Get Grounded: Expert Advice for Protecting Small Cells in Cities

Originally Published by agl Media Group
Author:Rohit Narayan

Sensitive equipment in small cells is susceptible to damage from electrical surges in power lines. Local grounding for small cells helps to protect them to ensure public safety and protect network infrastructure.

An insatiable demand for data is driving large-scale communications network densification in cities across the world. As consumer and enterprise use of mobile broadband accelerates, cellular carriers are racing to expand wireless connectivity to provide higher speeds and greater coverage.

Many carriers are already building hyperdense 4G wireless communications networks in metropolitan areas, a trend that is sure to intensify with the launch of 5G technologies. At the same time, carriers are eager to support new services by combining computing and storage resources with connectivity at the edge of the network. Edge computing, combined with small cell and shared spectrum solutions, can accelerate the deployment of dense, localized cellular networks.

Consider that a CTIA-commissioned analysis by Accenture Strategy found small cell deployments will escalate rapidly from about 13,000 in 2017 to over 800,000 by 2026. Prior research by Accenture Strategy estimated the U.S. wireless industry will invest as much as $275 billion to deploy 5G nationwide, requiring hundreds of thousands of modern wireless antennas to accommodate a crush of data traffic.

Approaches for Grounding

While the cellular industry navigates a host of regulatory requirements and reviews to roll out new network capacity solutions in urban areas, consideration for proper powering and grounding of small cells is also part of the challenge.

nVent Erico has engineered grounding, bonding, lightning and surge protection products for over a century and has studied powering and grounding options for small cells and associated deployments.

The following information article looks at options for powering and grounding small cells and related facilities including metered and unmetered AC power, line powering using ±190 volts, distributed powering and solar photovoltaic (PV) power.

Metered and Unmetered AC Power

AC powering and grounding challenges for metered and unmetered small cells are essentially the same. The only distinction between the two is that the unmetered system eliminates the tariff monitoring element of the power supply. It is arguable whether the unmetered installation offers any measurable benefit in instances where a fixed cost per month is applied. This is because the bulk of the installation expense is in the power cable reticulation and distribution as well as the real estate cost, both of which are roughly equivalent for metered and unmetered AC power.

The obvious benefit is that AC power is the most readily available power source on city streets. Additionally, there is no remote power source to manage, no cabling required from site to site and in most cases, AC powered solutions can be rapidly deployed.

The primary disadvantage of an AC powered site is that there is no backup power in the case of an AC main failure. At the same time, sensitive equipment is more susceptible to damage from surges in power lines. There is a clear need to provide local grounding to this type of facility to both ensure public safety and protect network infrastructure.

AC power can be brought to small cells in several ways where:

  • The small cell is mounted on a power pole with low-voltage (LV) distribution ― the power can be taken straight from the LV power distribution system to an AC power board or a meter board.
  • The small cell is mounted on streetlight poles ― the power can be taken from the same source that supplies the lights (often, an LV supply from a nearby pad- or pole-mounted transformer). Here, the cable reticulation system to the small cell is nearly always separate to the cable supplying the lights, though it is possible the underground ducts will be shared.
  • The small cell is mounted on a structure with no power ― this can require a considerable amount of cable reticulation work as power cables will need to be brought in, possibly in newly installed ducts from the nearest LV power supply or transformer.

When AC power is taken directly from a pole or underground infrastructure, the exposure to lightning surges and power system transients is higher than that expected in buildings. The prospective AC fault current at these sites may also be higher than in built structures. Figure 1 shows a typical grounding and surge protection scenario for an AC powered small cell.

About DC Line Powering

In this scenario, a 48-volt DC (VDC) power system resides in a central office or other remote premises where the power is taken up to ±190 VDC resulting in a 380 VDC line-to-line voltage at the supply end. This voltage is fed via twisted pair copper wires up to the small cell. Multiple twisted wires can be used in parallel to obtain a higher current rating.

The higher DC voltage can be transmitted for longer distances without losses. At the load end close to the small cell, the voltage is converted back to 48 VDC to supply the equipment. Thus far, DC line powering has predominantly focused on digital subscriber line access multiplexer (DSLAM) network devices where the required copper wiring already exists.

The advantages of line powering include:

  • There is no reliance on local AC power and associated installation costs.
  • The DC system has a large backup battery (often 4 to 10 hours) at the central office.
  • Allows use of existing copper wire infrastructure (previously used for fixed wire phones/ADSL).
  • The power can be transmitted locally to other nearby cells.
  • Equipment quite possibly does not need a local ground electrode (surge protection is needed however, because of the large amount of copper line that is exposed to weather).

The disadvantages of line powering include:

  • There may be no existing copper wire infrastructure or where infrastructure does exist, the wire insulation and conductor may be in poor condition.
  • There may be inadequate or missing records of cable pairs because of a declining emphasis on telephone line copper wire maintenance.
  • There may be limits on the amount of power being transmitted; however, this can be overcome by using multiple wire pairs (in general, small cells have relatively low powering requirements).
  • There may be industry concerns about 380 VDC present in phone wires. Typically, these concerns are mitigated by safety features built into the line powering equipment (using a ground fault interrupter circuit, for example).
  • Because of line resistance, attention must also be paid to power transfer curves, wherein there is a need to balance increases in current against voltage levels to optimize power transfer. Figure 2 shows a traditional line powering topology.

About Distributed Powering

In a white paper titled “Small cell siting challenges and recommendations,” 5G Americas and Small Cell Forum outline distributed cluster powering arrangements via hybrid fiber coax (HFC) as follows:

“In very dense environments, HFC can be used to deliver power and connectivity from a central location to a cluster of neighboring small cells. A suitable centralized location could be anywhere that has access to power and the optical network, such as an outdoor distribution cabinet, telecom closet or macro base station location.

“This approach takes advantage of advancements in DC power delivery. Such improvements have increased the efficiency of DC-DC conversion to more than 95 percent and enabled the use of higher voltage levels to transport more power over long distances more efficiently.

“HFC makes it possible to power and connect dozens of small cell locations — spaced 200 meters apart — from a single location with local grid power and room for power backup. This also cuts out the time and cost of a utility drop.”

Further advantages of distributed cluster powering via HFC include:

  • There is no need to connect each small cell to AC power.
  • There is some battery backup capacity in the outdoor distribution cabinet (typically a few hours).
  • There is low exposure to power surges and transients if surge protection is appropriately chosen for the outdoor distribution cabinet.

The fundamental disadvantage of this powering method is that it is limited to small areas with highly dense networks. A standard grounding and surge protection arrangement for such sites is shown in Figure 3.

About Solar Powering

Power can also be provided by a local solar PV array having regulated voltages and a backup battery. This method may be employed in remote areas, in temporary installations and in dense city locations where the previously mentioned powering solutions cannot be used for various reasons.

The greatest advantage of a solar PV system is that it can be readily deployed with no need for electrical connection to any reticulated power supply.

The main disadvantages are that solar panels require cleaning; also, charging and storage can be unpredictable depending on daylight conditions. For carriers, the presence of a battery backup system also means an added level of management. Maintaining hundreds of batteries along city streets where the temperature environment cannot be controlled can be costly and difficult. Figure 4 shows a conventional solar powered small cell grounding configuration.

Data Without Disruption

Densification will allow more citizens to stream data at incredible speeds, enable cities to unlock intelligent possibilities and lay the foundation for future connected services that can transform urban centers. Grounding and surge protection will be critical to protecting small cells and ensuring network reliability.

Only there is more at stake than faster downloads. The promise of dependable wireless service extends to economic development spurred by network expansion. Accenture Strategy’s recent analysis estimates the wireless industry’s investment in deploying 5G could create 350,000 new construction jobs and a total of approximately 850,000 jobs in the United States, and the broader economic benefits from 5G could create an additional 2.2 million jobs in communities across the country.

By protecting sensitive equipment from damaging voltages and surges, cellular carriers can grow their networks to better serve millions of consumers with fewer disruptions to service.

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