Belimo TRANE TAF Series, Technical Documentation Manual

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148
I20683 - Subject to change.
I-M. Multi-Conductor Wire Types
• “BELL WIRE” has parallel wires, which may act as an antenna and is therefore sensitive to elec-
trical noise. This type of wire should not be used for control circuits.
• “TWISTED PAIR” cancels out most of the electrical noise because the wires alternate their posi-
tions. This is the type of wire that is used for most control circuits.
• “SHIELDED WIRE” is a twisted pair that is surrounded by a metal foil or wire mesh which acts as
a shield and prevents electrical noise from reaching the wires inside.
Shielded wires are used for the actuators only if the electrical noise is very severe. Normally twisted pairs are sufficient.
Remember! The shield must be grounded in one point only!
I-N. Ground Loops
If a shield is grounded at both ends of a shielded wire, a ground loop is created. Ground loops will defeat the purpose of shielding,
and aggravate the electrical noise problem.
Ground loops can also be created by using more than one wire for signal common (COM
⊥
). The (-) signal common terminals on the
controller are usually interconnected. Therefore, a ground loop is formed when two or more signal common terminals of the controller
are wired to the same transformer. (See Figure 11-5 and 11-6, page 175.)
Signal common (COM
⊥
) is necessary, as a reference, but only one connection should be used.
Redundant signal common terminals should not be connected.
A ground loop acts as an antenna and will pick up electrical noise. This should be avoided, by using the correct wiring practice.
BELL WIRE
TWISTED PAIR
SHIELDED WIRE
SHIELDED WIRE
SHIELDED WIRE
SHIELDED WIRE
GROUND IN ONE POINT ONLY!
(CONNECT THE SHIELD TO GROUND)
GROUND ONE END ONLY! AT EACH JOINT, THE SHIELD HAS TO BE GROUNDED.
(BUT IN ONE END ONLY!)
THIS IS WRONG! THE SHIELD MUST BE GROUNDED IN ONE
END ONLY, OTHERWISE THERE WILL BE A GROUND LOOP.
GROUND LOOP!
Basic Electricity