Alpha Technologies FlexNet MPS48-12M - Technical Manual - Manual (Page 10)

10 021-514-B2-001, Rev. A
Grounding Connection Notes
In order to provide a ready, reliable source of backup power, it is necessary to establish a grounding system
that provides for the safety of service personnel responsible for operation and maintenance, and facilitates
the proper operation and protection of the equipment within the network. The grounding system must provide
protection with respect to operator safety, system communication, and equipment protection.
Safety Ground
The safety ground is a two-part system. The fi rst part is a return path for stray current back to the input
breaker, and the second is a return path from the Alpha enclosure to a second ground rod.
Typically, the safety, or utility ground, provides a return path to the input breaker or fuse panel by means
of a connection to an appropriate driven ground rod at the base of the power pole. This path must meet all
applicable codes to ensure the breaker will open, preventing unwanted current fl ow from posing a hazard to
service personnel.
The second part of the safety ground arrangement is the ground path between the Alpha enclosure and a
second ground rod located at least 6 feet away from the driven ground rod at the power pole. The second
ground rod and enclosure are connected via an AWG #6 solid copper wire buried at a depth of 8-12 inches.
The wire is connected to the cabinet by means of a ground lug on the back of the cabinet (for pole-mounted
enclosures), or to a ground lug inside the cabinet (for ground-mounted enclosures), and connected to the
ground rod by means of a listed grounding clamp suitable for direct burial, or exothermic weld. Normally it is
specifi ed that the impedance of this ground can be no greater than 25 ohms at 60Hz. If, however, dual ground
rods are installed approximately eight feet apart, it is not necessary to measure the impedance of the ground
rods to meet the maximum 25 ohms specifi cation—it is assumed that the impedance specifi cation is met.
Strike (Lightning) Ground
Lightning strikes, grid switching, or other aberrations on the power line have the potential to cause “fast rise-
time currents" that can cause damage to the powering system. Without a low-impedance path to ground,
the current, while traveling through wires of varying impedance, can produce high voltages that can damage
the powering equipment. The most viable method available to protect the system from damage is to divert
unwanted fast rise-time currents along a low-impedance path to ground. A low-impedance path to ground
prevents these currents from reaching high voltage levels and posing a threat to equipment. The single-
point grounding system provides a low-impedance path to ground, and the key to its success is the proper
bonding of the ground rods, so the components of the grounding system appear as a single point of uniform
impedance.