Here is our nomination for the good-sense statement of the week:
Connecting two supply sources is a task not to be taken lightly.
This comes from a recent article on the Web site for the National Electrical Code which is run by the National Fire Protection Agency. It covers two parts of the NEC that pertain to electric charging stations for electric vehicles and to connections between the electrical grid and alternative power sources -- principally, photovoltaic systems.
Article 625 is the part of the code that pertains to electric vehicle charging systems. It divides indoor charging systems into those that need ventilation and those that don't. Among the measures spelled out in this article is that chaging systems that need ventilation need more ventilation if there are multiple vehicles under charge. And there needs to be some means of cutting the system power if somebody drives away with the charging cord still connected.
There were revisions to this article during the last go-round that mainly pertained to definitions. But off-road vehicles, such as golf carts and fork–lifts are still outside its scope.
Article 705 is the one that pertains directly to PV installations. Among its requirements are that there be a safe means of interconnecting power sources in a variety of circumstances. For example, when there are two power sources interconnected, both systems need some kind of disconnect mechanism and overcurrent protection. Article 705 also provides details on where the interconnection can occur – whether on the supply side or load side of the service disconnecting means.
And there are special problems that can crop up if the grid suddenly goes down. In that case, the alternate source could attempt to supply the grid, so the inverter design must be such that it can protect the grid by immediately sensing this condition and disconnecting itself.
The full article is on the NEC site: http://www.necplus.org/Features/Pages/GreenTechnologies.aspx?sso=0
