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Wire being pushed into the back of a back-wired spring clip type receptacle (C) Daniel FriedmanElectric Wire Connector Clamping Force
Comparison of two back-wired electrical device connectors: push-in spring clip and screw connectors shows dramatic differences in clamping force that explains why some devices fail

  • BACK-WIRED ELECTRICAL DEVICES - CONTENTS: how to back wire an electrical receptacle (outlet) safely & which types of back-wired receptacles are unsafe. Some back-wired devices perform poorly in the field and risk circuit failure, overheating, or fire. Others may be fine. The distinction is described in this article.
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Electrical wire connectors at receptacles & switches: clamping force comparisons:

This article explains the calculation of clamping force in push-in type back-wired electrical receptacles or switches and offers a comparison of the relative clamping force of spring-clip wire connectors with that exerted by binding head screw or screw clamp connectors for electrical wires.

At page top we illustrate a #14 solid copper wire being pushed into the backwire opening of a common electrical receptacle. The wire has not been pushed fully into the connection as I wanted to show the diameter of the wire entering the push-in connector opening. Simple screw terminals are also visible in the lower left of the photo.

This article series illustrates and explains possible electrical failures and fire risk when using the push-in rear connectors on some back-wired electrical devices such as receptacles and switches. We illustrate the typical connector used in some receptacles and switches that accept a simple push-in connection usually found on the rear of the device. The rectangular opening is used to release an installed wire.



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Comparing Clamping Force in Two Electrical Wire-to-Connector Devices: Back-wired Spring-Clip vs. Binding Head Screw

Why is Electrical Connector Clamping Force Important?

Clamp type electrical wire connector on a receptacle (C) Daniel Friedman

[Click to enlarge any image]

Force in an electrical contact is important because as experts have pointed out, when we examine an electrical contact at very high magnification we learn that the contact surfaces are in fact not smooth but bumpy such that electrical current must actually flow through a myriad of bump-contact-points between the two surfaces.

Here we compare estimates of the connection force exerted by a spring clip wire connector such as the device shown abovce and a binding head screw wire connector. Stronger contact surface force means a more reliable contact and better connector performance, or as Dr. Aronstein puts it

The actual area of physical contact in the connections is directly proportional to the clamping force, exerted by either the screw or the spring or the screw. [Private email JA to DF 26 November 2015]

Relatively straight copper wire in a push-in backwired receptacle connector (C) Daniel Friedman

At BACKWIRED DEVICE SPRING CLIP DETAILS we describe the spring-clip connector used in push-in type back-wired electrical receptacles that rely on a spring rather than a screw to hold the electrical wire in place.

At WIRE-TO-CONNECTOR LIMITATIONS in PUSH-IN ELECTRICAL CONNECTORS we describe some limitations of spring-clip type electrical connectors including the risk of achieving only a rather small electrical contact surface between the electrical wire and the connector. In both of those articles we cite the critical importance of the combination of both contact area and contact force in establishing a good electrical connection between a wire and the connecting device.

At RECEPTACLE WIRE-TO-CONNECTOR CONTACT AREA SIZES we compare the areas of wire-to-receptacle contacts on electrical receptacles and in that article we noted that

The mechanical area of contact [between a wire and an electrical connector] is often very much less than that deductible from bulk properties.

and that a general way some engineers state the problem is

Contact Resistance is a function of (force x micro-contact points x other factors such as oxide resistance)

The indented italics above give three re-statements of the point that clamping force in an electrical connector is critical. So what is the connection's clamping force: the pressure exerted by the connecting device and the electrical wire?

With this question in mind we explored first a comparison of contact area comparing a screw connector with a spring-clip connector for electrical wire connections. At ELECTRICAL SCREW CONNECTOR TORQUE-FORCE I described some rough estimates of the torque force exerted when tightening a clamping-screw wire connector on a receptacle.

We continue that investigation with help from Jess Aronstein. Our correspondent expert, Dr. Jess Aronstein has kindly offered details about the measurement of the force exerted by the spring in a push-in type spring clip electrical connector.

The following is an excerpt from that correspondence in response to my question about the existence of research documenting the actual force exerted by the spring clip connector in a push-in backwired electrical receptalce or switch. Dr. Aronstein wrote:

I have not been able to find any data on the spring characteristics of the push-in receptacle contacts.

Estimates of the Contact Force or Clamping Force of Push-In Backwired Electrical Device Connectors

There is a simple way of estimating the contact force in the push-in backwired terminals, however. Measure the force needed to slide the wire in fully after it has initially been inserted enough to be under the spring. The initial insertion force is high, and then, once the spring is over the wire the force is essentially smooth and constant for the 1/4" or so of remaining insertion. That is the portion where you should measure the force.

Instead of a soft wire, however, use the smooth shank end of a hardened steel drill bit as a gage pin. Use a drill size that approximates the size of the wire that you are interested in. A 1/16" drill is approximately the same size as #14 AWG wire.

Using this method, I measured the four push-in terminals on each of two new receptacles that I happened to have here, one a P&S and the other a Leviton. The range was 2-1/4 lb to 3-1/2 lb, the average was about 3 lb.

The spring force is applied on one side of the wire and it is pressed against the contact plate with an equal force. The friction force that you have to overcome the sum of the resisting friction force at the two sliding points -- where the spring is pressing on the wire and where the wire is pressing on the contact plate. Assuming that the coefficient of friction is the same at both of these contact areas, then the push-in force that is measured is related to the spring force by the following equation:

F = 2uS 

where

u = coefficient of friction
S = Spring force against the wire

A typical published value for non-lubricated steel/brass sliding coefficient of sliding friction is 0.5. Using that value the equation reduces to simply

F = S (for u=0.5)

So, the spring force is essentially equal to the push-in sliding force.

Typical Contact Force of a Clamp or Binding Head Screw Terminal for Connecting Electrical Wires to Devices

Electrical contact points and area estimates for a binding head screw on an electrical outlet (C) Daniel Friedman

If we assume that the average torque applied to the screw (clamp or binding head) terminal 10 in-lb, then the clamping force is (according to the Kaiser Aluminum analysis) 225lb.

Comparison of Clamping Force: Screw Terminal Vs. Push-In Terminal

The actual area of physical contact in the connections is directly proportional to the clamping force, exerted by either the screw or the spring or the screw. Therefore, the ratio of actual area of contact at the current carrying interfaces is in the ratio of 225 to 3, which is to say that ...

There is of the order of 75x greater actual area of contact in a duplex receptacle wire termination at a screw terminal than in a push-in terminal of the types so far examined.

Resistance to Wire Pull-Out is Not a Predictor of Clamping Force

One question that is often asked is, "why it is so hard to pull the wire out, doesn't that show that the contact force is much higher?"

Notch in copper wire cut by spring of a push in backwire electrical receptacle (C) Daniel Friedman

The answer is in the geometry of the connection, by which the spring notches into the copper wire and prevents pullout. That does not increase the contact force unless the pullout force is maintained. With the hardened steel drill shank used in the test the spring does not create a notch, and the pullout force is approximately the same as the push-in force.

I hope that helped answer your question about the relative actual contact area in the screw terminal vs. push-in. - J.A.

Comments on Electrical Resistance & Contact Pressure

Reviewer Comment:

My curiosity in reading the section still leads me back to electrical resistance, which is directly related to heat generation. I would not expect electrical resistance to be linearly related to contact pressure. Rather, from previous tests I’ve run I’d expect that resistance would increase until some contact pressure, and then change little as pressure continued to increased. Thus the increased pressure from one connector to another may or may not influence heating. - Lee Shields. by private email to D.F. 2015/12/09. Mr. Shields worked as a forensic engineer in the automotive industry and is a member of the editor's family.

Reply:

I expect Jess may agree with you; he has done seminal work on "micro-arcing" that is the actual basis for the increase in heat resistance and overheating and ultimately fires in electrical wiring. The arcing burns the surface and increases the resistance.

I don't think Jess is as concerned about micro-arcing on copper to copper connections however. I don't think the resistance changes so much over time as with AL wire. The articles I reviewed were focused on the micro-contact areas and my lay interpretation was that the contact pressure affected the number of micro-contact areas and implicitly resistance that might be more problematic if more current were flowing through too-few points of contact.

Perhaps contact area and pressure and resistance are all together in one formula. I'm looking into Babu (2001) and Timsit (1998) cited below.

References & Research on Electrical Contact Pressure & Resistance & Performance

Article Series Contents

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Continue reading at RECEPTACLE WIRE-TO-CONNECTOR CONTACT AREA SIZES for a discussion of the areas of wire-to-receptacle contacts on electrical receptacles, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BACKWIRED RECEPTACLE FAILURE PHOTOS

Details about how to wire up an electrical receptacle are at ELECTRICAL RECEPTACLE CONNECTION DETAILS - where to connect black, white, red, green, ground wires.

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