10 High Frequency Shielding
Use of shielded cable is recommended when high frequency emissions or easily
disturbed signals are present. Examples are as follows:
- motor wires connected to inverters: connect shield to ground at both the inverter
and motor, NOTE the shield should never be used as the protective ground
conductor required by NEC. Always use a separate conductor for this.
- digital control wires: connect shield to ground at both ends.
- analog control wires: connect shield to ground only at the inverter.
The connection of meshed shields to the ground connection should not be done through
a single strand of the shield, but with metallic clamps to provide 360° contact around the
surface of the shield to the ground point. Connection with a single wire bundle from the
braided shield reduces the effectiveness of the shield 70%. Metal conduit clamps work
well for this. Be sure the fit is tight.
Ridged metal conduit can be used as the shield of the motor wires. Always observe the
following points :
- remove all paint from the control cabinet and motor housing where the conduit is
fastened
- securely fasten all conduit fittings
- run only the motor wires through the conduit, all other wires, high voltage AC
and low voltage signal, should be pulled through a separate conduit.
- connect the control panel to the Sub-panel with a heavy ground strap.
If KEBCO CE filters are used, they should be mounted to the inverter or as close as
possible to the inverter and on the same sub-panel as the inverter. Good metallic surface
contact to the sub-panel is required to provide adequate high frequency grounding of the
filter. Always use the shielding plate provided with the filter when connecting the filter
to the inverter.
Shielding of control wires:
If digital signal wires are terminated on a terminal block in the control panel, the
shields should be firmly connected to the sub-panel on both sides of the terminal
block.
The shields of digital signal wires originating outside the control cabinet which are not
terminated on a terminal block, must be connected to the sub-panel at the point where
the cable enters the control panel and at the inverter.
If the shield is terminated to the sub-panel with in 8 inches (20cm) of the inverter, then
the shield no longer needs to be connected to the inverter.
When using un-shielded signal wires, they should always be installed as a twisted
pair (signal and common).
Low voltage signal wires should cross high voltage wires at right angles.
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