Heating at Electrical Contacts

[bphgravity]

Here is an interesting article from FIRE TECHNOLOGY:

Abstract: Testing was performed using 120 volt, 15-amp receptacles and copper wire to determine the effect of torque and wire contact area on temperature elevation at receptacle screw terminals. Torque was varied on both the hot and neutral terminals from 0 to 12 in-lbs, and the apparent wire contact at the screw terminal was varied by 1/8″ and 5/8″. There was no significant difference in temperature when changing apparent wire contact. Increased temperatures were observed with reduced torque, however, they were not significant enough to initiate a glowing connection, nor high enough to cause rapid oxidation. Further testing showed that movement of a loose connection was necessary to cause significantly high temperature changes, arcing and sparking, rapid oxidation, and in some cases, glowing connections.

Here's the entire paper:

http://www.springerlink.com/content/y5838556404j3jv3/fulltext.pdf

 

[charlie tuna]

bryan,
sounds like a bunch of gobbily-gook to me. it says nothing. if you have a good surface area at a connection with proper torque --- you have no heating effects. if you double the surface area of that same connection, again, with proper torque --- you have no heating effects!!! whats the point?????????? proper connections are comprised of using proper materials for that connection and following the manufacturer's directions for torque requirements.

 

[bphgravity]

Here is the conclusion of the study:

Conclusion
It was evident that torque played a large role in the temperature rise at the contact. At torques of 4 in-lbs and higher on the hot screw, the temperature of the hot terminal did not exceed 40◦C, however at torques of 1 in-lb and lower the temperature varied greatly and in some cases reached as high as 90◦C. The torque on the hot terminal also affected the temperature of the neutral terminal. The neutral terminal reached a maximum of approximately 38◦C at hot terminal torques of 4 in-lbs and higher and up to approximately 80◦C at torques of 1 in-lb and less. The contact area was determined to not have a significant effect on the temperature rise at the terminals. The effects of torque on terminal temperature shows that a danger exists when torques are small, i.e. 1 in-lb or less. Leviton specifies a temperature rise of no more than 30◦C on the receptacle, as determined by UL 498. Tests showed that these temperatures where easily achieved at low torques. To achieve a glowing connection or extremely elevated temperature resulting in the melting and burning of combustibles, movement is required. Temperatures exceeded 300◦C on the screw heads when the wire at the connection as moved back and forward. It is important to note that the 15 A breaker in the circuit only activated under one condition when an arc occurred through char between the screw terminal and the ground blade of the plug in the receptacle. In all other cases, with and without movement, no breaker activation occurred.

The test series has shown that low torque on screw terminals produces temperatures exceeding those for which receptacles are rated. The hazard potential is significantly increased with the combination of a loose connection and movement.

It may be useful to characterize the vibration and movement that a household receptacle experiences over time to determine how the wire/screw terminal loosens over time. This research may ultimately determine the torques necessary to eliminate high resistance connections in residential receptacles, and will set the requirements for future installation
procedures.

 

[kingpb]

What's their point to all this, that you should pigtail and use the push back receps?

 

[wirenut1980]

I think the point to the article was to show that the amount of surface area of the wire making contact with the screw is not as critical to temperature rise, as is making sure that the connection has adequate torque. Inadequate torque can lead to the wire moving and to a high resistance connection leading to heat. Take it for what it's worth I guess...

 

[kingpb]

OK, now I'm going to ask, the receps stil have the push back, but can it be used? I can't see where it can't.

If so, wouldn't that get rid of the torque issue on the screw?

 

[LarryFine]

OK, now I'm going to ask, the receps stil have the push back, but can it be used? I can't see where it can't.

Yes, but you'll also notice that only #14 wire will fit; the holes have been sized to reject #12.