Substation Fixed Earthing

This equipment covers fixed devices for earthing 132kV substations having fault levels up to 5,000 MVA and 275 kV substations having fault levels up to 15,000 MVA. It was designed and developed by PBwel in conjunction with the CEGB and is manufactured to the CEGB Standard TIP 3/4 (Switchgear). Acknowledgement is due to the Electricity Supply Industry for permission to quote from the Standard.

Design and Construction

Each earthing device consists of a single-phase assembly shown typically in Fig.1 and having the following features: the operating pole is a glass fibre pole located vertically in two or more guides. The lower guide has a bolt located in the pole, thus preventing the pole from being raised. The bolt can be withdrawn only when freed by use of a conventional interlock key or padlock key.

A double hooked line end clamp is permanently attached to the top of the pole. One or two cables according to the rating are permanently attached to the line end clamp and to the nearest Earth bar.

A unique Earth bar clamp has been developed which is suitable for attaching to existing copper earth bars to give a fully rated permanently connection. A hole 8.73 mm diameter (11/32″) is required for each clamp. Ensure that the earth bar is clean back and front. If attaching to new earth bar secure aluminium or copper/aluminium compression terminal direct to clean (tinned in the case of copper) earth bar by 12.7mm (1/2″) diameter bolt through 8.73mm diameter hole ensuring that “shoulder” on fitting is hard against the edge of the earth bar.

The guides are attached to suitable steel adaptors which secure the equipment to convenient parts of structures provided for main equipment. They vary with the nature of the structure and are not supplied by PBwel, but a range of steel adaptors suitable for most applications is included in CEGB standard TPS 3/4. Interlock boxes for standard locks with 16mm (5/8″) long stubs are available for single way or three-way arrangements. The single way boxes will usually be fitted to two phases and the multi-way box to the third phase. Any number of locks can be added to the multi-way by means of extension boxes. If required, the equipment may be installed without interlocks but with provision for padlocking in the “open” position only.

The “lift” of the pole is limited by the distance between the bottom guide and the ground so that where the initial “lift” is insufficient, one or two extension poles are required. These extensions are of lightweight glass fibre with snap action sockets. A simple resting device is available which permits the pole to be supported at suitable locations so that both of the operator’s hands are free for fitting and removing extension poles. The line end clamp is suitable for rigid circular conductors or stubs of any diameter between 38 to 76mm (11/2″ to 3″) and is provided with one of three settings as required. The conductor or stub must be substantially horizontal. A series of busbar spigots are available each suitable for a prescribed position where the busbar is not substantially horizontal, or the busbar consists of a stranded conductor.

Special attention has been paid to the prevention of accidental or deliberate of the interlocking system. All current-carrying parts are of copper or copper-based alloys. Other metal parts are stainless or galvanised steel. It is not anticipated that there should be any deterioration of the equipment due to being located permanently outdoors, but glass fibre poles may require to be revarnished from time to time.

Technical Application

The equipment has been thoroughly tested for the fault ratings to be found on the CEGB’s systems. For fully interlocked devices, only one cable per phase is required irrespective of the fault level. For padlocked equipment, flexible earth leads must be provided according to the rating listed in Table 1.

Supply of Components

The components supplied by PBwel are as shown on figure on the right. The interlock locks and keys are not included. 

Table 1 - Performance and Characteristics

System Voltage kV Rated Fault Level Three Phase Symmetrical MVA Maximum Single Phase Fault Current r.m.s.kA No. of Cables Cross Sectional Area of Cable/s Sq. In.
132
2500
11.0
1
0.125
132
3500
15.4
1
0.125
132
5000
26.3
2
0.125
275
7500
15.8
1
0.125
275
15000
31.6
2
0.125

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