Downstream Switchboard

Energising a downstream switchboard throngh a transformer or plain interconnector. The upstream switching device is closed first. The downstream device is then closed. If either trips on fault then the other may be caused to trip by auxiliary circuits and relays. 

Very and extremely inverse relays are used in systems where the fault level downstream is low when compared with that available at the point of main supply, e.g. a main generator switchboard. They are also nsed where coordination with upstream or downstream fuses is necessary. 

The type of feeder cables described in this section are those between switchboards within an oil industry site, rather than those between a utility power plant and an oil industry site. These feeders may be described as primary feeders as opposed to secondary feeders downstream in the system. Feeders from a utility power plant or a transmission network have protective relaying systems that are more sophisticated than those described herein, e.g. multi-zone distance protection, admittance relays, carrier protection schemes. 

Each switchboard will supply power to each load connected to it and in many cases it will also supply power to switchboards or distribution boards immediately downstream. Hence the input power to a... 

Care should be taken in the design of systems such that critical circuits are not used with series-rated systems. In the case of our 200,000-AIC main, if there were a high current fault at a downstream sight, the 100,000-AIC circuit breaker would interrupt and it would also cause the 200,000-AIC main circuit breaker to interrupt simultaneously, which would terminate power to the entire distribution system. If this were used in, say, a hospital, you would not want a short circuit in a panel board or a downstream distribution switchboard to also cause the main circuit breaker to trip, taking aU power out of the hospital. In those types of critical situations, the minimum circuit breaker AIC rating should be no less than the available system fault current delivered by the utility. 

---