By Ron Burns
We concluded the previous article by discussing the requirements for adequately sized power supplies and the monitoring of cables running in the field.
Primary and secondary power supplies are addressed in paragraph 6 of the EN regulations. It becomes incumbent on the electrical engineer to have a working understanding of the electrical power supply requirements in terms of EN 12101:10 for the smoke ventilation system. Historically, this was only applicable to powered extraction systems (fans).
Today, with the reluctance of the fire engineer to install fail-safe roof ventilators, the supply of emergency power to all smoke ventilation systems is becoming a reality. Please note that fusible link ventilators are not ‘fail-safe’. In fact, fusible link ventilators fail to meet the requirements of any smoke ventilation system and should never be installed as part of a compliant smoke ventilation system.
Also read: Smoke exhaust – it’s serious – Power supplies for SHEVS systems
Fire engineers have moved away from fail-safe roof ventilators for one overriding fundamental reason: these ventilators open. These ventilators are designed to open when any malfunction occurs in the system, which results in the ingress of rainwater causing havoc, flooding within the building, and large insurance losses. Many attempts have been made to prevent the ventilators from opening. Maintenance, although the best preventative measure, is the most overlooked aspect of many smoke ventilation systems. After trying all sorts of ‘not nice’ remedies, the industry has addressed the problem by installing power closed / power open systems. Enter the electrical engineer — or perhaps not.
Let’s explore the system a bit to appreciate what can go wrong.
How does the engineer guarantee the power supply to the actuator? (And yes, the engineer has to.) The control panel requires two power supplies — EN 12101:10 stipulates this. This is the starting point, not the end point. The engineer cannot simply supply a dual power supply electrical control panel and not consider other important items. How is the power supplied to the actuator? Often, fire-rated cabling is installed — nice, but not a requirement. Fire-rated cabling is a belts and braces approach to a perceived problem that is often outside the ambit of EN 12101 requirements. Should the room being protected be fitted with a smoke detection system, the time from detection of the smoke (approximately 90 seconds) and the activation of the roof ventilators, is deemed sufficiently quick enough to allow the ventilators to open without the cabling succumbing to the heat of the fire. It is therefore unnecessary to spend the client’s resources on product, which although sounds good, offers no advantage to the system.
What happens to the system if the cable loses its integrity? If the designer has installed fail-safe ventilators, the result would be the opening of the ventilators. The system is in perfect fire-state working order. The most common risk in South Africa relates to Eskom: Load-shedding affects the battery life with the battery constantly discharging; added to this, the lack of maintenance results in unmonitored wear on the batteries as a common result of load-shedding. The ventilators now remain closed for shorter periods during power outages, resulting in open ventilators.
The second-most common risk is lightning strikes. Lightning is often associated with rain; in many storm conditions, the lightning creates a power outage and often, due to the maintenance challenges, this results in open ventilators and the ingress of water. Stock damage through ventilator flooding results in unnecessary and preventative product loss. Although the prevention of unnecessary financial loss is important, one cannot forsake the safety of the building occupants in favour of product loss. The loss of cable integrity can create the scenario where the smoke ventilation system is rendered inoperable due to a perished cable. To avoid this, the panel needs to be designed with cable monitoring functions. Cable health, fire signal health, and battery health should be constantly monitored and reported upon.
As discussed last month, the duration of standby battery requirements for a power close / power open system may be reduced from 72 hours based on additional criteria; so, too, must the complete system health be monitored and attended to immediately, ensuring comprehensive protection should there be any indication of system failure due to cable health, fire signal health, and/or battery health.
I think it is appropriate to touch on the term ‘fire signal’. Smoke control/ventilations systems do not respond to a fire signal. The control panel should receive a permanent signal from the detection source (fire signal), which informs the smoke panel that no fire condition exists. The panel reacts and goes into fire mode when the signal is no longer present. The panel operates on the interruption of the fire signal — this makes the control system ‘fail-safe’. Too many smoke detection systems issue a signal to activate the smoke control system.
What happens when the signal cable perishes? How is the smoke control system activated? Manually? By whom? Does the system automatically reset once running if the smoke detection system loses power?
We have drifted away from the power supply topic; however, the importance of supplying power to the control panel is only a small part of ensuring system functionality.
“Fusible link ventilators fail to meet the requirements of any smoke ventilation system and should never be installed as part of a compliant smoke ventilation system.”
When considering the power requirements for standby generators, ensuring compliance with this part of the EN 12101:10 code is critical to compliance of the entire smoke ventilation system. I have often responded to the request to forward the electrical loading to the electrical engineer; however, there is more to be considered than simply sending through the loadings. The compartmentalisation of the building is important. How many systems are going to operate simultaneously?
Although it has merits, the theory of designing for one fire only cannot be used as reason for considering a single panel operation. When designing complex systems, such as shopping malls, consideration to the fire location needs to be given. It is possible, due to building geometry, that a single fire will be contained within a single smoke ventilation zone. Smoke may spill into two or three smoke zones that share common divisional boundaries. This can easily result in a single smoke zone system containing 55kW of smoke extraction fans, approximately 110A, 400V, 50Hz activating. The combined current to effectively provide operating current to the smoke ventilation system shall now require a standby generator of 330A, 400V, 50Hz — a beast in any engineer’s language. Failure to meet these requirements can result in the entire standby power system becoming overloaded and failing, rendering the smoke ventilation system unusable.
A consideration often overlooked is the attempt to save costs by supplying the minimum. Although I am not advocating the unnecessary expenditure of resources, it is important that the minimum requirements for each system be met. As an example, if the building is too small for a smoke detection system or sprinkler system, the ventilators or smoke extraction fans cannot be activated; therefore, the minimum requirement of these ancillary systems falls away to facilitate the activation of the smoke ventilation system. So, too, when it comes to the requirements for the standby generator. The standby generator needs to react within the time constraints of the minimum ‘power downtime’. The generator set shall automatically provide full output power within 15s of failure of the primary power supply.
The following extract from EN 12101:10 is a stringent requirement:
6.3.3
If the generator set is dedicated to the building life safety systems and will only start in case of a fire signal and provides fault indication to a permanently manned control room, the generator set shall incorporate a fuel supply capable of supplying the generator set for a minimum of 4 h at full output. If the generator set operates whenever the primary power source fails and provides fault indication to a permanently manned control room, the generator set shall incorporate a fuel supply capable of supplying the generator set for a minimum of 8 h at full output. Otherwise it shall be capable of 72 h supply at full output.
Note if the c.p. requires an external power supply to be maintained at all times then the generator set should operate immediately on loss of the primary power supply, regardless of fire or standby condition.
[EN 12101-10: 2006; p. 20]
The duty to provide a fully functional emergency system to a building does not rest solely on the fire engineer. The fire engineer has the responsibility of working together with the entire professional team. All emergency systems and components as well as subsystems of the emergency system need to interact harmoniously to provide complete system functionality in an emergency, especially those systems relating to fire and the safe evacuation from the building.
Whether it be the wind loading affecting the roof ventilator operation, the cable quality supplying power to the powered extraction equipment, or the external wind load applying forces to windows and inlet air louvres. Each component needs to work together in a co-ordinated manner to ensure the safe evacuation of the building occupants and the necessary aid to the fire fighters arriving on the scene to fight the fire and prevent additional loss to the building or life. When the building is in an emergency condition, all parties involved in the design of the building are responsible for providing adequate means of escape and protection of building occupants.
