This applies to all high current appliances, not just space heaters though these are most likely to be afflicted since they are likely to be run for extended periods of time. Of course, if the problem is with an *extension* cord, then either it is overloaded or defective. In either case, the solution should be obvious. Some cords will run warm just by design (or cheapness in design using undersized conductors). However, if it gets hot during use, this is a potential fire hazard. If it is hot mainly at the plug end - get a heavy duty replacement plug - one designed for high current appliances using screw terminals - at a hardware store, home center, or electrical supply house. Cut the cord back a couple of inches. If the entire cord gets warm, this is not unusual with a heater. If it gets hot, the entire cord should be replaced. Sometimes with really old appliance, the copper wires in the cord oxidize even through the rubber insulation reducing their cross section and increasing resistance. This leads to excessive power dissipation in the cord. Replacement *heavy duty* cordsets are readily available. Extension cords We treat extension cords too casually - abusing them and using underrated extension cords with heavy duty appliances. Both of these are serious fire and shock hazards. In addition, the use of a long inadequate extension will result in reduced voltage due to resistive losses at the far end. The appliance may not work at full capacity and in some cases may even be damaged by this reduced voltage.
Extension cord rules of use:
1. The capacity must be at least equal to the SUM of the wattages or amperages of all the appliances plugged in at the far end. Larger is fine as well and is desirable for long extensions. Check the rating marked on the cord or a label attached to the cord. Thickness of the outside of the cord is not a reliable indication of power rating.
2. Use a type which is the most restrictive of any appliances that will be plugged in (e.g., 3 prong if any are of this type, 2 prong polarized otherwise unless your outlets are non-polarized (old dwellings).
3. Use only as long an extension as required. For very long runs, use a higher capacity extension even if the power requirements are modest.
4. NEVER run extensions under carpeting as damage is likely and this will go undetected. Never run extensions inside walls. Add new outlets where needed with properly installed building wire (Romex). This must be done in such a way that it meets the National Electric Code (NEC) in your area. It may need to be inspected if for no other reason than to guarantee that your homeowner's insurance won't give you a hard time should any 'problems' arise. Surface mount outlets and conduit are available to extend the reach of existing outlets with minimal construction if adding new ones is difficult or too costly.
5. Don't use heavy duty extensions as a long term solution if possible. Similarly, don't use extensions with 'octopus' connections - install an outlet strip. Extension cords of any type, capacity, and length can be easily constructed from components and wire sold at most hardware stores and home centers. This is rarely economical for light duty polarized types as these are readily available and very inexpensive. However, for heavy duty 3 prong extensions, a custom constructed one is likely to save money especially if an unusual length is required. Making up a heavy duty extension with a 'quad' electrical box with a pair of 15 amp duplex outlets is a very rugged convenient alternative to a simple 3 prong socket.
Common problems: internal wiring conductors broken at flex points (socket or plug). With yard tools, cutting the entire cord is common. The connections at the plug may corrode as well resulting in heating or a bad or intermittent connection.
Testing: Extension cords can always be tested with a continuity checker or multimeter on a the low ohms scale. Determining the location of a break in an extension cord This isn't worth the time it would take to describe for a $.99 6 foot K-Mart special but it might make sense for a 100 foot heavy duty outdoor type. If the problem is near one end, a couple of feet can be cut off and a new plug or socket installed. If more towards the middle, the wires can be cut and spliced or two smaller cords could be made from the pieces. But, how do you locate the break? * Use a Time Domain Reflectometer (TDR). Oops, don't have one? And, you probably don't even know what this means! (Basically, a TDR sends a pulse down a wire and measures how long it takes for a reflected pulse to return from any discontinuities. The delay is a measure of distance.) Don't worry, there are alternatives :-). * If there is no obvious damage - you didn't attempt to mow the cord by accident - the most likely location is at the end where the plug of socket strain relief joins the wire. Squeezing, squishing, pushing, etc., with the cord plugged into a live outlet and lamp or radio plugged into the other end may reveal the location by a momentary flash or blast of sound. * Try a binary search with a probe attached to a straight pin. This works best with a cord where the wires are easily located - not the round double insulated type. Attach one probe of your multimeter to the prong of the plug attached to the broken wire. Start at the middle with your pin probe. If there is continuity move half the distance to the far end. If it is open, move half the distance toward near end. Then 1/4, 1/8, and so forth. It won't take long to located the break this way. Of course, there will be pin holes in the insulation so this is not recommeded for outdoor extension cords unless the holes are sealed. * You may be able to use one of those gadgets for testing Christmas Tree light sets - these inexpensive devices sense the AC field in proximity to its probe. Plug the cord in so that the Hot of your AC line is connected to one of the wires you know is broken (from testing with an ohmmeter) and run the device along the cord until the light changes intensity. This also works for appliance cords where you are too lazy to go inside to check continuity. You may need to try both wires in the cord to locate the broken one. * If you have some real test equipment (but not a TDR!) attach the output of a frequency generator to the prong of the plug for the wire you know is broken. Use an oscilloscope as a sensor - run the probe along the cord until the detected signal abruptly drops in intensity. An AM or multiband radio may also be suitable as a detector.