GROUNDING Considerations

The first step in providing effective personnel and equipment protection is preparing a low resistance grounding electrode or grounding electrode system at each equipment housing room.

Once the low resistance earth ground is established as described above for a signal housing the apparatus in the house should be connected to the earth ground as described in the following sections.

6.2.3.1 Earth Rods

When installing the outside earth rods, begin by auguring out the holes for the earth rods. Use a 10 ft rod, which is a standard length and practical for most installations. Augur the holes using an 8-inch diameter bit and drill for a depth: of 8 ft. Place the rod in the center of the hole and drive it into the earth 3 ft. This will put the head of the ground rod about 1ft below the ground surface. A trench of 24 inch deep and 1 ft wide should be made between all rod holes.

The rods will sit concentrically in the augured holes and remain in place by the 3 ft driven depth at the bottom of the hole. Please refer fig. 6.1

Earth rods are subject to corrosion and installation procedure. As the earthing system is to have a long life span, comparable to that of installation, all earth rods should be to international standards. The earth rods should be molecularly bonded copper rods of size 50 mm dia.

Coupling between successive ground rods for greater depths shall be via Exothermic welding to UL 467 for greater reliability and a longer life span. Once the joints between Exothermic welding rods are secure, the augured holes and interconnecting trenches are to be filled with a sodium bentonite (or similar proprietary compound) water mix. The sacks of sodium bentonite are poured into the holes simultaneously with water until the top of the ground rods are just covered. The trenches are then filled with sodium bentonite and water mixture also to just cover just up to the copper straps. The remaining trench depth can be filled with ordinary soil.

The distance between adjacent earth rods shall not exceed twice the driven depth. In our case the distance between two earth rods should be less than 20 ft. Such earth rods installed along the perimeter of the equipment room and power room will be coupled together as explained in the following sections forms the building perimeter earth. This ring should be at a distance of 4-6 ft from the outer wall of the building.

Apart from the earth rods that form the perimeter earth, a separate earth rod is to be installed near the equipment room of the building. This earth rod forms the Low Voltage Earth Pit, which will be connected to the star connected equipment earth and perimeter earth. Please refer fig.6.2

Another earth rod shall be installed near the panel room of the building in case of signal installation where operator panel is housed in a separate building. This earth rod and the equipment room perimeter earth are coupled through copper tape as explained in the following sections.

It is recommended that all connections underground be exothermic bonded to UL 467 for long-term reliability.

LIGHTNING AND SURGE PROTECTION

Fig 6.1 installation of out side earth rod

PARIMETER EARTH & REB-REP BONDING

Fig: 6.2 Parimeter earth & REB-REP Bonding

LIGHTNING AND SURGE PROTECTION

6.2.3.2 Earth Resistance & Transient Impedance

The following values stated are the minimum earth resistance requirements prior to bonding the Low voltage earth pit (LV earth pit) to the building perimeter earth. The values apply to the readings obtained at the test point of each earth conductor.

Earth measurement points Resistance Surge Impedance *

LV Earth pit 1 Ohm 30 Ohms

Building perimeter Earth 1 Ohm 30 Ohms

Panel room Earth pit 1 Ohm 30 Ohms

Notes:

* In areas of high lightning activity it has been found that the surge impedance of the earthing system is a very critical actor. The surge impedance stated above is a recommended value.

For lightning, the critical factors are the rate of rise of current, which is of the order of microseconds. Therefore, the inductance of the earthing system is the predominant factor (in comparison to resistance) during the rising stage of the surge. Hence, in an installation with sensitive electronic equipment, the surge impedance needs to be kept as low as possible. To minimize the surge impedance, it is strongly recommended that.

9 A radial earth system should be employed at the points, where earth conductors terminate at earth pits.

9 Copper tapes, which have a much lower inductance than copper cables be used as the bonding conductors (equivalent in circular cross sections to the bond wires recommended, this is to minimise the skin effect and flux exclusion effect).

6.2.3.3

Room earth bar is provided in each room for terminating directly. The ground connections are connected to all the equipments in the room and form the interface point with the external earth system. The room earth bar (REB) shall be a tinned copper bus bar to 881432 Grade C101 having the following dimensions 675 x 50 x 6 (mm). The REB shall be predrilled with 12 holes of M10 size suitable for termination of bolted cable lugs. This size will accommodate the required number of connections with spare capacity as required.

There shall be one REB for each room, which includes the Panel room; Equipment room and Power supply room.

(a) To avoid circulating earth loops, the REBs shall be insulated from the building structure. Each REB shall be installed against the wall, with low voltage insulator spacers of height 60 mm and 6 KV insulation. Insulators shall be of halogen free conforming to UL94 fire rating. The REBs shall be installed at the following heights in the respective rooms:

(b) Panel Room at 0.5 metres.

(c) Equipment room at 0.5 metres.

(d) Power supply room at 0.5 metres.

All terminations on the REB shall be by bolted lugs with spring washers. The earth leads shall however be bonded to the lugs by cad welding (exothermic bonding) to UL467.

RES-REB BONDING CONDUCTORS

6.2.3.4

To minimize the effect of circulating earth loops, noise pick-up and to provide equipotential bonding, star point earthing is required which is to be achieved as follows.

Equipment room and Power supply room REBs mentioned above should be directly connected to the REB within the Power supply room. To facilitate this, it is proposed that a REB be installed within the Power supply room at a height of 0.5 m.

For low inductance and to minimize skin effect, flat copper tape to BS1432 Grade C101 shall be used as the bonding conductors. Furthermore, for mechanical rigidity, the copper tape shall have a cross section of 6 x 25 mm.

To avoid circulating earth loops, the bonding conductors shall be insulated from the building structure. The bonding conductor shall be installed alongside the wall and ceiling with low voltage insulator spacers of height 60 mm, 6 KV insulation, at an interval of every 0.6 meter.

The insulators shall be of halogen free conforming to UL94 fire rating. When the conductor passes through the wall it shall be sleeved and insulated accordingly.

The installation of the bonding conductors shall have a bending radius not less than 200 mm. These bonding conductors will be routed as illustrated in fig. 6.2

The bonding conductors shall be bonded to their respective lugs by exothermic bonding to UL467.

6.2.3.5. Earth Conductors

Earth conductors are typically those conductors that carry the surge to the main point of injection into the ground. These would typically be,

(a) Copper conductors for equipotential bonding or fault protection, that come out of the building and which are finally terminated in an earth pit.

(b) The copper conductors that come down the sides of a building (for lightning protection) and which are finally terminated in an earth pit.

It is recommended that the earth conductors are copper tapes instead of copper cables to minimize the effect of inductance and skin effect.

The earth conductor from equipment room REB to the LV earth pit should not be less than 3 x 25 mm copper tape to SS 1432 Grade C101. Similarly, the earth conductor from panel room REB to its earth pit should be 3 x 25 mm copper tape. Where this earth conductor is buried in the ground it shall be mechanically protected. For diversity, it is proposed that there shall be two such earth conductors up to the first LV earth pit and then on to the nearest building perimeter earth pit.

It is recommended that the termination of earth conductors to the earth pit of the building perimeter earth be exothermic welded to UL467 to achieve long term reliability.

6.2.3.6 Equipotential Bonding of Earth Systems

Bonding of the LV earth pit to the building perimeter earth should be done external to the building i.e. neither earth pits nor lightning earth conductors should be brought into the building.

LIGHTNING AND SURGE PROTECTION