Circuit integrity

ASTM E1725 uses cable tray temperature as the failure criterion because circuit integrity has been shown to be unreliable as an indicator of failure of a cable for hydrocarbon fires.This is the result of research that indicated cables believed to be of identical composition and rating had different functional failure temperatures. This means that circuit integrity failure cannot be assumed to reliably occur at the same time point in a fire. Therefore, it is more reliable to establish a maximum temperature beyond which any cable can be considered to be at risk of failure. Using cable temperature as the failure criterion ensures that the maximum temperature at which functionality of any cable can be assumed to be maintained is not exceeded, even if circuit integrity can be maintained by the sample cable used in the fire test. Furthermore, the measurement of tray rather than cable temperature, with an empty tray, allows any cable loading to be used in practice.

Importantly the cable tray is tested for structural stability which often is found to fail before the circuit integrity in Hydrocarbon fire cases. Hence cable enclosures ensures cable tray is tested along with cables and recommended as a better option.

The other grandfathered approach is drywall shaftwall systems. Drywall shaftwalls were tested as a flat wall, no corners, no turns. This approach has pretty much been negated for use around ductwork (i.e. pressurisation and grease ducting, which are required to have a fire-resistance rating) since the adoption of the more suitable ISO6944 test regime by ULC as well as Underwriters Laboratories, whereby a duct is suspended from a full scale floor slab and the enclosure is built around the duct (or an inherently fire resistant duct is similarly tested without an enclosure, since it already contains a layer of insulation), for a more realistic 3D configuration and exposure. Drywall shaftwall systems were entirely grandfathered for this application and ceased to be legally representative of due diligence the instant a properly and purposely tested system with bona fide listings became available. The same thing applies to circuit integrity enclosures.

For the mechanical ductwork, a Canadian entrepreneur got ISO6944 passed by the ULC Standards Council and then performed testing. This made all grandfathered systems legally indefensible.

This has yet to occur in Canada for circuit integrity, but it has long been standard construction work in Europe and also in the US, through work done by UL and other laboratories. Since UL is accredited by the Standards Council of Canada in Canada and its listings are considered public record up all over North America including Canada, one is ill-advised to use grandfathered systems for circuit integrity anywhere.

Importantly, drywall shaftwall systems have only been qualified as straight walls in panel furnaces, not 3D enclosures with corners.

Germany has standardised this sort of testing via DIN4102 Part 12, dated January 1991, Fire behaviour of building materials and elements, Fire resistance of electrical cable systems, Requirements and testing. Part 12 encompasses both enclosures for cabling and bus ducts, as well as inherently fire-resistive cables, such as mineral insulated cables. Enclosures for ductwork as well as wiring are a regular part of passive fire protection there. It is also not nearly as expensive as North American qualified approaches. Typically, lightweight mineral boards are used, such as calcium silicate and sodium silicate bonded vermiculite.

The North American state of the art is UL1724 Standard for Tests of Thermal Barrier Systems for Electrical System Components as well as its cousin, UL2196 Standard for Tests of Fire Resistive Cables. UL1724 had its origin with USNRC Generic Letter 86-10 Supplement 1, issued by the Nuclear Regulatory Commission. "Supplement 1" was to address lessons learned from the widely publicised Thermo-lag 330-1 scandal, following disclosures by whistleblower Gerald W. Brown, which resulted in Congressional hearings and a large amount of remedial work.

Supplement 1 is a particularly difficult and expensive test to pass. No testing is done in anything less than a full scale fire test, running easily into 6 figure costs per burn multiplied by all the applications one desires to test. In order to pass, one must test the smallest as well as the largest application (12" and 36" cable tray, 1/2" and 6" conduit). Accordingly, the approved materials are costly, as manufacturers must get a return on the large test investment.

In concept, it is simple to devise systems that will pass the test. As far back as the 1970s, it was apparent that when one uses enough high temperature qualified insulation, such as ceramic fibre, one is assured of a rating. However, this comes at the price of significant ampacity derating. Also, the concept that more fireproofing is better, was defeated by industry tests of Thermo-lag 330-1 (which is not a fibrous insulation). No matter what was done to this material (used for fireproofing purposes over electrical circuits in full scale fire testing) by various nuclear power plant owners (USNRC licensees) who sponsored extensive testing, where more of the old Thermo-lag was applied onto the older substrate, no satisfactory results were achieved. In order for licensees to come into compliance, other methods, replacements, overlays and MI Cable were used to fix the problem. Also, since the forerunner of this testing was the USNRC, and the commercial version of it (UL1724) has undergone various revisions, the UL systems listed in the UL Building Materials directory are not necessarily qualified to the latest USNRC compliant or the latest UL version. But that does not mean that the older listings are simply discarded or that the manufacturers performed all new tests. Therefore, users must closely review the versions of the tests deemed acceptable in an end-user facility.

Ampacity derating refers to the reduction of the ability of a cable to conduct electricity. It can be tested through the use of IEEE 848 Standard Procedure for the Determination of the Ampacity Derating of Fire-Protected Cables. The more one insulates a conductor, the less current it can conduct without damage from overheating. The result of the test referenced herein is quantified in terms of percentage. If a cable is derated by 30%, it can be used to conduct only 70% as much current, thus cable of greater cross sectional area is often needed to conduct a given amount of power. The use of intumescent "windows", which shut in case of a fire, can reduce or negate the effect of ampacity derating, subject to listing and approval use and compliance.