The next generation of millimeter-wave (mmW) 5G wireless communications technology deployment, will spur the use of short-range, small cell structures, mostly in the form of integrated street poles, in urban areas and cities.
These structures, often referred to as “smart” or “small cell” poles, usually comprise pole assemblies densely populated with electronic systems. The small cell sites can be built on existing or new metallic street lighting poles, either partially concealed or fully concealed, and on existing wooden utility poles. These electronic systems typically include:
In more sophisticated instances, these smart poles will also integrate smart city hubs containing sensors, such as high-resolution concealed cameras, gunshot detection microphones and atmospheric sensors for calculating the ultraviolet (UV) index and measuring solar brightness and solar radiation. In addition, the poles may accommodate additional structural subassemblies, such as support arms for LED street lighting, conventional sidewalk luminaries and receptacles for electric vehicle charging.
A centralized equipotential bonding system is usually provided within the pole via strategically positioned grounding bars, to which the different radio systems are connected. Typically, the neutral conductor of the incoming utility power supply also is bonded to ground at the energy meter’s socket, which in turn is bonded back to the main grounding bar. The pole’s external system ground is then bonded to this main grounding bar.
The simple light pole seen along sidewalks and city pavements is changing and will soon become a pivotal component of the new 5G wireless infrastructure. These systems will have paramount importance because they support the new technological layer of cellular networks for high-speed services. No longer will such pole structures simply accommodate incandescent light fixtures. Instead, they will become the core of a highly sophisticated technology. With this advance in integration, capability and reliance comes inevitable risk. Even with their relatively low heights compared to macro cell sites, such sophisticated electronic subsystems are set to become exponentially more susceptible to damage from overvoltage surges and transients.
The importance of these small cells in the 5G infrastructure cannot be underestimated. Far from just being used to fill gaps in radio coverage and increase capacity, in 5G networks small cells will become the radio access network’s primary nodes, providing high-speed services in real time. These technologically advanced systems may well provide critical gigabit service links to customers where outages cannot be tolerated. This necessitates the use of highly reliable surge protection devices (SPDs) to maintain the availability of these sites.
The source of such overvoltage risks can broadly be categorized into two forms: those caused by radiated atmospheric disturbances and those caused by conducted electrical disturbances.
Let us consider each in turn:
Overvoltage protection (OVP)
Standards such as IEC 61643 describe the use of surge protective devices to mitigate the effects of such overvoltages. SPDs are classified by test class for the electrical environment within which they are intended to operate. For example, a Class I SPD is one that has been tested to withstand — using IEC terminology — “a direct or partial direct lightning discharge.” This means that the SPD has been tested to withstand the energy and waveform associated with the discharge most likely to enter a structure in an exposed location.
As we consider the deployment of small cell infrastructure, it is clear that the structures will be exposed. Many such poles are expected to appear along residential curbsides and pavements of metropolitan cities. It is also expected that such poles will proliferate in communal gathering places, such as indoor and outdoor sports stadiums, shopping centers and concert venues. Thus, it is important that the SPDs selected to protect the primary service entrance utility feed are suitably rated for this electrical environment and meet Class I testing, i.e., that they can withstand the energy associated with direct, or partially direct, lightning discharges. It is also recommended that the SPD selected have an impulse withstand level (Iimp) of 12.5 kA in order to safely withstand the threat level of such locations.
Selection of an SPD capable of withstanding the associated threat level is not in itself enough to ensure the equipment is afforded adequate protection. The SPD must also limit the incident conducted surge to a voltage protection level (Up) lower than the withstand level (Uw) of the electronic equipment within the pole. IEC recommends that Up< 0.8 Uw.
Raycap’s patented Strikesorb SPD technology is purposefully designed to provide the required Iimp and Up ratings to protect sensitive mission critical electronic equipment found in small cell infrastructures. Strikesorb technology is considered to be maintenance-free and can withstand thousands of repetitive surge events without failure or degradation. It provides a highly safe and reliable solution that eliminates the use of materials that could burn, smoke or explode. Based on years of field performance, Strikesorb’s expected lifetime is more than 20 years, and all modules are supplied with a 10-year limited lifetime warranty.
The products are tested according to international safety standards (UL and IEC) and offer unparalleled performance against lightning and power surges. Furthermore, Strikesorb protection is integrated into a compact AC distribution enclosure suitable to being installed within the small cell poles. This provides overcurrent protection to the incoming AC service and outgoing distribution circuits, thereby providing a convenient point at which the utility service from the electric meter can enter and distribute within the pole. These AC distribution enclosures are designed to meet the requirements of the National Electrical Code (NFPA 70) in order to be classified “suitable for use as service equipment” (SUSE) and are listed under UL 67.
This article ran in the June 2019 issue of AGL Magazine.