Safety equipment and safety circuits in commercial buildings are likely to stop functioning in the event of a fire, even if they are designed and installed according to state-of-the-art methods and current regulations. This is due to the increased resistance at the temperature at which these circuits are required to function, according to the findings of a case study of a modern shopping mall in Tarnovia, Poland. This paper summarizes the case study results and provides a framework for the proper selection and sizing of conductors supplying electrical equipment that must remain functional during a fire.

1. STATE-OF-THE-ART SAFETY CIRCUITS LIKELY TO FAIL DURING A FIRE
Proper methodology for cable selection and sizing provides
solution
Dr. Jacek Wasilewski and Dr. Wojciech Wiechowski
WTW Power Solutions

2. 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

3. 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.
State-of-the-art safety circuits likely to fail during a fire

4. 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

5. 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
State-of-the-art safety circuits likely to fail during a fire

6. 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

7. Solution: Increasing the size of cable conductors
In 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

8. 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.
State-of-the-art safety circuits likely to fail during a fire

9. 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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