Central Battery System Design
When it has been decided that a central battery system is the most suitable system of emergency lighting for a particular site, the designer needs to give consideration to the following:
• Lighting design
• Type of system
• System control & mode of operation
• Battery type
• System sizing
• Battery room ventilation

This section of the website provides a guide to how to choose the most suitable type of central battery system and then how to ensure it will meet the installation requirements. Technical assistance is available to help you with selecting and designing a system correctly. Contact the Cooper Lighting and Security Central Systems Technical Sales department, Tel: 0113 3853501/2 or E-mail: central.systems@cooper-ls.com
LIGHTING DESIGN CONSIDERATIONS
Current legislation and design increases the attraction of using central battery systems to provide emergency lighting in a building. In particular, an increase in the use of static inverter systems, which provide an alternative source of power to normal mains luminaires. These considerations can be summarised as follows:
1. BS 5266 part 7 (EN1838) specifies increased emergency light levels than previous standards
2. Slave luminaires, operating from AC/DC and AC/AC central systems, offer a higher light output and improved spacing characteristics over comparable self-contained versions of the same luminaire
3. Compact fluorescent lamps make ideal slave luminaires, offering high efficiency and appropriate light output for areas with low ceilings
4. There is an increasing requirement from architects and users to make emergency lighting as unobtrusive as possible, so utilisation of the normal mains luminaires is an ideal solution
Through the use of dedicated slave luminaires and conversion modules for mains fluorescent luminaires, these considerations can be catered for by both AC/DC and AC/AC central systems. An illustration of the increased output that can be expected from 8W slave luminaires compared to self-contained versions is shown in figure 1.

When performing photometric calculations for converted mains luminaires with static inverter systems, the full design lumen output of the luminaire must be taken into account, as the lamps are powered by conventional ballasts. It is important to ensure that the use of such high output luminaires in low ceiling areas does not exceed the uniformity factor limitations. The utilisation factor should be taken at zero reflectance in line with BS5266 Pts. 1 and 7 1999.
Fig. 1 Light output of different types of luminaire (nominal lamp lumens based on standard 8 Watt fitting)
TYPE OF SYSTEM
There are numerous different combinations of central battery system type and the correct choice depends as much on customer preference as on design criteria. The selection chart below gives some general guidance. Should you wish to discuss a proposed system type for a particular application, our technical department is available to provide assistance. Contact the Central System Technical Sales department, Tel: 0113 3853501/2 or E-mail: central.systems@cooper-ls.com.
NOTES
1. Conversion modules are designed to be incorporated into a conventional mains luminaire. During normal conditions the luminaire operates at full brightness (using the normal switched mains supply and conventional control gear). In emergency conditions the luminaire continues to operate at reduced brightness (with the emergency lamp being powered from the conversion module instead of the conventional control gear). Conversion modules are ideal for use with mains luminaires which have louvres with a sharp cut off angle, or for projects where the mains luminaires have multiple tubes, but only one tube is required to be illuminated during emergency conditions.
2. Static inverters provide mains voltage output during both normal and emergency conditions. They are designed to run conventional mains fittings at full brightness even in emergency conditions. Static inverters are ideal for projects with large open areas, or hazardous areas requiring higher than normal emergency lighting levels, or for powering compact fluorescent luminaires where there is often insufficient space within the fitting to accommodate a conversion module.
3. Static inverter systems operate the emergency luminaires at full brightness throughout the emergency autonomy period, which usually results in significantly improved luminaire spacing for mains slave luminaires compared with an equivalent low voltage AC/DC unit. In addition, the combination of higher supply voltage and the resultant reduced input current reduces installation costs by allowing the use of smaller distribution cables than would be required with a lower voltage AC/DC system.

Please note - BSEN60598-2-22 prohibits the use of glow starters in fluorescent luminaires used for emergency lighting.
SYSTEM CONTROL & MODE OF OPERATION
It is a requirement of any well designed emergency lighting system that the emergency lighting is activated both in the event of complete mains failure, and also in the event of a local mains failure. The emergency lighting system can have luminaires that are maintained or non-maintained. Similarly, the central battery unit can also be maintained or non-maintained operation. The following diagrams explain how activation of the emergency lighting is achieved, using the main types of central battery systems.

Central systems with dedicated slave luminaires
a) Non-maintained central battery unit with sub-circuit monitors.
With this method, relays are used to monitor the normal lighting supplies. The contacts of these relays are wired in a series loop such that in the event of failure of any of the normal lighting supplies, the loop is broken, sending a signal to the central battery unit to activate all of the emergency luminaires. Details of purpose-made remote sub-circuit monitor units can be found in the Loadstar product section.
Normal mains healthy condition Failure of normal lighting final circuit Total mains failure
b) Maintained central battery unit with the maintained circuit continuously energised. A simple installation where emergency luminaires are illuminated at all material times irrespective of the status of the normal lighting. In the event of a complete mains failure, the slave luminaires are illuminated from the battery supply.
Normal mains healthy condition Failure of normal lighting final circuit Total mains failure
c) Maintained central battery unit with remote hold off relays.
The maintained output from the battery unit is fed to a number of remote hold off relays throughout the building. The coil of the hold off relay is connected to the unswitched side of the local normal lighting supply. Assuming this supply is healthy, the relay will pull in, opening the contacts and preventing power from reaching the slave luminaires.

In the event of a local mains failure, the relay drops out, the contacts close and the emergency luminaires in that particular area are illuminated from the maintained circuit of the battery unit. In the event of a complete mains failure, the system operates in a similar manner, except that the slave luminaires are illuminated from the battery supply. Details of purpose-made remote hold off relays can be found in the Loadstar product section.
Normal mains healthy condition Failure of normal lighting final circuit Total mains failure
d) Maintained AC/DC central battery with conversion luminaires
With this option, the normal mains luminaires are fitted with a conversion module, enabling them to also operate as emergency luminaires in the event of mains failure. Each conversion module includes a changeover relay which, under normal circumstances, is energised by a permanent supply from the unswitched side of the normal lighting circuit.

Whilst energised, it connects the lamp to the conventional mains control gear within the luminaire allowing it to operate as a standard mains fitting, powered via a switched live connection to the mains ballast. Should the normal lighting fail, the relay within the conversion module drops out, disconnecting the lamp from the conventional control gear and connecting it to the inverter within the conversion module. This illuminates the lamp at reduced brightness. In multi-lamp luminaires, the conversion module only operates a single lamp in the emergency mode. All other lamps will extinguish upon mains failure.
Normal mains healthy condition Failure of normal lighting final circuit Total mains failure

OR
e) Static inverter unit with conventional mains fittings
A static inverter runs conventional mains luminaires at full brightness during both mains healthy and mains failure conditions. However, there is usually a requirement for local switching of the luminaires during mains healthy conditions, with automatic illumination in the event of mains failure.

Local switching with automatic illumination in the event of mains failure can be easily achieved by use of the Menvier ACM1 module, which is purpose-designed for this application.
Normal mains healthy condition Failure of normal lighting final circuit Total mains failure

OR
BATTERY TYPE
Cooper Lighting and Security offer a choice of five different battery types:

• Valve regulated lead acid (10 year design life)
• Valve regulated lead acid (3-5 year design life)
• Vented nickel-cadmium
• High performance plante lead acid
• Flat plate lead acid

Each battery type has specific characteristics. In order to assist with the choice of battery, full details of the characteristics and benefits can be found in the Loadstar and Static Inverter System product pages. The table below (fig. 2) provides a comparative guide to these characteristics

The most popular battery type is valve regulated lead acid with a 10 year design life. This type of battery is used on approximately 90% of projects due to its competitive cost, good life characteristics, ease of maintenance and compact size.
Fig. 2 Comparison of Battery Characteristics
Characteristics Valve Regulated Lead Acid (10 year life) Valve Regulated Lead Acid (3-5 year life) Vented Nickel Cadmium High Performance Plante Lead Acid Flat plate Lead Acid
Expected Life
Capital Cost
Maintenance
Resistance to damage & abuse
Through Life costs		 ***
****
*****
*
****		 *
*****
*****
*
***		 *****
*
**
***
**		 *****
*
**
*
**		 ***
***
**
*
**
BATTERY ROOM VENTILATION
Vented batteries, such as nickel cadmium, plante and flat plate lead acid emit potentially explosive gases under charge conditions. Therefore it is important when selecting rooms for emergency lighting central battery systems with these types of battery, to calculate the amount of ventilation required. The required number of air changes per hour (A) is given by the following formula:

A = 0.045 x N x I
V

Where:
N = Number of cells in the battery
V = Volume of room in cubic metres
I = Charge rate in Amperes

This formula will give the number of air changes per hour required during boost charge conditions. On float charge (systems are on float charge for most of their service life), the amount of gas emitted is approximately 1.5% of that liberated whilst on boost charge and under most circumstances this will be dissipated by natural ventilation, and will not present a hazard. However, we recommend that the boost charge condition is allowed for at the design stage to ensure the appropriate decision on ventilation requirements is made.

Although Valve Regulated Lead-Acid Batteries require little ventilation under normal operating conditions, it is good practice to apply the formula to calculate the number of air changes required to achieve minimum risk under battery fault or failure conditions.
SYSTEM SIZING
When sizing the system, it is important to allow for the full input requirement of the light fittings rather than the lamp wattages.

AC/DC systems
When using conversion modules fitted to conventional mains fittings, the lamp will be illuminated directly from the mains ballast during normal mains healthy operation and via the inverter during emergency conditions. When being driven from the battery unit via the conversion module, the emergency lamp will be illuminated at less than full output, and as a result, the fitting will consume a reduced input power.

AC/AC systems
When utilising a static inverter system, the fitting operates at full output during both mains healthy and mains failure conditions. When sizing a suitable static inverter to power a particular load, it is important to consider the input VA and the input (not lamp) wattage of the emergency luminaires. The total VA requirement defines the inverter module size, and the total input wattage defines the battery size.

Therefore, to establish the correct inverter module size, the power factor correction (PFC) rating of the luminaires must be considered in addition to lamp wattage and control gear losses. High frequency control gear circuits have excellent PFC ratings, usually of around 0.96 to 0.98. This compares with 0.85 to 0.9 for equivalent lamp magnetic control gear circuits. Care should be taken when low wattage compact fluorescent lamps are used, utilising high frequency gear or high PFC versions where possible. Low power factor versions can have PFC ratings of only 0.45 to 0.5, thereby greatly increasing the inverter rating required for the system. If utilising low voltage lighting powered via step-down transformers, it is essential to allow for the efficiency and power factor of the step-down transformers.

Table (fig. 3) and graph (fig. 4) illustrate the relationship between wattage and VA rating for a typical system.
Fig. 3 Typical System. VA rating with and without power factor correction
Qty of Luminaires Description Total Circuit Watts VA Rating (Compact lamps without PFC) VA Rating (Compact lamps with PFC)
25
40
15
15
5		 1x58W T8 (wire wound ballasts)
1x28W 2D (wire wound ballasts)
1x16W 2D (wire wound ballasts)
1x13W TC-D (wire wound ballasts)
1x40W GLS incandescent

Inverter Rating =		 1725
1360
315
270
200

 3870		 1925
2960
690
600
200

 4375		 1925
1560
375
315
200

 6375
Note: Use of compact fluorescent luminaires with power factor correction (PFC) leads to a reduced inverter module size and therefore savings in space and capital costs
Fig. 4 Typical System. VA rating with and without power factor correction
ADDITIONAL CONSIDERATIONS
Spare capacity
With any central battery system it is important to bear in mind that it is difficult to extend the system at a later date unless capacity has been allowed for at the design stage. For this reason, we would strongly recommend that some spare capacity is included when selecting the central battery system rating. Our technical department is available to provide assistance. Contact the Central System team, Tel: 0113 3853501/2 or E-mail: central.systems@cooper-ls.com.

Fire protection of cables
Cables should be routed through areas of low fire risk.
The following cables and wiring systems should be used.
a) Cables with inherently high resistance to attack by fire
i) Mineral-insulated copper-sheathed cable in accordance with BS6207: Part 1
ii) Cable in accordance with BS6387. The cable should be at least category B

b) Wiring systems requiring additional fire protection.
i) PVC-insulated cables in accordance with BS6004 in rigid conduits
ii) PVC-insulated cables in accordance with BS6004 in steel conduit
iii) PVC-insulated and sheathed steel wire armoured cable in accordance with BS6346 or BS5467

Systems should be installed in accordance with IEE Regulations and BS5266. Additional fire protection may apply. For example, if cables are buried in the structure of the building.
Cable sizes
When selecting cable sizes, due regard should be paid to limitations imposed by voltage drop and physical strength. Each conductor shall be of copper, having a nominal cross-sectional area of not less than 1mm2. BS 5266 states that the voltage drop in cables connecting a central battery to a slave luminaire should not exceed 4% of the system nominal voltage at maximum rated current.

Using copper conductors, volts drop can be calculated per pair of conductors as shown in table fig. 5.
Total volts drop on a circuit can be calculated according to the formula:

VDT = I x VDM x D

Where:
VDT = volts drop total
I = maximum load current
VDM = volts drop per amp per metre (obtained from fig. 5)
D = cable run in metres
Fig. 5
Nominal Cross Sectional Area Maximum Current Rating Volt per Drop per Metre
1.0mm²
1.5mm²
2.5mm²
4.0mm²
6.0mm²
10.0mm²
16.0mm²		 14 amps
17 amps
24 amps
32 amps
41 amps
55 amps
74 amps		 42mV
28mV
17mV
11mV
7.1mV
4.2mV
2.7mV
The problems of volt drop can be overcome by:

• Using higher system voltages (= lower currents and therefore lower volt drop)
• Using larger cables (= lower resistance and therefore lower volt drop)
• Using multiple outgoing circuits (= less current per circuit and therefore lower volt drop)
Example:
Fig. 6 & 7 show an example comparison for a central battery system with a total connected load of 1500W and a 50m run of 16mm² cable supplying the luminaires.

This example shows that for this configuration, a 230V system would be most suitable to meet the requirements of BS 5266.
Fig. 6
Comparison Data 24V System 50V System 110V System 230V System
Max. permissible Volt drop (BS5266)
Total current for total connected load of 1500W
Actual volt drop for 16mm cable with 50m² length		 0.96V
62.5A
8.43V		 2.0V
30A
4.05V		 4.4V
13.6A
1.84V		 9.2V
6.52A
0.88V
The use of larger cables or multiple outgoing circuits may permit the use of 24, 50 or 110V systems in the above example.
Fig. 7