Short circuiting straps (Shorting Strap) are used to short the positive and the negative circuit of a D.C. feeder section. In the event of an emergency or inadvertent re-energization of the supply the devices cause the supply protection to operate.
Each short circuiting strap consists of three J117-NSE rail clamps fitted with suitably rated interconnecting flexible copper leads. The clamps are constructed from steel and are zinc galvanized to provide protection from site atmospheres.
The body has an insulating shield fitted to prevent welding to the rail should the clamp be subjected to a fault current.
Secure location upon the foot of both ‘flat-bottomed’ and ‘bull-head’ rail types is provided by a hardened tip above the rail and a grooved block beneath the rail. The hardened tip fits into the operating screw with an insulating bush to ensure that the correct contact path is made.
The grooved block fits directly on top of the copper busbar ensuring a very low contact resistance. The busbar sits upon a stack of disc spring washers, which should be fully compressed when the clamp is applied. These washers apply a pre-loaded pressure upon the rail in order to maintain firm contact.
Short circuiting straps are to be used in accordance with the requirements of Railway Group Standard GO/RT/3091 and Railtrack Working Instructions for the Drayton Park – Moorgate (BR29987/9) and Working Instructions for DC Electrified Lines in Liverpool area (BR29740).