Ground Wiring - Lenel OnGuard Hardware Installation Manual

Hardware Installation Guidelines
2.2.4 Cabinet Tamper
Cabinet tamper for all enclosures must be connected and programmed for UL installations.
2.3

Ground Wiring

Each hardware product must be grounded to provide ESD protection, personnel safety, and signal reference
for devices which communicate with each other. Grounding provides a good shield against external
transients. See the installation manuals for the grounding point of each product.
There are three types of circuit grounds in systems using hardware products:
• DC negative
• RS-485 signal ground
• Safety ground
2.3.1 DC Negative
The DC ground provides signal reference for devices to communicate. It is the DC return from the power
supply.
2.3.2 RS-485 Signal Ground (SG)
The RS-485 signal ground is connected to the DC ground internal to a device through a current limiting
resistor. It provides a signal reference for the RS-485 interface.
2.3.3 Grounding System
A grounding system can be viewed as two subsystems: the DC system and the ground system. The DC
system consists of all interconnected power supply returns, DC distribution wiring, and load devices. The
principal function of the DC system is to provide signal reference for communication. The ground system
consists of all chassis grounds for power supplies and other devices, safety grounds, and AC grounds.
Ground connection should be made to avoid ground loop problems.
Ideally, there should be ONLY ONE ground return point in a power supply system.
2.3.4 Safety Ground
Safety ground (copper wire of 16 AWG minimum) is part of the AC power system. To avoid ground loop
current, there must be NOT more than one point at which the safety ground connects to the DC ground.
The RS-485 signal ground must be isolated from the safety ground. This means that the RS-485 cable shield
must be insulated so that it will NOT accidentally short circuit to the conduit in instances where the conduit
is connected to the safety ground.
The National Electrical Code and other safety regulations require that all equipment chassis and or
enclosures be grounded in order to prevent shock hazards. Each device must have a green wire safety
ground. The function of the green wire safety ground is to provide a redundant path for fault currents and to
insure that the circuit breaker will open in the event of a fault. In addition, grounding the enclosure provides
a path for ESD dissipation, thus protecting sensitive electronic devices.
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