Page last updated Oct 20, 2021 @ 10:04pm


A backup power system must be installed and used properly to avoid personal injury and/or damage to property.



Backfeed Safety

Special precautions must be taken whenever an independent electrical power source is connected to a building electrical system.  This is because any inadvertant backfeed into the utility system would be a hazard to repair personnel and possibly to neighboring customers.   It would also be dangerous when the utility power is restored since the relatively small backup generator could be destroyed.  For example, consider the situation of a home that has lost power due to a fallen tree.  In the figure below, the tree has taken down the high voltage line near the service to this home.

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The home generator has been started and is providing power to the electrical circuits in the home.  However, if there is no transfer switch and the main service disconnect is left in the closed position, voltage will be conducted outside and through the service drop up to the pole-top transformer.  Utilities use a number of standard distribution voltages depending upon the area of the country.  If the voltage in use is 12,470/7,200 V, the transformer will step the 120/240 volt service back up to 7,200 volts!   In the figure above, this voltage would appear on the normally bare, but in this case broken and dangling, conductor creating an extremely dangerous shock hazard.  A utility worker, and anyone else who happens to touch that conductor would most likely be killed.

Even in cases where there is no downed wire, there are risks if a transfer switch is not used.  For example, assume the fuse on the pole-top transformer opens causing a loss of power, and the home generator is started and connected without opening the connection to the pole.  A utility worker could arrive, replace the fuse, and restore the service.  The restored utility power would at a minimum damage the generator, and at worst cause a fire.

Various hypothetical conditions can be imagined to create other similar backfeed hazards, even for underground services powered from pad mounted transformers.

In some localities, it is actually against the law to connect a permanently installed or portable generator to another power source.  Article 702.6 of the National Electric Code requires the use of a transfer switch to prevent inadvertent backfeeding of electric power from generator into utility system which can be a safety hazard.  If you own or operate a backup power source, you are responsible for ensuring that its power cannot backfeed into the utility system.

An additional consideration is the homeowner's insurance.  If the home is damaged by fire or someone is injured or electrocuted as a result of improperly connecting a generator without a transfer switch, insurance claims might not be paid.

In conclusion, if you have a backup power source, there are only two correct choices:

  1. Do not connect it to your building's wiring at all--just use extension cords, or
  2. If the source will be connected to the building's wiring, install a two position
    transfer switch or transfer panel to prevent backfeed into the utility.

Have a licensed electrician install your backup system, or at least have it inspected by a licensed electrician or your local authority.  Some utilities also request that you notify them if you have a backup power system.


Engine Operating Safety

Whenever an engine is used, there are hazards associated with the flammable fuel and with the combustion products, particularly carbon monoxide.  There is some degree of risk also related to contact with hot parts of the engine, fast rotating parts, etc.



Electrical Safety

Even though your backup power source is most likely limited in capacity, it can still cause electrocution and overheating if not used properly.

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Battery Safety

Batteries are usually installed and forgotten until they need to be replaced.   They normally operate quietly and reliably.  This is true of both dry cells and for lead acid batteries used in vehicles and in deep cycle applications.   However, lead acid batteries can be dangerous due to the fact that they contain sulfuric acid and because they produce explosive hydrogen gas inside the case when charging.  When they are used properly and as long as the case and caps or vents are intact they cause no problems.  However, if the case has cracks or gaps around the posts, or if the vents or caps allow a path for hydrogen and external sparks to meet, there can be a hydrogen explosion.  Therefore, precautions should always be taken when working around lead acid batteries especially when making or breaking electrical connections for powering inverters.


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