Future Design MCT-CM, User Manual

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MCT-CM
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3.2 Grounding Requirements
Proper grounding is critical to the performance and reliability of the MCT. Most of the problems in electrical
systems can be attributed to poor or improper grounding techniques, as well as poor earth grounds at the
installation site. Control systems like the MCT, which contain sensor and communication wiring as well as
sensitive electronics, are particularly acceptable to stray electrical transients.
Proper wire routing, away from sources of high-power AC or inductive DC circuits, and the use of shielded
cables for sensor and communications wiring must be considered carefully. Ground conductors should be as
short and as large in size as is feasible. Each ground conductor should be individually run back to the central
earth grounding point for the system. Ground conductors should not be daisy-chained or branched off from
one device to another. This can cause an imbalance in the ground potential at each device in the system,
which can induce electrical currents to flow between devices along the ground path and result in damage to, or
malfunction of the device.
3.2.1 Special Considerations for Low Voltage System Grounding
A typical installation of the MCT controller will result in a combination of both high voltage (possibly 120Vac or
higher) and low voltage (24Vdc) components and wiring. While all components that are required to be grounded
must all attach to the central ground point of the system, randomly combining the grounds from both AC and
DC components to a central ground bar can introduce noise into the DC grounds. This is due to small ground
loop currents that can come across the ground bar due to leakage from AC components such as variable speed
drives.
It is good design practice, and is recommended if the installation includes variable speed drives, to install a
separate DC, isolated ground bar that is then connected to the central earth ground of the equipment through
a single conductor. This prevents ground currents of AC equipment from traveling across this ground bar and
inducing small voltage imbalances across the DC component grounds. While this may not be practical in all
installations, if problems arise in the system due to electrical interference, installing this type of grounding
system may reduce or eliminate the source of the problem.
NOTE: Even though the DC power supply for the MCT is powered from an AC source, the ground of the
power supply should be connected to the DC ground bar. This e
nsures that any electrical ‘noise’ or
stray voltages that come across the AC power wiring or DC power wiring is distributed evenly across
the DC ground bar to reduce the chance for voltage imbalances between different components of the
control system.