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Power networks

In our growing industry there are various security devices and device power options available, writes Tim Scott, of Dantech Electronic Engineering.

Security system components may require a supply of direct mains, specified DC or AC low voltage, or standard compliant power over ethernet (PoE). Often systems will use a mixture of device supply voltages due to the requirements of individual components. Power distribution decisions should however be based on the consideration of key factors such as reliability, continuity, accessibility, compliance, physical security and cost; rather than simply on individual product requirements.

System components
Helpfully, many system devices are capable of functioning with more than one type of supply. For example a surveillance camera might accept a specified low voltage AC or DC input as well as PoE power. Voltage converters and inverters are helpful options; for example allowing two devices with dissimilar input specifications to be supplied from a single battery maintained power unit where necessary, albeit at some efficiency cost. These features and device options aid design flexibility.

Electronic security systems have always been networks, even before the advent of IP technology. They consist of pathways carrying information between system components and pathways distributing power to them. With modern IP networked systems it remains the case that these two types of interconnecting path can be located separately, follow a similar route or be physically combined; depending on the system design and component technology used.

All components within the security system should be viewed as critical components, with a robust strategy used to protect their operation. This differs from IT systems that may include a mixture of critical and non-critical device types. Whatever your physical implementation, the reliable connection of power and data are fundamental requirements for the system to function. For the system designer, separate visualisation of the power network from the communication network is helpful at the planning stage, giving clarity and allowing both to be defined on their own merits.

Network design
System power can be planned much like a simple data network, with the incoming mains point branching out, via power supplies and power storage devices, to the various system components. The pathways between each device will consist of existing or new wire conductors, carrying either mains or a lower voltage to each system device location. A good design will include the minimising of power transmission bottlenecks, the reduction of single points of failure and the reduction of consecutive points of potential failure within each path.

Mains backup generators and UPS systems of various capacities may be pre-existing at a location or specified for installation. These can be helpful but should be given some scrutiny to judge their acceptability, including consideration of the effect of a unit failure. Marketing terms like ‘computer grade power’ are not technically meaningful and are not a replacement for accurate device technical data. Failure of continuous conversion type UPS units will result in system power loss regardless of mains state and they are relatively inefficient due to first down-converting to a storage battery and then up-converting and inverting their output, all of the time. Changeover type UPS units and standby generators cause a momentary break in output power upon mains failure, or carry the possibility of a stepped change in output voltage; either of which causing possible system interruption.

IP security devices such as surveillance cameras tend to ‘reboot’ following a power interruption for a period that might range from 30 to 90 seconds, dependent on type. The use of a dedicated maintained power supply local to load devices, either of secure PoE midspan or conventional type, ensures seamless operation. The prevention of device reboot downtime makes a powerful case for the use of battery maintained power supplies in all security applications. So too does the ability to continuously provide functions like edge storage recording, door entry services and call-point access, regardless of mains state, and potentially regardless of network state.

Just as it is not physically secure to use power supplying equipment that can be unplugged during operation, consideration should also be given to the risk of remote disconnection. While individual system devices may have remote control interfaces, which carry with them their own security implications, it is not recommended that power supplies with remote activation function are specified. The risk of accidental or deliberate interference outweighs potential benefits, which can instead already be achieved through load device interfaces. The added risks and setup complexity of power supplies with software management seem inappropriate for security applications under normal circumstances.

Secure power
It is more reliable to use power supplies that are separate from network infrastructure, for many reasons. Separation of power provision removes single points of failure in the system, reducing both the likelihood and the magnitude of system failures. Good dedicated power units normally have higher life expectancies than network switching equipment. All power supplies have a conversion efficiency loss which is shed in the form of heat; potentially reducing the reliability and lifespan of other nearby equipment.

Network devices with integrated power outputs are subject to higher component counts as well as dissipated heat from power conversion, having shorter life spans than those without integrated power. Some such devices have a ‘power budget’, limiting the defined output power available to a specified total figure, rather than providing full capability to all outputs. This reduces cost, size, power and therefore heat generation of the integrated supply, but allows the possibility of load devices being connected with insufficient power available. With some supply products of this type it is possible to overload the integrated power supply, resulting in total shutdown of all outputs until the overload is removed. As some common load devices have variable power consumption during operation, for example PTZ cameras and camera housings, it is a risk that such systems will appear to function normally until loads demand higher power levels, leading to failure. While convenient, the use of integrated power outputs for critical operations is questionable. The specification of separate power supply units designed for continuous operation will improve system reliability. Using multiple edge located power supplies, each with limited numbers of attached load devices, will further reduce risk by minimising the percentage of a system that is lost during any single PSU failure.

Power delivery
Given sensibly specified power supplying equipment, bottlenecks in power distribution paths are a function of voltage supplied, power demanded and wire conductor size used. These bottlenecks physically manifest as resistive ‘volt drop’, frequently discussed and encountered by installers. Conductors have an inherent resistance at a given temperature and the degree of volt drop is inversely proportional to the voltage supplied. Ohm’s law dictates that for any known conductor and power conducted a doubling of supply voltage will half the current; hence also halving the voltage drop. Importantly, the resulting transmission power loss is actually quartered as a result of doubling the voltage; due to the halving of the current conducted and the resultant halving of the volt drop. This shows that for efficiency it is preferable to use available higher voltages until within the area of load devices, which commonly means using mains power. At the edge load device area, a secure conventional power supply or secure PoE midspan can be sited, allowing installer access and including battery backup if required. Using wire of adequate specification is important throughout to ensure minimised losses, as is ensuring reliable wire provenance; following recent reports of sub-standard and incorrectly badged imports.

The IEEE standard requirements of compliant PoE devices ensure that where correct wiring is used, power losses will be acceptable within a 100M span. Non-compliant PoE devices of higher power levels or of different technical function are an unknown and a potential hazard. Thorough investigation is necessary and advice should be sought to determine whether such devices should be used and whether they have special installation requirements such as the use of cable with larger wire conductors.

Operational standards
To reduce points of potential failure in a power path it is necessary to minimise the number of power conversion stages used. Supplying mains voltage to the load area, (or where not possible the next highest available voltage) then using one or more low voltage power supplies keeps the path as simple as possible. This reduces the number of possible failure points. For normal practical purposes and as a standard requirement for PoE supplies, installation of the low voltage PSUs will be within 100M of the load devices. If necessary, these can be sited alongside network apparatus such as edge switches and media converters, as well as being status monitored dependent on system requirement. Power repeater products designed for consecutive placement in a power path can save up-front cost but give multiple failure points in the path. They are also an inefficient solution, having a conversion efficiency loss at every stage. Selection of these is therefore a compromise, useful when other options are impractical.

The applicable standard defining the requirements of all compliant PoE devices is IEEE802.3:2012. This incorporates the earlier PoE related standard amendments 802.3af (2003) and 802.3at (2009). The ‘af’ and ‘at’ references continue to be seen, frequently used to indicate power capability as well as standard compliance. Two device power categories exist, ‘type one’ up to 15.4W (‘af’) and ‘type two’ up to 30W (‘at’, also known as PoE+). Load power levels within these limits are automatically managed. For reliability, longevity and adaptability it is sensible to specify the newer type two 30W supplies; allowing the connection of any PoE standard compliant device. This is no less efficient due to inbuilt power management and normally ensures headroom so that power units are under-stressed. This gives the added benefit of allowing for future device changes and system expansion.

Where the option exists, the choice of conventional versus PoE power is open to debate. There is a definite need to specify reliable security equipment rather than sub-standard imports and generic IT offerings. Beyond that, both methods are valid, subject to addressing the physical security of enclosures and interconnections. Whichever method is used, the consideration of minimising losses, separating power, removing potential failure points and spreading risk between multiple edge power supply units will enable the design of far more reliable systems.


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