Page 461 - IRSEM_Main Book

Section 11: Earthing for Outdoor Signalling Equipment

19.11.1 Types of Earths

(a) Earths are of two types

(i) Functional Earth: The earth return used for block instruments is a
functional earth which is used to conduct the current through earth
during normal operation/function of equipment.

(ii) Protective Earth: This earth for dissipating surges, which comes in
contact with equipment’s connections through Surge Protection Devices
(SPD) during Lightning or surges so as to protect the equipment.

(b) Perimeter Ring Earth (PRE) shall be provided around building housing
signalling equipment. Chasis of all S&T equipment shall be bonded to BRC
(Bonding Ring Conductor) provided inside the Power Equipment Room or
Signal Equipment Room. BRC is to be connected to MEEB in Power room
and from there to PRE as per extant guidelines. In case, no BRC exists,
chassis of the equipment shall be connected directly to MEEB (Main Equi-
potential Earth Busbar).

(c) Earthing shall be provided as per approved specifications, drawings and
code of practice issued by RDSO.

(d) The maximum earth resistance shall be specified as per OEM & RDSO
recommendations.

19.11.2 Purpose of Earthing

Earthing of cables, equipment, buildings and structures is done for one or
more of the following purposes:

(a) Lightning & surge protection of equipment.

(b) Earthing of metal screens of telecommunications cables and equipment for
reducing Electromagnetic interference.

(c) Human safety.

19.11.3 Earthing to be provided at

(a) The lever frame and other metallic frames of the cabin shall be earthed.

(b) The earthing shall be provided at every location box where cables
terminate.

(c) The earthing shall be provided at each signal.

(d) Wherever possible, the common earthing system to be provided for closely
located location boxes, signal posts, etc. Separate earth is required for
equipment requiring functional earth only.

Chapter 19: Installation, Testing & Maintenance of Signalling Equipment Page 409 of 535