Karins Notes - Electrical



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    Clayton Cowell

    Every building must have a disconnect. (there are fine points to this but just know you have to be able to turn the power off to every building)

    In “small” buildings that have a small service (like a house size) the meter is capable of being the disconnect.  (all the electricity flows through the meter, so removing the meter disconnects power to the building)  We would normally also have a disconnect inside the main panel (main circuit breaker) since the utility would get mad that you broke the seal and removed the transformer.

    In medium and large buildings the meter simply is a readout of the CT(current transformers) that the main electrical wires pass through before going into the building.  That is why you see a large CT cabinet outside buildings with a meter nearby.  They have to run all the wires through the CT’s inside that box.  From there the power then typically goes to a single large disconnect or main circuit breaker.

    Some utilities will provide some kind of switch in the primary system at High Voltage for their own maintenance purposes but it is not operable by the building personnel.

    I have a few issues with the diagrams.  Keep in mind for 98% of instances the main switch is actually the main circuit breaker in the main switchboard or panel in a building.  Just remember that you always need a disconnect (main circuit breaker or fused switch) between the utility transformer and the building which is 98% of the time a main switch in the main electrical gear.

    If you have a super big building you may be fed with primary voltage from the utility (in which case you may own the high voltage transformer and its associated maintenance which is a big deal)  You would then have a very similar set up for a main switch or switches in high voltage gear which then would feed the transformer then through another main disconnect switch which would then in turn feed the building electrical loads.  It is extremely rare to meter high voltage.  Metering is typically done after the utility transformer and just ahead of the disconnect switch.

    Disconnects generally are manual.  They should operate automatically for overcurrent or a ground fault.

    A disconnect switch is the means to protect the building electrical system from fault or overcurrent as well as servicing/maintenance in the building.  An automatic transfer switch senses if the normal electrical service is active.  If it senses a loss of power it will signal the generator to start and after the generator is up and running to tolerance it will internally disconnect from the normal/utility service and transfer to the generator power so that loads connected to it are provided with power from the generator.  It is normally fully automatic, especially if it is a life safety system.  Manual transfer switches are rather rare and certainly only optional standby power loads.  A manual switch is mostly the same guts and operation it just doesn’t have the logic to tell a generator to start, etc.  It simply has a button that operates the internal switching.

    The confusing part in this diagram is that the meters in the medium and large buildings are not diagrammatically correct.  Meters are 99% after the utility transformer.

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    Jennifer Schuler

    Clayton, thank you. This is extremely helpful!! 

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