Circuit integrity - AbsoluteAstronomy.com

Circuit integrity refers to the operability of electrical circuits during a fire

Fire

Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....

. It is a form of fire-resistance rating

Fire-resistance rating

A fire-resistance rating typically means the duration for which a passive fire protection system can withstand a standard fire resistance test. This can be quantified simply as a measure of time, or it may entail a host of other criteria, involving other evidence of functionality or fitness for...

. Circuit integrity is achieved via passive fire protection

Passive fire protection

Passive fire protection is an integral component of the three components of structural fire protection and fire safety in a building. PFP attempts to contain fires or slow the spread, through use of fire-resistant walls, floors, and doors...

means, which are subject to stringent listing and approval use and compliance.

Fireproofing

Providing fireproofing

Fireproofing

Fireproofing, a passive fire protection measure, refers to the act of making materials or structures more resistant to fire, or to those materials themselves, or the act of applying such materials. Applying a certification listed fireproofing system to certain structures allows these to have a...

for cables, cable tray

Cable tray

In the electrical wiring of buildings, a cable tray system is used to support insulated electric cables used for power distribution and communication. Cable trays are used as an alternative to open wiring or electrical conduit systems, and are commonly used for cable management in commercial and...

s, or electrical conduit, is meant to keep cables operational during a specified fire exposure and time. This can be done in two different ways:

Cable coating is generally considered a fire retardant
Fire retardant
A fire retardant is a substance other than water that reduces flammability of fuels or delays their combustion. This typically refers to chemical retardants but may also include substances that work by physical action, such as cooling the fuels; examples of these include fire-fighting foams and...

, which lowers the spread of flame and generation of smoke
Smoke
Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...

along the combustible cable jacketing. Some cable coating systems are able to achieve a measure of circuit integrity, which is demonstrated and quantified through certification listing
Certification listing
A certification listing is a document used to guide installations of certified products against which a field installation is compared to make sure that it complies with a regulation, such as a building code...

and listing and approval use and compliance.
An enclosure can be provided. Calcium silicate
Calcium silicate
Calcium silicate is the chemical compound Ca2SiO4, also known as calcium orthosilicate and sometimes formulated 2CaO.SiO2. It is one of group of compounds obtained by reacting calcium oxide and silica in various ratios e.g. 3CaO.SiO2, Ca3SiO5; 2CaO.SiO2, Ca2SiO4; 3CaO.2SiO2, Ca3Si2O7 and...

board can be used, or other methods including boards made of vermiculite
Vermiculite
Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite...

, bonded and pressed with sodium silicate
Sodium silicate
Sodium silicate is the common name for a compound sodium metasilicate, Na2SiO3, also known as water glass or liquid glass. It is available in aqueous solution and in solid form and is used in cements, passive fire protection, refractories, textile and lumber processing, and automobiles...

, flexible wraps made of ceramic fibre and rockwool, or ceramic fibre wraps treated with endothermic
Endothermic
In thermodynamics, the word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. Its etymology stems from the prefix endo- and the Greek word thermasi,...

materials. In all cases, the installed configuration must meet the certification listing
Certification listing
A certification listing is a document used to guide installations of certified products against which a field installation is compared to make sure that it complies with a regulation, such as a building code...

of the tested system. Alternatively, cables that achieve fire-resistance ratings on their own can be used, such as Mineral-insulated copper-clad cable
Mineral-insulated copper-clad cable
Mineral-insulated copper-clad cable is a variety of electrical cable made from copper conductors inside a copper sheath, insulated by inorganic magnesium oxide powder. The name is often abbreviated to MICC or MI cable, and colloquially known as pyro...

, or MI cable. Mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

insulated cables have also demonstrated a measure of circuit integrity for small cables.

Testing and certification

In Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, testing is run in accordance with ULC-S101, as required by the local building code

Building code

A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...

. Unfortunately, S101 is ill equipped to deal realistically with circuit integrity, particularly for enclosures. For circuit integrity cables, one simply uses a full scale wall panel test, loops the cables through the fire, energises the cables and quantifies the current carrying capacity of the cables during the fire.

There are two ways of achieving circuit integrity. One may either choose mineral insulated or otherwise fire-resistant (tested for that purpose) cables, or one may use an enclosure that was tested for that purpose. This is where "grandfathered" systems still find acceptance in certain parts in North America. A prime example of this is Canada

Canada

, where the code indicates that 2" of concrete

Concrete

Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

coverage over or around electrical circuits is sufficient to obtain an unquantified duration of circuit integrity. No testing documentation exists to qualify this measure, according to the Institute for Research in Construction, a part of the National Research Council of Canada

National Research Council of Canada

The National Research Council is an agency of the Government of Canada which conducts scientific research and development.- History :...

. 2" of concrete, regardless of the conductor configuration, percentage fill, etc. is of course a judgment call.

Inherently fire resistive cables can be tested to UL 2196, Tests for Fire Resistive Cables, whereas enclosures for cables that are not inherently fire resistive can be tested to UL

Underwriters Laboratories

Underwriters Laboratories Inc. is an independent product safety certification organization. Established in 1894, the company has its headquarters in Northbrook, Illinois. UL develops standards and test procedures for products, materials, components, assemblies, tools and equipment, chiefly dealing...

1724 or USNRC

Nuclear Regulatory Commission

The Nuclear Regulatory Commission is an independent agency of the United States government that was established by the Energy Reorganization Act of 1974 from the United States Atomic Energy Commission, and was first opened January 19, 1975...

Generic Letter 86-10, Supplement 1 in North America, or BS476 in the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

or DIN4102 in Germany

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

The mechanical ducting precedent

The other grandfathered approach is drywall

Drywall

Drywall, also known as plasterboard, wallboard or gypsum board is a panel made of gypsum plaster pressed between two thick sheets of paper...

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

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 indefencible.

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

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.

Current test methods

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

Calcium silicate

Calcium silicate is the chemical compound Ca2SiO4, also known as calcium orthosilicate and sometimes formulated 2CaO.SiO2. It is one of group of compounds obtained by reacting calcium oxide and silica in various ratios e.g. 3CaO.SiO2, Ca3SiO5; 2CaO.SiO2, Ca2SiO4; 3CaO.2SiO2, Ca3Si2O7 and...

and sodium silicate

Sodium silicate

Sodium silicate is the common name for a compound sodium metasilicate, Na2SiO3, also known as water glass or liquid glass. It is available in aqueous solution and in solid form and is used in cements, passive fire protection, refractories, textile and lumber processing, and automobiles...

bonded vermiculite

Vermiculite

Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite...

.

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

Nuclear Regulatory Commission

. "Supplement 1" was to address lessons learned from the widely publicised Thermo-lag 330-1 scandal, following disclosures by whistleblower

Whistleblower

A whistleblower is a person who tells the public or someone in authority about alleged dishonest or illegal activities occurring in a government department, a public or private organization, or a company...

Gerald W. Brown

Gerald W. "Jerry" Brown is an American whistleblower who concerned himself with deficiencies in passive fire protection systems in US and Canadian nuclear power plants.-Thermo-Lag scandal:...

, which resulted on Congressional hearings and a large amount of remedial work.

Supplement 1 is a particularly difficult and expensive test

Fire test

A fire test is a means of determining whether or not fire protection products meet minimum performance criteria as set out in a building code or other applicable legislation. Successful tests in laboratories holding national accreditation for testing and certification result in the issuance of a...

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

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

Intumescent

An intumescent is a substance which swells as a result of heat exposure, thus increasing in volume, and decreasing in density. Intumescents are typically used in passive fire protection and, in America, require listing and approval use and compliance in their installed configurations in order to...

"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.

What circuit integrity methods are used where

Ordinarily, small runs of cables are individually run with cables that are fire-resistance rated on their own. Larger bundles and trays full of wiring may be less expensive to clad or wrap on the outside. The concrete cover method is most often used in Canadian construction, as the code and common practice permit this, despite the absence of testing data that gives the required "carte blanche" for all cables and indefinite ratings.

Cladding and wrapping considerations

The added weight of the wrap systems must be included in static and seismic calculations. Fireproofing of the hanging system must also be considered. Regular maintenance must be considered, as cladding and wraps are not load-bearing and can be damaged during normal building or facility operations. Ampacity-derating may be mitigated by the use of purpose-designed intumescent or mechanically/electronically activated "windows" that permit heat venting. Like everything else in passive fire protection, all such methods are subject to stringent listing and approval use and compliance.

Circuit integrity cable considerations

Termination points and junction boxes, in other words the entire circuit, must be completely protected. Often, termination points are left out, providing a weak link. Therefore, some enclosures are needed to be used in conjunction with MI cables. One may run MI cable into a box in an electrical room. However, just because that room may be a "service room" and may be subject to compartmentalization (fire protection)

Compartmentalization (fire protection)

Compartmentalization in structures, such as land-based buildings, traffic tunnels, ships, aerospace vehicles, or submarines, is the fundamental basis and aim of passive fire protection....

, this does not mean one no longer requires a rated box or wrap around the electrical outlet box or junction box where the wiring is terminated because that box may be disabled as a result of a fire within the room. The probability of electrical fires are a strong motivating factor for compartmetalisation to begin with. The cable may thus be operable but the circuit as a whole may be defeated because the junction box would not have been protected. Such omissions are not entirely uncommon in the field.

External links

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Fireproofing

Testing and certification

The mechanical ducting precedent

Current test methods

Ampacity derating

What circuit integrity methods are used where

Cladding and wrapping considerations

Circuit integrity cable considerations

See also

External links