Frame Leakage Protection of Busbar or Fault Bus Protection:

Frame Leakage Protection of Busbar or Fault Bus Protection is one of the most simple form of protection and is applicable to small size metal-clad switchgear. This method consists of insulating the bus-supporting structure and its switchgear from ground, interconnecting all the framework, circuit breaker tanks etc. and providing a single ground connection through a CT that feeds an overcurrent relay as illustrated schematically in Fig. 13.10. The overcurrent relay controls a multi-contact auxiliary relay that trips the breakers of all circuits connected to the bus, as illustrated in Fig. 13.11. Sometimes an impedance is connected in the earth connection to limit the short-circuit current during line-to-earth fault. Also it is necessary to isolate the switchgear framework from lead cable sheaths, cable boxes and the conduit fittings so that when a leakage to the framework occurs, the only path for the leakage current is through the connection from the framework to earth. An external flash­over of an entrance bushing will also improperly trip all breakers unless the bushing support is insulated from the rest of the structure and independently grounded.

In case of a sectionalized bus structure, the housing of each section must be insulated from adjoining sections, and separate fault bus-relaying is provided for each section. The fault-bus scheme does not provide overlapping of protective zones around circuit breakers; and consequently supplementary relaying is required to protect the regions between bus sections.

In the schematic arrangement of Frame Leakage Protection of Busbar illustrated in Fig. 13.10, the metal supporting structure or fault bus is grounded through a CT, the secondary of which is connected to an overcurrent relay. Under normal working conditions, the relay remains inoperative but fault involving a connection between a conductor and grounded supporting structure will result in current flow to ground through the fault bus, causing the relay to operate. The operation of the relay will trip all the breakers connecting equipment to the bus.

Figure 13.11 illustrates the scheme in which the station bus-bars are supplied from a power transformer having star- connected secondary. A CT is also connected in the earth connection of this star-connected secondary. The secondary of CT is connected to the operating coil of the check relay.

From the scheme illustrated in the figure it is obvious that the multi-contact relay is energized only when both the leakage and check relay contacts are closed i.e., there is an earth fault within the protected zone. However, if the earth fault occurs outside the protected zone, the earth leakage current will pass through only the earthed secondary of power transformer and the check relay contacts will close but the frame leakage relay contacts will not close and thus multi-contact trip relay will not operate.

Fault-bus protection is more favourable to indoor than to outdoor installations. Certain existing installations may not be adaptable to such protection, owing to the possibility of other paths for short-circuit current to flow to ground through concrete reinforcing rods or structural steel.