Romex Shorting (10-23-2K)

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August 15, 2000


Submitted By: Paul Keleher, 35 Barnes Road, Berlin, MA, Electrician's License # A8471

To: Office of the State Fire Marshall, Stow, MA 01775

Subject matter of report: "SHORTING ROMEX"

This report concerns 2 separate observations of non-metallic sheath cable, TYPE NM-B, 600volt, (90 deg C rated), 14/3 & 12/3 (These are standard, round, residential electrical power cables with a black, red, white and bare grounding conductor.) One of these cables was install @ 55 Hunt Road, Chelmsford, MA in 8/97, and was removed in 9/98 (est.) because it was shorting. The other was installed in 2/00 @ 65 Lincoln Road, Sutton, MA, and was removed because it was shorting in 6/00.

Both cables were installed by myself or my helper, stapled using conventional insulated cable staples onto wood joists, and metallic "romex" connectors, 1/2" trade size. Both circuits operate @ 240 volts, but with very low load, protected by a 2 Amp fast-blow fuse. It was the blowing of this fuse in each case, which called the problem to my attention.

The cause of failure @ 55 Hunt Road, was short-circuiting due to charred insulation on more than one conductor over a span of 2-4 inches, under each of 5 staple points. Nothing was visible on the exterior of the cable. The jacket had to be slit open to see the burned insulation. Rather shocked at this observation, I kept the 15' piece of cable with 5 burns for over a year. Finally, assuming it was an isolated fluke of some kind, I discarded the evidence in 4/00, 2 months before the second observation!

The cause of failure @ 65 Lincoln Road was short-circuiting from the insulated black conductor to the bare grounding conductor within the cable, at the point where the cable passed through the throat of a 1/2" trade-sized 2-screw clamp connector. The cable was properly installed in the connector: it was not pinched, and was squeezed snug, and again, the burn from the short was not visible on the exterior of the cable.

In 25 years of electrical service in existing homes in this area, I've never seen romex cable undergo insulation breakdown under staples, normally installed, as this was. I have seen 1 other occurrence of romex shorting within a metallic connector. That was a 25 year-old cable in a tightly squeezed connector bearing a substantial motor load.

Within the past 5 years, non-metallic sheathed cable has been manufactured using Type THHN insulation without paper padding around the conductors. THHN is a very thin insulation with an outer coating of Teflon used to give it its 90 degree C temperature rating.

Both of these cable faults have occurred at pressure points (staples and connectors). Both cables are recent constructions made with no paper packing, and with THHN insulation.

Teflon is known to possess what are called "cold-flow" properties: i.e., when squeezed under mechanical pressure, it melts and flows momentarily.

Romex staples, although insulated so that the conductive metal staple does not contact the cable, have no mechanism to limit the pressure applied to the cable from hammering. The same lack of clamping limit applies to most UL listed non-metallic sheath cable connectors. And both are required by the NEC to "securely fasten" the cable in place. Both of the above described fault observations occurred inside the cable interior, not visible until slit open.


Is thinning of conductor insulation, use of Teflon and elimination of packing papers combining to reduce the pressure tolerance of non-metallic sheath cable to a level which threatens its voltage rating under normal installation and use?

Have manufacturers of non-metallic sheathed cable conducted insulation testing under the mechanical stresses of installation to certify its ratings under stress? Does the UL standard for these cables require such testing?

Have there been other similar discoveries?

Should more cables be studied either in lab tests or in the field?


TEL: 978/838-2457

FAX: 978/838-2105


MA LIC: A8471




Two failures observed in past 2 years. Both cables were 3-wire round Type NM-B Manufactured by Essex Group, Inc., Wire & Cable Division, Fort Wayne, Indiana, and were installed new within the last 3 years. One fault occurred at several staple points under staples normally installed on wood framing, the other within the clamp of a 2-screw sheath cable metallic connector. Both failures were ground-faults apparently caused by insulation failure at pressure points (staples and connectors) within the cable jacket, not from any contact with an external object.

The available sample is the connector fault, which was located within the throat of the connector, and the cable was properly centered in the connector. The fault current appears to have passed from the black conductor to the bare equipment grounding conductor.

The staple fault, unfortunately is not available, as it has been discarded. No damage was visible on the jacket exterior; however, upon slitting open a few inches of jacket around each of 5 contiguous staples over a 20 foot run, and peeling open the jacket, 2-4 inches of the cable interior had been completely blackened by carbonization. All conductors appeared black, and the cable was shorted. I subsequently checked all accessible staples in that installation, but found no other occurrences of this damage.

Both cables were constructed with type THHN (Teflon-coated) insulation. Teflon has recognized "cold-flow" properties: i.e., when placed under pressure, it reacts chemically to liquefy momentarily and if space permits, will flow, or move. In the case of its application as part of conductor insulation, this leaves behind a spot of only the very thin coating of thermoplastic beneath the Teflon. Neither cable contained any paper packing between the conductors and the jacket.

Both cables were part of the field control cable wiring in an electric heating system (Electric Thermal Storage Heat) and were protected by a 2-amp fast-bow fuse, in a control panel which upon opening causes shut-down of the heating system and several major household appliances. This shutdown usually triggers an immediate service call from the homeowner. Thus, in both cases, the fuses acted as an "early warning signal" calling immediate attention to these particular failures.

I have checked with several local electricians whom I know, and also with 2 nationally known electrical consultants in an effort to learn whether anything like this has been discovered elsewhere to-date. None of my sources has heard of anything like this before my asking. This does not surprise me, as I have tested the short-circuit/ground fault response of many circuit breakers. My studies show that fault currents must reach levels of 300 to 500 amperes before conventional circuit breakers will operate in magnetic (instantaneous) mode.

As an electrician I have specialized in the study of the short-circuit response of conventional 15 & 20 amp circuit breakers. I have developed and patented a device for conducting such tests, and am presently developing a plan to commercialize the testing of installed 15 & 20 amp circuit breakers on a widespread basis.

I do not know if these findings are an isolated occurrence, or a particular cable manufacturer's error, or an oversight in the standards, or if I over tighten connectors and drive staples too tight, or if it is symptomatic of a larger problem: that the margin of safety between an application of sufficient pressure during installation of non-metallic sheath cable to ensure an installation which is workmanlike and code-compliant, and the point at which the insulation suffers breakdown under pressure has become dangerously narrow. What testing of this margin has been done?

Mike Holt's Comment: If you have any additional comments, please notify me ( and Paul Keleher (