April 2010 Electrical Apparatus

April 2010 Electrical Apparatus

This is a summary of the Electrical Apparatus April 2010 featured technical article,  by Richard L. Nailen, P.E.    

The speed-torque curve is the essential indicator of an a-c motor’s ability to accelerate its load. It’s a plot of available shaft torque at all values of RPM from standstill to the maximum running speed allowed by the connected load. Except for those two end points, the curve displays only transient conditions, because the motor will not remain in stable operation at any intermediate speed unless forced to by load counter-torque.

From its 1997 introduction to the U.S. market, the arc fault circuit interrupter, or AFCI, has been controversial. Among those favoring its use to protect residential receptacle circuits are the National Electrical Manufacturers Association, the National Fire Protection Association, and Underwriters Laboratories.

The purpose of an AFCI is to detect any arcing fault on the protected circuit, whether between conductors or across the ends of a separated conductor, and quickly operate a breaker to open the circuit. Key to AFCI operation is electronic circuitry that distinguishes between arcing faults and such normal disturbances as brush sparking in an appliance motor.

Widespread AFCI use is expected to reduce the incidence of home fires in which several hundred lives are lost annually. The National Electrical Code therefore mandates AFCI installation on all residential branch circuits, whereas early codes covered only bedroom circuits. Several types have been developed, for installation at panelboards, on outlet boxes, or as plug-in devices. Whereas two receptacle circuits formerly required two separate AFCI’s, a single two-pole device is now available for such applications.

Although UL testing subjects an AFCI to a wide variety of possible arcs, to ensure device operation only on a genuine fault, user complaints indicate that nuisance tripping still occurs. Electricians and home builders complain that AFCI installation adds too much to new home cost, or that field experience doesn’t yet justify the requirement.

Other concerns: although users are urged to test their AFCI’s monthly, test methods have caused controversy. An AFCI that fails test should be replaced–but that requires a qualified electrician. Some arcs can cause fire to break out before the AFCI has time to operate. An AFCI cannot protect against overheated terminations in the absence of an arc. Finally: how many fires do result from arcing, and how long may it be before all circuits are protected?

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