Tinned Copper

[Tom Rees]

Jerry, Do you know of a good site for tinned copper problems, i.e. insulation drying out etc.

[Jerry Peck - Codeman]

Hi Tom,

No, I don't know if there is a good site about tinned copper, however, the reason the copper was tinned was the copper conductors reacted with the rubber insulation, so if you are finding tinned copper you know you have rubber insulation.

All rubber, including the rubber used for insulation on tinned copper conductor, dries out, cracks and become brittle. That is a well know and understood problem. To make the matter worse for electrical installation is that the dielectric insulative value breaks down with the drying out and breaking down of the rubber, and, of course, when the rubber cracks there is no dielectric insulative value at the locations of all the cracks.

When you find tinned copper/rubber insulated conductors in the panel, I would recommend re-wiring. If re-wiring is not desired then those circuits should be hi-pot (high potential) tested with a megger to verify the dielectric property of the insulation is still adequate.

Keep in mind, though, that even if those conductors pass a megger hi-pot test that there may be problems with the insulation/ Typically rubber insulation was used with two-wire ungrounded circuits and there may be a failure of the insulation not adjacent to the adjacent conductor which does not indicate a failure to ground as there may not be any ground near the failed insulation.

If the rubber insulation does not pass the megger test, then it has been confirmed to no longer be adequate in dielectric insulative values. If the rubber insulation does pass the megger test I would still hold the rubber insulation's dielectric value in suspect and recommend replacement. Typically you will find rubber insulation in pre-1950s homes, sometimes pre-1940s, which makes that rubber almost 60 years old to 70 years old.

[Joe Tedesco]

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