STATE-OF-THE-ART SAFETY CIRCUITS LIKELY TO FAIL DURING A FIREProper methodology for cable selection and sizing provides solution

Dr. Jacek Wasilewski and Dr. Wojciech Wiechowski

WTW Power Solutions

State-of-the-art safety circuits likely to fail during a fire

Background

• The maintenance of safety services in a building during an emergency is vital

• During a fire, cables are expected to retain their functionality and provide power to essential equipment

• The designer must take into account the increased electrical resistance at elevated temperature and thus the correct sizing of the conductor

• Currently, cables are specified in terms that reflect their normal operational conditions, but design parameters under fire conditions are rarely specified

State-of-the-art safety circuits likely to fail during a fire

Background

Graph from Dutch Copper Institute (Nederlands Koperinstituut, Uitgave No 3, Koper en Koperlegeringen, 1964, p 3-12):

The conductivity of copper is a factor of 5 less during fire. This is not taken into account in current fire safety standards for cables.

Case Study

• This study was based on the design data of an actual large building, the Galeria Tarnovia Shopping Centre in Tarnow, Poland

• First, conductor sizing for selected fire safety circuits based on selection criteria described in IEC 60364 was verified

• In the second step, the steady-state and dynamic performance of selected fire safety equipment and the protective relays were verified through detailed dynamic simulations with a professional simulation tool

| State-of-the-art safety circuits likely to fail during a fire

State-of-the-art safety circuits likely to fail during a fire

Case Study Results

Under normal operating conditions (no fire): cable size selection is correctly made according to state-of-the-art methods and current regulations, e.g. the IEC standard 60364-4-41.

For a maximum value of temperature for the standard fire temperature–time curve:

• The maximum allowable times of automatic disconnection of supply may often be exceeded

• The maximum levels of voltage drop across circuits are often exceeded as well, which results in large currents that cause induction motor overload but not enough for the activation of thermal protection in the investigated scenarios

Case Study Results

• Motor startup under fire conditions is often impossible as currents are large due to a decrease in the supplying voltage, as well as motor stall

• This implies that problems with operation of fire safety equipment, as well as with fast disconnection of fire safety circuits by protective relays, may occur when the temperature of wires of the supplying circuits is high due to a fire

| State-of-the-art safety circuits likely to fail during a fire

Solution: Increasing the size of cable conductorsIn order to ensure the correct operation of fire safety equipment during a fire, it is essential to:

• Increase the cross-section of conductors of the supplying cables - to decrease voltage drop

• Increase cable cross-section of the conductors of supplying cables and the protective conductor (or apply supplementary equipotential bonding) - to decrease phase-to-ground impedance and ensure sufficiently fast automatic disconnection of supply

• Set thermal protection current settings to no less than 1.1 times the rated motor current in order to minimize the likelihood of circuit disconnection in cases of slight motor overloading.

| State-of-the-art safety circuits likely to fail during a fire

State-of-the-art safety circuits likely to fail during a fire

Conclusion and recommendations

• It is essential to increase the cross-section of cable conductors to ensure the correct operation of safety equipment during a fire

• It is necessary to elaborate and evaluate standards and recommendations for fire safety circuits that include clear cable sizing criteria for the expected fire conditions

• The longer the safety services remain operational, the better the chance of bringing a fire under control and reducing property damage.

More information?• Read the full report and the application note at Leonardo Energy:

http://bit.ly/12mrSzW

• Contact : Stefan Fassbinder – stefan.fassbinder@copperalliance.de

State-of-the-art safety circuits likely to fail during a fire

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