Because of the sensitive nature of the vital and essential consumers with regard to personnel safety and production continuity, it is established practice to supply their associated switchboards with dual, or occasionally triple, feeders. For non-essential switchboards it may be practical to use only one feeder.
For switchboards other than those for the generator or intake feeders it is established practice to add some margin in power capacity of their feeders so that some future growth can be accommodated. The margin is often chosen to be 25% above the TPPL. If the feeders are plain cables or overhead lines then it is a simple matter to choose their cross-sectional areas to match the current at the 125% duty.
For transformer feeders there are two choices that are normally available. Most power transformers can be fitted with external cooling fans provided the attachments for these fans are included in the original purchase order. It is common practice to order transformers initially without fans and operate them as ONAN until the demand increases to justify the fan cooling. Thereafter the transformer is operated as ONAF, see sub-section 6.5. Adding fans can increase the capacity of the transformer by 25% to 35%, depending upon the particular design and ambient conditions. The alternative choice is simply to rate the ONAN transformer for the 125% duty, and initially operate it at a lower level. The decision is often a matter of economics and an uncertainty about the future growth.
When standby or future capacity is required for transformers it is necessary to rate the secondary cables or busbars correctly at the design stage of the project. Likewise the secondary circuit breakers and switchgear busbars need to be appropriately rated for the future demand. The decision to over-rate the primary cables or lines may be made at the beginning of the project or later when demand increases. Again this is a matter of economics and forecasting demand.
For switchboards other than those for the generator or intake feeders it is established practice to add some margin in power capacity of their feeders so that some future growth can be accommodated. The margin is often chosen to be 25% above the TPPL. If the feeders are plain cables or overhead lines then it is a simple matter to choose their cross-sectional areas to match the current at the 125% duty.
For transformer feeders there are two choices that are normally available. Most power transformers can be fitted with external cooling fans provided the attachments for these fans are included in the original purchase order. It is common practice to order transformers initially without fans and operate them as ONAN until the demand increases to justify the fan cooling. Thereafter the transformer is operated as ONAF, see sub-section 6.5. Adding fans can increase the capacity of the transformer by 25% to 35%, depending upon the particular design and ambient conditions. The alternative choice is simply to rate the ONAN transformer for the 125% duty, and initially operate it at a lower level. The decision is often a matter of economics and an uncertainty about the future growth.
When standby or future capacity is required for transformers it is necessary to rate the secondary cables or busbars correctly at the design stage of the project. Likewise the secondary circuit breakers and switchgear busbars need to be appropriately rated for the future demand. The decision to over-rate the primary cables or lines may be made at the beginning of the project or later when demand increases. Again this is a matter of economics and forecasting demand.
