Signs of Heat
The items in this photo had reached a point that they stopped the operation of their part of an electrical circuit. They represent damage that had gone as far as it was going to be able to go -- once the circuit is interrupted, there is no more current to create any heat.
From Loose Electrical Connections
[Also see my Diagram of connection problems and Signs of a bad connection].
A. This smallest object on the page, tucked next to a receptacle with push-in wire receivers (B), comes first on my list because it shows the most common connection malfunction. The others items are more dramatic visually, but you can barely see the dark spot not far from the end of this piece of aluminum wire. That spot it where a receptacle similar to B was holding wire A by means of springy receivers, which bit into the soft aluminum. So the amount of pressure on the wire at that point was not as much as the receptacle was designed to give (designed, namely, for copper only). In addition, the aluminum and the brass alloy of the receiver will corrode somewhat when in contact with each other, especially under conditions of current and the heat it produces. The dark spot was mostly created by the extreme heat of current arcing past the deteriorated or depressurized metal-to-metal contact point. Copper will often show this problem too. In the case of copper, pressure may be adequate, but poor contact can still be from surface oxidation on the copper or simply from the surface area designed for the contact being too small to sustain varieties of current in varieties of temperature and humidity over the years. Some people speak of wires working their way loose from current's heat expanding and contracting them. I think this could be confirmed for connections being held by screws. In the case of these push-in type, I think it is sometimes due to careless installation, including putting too much stress on the receivers and "despringing" them. As I say, these minor looking arc points toward the bare wire-end are the most common cause of both partial circuit outages and the flickering that often precedes them.
B. This push-in style receptacle (see A above) is shown from its back side. The darkness around the push-in holes tells us that heat had developed at both "hot" and neutral wire connection points. This is not very common, but we can imagine that the heat starting on one side can hasten metal-surface deterioration on the other.
C. The "hot" side of these two receptacles did get hot. Both were using screw terminals to hold their aluminum wires. The plastic of the one on the left suffered so much that the screws and metal plug-receivers that were usually held captive pulled right out. The one on the right is a fairly new receptacle used to replace an older one, but since the wires were aluminum, which don't get along well with the alloys of most receptacles, these connections didn't last long. Only receptacles that say CO/ALR (copper/aluminum rated) are able to connect aluminum wires directly to their terminals.
D. These two aluminum-wired receptacles suffered their heat on the neutral side. True, their wires don't look white like neutrals should, but they were at one time. The insulation on the wires of the left hand receptacle melted and vaporized back about two inches from the source of the heat (the screws). In my experience three inches is as far as I see things go. By that time the connection has gotten so poor it doesn't let current flow through it anymore. So the heating process is self-defeating -- usually. Under some conditions, the heat does reach and ignite combustible materials; I am not called to these cases, firemen are.
E. With these two receptacles we are back to trouble on the "hot" side, but now it is copper wire. What caused the copper connections to go bad? I know of two common reasons. One is that the person installing did not seat the loop of copper evenly and directly under the screw, did not tighten the screw enough, or curled the copper counterclockwise under the screw (so that tightening actually squeezed the loop apart and outward). The other reason is similar -- the installer was not careful to insure that only metal ended up trapped under the screw. When the loop under the screw includes any insulation, the screw cannot be holding the metal as tightly as it should, even though it feels tight. One reason people do this by accident is that they are replacing receptacles in the home that had the wires connected push-in style in the back. They are going to improve things by having their new receptacles connected with the more reliable screws. Trouble is, the push-in wires never needed to have their insulation stripped back very far to do their job, but few of these have enough length for a full loop that will be held under the screw without catching some insulation under it. Strip more off before putting them under screws.
F. Here copper had a poor connection on the neutral side.
G. These are the ends of three neutral wires. I don't recall whether they had been attached to receptacles or whether the poor contact was in a wire connector ("wirenut").
H. Likewise, here are two "hot" wires that led dysfunctional lives.
I. The discoloration of this receptacle at one of its straight slots is typical of poor pressure being given by the receptacle's receivers on the prongs plugged into them. Many people are aware of receptacles in their homes that do not hold on strongly. When this is so, heat can develop there, especially for heavier watt-users. The receivers get sprung loose this way by repeated plugging in and out, and more so when a cord is pulled out a bit sideways, as when a vacuumer has gone too far afield. Receptacles with this kind of appearance should be replaced. The kind of arcing that can go on when a plug is gradually falling out while still running a lamp, has been known to start fires and is the kind of hazard which arc-fault interrupters are supposed to prevent.
J. This monstrosity is a clothes dryer cord-plug (on the left) and what is left of one receiver (on the right) of that dryer's receptacle. Though signs of heat are evident on everything here, the source was the receiver being sprung apart too far. Maybe sometime in the past someone moved the dryer too far while it was still plugged in. This is the same situation as I just described above in "I", but on a 240-volt 30-amp scale.
K. This is the socket of a common porch light. The heat it suffered was not from a poor connection, the way everything described above was. It was from running 75- or 100-watt bulbs in a fixture that was only designed to dissipate the heat of a 60-watt bulb. The result is the same: the thing stopped working. But it might have shorted its whole circuit out first.
L. This chunk of underground cable was shorting its circuit down in the ground, helped by water and soil, but started by a shovel's nick. This too is not a case of heat from a poor connection, but I wanted to show that there are other ways wires can get looking ugly. I find that an underground cable like this one doesn't short into the actual earth significantly -- it is enabled to short from its exposed hot over to its own ground wire, by the conductivity of the moisture and soil that is right along the surface of the cable.
©2006 Larry Dimock