An Exploration of Emergency Lighting Slave Units

The term slave unit is one that baffles even experienced fire safety experts. The following article is designed to demystify this term and explain the related emergency lighting systems containing a central battery for power supply.

At Safelincs, we listen to our customers and constantly strive to provide the resources you need to make responsible fire safety decisions. As we sell numerous slave emergency lights, we have, over the years, received many questions asked about emergency lighting slave units and systems. Regular enquiries such as "can slave units be switchable?", "are your slave units compatible with my system" and "how easy is the system to maintain?" have led us to compile this article to answer these questions, and more.

The Basics

We need to first understand how slave units differ from the more commonly encountered self-contained types of emergency lights. The key difference is the location of the battery providing the power needed to illuminate the device in the event of a mains power failure.

Self-contained devices are, as the term implies, capable of functioning as an independent entity. They include their own backup battery within the fitting, and constantly monitor (and are wired directly in to) the mains power of the building. By contrast, slave units are linked to a central battery bank which acts as the backup power source for all connected emergency light fittings in the event of power loss. The name 'slave' was presumably chosen to express the fact, that individual slave units have no input whether the emergency light comes on or not. That 'decision' is made through a central monitoring panel.

The Central Battery System

Whilst designs vary, most central battery systems comprise of the same basic features. They contain backup batteries cells, transformers and a charging system, as well as a control panel which monitors the mains power of the building.

Battery types depend on the energy storage capacity needed for the installation, but regardless of the composition of the battery, they all serve the same purpose. Most battery compartments require ventilation and temperature control.

The control panel ordinarily features a display giving real-time information about the system's status, and will allow for a wide range of testing functions appropriate to the scale and design of the system.

Central battery systems should be manufactured to EN 50171, and a further sign of quality and conformance to the relevant standards is accreditation by the Industry Committee for Emergency Lighting (ICEL). If looking to purchase a central battery system, look for products from manufacturers registered under the ICEL 1009 scheme.

Triggering Slave Luminaires

Much like self-contained emergency lighting luminaires, a central battery system constantly monitors the presence of regular mains power within the building. This can either be set on a building-wide level (usually for smaller premises), or the power can be monitored for individual zones in more extensive systems. When the regular power supply fails, the central battery system engages and illuminates all of the connected slave luminaires.

Where Slave Luminaires Can Be Found

Slave lighting systems are mainly used within large scale premises, or within buildings where environmental factors would adversely affect the performance of backup batteries sited in certain areas. For example, railway stations, power generation facilities and large scale factories will more often than not make use of slave luminaires rather than self-contained types.

A common cause of shortened battery life in self-contained emergency lighting luminaires can be temperature fluctuation. By locating a luminaire's backup battery remotely, it is possible to install slave luminaires in areas with varying temperatures, making them ideal for industrial applications. This ability to withstand extreme temperature fluctuation is unique to slave luminaires.

Central battery systems are particularly suited to larger premises as they provide a single location for maintenance and battery testing. Central battery systems in large buildings, such as hospitals can therefore reduce the cost for battery testing and battery replacement.

Strengths and Weaknesses

Central battery systems, due to the initial investment costs, are not suitable for every building. The tables below condenses some of the pros and cons of slave luminaires compared to self-contained emergency lights:

With all of this in mind, let's tackle our most commonly encountered questions about slave units.

Are the Safelincs slave units compatible with my central battery system?

In the majority of cases the answer would be yes, but it is crucial to be mindful of the variables. Safelincs' Orbik slave luminaires are available in 24V, 50V or 110V DC versions, and care must be taken to ensure that any replacement units you purchase match the output of your central battery bank.

Our UK-manufactured slave units produced by Orbik are designed to be compatible with central control systems which are kitemarked to BS EN 50171. All equipment should be installed in line with the requirements of BS 5266 by a qualified person.

Assuming that your system was installed in line with regulations in the first instance, it should be possible to simply replace any faulty slave lights with a new fitting from our range of slave units.

What do the different voltages mean?

The slave units we sell are available in either 24V, 50V and 110V DC versions. The reason for this range of options is to do with the power distribution requirements dictated by the overall scale of the installation. Smaller installations require a less powerful output as the battery load is distributed to fewer devices, and usually over smaller distances. Larger premises will require a greater number of luminaires, and these can often be located far from the central battery bank. To overcome potential voltage drop-off over large distances, a higher voltage output is required. In these cases it is necessary to install luminaires designed to accept the higher voltage.

How easy is the system to maintain?

This is one area where central battery systems and slave lights really excel. In comparison, self-contained emergency lights require maintenance on an individual basis which can be time consuming as each luminaire's backup batteries must be individually tested for endurance (usually for a period of three hours) and bulbs must be replaced as necessary if they show signs of aging.

Slave units, on the other hand, require less maintenance. Whilst the bulbs in slave units must also be monitored and refreshed when needed, the test of the backup power system only requires a visual inspection of the battery system in one location. In addition, the expected lifespan of a central battery bank can be more than fifteen years! This far exceeds the lifespan offered by batteries found within self-contained emergency lights. As a result, a system of centrally powered emergency lighting slave units requires less battery maintenance than an equivalent network of self-contained fittings.

Each system will have its own guidance regarding what exactly to do during inspection, but in general terms, central batteries should be inspected and cleaned on a regular basis. It is also important to maintain electrolyte levels within the battery cells where appropriate in line with the manufacturer's guidance.

Can slave units be switchable?

The simple answer is potentially yes with special provisions in place, but this isn't a common feature.

The term 'switchable' warrants explanation, as it is not something that everyone is familiar with. Self-contained emergency lighting can usually (depending on the model being installed) be installed in such a way that the luminaire can be turned on and off by a standard light switch without compromising the functionality of the light in an emergency. Regardless of whether the light is switched on or off prior to a mains power failure, the backup batteries kick in when needed and illuminates the fitting. This functionality is referred to in the industry as a light being 'switchable'.

The nature of slave unit systems and the way they are wired prevents this function from being straightforward. Whilst it is possible to wire many self-contained emergency lights to be switchable as mentioned, slave units bypass local lighting controls, and are controlled from a central location. This means that additional circuitry is needed in order to facilitate this function.

If you are looking to replace an emergency slave light which is currently switchable, please contact us via support@safelincs.co.uk for further advice.

Why aren't slave units available in a choice of maintained or non-maintained?

Whether a centrally powered emergency lighting system is maintained or non-maintained is dictated by the setup in the central battery bank. As such, slave units when purchased are neither maintained nor non-maintained in themselves, but can be used as either depending on the system they are intended for. Some central battery banks allow a dual output enabling separate maintained and non-maintained loops to be used. If you are looking to replace a faulty slave light then the variable to check is the voltage of your system, not whether the unit is maintained or non-maintained.

Are slave units available with LED fittings?

In short, yes. LED (light emitting diode) fittings are becoming increasingly popular due to their reduced energy consumption. This feature is available for slave lighting units, with a large selection of Orbik lights being available for manufacture to order as LED slave units, such as the Arc, the Calabor and the Pico designs. If you would like a quotation for LED slave lighting units, feel free to contact us via support@safelincs.co.uk.

If you have a question that is not covered above, or would like further information, please feel free to contact our friendly and helpful customer service team via support@safelincs.co.uk.

(doc:530 V1.0). Our articles are reviewed regularly. However, any changes made to standards or legislation following the review date will not have been considered. Please note that we provide abridged, easy-to-understand guidance. To make detailed decisions about your fire safety provisions, you might require further advice or need to consult the full standards and legislation.